Quaternary History, Isostatic Rebound and Active Faulting in the Lake Lahontan asin,

1996

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Table of Contents

List of Figures ..................................................................................................................... .ii

List of contributed articles ................................................................................................... .iii

Introduction .......................................................................................................................... 1

AcknowledgITlents ....................................................................................................... , ........ 3

Field Trip Guide:

Field Trip Guide:

Field Trip Guide:

Day 1- The Jessup Embayment to the playa fringing dunes in the northeast Carson Sink ...................................................................................... 5

Day 2- Playa fringing dunes to Weber Reservoir ........................... 25

Day 3- Weber Reservoir to the Thome Bar .................................... 43

Appendices ................................................................................................................... , ..... 57

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List of Figures

1 Stratigraphy of the Lake Lahontan basin .................................................................. .2

2 Route map for Day 1 ................................................................................................ 7

3 Air photo of the head of Jessup Embayment. .......................................................... 8

4 Air photo of the barriers near the mouth of Jessup Embayment.. ......................... .l4

5 Air photo of the Grimes Canyon area ..................................................................... 21

6A Route map for Day 2 (northern half) ...................................................................... 26

6B Route map for Day 2 (southern half) ...................................................................... 27

7 Air photo of the Cox Benchmark site ..................................................................... 28

8 Contour plot of the Lahontan basin showing deformation since

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the Sehoo highstand ................................................................................................ 31

Bathymetric plot of the Lahontan basin showing approximate depth of water during the Sehoo highstand ........................................................................... 32

Map of the Lahontan basin showing the distribution of active faults ..................... .3 3

Route map for Day 3 .............................................................................................. 44

Air photo of the Weber Dam and Walker River Narrows area ............................. .46

Map of Lake Lahontan showing net shore drift directions during the Sehoo highstand ................................................................................................ 48

Geologic map of the Thome Bar ............................................................................ 51

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List of Contributed Articles

Appendix 1: Adams, K.D., and Wesnousky, S.O., 1996a, Shoreline Processes and the age and elevation of the Lake Lahontan highstand in the Jessup Embayment, Nevada.

Appendix 2: Adams, K.D., and Wesnousky, S.O, 1996b, Soil development, spatial variability and the age of the highest late Pleistocene Lake Lahontan shorelines, northwestern Nevada and northeastern California.

Appendix 3: Cupp, K, 1996, Lake Lahontan geology of the Fernley Sink, Nevada: Preliminary results from lake shoreline profiles.

Appendix 4: Ritter, J.B., Coonfare, C., Miller, J.R., and Husek, J., 1996, The alluvial fan stratigraphy of Buena Vista valley, north central Nevada: Implications for a synchronous geomorphic response on alluvial fans in a semiarid climate.

Appendix 5: Lancaster, N., and Cupp, K., 1996, Eolian deposits of the Carson Sink.

Appendix 6: Harvey, A.M., and Wells, S.O., 1996, Relations between alluvial fans and Lake Lahontan shorelines: Stillwater mountain front, Nevada.

Appendix 7: Reheis, M.C., 1996, Old, very high pluvial lake levels in the Lahontan basin, Nevada: Evidence from the Walker Lake Basin.

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Introduction

Welcome one and all to this year's Pacific Cell FOP. We hope that the stops put together in this guidebook will be interesting, enjoyable and maybe just a little bit thought provoking. The research presented in the following pages is all new. None of it has been published, except in a few meeting abstracts. I can't speak for all the co-leaders on the trip, but I would appreciate feedback on the research I have undertaken. By incorporating your comments and criticisms into the ways I think about Lake Lahontan, I hope to also learn much on this trip.

The Lahontan basin covers a huge area of northwestern Nevada and adjacent northeastern California. Simply judging by the number of researchers who have worked here as well as the number of controversies that have erupted over the years, the geology and history of Lake Lahontan is not simple. However, there have been many outstanding scientists who have contributed much to our understanding of this complex lake system. And to these people, we owe a great debt.

I.C. Russell performed the seminal research in the Lahontan basin and to this day his study remains the only comprehensive work covering the entire Basin. His results were published in 1885 as U.S.G.S. Monograph XI. I have read this tome several times, and am still amazed at his insight and attention to detail. He wrote about certain aspects of the Lake's history that have not been addressed since. He also worked out the basic stratigraphic framework that was later expanded upon and formalized by Roger Morrison.

Roger Morrison has worked in this Basin longer than anyone ever has, and probably ever will. He published his landmark professional paper on the stratigraphy of Lake Lahontan in 1964 and more recently has updated the stratigraphic nomenclature for the Basin in the 1991 GSA DNAG volume "Quaternary nonglacial geology: Conterminous U.S.". These papers provide a valuable framework with which to interpret deposits in the Basin. A summary of the stratigraphic nomenclature as it exists today is presented in figure 1.

I never had the opportunity to meet Jonathan Davis but I know that a great many of you valued him as a colleague and as a friend. His work on the tephrochronology of the Lahontan basin as well as the general history has also proved invaluable to the rest of us. The time lines represented by the tephras that he so carefully characterized and catalogued are oftentimes the only hope one has for dating isolated outcrops.

Marty Mifflin and Meg Wheat have certainly made my job easier in attempting to characterize the isostatic rebound of the Lahontan basin. In the late 1960's these two spent some five years traveling around the Basin measuring the elevations of high shorelines. However, they did not have the modern surveying equipment that was my lUXury. Instead, they literally marched across the desert floor hand leveling and leapfrogging their way up to a high shoreline and then back down again to some known elevation. I applaud their efforts because even with the relative ease of a total station surveying instrument, the work was at times arduous. Unfortunately, their results on the isostatic rebound are limited to an abstract and a small figure in a guidebook. However, I have received a great deal of sage advice from Marty over the last few years and know that his experience and suggestions have made my research more efficient and better focused.

In terms of dating deposits in the Basin, no one has generated more dates than Larry Benson. His lake level curves are the standards against which every new date in the Basin must be compared. If a new date and elevation doesn't match with his curves, one had better be able to explain why. The sheer number of radiocarbon dates generated by Larry is unsurpassed in most other paleoenvironmental reconstructions.

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Allostratigraphic and Pedostratigraphic Units

Eetza Alloformation (Sediments of three deep lake cycles)

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Paiute Alloformation and Cocoon Geosol complex (Alluvium and eolian sand with many paleosols)

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To all of these people and more that I haven't mentioned, those of us who work in the Lahontan Basin owe their thanks. It is their ideas and interpretations that sometimes guide our own research endeavors. Even though I do not always agree with previous interpretations, I respect them for what they are-- someone else's hard-earned thoughts, ideas and insights. One day, if I'm fortunate, someone may come along and challenge my ideas about Lake Lahontan, at least then I'll know that they were worth something.

Acknowledgments

KDA Reno, NV September, 1996

Without the help of a great many people, this trip would have never happened. The other trip leaders have been great in their support and enthusiasm for this trip. Others who have helped me over the last few years are too numerous to mention but I thank all of them. Those who deserve special mention are Ryan Aglietti who helped measure more shorelines than I care to remember, John Caskey who served as a solid sounding board and helped reel me in when I got a little too far afield, Mark Stirling, John Oswald and Chris Willoughby, who have tolerated my sometimes incessant ramblings and challenged my sometimes too imbedded ideas, and Alan Ramelli, John Taylor, and Jim Humphrey who still appeared interested even while climbing the glittering slopes of Galena.

I thank my advisor, Steve Wesnousky, who allowed me to pursue my own interests, but somehow has kept me focused. Without his support and encouragement, this project would never have gotten off the ground.

Finally, I thank my wife, Sheryl Fontaine, whose unwavering support, never ending patience and overriding common sense have kept me on track and sailing forward with an even keel.

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Day 1

Jessup Embayment to the playa fringing dunes in the northeast Carson Sink

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Stop l-IA (8:00 -10:30 am): Park near the head of the Embayment, immediately downstream of the incised bedrock canyon along the road in the main wash. This is a walking tour of the Jessup Playette, progradational barrier complex (PBC) and lower barriers 3 and 4. Begin by walking uphill (south) toward Jessup Playette (Figure 1).

The Jessup Embayment was a small bay in the northwest Carson Sink subbasin of Lake Lahontan (Figure 1, Appendix 1). Because of the large fetch to the southeast, well­developed shorelines are found from the highstand at about l340 m down to an elevation of about 1227 m (Figure 2, Appendix 1). We have interpreted the recent pluvial history of the Jessup Embayment through detailed mapping, numerical dating, stratigraphic and sedimentologic descriptions and soil analyses (Adams and Wesnousky, 1996a, Appendix 1, this volume). The soils study has been particularly useful because the character and spatial variability of Sehoo-age soils documented here has served as a known age standard with which we could compare other highstand soils from around the Lahontan basin (Adams and Wesnousky, 1996b, Appendix 2, this volume).

Jessup Playette The first stop on this walking tour is the Jessup Playette. This small playette was formed by the emplacement of the barrier that lies along its southeast side during the highstand of the Sehoo Lake cycle. The crest of the predominately swash-aligned barrier lies at an elevation of l339.9 m and is composed of coarse beach gravel and cobbles. Figure 9 from Appendix 1 is a topographic map of the Jessup Playette showing the locations of the trench and adjacent soil pit. The areal extent of the drainage basin that was available to fill the closed depression formed by the emplacement of the barrier is about 6000 m2. It is possible that the drainage along the southwest side of the map flowed into the lagoon at times, but there is no sedimentologic evidence to support this.

We excavated the trench in hopes that there would be datable material which would constrain the age of the highstand barrier. Figure 10 from Appendix 1 is a log of the trench which shows the relationships between the different stratigraphic packages. The oldest unit exposed is poorly-sorted, angular alluvium at the base of the trench. The alluvium is overlain by well-sorted sands which show cross-bedding near the central part of the exposure. We interpret these sands to represent deposition in a back-barrier lagoon. The lagoonal sands are interfingered with the distal (landward) end of the barrier gravels which demonstrates that the sands and beach gravels are coeval.

The foresets, topsets and backsets of the barrier can clearly be seen in the exposure. This arrangement of strata reflects the formation and migration of swash-aligned barriers. As waves washed over the crest of the barrier, the onrush of water moved the gravel over the crest and down the backside of the barrier. The overall effect of overwash was to cause the landward migration of the barrier in a process called barrier rollover. Evidence for this migration can be seen near the bottom of the trench where the gravel overlies the lagoonal sand and individual sand layers ramp up into the backsets (Figure 10, Appendix 1).

Figure 3. Air photo showing the head of the Jessup Embayment and features discussed on Stop I-IA.

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The backsets dip to the northwest at about 33-34° which is near the angle of repose. Because oftheir tabular nature and asymptotic (non-erosive) basal contact with the lagoonal sands, we interpret the backsets to have been deposited in standing water in the lagoon. Water in the lagoon probably stood between 1338 and 1339 m during the highstand which is the elevation of the "hinge point" where the topsets roll over into the backsets (Figure 10, Appendix 1). Water level in the lagoon probably closely tracked lake level because of the high penneability of the coarse clastic barrier.

The youngest package of sediment in the exposure is referred to as the playette-fill deposits (Figure 10, Appendix 1). This package consists of thin alternating beds of alluvial sheet wash layers separated by fme-grained "dust" layers. The sheet wash layers generally have erosive bases and abundant cross-bedding and are interpreted to have been generated by runoff from the local hillsides of sufficient magnitude to entrain coarse sand and small pebbles. The "dust" layers are composed primarily of silt with lesser amounts of fine sand and clay (Table 1, Appendix 1) and are also interpreted to result from runoff from the surrounding hillsides, but of a lesser magnitude than the events which generated the sheet wash layers. Water moving onto the aggrading playette surface carried the fine­grained material in suspension and then, over time, the material settled out of the standing column of muddy water. In essence, this process transported the dust that had been deposited on the surrounding hillsides and concentrated it in discrete layers in the playette. Through repeated runoff events of greater and lesser magnitudes, the playette surface aggraded. A curious fact about the playette-fill sediments is that only the upper 50 cm or so contains carbonate dust. None of the layers were found to effervesce below the obvious surface soil. This might be due to changing dust sources through the Holocene or some other process which we do not yet understand.

The distal end of a radioulna and a metacarpal from a camel (Camelops hesternus) were found at the contact between the lagoonal sands and the playette-fill sediments. Collagen extracted from the radioulna yielded an AMS 14C age of 12,690 ± 60 yr B.P. (Figures 10 and 11, Appendix 1). We interpret this date to closely constrain the timing ofthe last highstand of Lake Lahontan. This age is a minimum limiting age, but we place a maximum age on the highstand of about 13.1 ka based on dating and interpretations of the progradational barrier complex, located at the head of the Embayment (Figures 8 and 12, Appendix 1).

Three soil profiles were described and analyzed across the crest of the barrier to examine the effects of topographic position on soil development in these coarse clastic landforms. Two additional profiles were described in the soil pit, making a total of five profiles from this single-age surface (Adams and Wesnousky, 1996b, Appendix 2). There is a certain amount of variability in all of these profiles in tenns of thickness, texture and other field properties (Table 1, Appendix 2). However, the profiles are more similar than they are different.

We interpret virtually all of the fines in the soil profiles to be eolian material introduced into the generally coarse clastic barrier gravel. Eolian addition of fines will be a recurring theme on this trip when examining soil development on what we interpret to be young (- 12.7 ka) highstand deposits. The ubiquitous Av horizons found on many of the surfaces of pluviallandfonns provide strong evidence that much of the soil development is a product of eolian additions. Once the fines accumulate on the surfaces of these

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landfOlIDs, they can be introduced into the profile in a number of ways. Thefine sand, silt and clay can simply fall into the interstices between the oftentimes clast-supported gravels or fines can be washed in by water. There is also evidence that plants and burrowing animals greatly influence the distribution of fines in the soil profiles. The depth of bioturbation in the barrier exposed in the trench is closely related to the depth of soil development (Figure 2, Appendix 2). The clasts at the southeast end of the trench are well-sorted, coarse (::; 25 cm) disc-shaped cobbles and appearto have limited the depth and amount of rodent burrowing. Consequently, the soil developed on this part of the barrier is relatively although the particle size distribution of the fines is similar to the other barrier profiles.

The mixing of the barrier gravels with the playette-fill sediments in the vicinity of Profile 3 (Figure 2, Appendix 2) also probably reflects bioturbation. Through time, the northwestern edge ofthe barrier gravels has migrated onto the playette surface giving the false impression that the crest ofthe barrier is wider than it actually is. Bioturbation has probably led to minor lowering of the barrier crest by redistribution of the barrier gravels toward the playette.

The Progradational Barrier Complex A progradational barrier complex (PBC) is simply a group of barrier ridges formed at about the same elevation because of a relatively stable water level and abundant sediment supply. These features are referred to as progradational because through time they build lakeward and increase the land area. Figure 6 of Appendix I defines the terminology used for shore features in this guidebook.

Along the way to the longitudinal section of the PBC at the head of the Jessup Embayment, we walked down a spit which began to form at the highstand and was lengthened as the lake began to recede (Figure 7, Appendix 1). The distal end of this spit was then truncated by waves when the water level was between 1332 and 1334 m, about 6 to 8 m below the highstand. A spit was then built upstream from the PBC, but at about the same elevation as the PBC surface ridges. Through continued longshore drift from the south to the northeast, the surface ridges of the PBC were built across the head of the Embayment (Figure 7, Appendix 1). The cross-cutting relationship evidenced by the truncation of the highstand spit demonstrates that the surface ridges of the PBC postdate the highstand.

An intermittent stream has eroded the backside of the PBC exposing a beautiful section of beach gravels which is shown in figure 8 of Appendix 1. This section displays a record of alternating shore drift directions and different energy regimes within the surf zone. Note that there is a tendency for the discrete packages of gravel and cobbles to be size-sorted as well as shape-sorted. These two characteristics are common in coarse clastic beach systems, but are not well understood.

Three discrete layers within the section were sampled for tephra. The lowest layer is near the base of the exposure in unit 1 (Figure 8, Appendix 1) and the middle layer is the prominent fine-grained unit between units 2 and 3 (Figure 8, Appendix I). The upper layer consists of a whitish matrix between coarse cobbles directly below the cellular tufa and beachrock horizon at about the 15 m mark. Each of these layers contain from 7 - 20% glass shards, suggesting that instead of tephras, these layers

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should be referred to as ashy clastic sediments (Andrei Sarna-Wojcicki, written comm. 1995). The glass shards in all three of the ashy clastic sediment samples best correlate to one another and to a group oftephras known as the Walker Lake-Negit Island Causeway set of "proto" Mono Craters layers, which are estimated to be between ~65 to ~80 ka in age (Andrei Sarna-Wojcicki, written comm. 1995). These correlations indicate that the "tephra" layer or layers were originally erupted in early Wisconsin time and not during the late Wisconsin or Sehoo time. However, it is clear from the sedimentology of the layers, as well as the glass shard concentrations, that all three of the ashy clastic sediment horizons have been reworked and do not represent original airfall. The question is, how much time elapsed between the original eruption of these tephras and their incorporation into the PBC?

Data bearing on this question are in the form of AMS radiocarbon dates from gastropod shells collected from the ashy clastic sediment layers. We collected Vorticifex (Parapholyx) solida shells (Burch, 1989) from both the upper and middle ashy clastic layers for radiocarbon dating and additional shells from throughout the section for X-ray diffraction studies. The dates were provided by the Swiss Federal Institute of Technology in Zurich. Shells from the upper ashy clastic sediment layer date from 13, 110 ± 110 yr BP, while shells from the middle ashy clastic sediment layer date from 13,280 ± 110 yr BP (Figure 8, Appendix 1; Irka Hajdas, written comm., 1994). Although these radiocarbon estimates are in stratigraphic order, they certainly do not agree with the age estimates provided by the tephra correlations.

This situation implies one of two possibilities: 1) the shells and glass shards were deposited coevaUy with the beach gravel, sometime between 60 and 85 ka and then at some later date the shells were recrystallized, thereby incorporating young carbon and providing anomalously young ages, or 2) the beach gravel, shells and glass shards were all deposited at approximately 13.1 to 13.2 ka, which implies that the glass shards were derived from a deposit in the area. To ascertain which of these scenarios is most likely correct, we used X -ray diffraction to examine the composition of shells from each of the ashy clastic sediment layers, shells from the upper part of the sequence which we interpret to post date the highstand (~ 12 .7 ka) and shells from the shore of modern Pyramid Lake which we interpret to represent recently living examples of Vorticifex. According to B<j>ggild (1930), freshwater pulmonate gastropods (including Vorticifex) are composed of aragonite when living. All of the shell samples examined were composed of aragonite. Calcite was not present in any of the shell samples. This implies that the shells from the ashy clastic sediment layers have not been recrystallized and that their radiocarbon ages represent the true age of the lower or transgressive part of the progradational barrier complex.

As explained above, the surface ridges of the PBC post-date the highstand, but the shell dates indicate that the lower part of the PBe exposed in the wash pre-dates the highstand. Therefore, we interpret the sediments exposed in this cut to have been both deposited during the transgression to the highstand and during the regression from the highstand. The contact between the transgressive and regressive packages is placed at the cellular tufa and beach rock horizon (Figure 8, Appendix 1). Sediments above this horizon comprise the surface ridges of the PBC. Note that the shell dates were obtained from below this horizon. We interpret the cementing and tufa precipitation to have occurred during the time of the highstand when lake level was about 7 or 8 m above this horizon.

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When lake level dropped and stabilized at the elevation of the PBC, the regressive spit and surface ridges of the PBC were emplaced (Figure 7, Appendix 1).

Using the dates and elevations from this study we propose that the magnitude of the highstand was higher and the timing younger than that recently proposed by Benson, et al (1995). We place a minimum age of about 12.7 ka and a maximum age of about 13.1 ka for the Sehoo highstand, which reached an elevation of about 1339 m in the Jessup Embayment (Figure 12, Appendix 1).

Lower barriers 3 and 4 When lake level continued its overall decline from the highstand, a series of over 20 regressive barriers were formed, with each barrier representing a momentary stillstand in the overall regression. The elevations of 12 ofthe more prominent barriers are shown in figure 5 of Appendix 1. Lower barriers 3 and 4 lie at elevations of about 1320 and 1317 m, respectively. Both lower barriers 3 and 4 are composed of gravel, but the surface gravel oflower barrier 4 overlies a well-sorted sand which we interpret to be the upper edge of the Qss unit (Figure 2, Appendix 1). The Qss unit consists of well-sorted sand, probably winnowed from the sulf zone when the lake was at higher levels and then subsequently concentrated in the central part of the Embayment in relatively calm, deep water. The Qss unit is also found at the base of steeper slopes and small hollows in other parts of the Embayment (Figure 2, Appendix 1).

The soils developed on lower barriers 3 and 4 have less silt in the B horizons than the soils developed on the Jessup Playette barrier, but are generally similar in terms of thickness and character (Tables 1 and 2, Appendix 2). Note the lateral variability in the thickness of the B horizons in each of the soil pits .

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Begin road log by setting odometer to 0.0 at easternmost bedrock outcrop immediately downstream from incised bedrock canyon on north side of main wash. Proceed southeast toward 1-80 in road following main wash.

Turn left on road exiting main wash. Continue southeast.

Begin parking for Stops I-IB and l-IC (11:30-12:30 PM). For Stop I-1B walk about 300 meters upstream (north northwest) in gully to view cemented beachrock and buried soil developed in beach gravel and covered by younger beach gravel.

The paleosol here is developed in beach gravels and is buried by younger beach gravels, most likely of the Sehoo Alloformation (AF). A field description of the soil is found in Table 4 of Appendix 2, but in general this paleosol is much better developed than the surface soils in the Embayment in terms of total clay accumulation, structural grade, consistency and other field properties. I have not yet analyzed the particle size distribution in the lab. The interval oftime over which this paleosol developed is probably much longer than the interval of time since the last highstand (-12.7 ka), judging by the better development of the paleosol than the surface soils in the Embayment. However, the paleosol here at Jessup is not as thick as the one that we will examine later today at Grimes Canyon (Stop 1-2) . It is not known whether the two soils were developed on beach deposits of the same age.

For Stop 1-1 C walk downstream in the wash about 600 meters to the dissected tufa covered barriers and Lower Barrier 11. You will walk past the parking location.

The barriers in this area are covered by cellular tufa and host many small to moderate sized tufa domes. However, in cross-section the tufa can be seen to rise from a single horizon; Below the horizon, the gravel is cemented but tufa is either scarce or absent. At and above the horizon, cellular tufa is abundant. Conduits leading up to the small tufa domes and heads have not been observed.

My current hypothesis for tufa formation at this site is that lake levelwas at or slightly above this level and that ground water traveling through the beach gravels and sand in the upper parts of the Embayment was forced to the surface here in the forms of seeps and springs. The interaction of the ground water and the lake water caused the precipitation of the tufa. However, I have not studied the geochemistry behind this interaction. The distribution of the tufa provides observational evidence in support of this idea. Tufa domes are prominent here in the central part of the basin where the surficial deposits are presumably thick, but the tufa becomes more rare or is absent at this level in areas adjacent to bedrock slopes. This is probably related to the distribution of ground water.

Figure 4. Air photo showing the barriers near the mouth of the Jessup Embayment and features discussed in "Stops l-IB and I-Ie.

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The tufa covered barriers are at an elevation of about 1235 to 1240 m. Other tufa covered barriers at the same elevation can be seen both to the north and south of the Jessup Embayment and are probably related to the same lake stand. The elevation of the barriers is roughly the same elevation as the sill dividing the Carson Sink.from Rawhide Flats when corrected for isostatic rebound. Rawhide Flats is located off the southeast comer of the Carson Sink and may have acted as an evaporative pond temporarily stabilizing lake level in the Carson Sink. .

Lower barrier 11 stratigraphically overlies the tufa covered barriers but consists of well­sorted clean gravel with only occasional tufa pieces scattered about. The relationship between the tufa covered barriers and lower barrier 11 can be seen in several small gullies and in the main wash. After the formation of the tufa barriers the lake probably continued to recede to some unknown level but then retransgressed to about the same level (~123 5 m). This intetpretation is based on the very different character of the tufa barriers and the clean gravel barriers. The coincidence in elevation between the two barriers again argues for some external lake level control, possibly in the form of the sill leading to Rawhide Flats.

The soil developed on lower barrier 11 is again similar to the other soils developed on features of Sehoo age in the Embayment (Table 2, Appendix 2). Note that the weight percent of clay in the 2Btk horizon of one of the profiles described here is higher than in any other B horizon described on Sehoo deposits, despite the fact that this is the youngest barrier upon which soils were described (Tables 1 and 2, Appendix 2).

After Stops I-IB and I-IC, continue toward 1-80 in vehicles.

Road meets again with main wash. Passing by Stop 1-1 C. Continue southeast toward highway.

Pass the lowest group of barriers in Embayment at an elevation of about 1227 m. This group of barriers can also be thought of as a progradational barrier complex and indicates a momentary stillstand in the overall regression of the Lake.

Stop sign at freeway interchange. Pavement begins.

Reset odometer.

After resetting odometer at stop sign, proceed east beneath freeway and turn left (north) onto 1-80 East toward Lovelock.

Mile marker 79. The White Plains section of the Carson Sink is to the southeast (right). The Mopung Hills are located at about 2:30. These are the multi-colored hills which fOlID the southern tip of the West Humboldt Range. At the highstand the Mopung Hills formed an island. A small tombolo on the north side was measured at 1338.2 m. Soil developed on thistombolo is relatively weak and probably dates from the Sehoo highstand (Appendix 2, Table 3, Site CS-27). The smaller multi-colored hills located to the north (left) of the Mopung Hills are known as the Niter Buttes.

Exit 83. US 95 South interchange to Fallon and Las Vegas. Continue East on 1-80.

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6.0 You are now beginning to drive up the southern slope of the Humboldt Bar (Russell, 1885). The Humboldt Bar is a large cuspate barrier complex which extends across the lake floor to the Niter Buttes and separates the Humboldt Sink from the Carson Sink. There is a small channel, which has been artificially enlarged, through the barrier near its center, probably indicating that water overtopped the barrier, flowing from the Humboldt Sink to the Carson Sink.

6.5 Mile marker 85. You are now near the top of the Humboldt Bar. Note that there are many small playettes and subsidiary beach ridges that together make up the barrier complex. Most of the surface ridges that you see were probably formed during the recession from the Sehoo highstand. The highest beach ridge at the north end of the complex was measured at 1337.8 m.

7.9

9.0

9.4

10.2

14.6

There is a buried soil exposed on the north side of the westbound lanes at about 9:00 that appears as a reddish zone. The paleosol is developed in beach gravel and buried by younger beach gravel, probably ofthe Sehoo (AF). This relationship shows that the Humboldt Bar is a compound feature built during more than one lake cycle.

The Humboldt Sink lies to the east and southeast (right) of the freeway. The Sink is the terminal basin of the Humboldt River which rises in northeastern Nevada and flows west to end here. However, much of the water is div~rted for agricultural use before it arrives and only occasionally does a lake occupy the Sink. The steep face of the West Humboldt Range can be seen on the far side of the Sink. At the change in slope is a steep escarpment mapped as a Holocene rupture by Russell (1885). Since Russell's time, this feature has apparently not been mapped. The inscribed mark of the high shoreline is evident on the mountainside above the fault.

The Humboldt Range is on the far skyline at 12:00. The range reaches an elevation of 2997 m (9834 ft) at Star Peak near the northern end of the range.

Pershing County line.

The West Humboldt Range continues north to where it connects to the Humboldt Range near Coal Canyon where we will be leaving the freeway and heading east into the Carson Sink. At the Sehoo highstand, the West Humboldt Range formed three large islands separated by narrow straights. In two places water just topped the crest of the range forming highstand barriers on opposite sides of small playettes. The barriers on the northwest sides of the playettes were formed by waves coming out of the Humboldt Sink, whereas the barriers on the southeast sides of the playettes were formed by waves coming from the Carson Sink. The Carson Sink barriers are about 1.5 to 2.0 m higher than the Humboldt Sink barriers indicating that waves traveling north across the Carson Sink were larger than waves traveling south or west across the Humboldt Sink. The difference in wave size and hence barrier size, is probably related to the greater fetch of the Carson Sink.

Exit 93 Toulon. Continue east on 1-80. The large embayment to the north and west (left) of the freeway also has a series of tufa covered barriers and tufa mounds at about the same elevation as those in Jessup and probably represent the same lake level. Note

17

19.0

20.5

23.4

26.3

that the tufa mounds are more common in the center of the embayment where there are presumably thick accumulations of sediment. The tufa is less common or absent near the southwest and northeast edges of the embayment and near bedrock exposures. This pattern is also similar to that at Jessup and further supports a ground water origin for the formation of the tufa.

To the northwest (left) at about 9:00 is a well-developed wave cut platform at about the same elevation as the tufa covered barriers at Jessup and in this embayment.

Coming into view from about 10:00 to 11 :00 is a feature known as Boothill which is a large complex barrier structure cored by bedrock, as evidenced by bedrock exposures on the crest of the complex. The elevation of the highest beach ridge on Boothill is about 1334.6 m. The Boothill complex was studied by Meg Wheat and her observations are in an unpublished report from 1961. She concluded that the highest lake level recorded at Boothill dates from the Eetza lake cycle. However, based on relatively poor soil development we have interpreted the highest beach ridge to date from the Sehoo lake cycle (Appendix 2, Table 3, Site LA).

Entering Upper Valley. Lone Mountain is located to the northwest (left) in the mid ground at about 10:00 and is a good example of how contrasting lithologies can control shore terrace development. The south side of Lone Mountain is composed of granitic bedrock, whereas the north side is primarily composed of intermediate volcanics. Well­formed terraces yhat are cut into the volcanics can not easily be traced into the granitics, which implies that substrate material (bedrock) exerts some control on shoreline development. However, aspect might also playa role here as the southwestern side of Lone Mountain would have been relatively protected from waves coming from the north, which is the direction of greatest fetch.

Exit 105. First Lovelock exit.' Continue east on 1-80.

27.5 Exit 106. Downtown Lovelock. Exit here if you need beer, gas, food or water for tonight. It is about 75 miles to the next opportunity in Fallon, which we won't be driving through until tomorrow afternoon. Lovelock has a Safeway (located just to the west of downtown), several gas stations, restaurants and of course, casinos.

28.9 Crossing the Humboldt River. Not much to look at these days.

30.7 Mile marker 109. The ilL" for Lovelock is to the east (right) at about 3:00. A Holocene rupture lies immediately below the ilL" and continues to the north and south. Shorelines along this stretch of range front are faulted and vertical separation measured across an offset barrier is about 3.2 m. The scarp appears to have been produced in a single event. The elevation ofa high barrier (1337.6 m) located on the footwall of the fault,just to the north of the "L" is about 4 m higher than the elevation ofa high barrier (1333.7 m) located on the hanging wall of the fault a few kilometers to the north. Given that the natural variability of shoreline elevations is taken to be ± 3 m, it would appear that in this location, localized offset along a fault may be discerned from shoreline elevation patterns.

31.9 Sign that reads "Coal Canyon, 2 miles". Prepare to exit. The brand new Nevada State Prison is to the right (east) at about 2:00, but there were no residents as of August, 1996.

18

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33.9 Exit 112, Coal Canyon. Leave I-80, proceed to the end of the off ramp and turn east (right) at the stop sign. Head east toward the West Humboldt Range on Coal Canyon Road.

The escarpments at the base of the West Humboldt Range are probably a combination of both wave-fonned and fault-fonned features.

36.1 Passing a dissected highstand barrier on the north (left) side of the road at the mouth of Coal Canyon. This feature has two closely spaced barrier crests with the higher ridge measured at 1333.7 m. Continue east. Watch for livestock on the road. This is open range.

41.2

43.6

45.5

47.4

48.9

50.0

53.5

54.0

This part of the West Humboldt Range is composed of Triassic-Jurassic metasedimentary rocks overlain by Tertiary sediments and volcanics.

Crest of the West Humboldt Range. Coming into view is the southern end of the Humboldt Range to the east in the mid ground.

Entering Packard Flat. The Stillwater Range is ot). the far skyline to the east and is the focus of the stops for the next day and a half.

The Carson Sink stretches out to the southwest (right) of us here for about 100 km. The small outcrop on the playa is known as Lone Rock and serves as a bombing target for Navy jet fighters. Lone Rock is about 30 km to the south of us.

The high shoreline at the north end of the Carson Sink in this area is about 13 3 3.1 m -only about 60 cm lower than the high barrier on the west side of Coal Canyon.

Intersection with paved road to McKinney Pass. Continue forward. The sign for this intersection reads "Dago Pass, Pleasant Valley, Dixie Valley".

Chocolate Butte is located at 12:00 in the mid-ground below the skyline. Chadwick and Davis (1990) wrote about soils developed here in highstand Lahontan deposits, but I was unable to locate their sites from maps and descriptions.

Intersection with road toward Chocolate Butte. Stay on paved road to the right

The east side of the West Humboldt Range lies to the west (right) of us here from about 2:00 through 6:00.

Pavement ends. Proceed forward.

Chocolate Sm (~1262 m) which separates the Carson Sink and Buena Vista Valley is located at about 10:00. In this context, a sill is the lowest point on a divide between two adjacent basins. The morphology of Chocolate Sill and a channel partially cut through it

19

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55.0

56.0

58.0

61.0

65.0

66.4

on the Buena Vista Valley side suggests that when water topped this divide, flow was from the Carson Sink: into Buena Vista Valley.

F or an interesting discussion of the alluvial fans of Buena Vista Valley and their relationship to shorelines, see Ritter et al (1996, Appendix 4, this volume).

Large playa-fringing dunes which will be discussed at Stop 1-3 later today are located to the south (right) of us from about 12:30 to 3:00.

The Buena Vista (BY) Hills are located to the left from about 9:00 through 11 :00. The BV Hills are composed primarily of a Jurassic gabbroic complex and associated mafic volcanic rocks.

Intersection with road that continues south along east side of Carson Sink. Turn right (south), This is the more well-traveled route.

The Stillwater Range to the east (left) is similar here to the BV Hills in that it is primarily composed of the same Jurassic gabbroic complex with associated mafic volcanic rocks, but here it is also capped by Tertiary silicic volcanics. Note that the shoreline here is very crenulate and irregular fonning effective sediment traps to the dominantly south to north longshore transport.

The prominent escarpment on the piedmont to the east (left) from 9:00 to 11 :00 may be a compound fault scarp accentuated by shore processes. A Holocene rupture trace appears to trend into this feature from the south.

Turn left (east) on road heading up the fan toward Grimes Canyon and the range front.

67.4 Begin parking for Stop 1-2. Make sure you park off of the road and allow others to pass. Parking is tight here but if we are patient and creative in our parking, everyone will fit.

Walk east up four wheel drive track to the crest of the shoreline complex at Grimes Canyon for Stop 1-2.

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Grimes Canyon is a small embayment located on the Stillwater Range front in the northeastern Carson Sink: where the fetch was very large and the wave energy must have been tremendous. To the south of Grimes Canyon the Stillwater Range presents a steep, linear erosional front with triangularly faceted spurs, steep wineglass valleys and a Holocene fault rupture, all indicative of active uplift, although wave beveling has also

Figure 5. Air photo of the Grimes Canyon area showing the excellent development of shore features in this area. Also note the compound scarp fronting the Grimes Canyon Embayment as well as locations of the surface soil pit and the buried soil exposure.

20

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modified this range front. The shorelines of Lake Lahontan south of Grimes are generally in the form of erosional or built terraces up to and including the high shoreline. However, from Grimes Canyon north to the BV Hills (- 9 km), the high shoreline is very crenulate with many small embayments and protruding headlands. The large scale change in range front morphology is probably related to differences in bedrock composition. To the south of Grimes, bedrock consists primarily of sedimentary and metasedimentary rocks of Triassic and Jurassic age, whereas from Grimes north to the BV Hills, bedrock is composed primarily of a Jurassic gabbroic complex capped by Tertiary volcanics (Wilden and Speed, 1974).

The change in range front morphology from steep and fairly linear to less steep and highly embayed has resulted in wholesale changes in shoreline development. The deeply embayed shore from Grimes to the BV Hills provided efficient sediment traps to longshore drift moving from south to north. Most of the sediment comprising the constructional shorelines in Grimes Canyon consists of sedimentary and metasedimentary rocks even though there is limited outcrop of these units within the Grimes Canyon drainage. Sediment in the fans graded to the high shoreline as well as alluvium in the modem drainage of Grimes Canyon consists primarily of Tertiary volcanics with lesser amounts of carbonates. Differences in composition between the lacustrine gravel (primarily metasediments) and alluvial sediment (primarily volcanics) of Grimes Canyon suggest that most of the lacustrine gravel was transported into the Grimes Canyon embayment via longshore transport from the south. The morphology of high constructional shorelines from Grimes north to the BV Hills also display strong directional components in that several spits were built from south to north. This evidence indicates that, much like today, prevailing winds were from the west-southwest during the Sehoo highstand (~13 ka).

The shorelines at Grimes Canyon are another spectacular example of a well-developed progradational barrier complex (PBC). Unlike the PBC at the head of the Jessup Embayment, the PBC at Grimes was formed during the Sehoo highstand and not during the regression. Contrast the development of the PBC at Grimes with the highstand features observed at Jessup. There, the highstand features are relatively small pocket barriers and spits. However, at Grimes the highstand PBC is a massive feature which contains a very large volume of gravel. Because all highstand features formed during the same length of time, some factor other than time must be responsible for the marked differences in shoreline development between the Jessup Embayment and Grimes Canyon.

The magnitude and frequency of Pleistocene storm events affecting Grimes and Jessup probably were a controlling factor in shoreline development. Jessup would have been protected from storms blowing from the west or southwest, whereas Grimes would have received the full force of waves generated across the width of the Carson Sink. Conversely, Grimes would have been protected from storms blowing from the east or southeast and Jessup would have received wave energy from that direction. Assuming that at least some of the highstand features in Jessup had adequate accommodation space and sediment supply, the differences in development must have been due to larger and/or more frequent storm events from the west southwest during the period of the highstand.

22

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Another factor to consider is that the PBC at Grimes is a compound feature built during at least two lake cycles both rising above 1325 m. There is a very well-developed buried soil exposed in a stream cut on the backside of the Grimes PBC (Appendix 2, Table 4). The paleosol is developed in well-rounded beach gravel and demonstrates that a pre­Sehoo lake reached a level at least as high as 1325 m but does not represent the highstand. For comparison, the soil developed on the upper surface of the PBC, about 9 m higher than the top of the paleosol, is much less developed (Appendix 2, Table 3, Site CS-Il b). The Grimes surface soil is similar in thickness, character and development to the Sehoo-age soils from Jessup (Appendix 2, Tables 1,2 and 3). Although shoreline development at Grimes is stronger than at Jessup, preservation of shore features is similar. Therefore, based on soil development and preservation of morphology, we interpret the upper surface of the high PBC at Grimes to also have formed during the Sehoo highstand at about 12.7 ka.

Turn vehicles around and retrace route back to the main road.

Main road intersection. Turn a dogleg right (north) 30 m and then left (west) onto road heading out towards playa-fringing dunes.

Road bears left and proceeds up and over fIrst ridge of dune complex.

"

Park to camp in large flat between dune ridges.

Stop 1-3 (5:00- 6:00 PM). Walking tour of the dunes with Lancaster and Cupp. A large complex of lunette dunes that consists of two and locally three ridges up to 1000 m wide and 40 m high extends for 35 - 40 km around the east and northeast margin of the Carson Sink playa. The lunette dunes are very similar to those described from Australia and southern Africa and overlie, with an erosional contact, late Pleistocene saline lacustrine clays of paleolake Lahontan. They represent at least two episodes of mid-to late-Holocene deflation of sediments from beaches of the Carson Sink playa.

The outermost dune ridge consists of a core of poorly sorted medium and fine sand partially cemented by saline clay, gypsum, and silt overlain by 2 - 5 m of unconsolidated poorly sorted very fine and coarse sand. The inner, or south-westerly, ridge is up to 5 m high and consists of both partially indurated and mobile sand. At a number oflocalities, the indurated core of the outer dune ridge is carved into 1 to 4 m-high yardangs (streamlined small hills with a lemniscate shape that results from wind erosion of homogenous sediments). The yardangs appear to be actively forming today.

The formation of playa margin or lunette dunes requires a restricted range of environmental conditions. Sand-rich lunettes form during periods of high lake levels with a relatively low salinity, whereas clay-rich lunettes form in low lake level, high salinity times, when clay pellets are deflated from a zone of efflorescence where the capillary fringe of a shallow ground water table intersects the playa surface. The Buena Vista dunes represent an example of the former conditions, with the dunes being fed by deflation of sands from an adjacent beach. The poor sorting ofthe eolian sands suggests a low energy lacustrine wave regime and probable local sediment sources in nearby washes and alluvial fan systems.

23

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After the stop, walk back to camp, cook some dinner and join the party.

End Day 1 Road log.

24

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Playa fringing dunes to Weber Reservoir

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To begin Day 2 road log, retrace route from dunes campsite to main range front road. Turn right (south).

Approximate milea2e

0.0 Reset odometer at the intersection of the dunes road and main range front road. Proceed south along main road.

South of Grimes Canyon, the Stillwater Range front is very linear and is composed primarily of Triassic-Jurassic metasediments. As discussed yesterday afternoon, most of the beach sediment in the Grimes Canyon shore complex was derived from the south via longshore drift.

2.5 There is a fault scarp at the range front that can be traced from at least 8:00 through 10:00. This scarp shows up well in the early morning light.

5.1

5.5

10.l

Mine on the left (east) at 9:00 and ore processing tanks to the right (west) of the road.

To the left (east) there is a scarp on the piedmont exposing beach gravels in its steep face. This feature might also be a wave-accentuated compound scarp. Note the well-developed beach cliff at the base of the range.

Turn left (east) on road heading up the fan toward the range front.

10.6 Begin parking for Stop 2-4. Walk south (to the right as you are facing up the fan) across the wash and up onto the shore platform following a four wheel drive road. Stop 2-4 will begin on top of the platform. This is known as the Cox Benchmark Site. (8:00 - 10:30 am)

Stop 2-4A. Alluvial fan stratigraphy. Harvey and Wells. Figure 5 in Appendix 2. A complex of lake sediments is seen here. We do not understand the relationships, but exposed in a trench north of the headland are what appear to be two sets oflake sediments; an older, partially cemented set which is apparently unconformably overlain by a younger non-cemented set. In addition, interbedded fan and lake sediments are exposed on the north side of the trench. The younger lake sediments at the highest lake levels form a bar and flat complex linked to the rock headland. These are overlain by a thin veneer of intermediate (group 2) fan deposits. The whole set is faulted by a small arcuate normal fault. Above the highest shoreline are pre-lake surfaces of uncertain origin. The lake sediments are deeply trenched, and young fans (group 3) issue from the trenches.

Figure 7. Air photo of the Cox Benchmark site showing the relationships between shoreline deposits, alluvial fans and Holocene faulting. Note that the prominent spits built off the rocky headland to the southeast do not represent the Sehoo highstand. Instead the highstand here is represented by an erosional shoreline located to the east of the Holocene fault scarp and is partially buried by group 2 fans.

29

Stop 2-4B. Isostatic rebound and active faulting. Adams, Wesnousky and Bills. When the high shoreline of Lake Lahontan fanned about 12.7 ka, it essentially represented a horizontal surface that extended through about 3° of latitude and longitude. The natural variability of this near horizontal surface is taken to be ± 3 m. Over the last three years we have measured the elevations of over 180 highstand features from throughout the Basin. The once horizontal plane of the high shoreline is now deflected some 20 m from the horizontal. In other words, the difference in elevation between the highest elevated high shorelines and the lowest high shorelines is about 20 m The results are presented as a contour plot in figure 8 and show a clear signal of isostatic rebound with the areas of highest rebound occuning adjacent to the largest water loads in the Carson Sink and Black Rock Desert. In striking contrast, high shorelines in the far northern arms ofthe lake where water was relatively shallow are up to 20 m lower, not having been deflected as much. Figure 9 is a bathymetric plot of the depth of water at the Sehoo highstand. Note the close correspondence of the pattern of rebound with the bathymetry of the Lake.

An obvious question to ask is how much of an effect have the active faults in the Basin had on the defonnation of the high shoreline? Figure lOis a plot of the active faults showing their spatial distribution with respect to the high shoreline of Lahontan which is represented by the blue outline. Because the high shoreline only reflects defonnation since about 12.7 ka, only late Pleistocene, Holocene and historic ruptures could significantly affect this surface. We do however, recognize that off-fault deformation occurs on a more continuous time scale, and are actively researching the possible effects on the overall defonnation pattern.

The Lahontan basin straddles the boundary between the Walker Lane Belt which is zone of northwest trending dextral shear, and the Basin and Range Province which is characterized by extension along north-northeast trending ll(innal faults (Figure 10). Within the Walker Lane Belt, both the Pyramid Lake and the Honey Lake fault zones have had multiple Holocene ruptures but because they are dominantly right-lateral faults, little vertical defonnation accompanied these events (Anderson and Hawkins, 1984; Wills and Borchardt, 1993). Holocene ruptures can be seen to cross or closely approximate the high shoreline of Lahontan in several places in figure 10. However, in most places where interaction occurs between faults and shorelines it is difficult to measure the absolute defonnation of the shoreline. Two notable exceptions are along the West Humboldt Range near Lovelock (3.2 ± 0.2 m of vertical separation) and here at the Cox Benchmark site.

This is one ofthe better places to view offset shorelines, although the spits that we are standing on do not represent the highstand of Lake Lahontan. At this site, the highstand is represented by an erosional shoreline that is mostly buried by group 2 fans of Harvey and Wells (1996, this volume). The high shoreline can be traced across the shore platform east of the rupture trace in figure 7. A constructional high shoreline feature is offset by the fault about 150 m north of the incised trench on the north side ofthe shore platform. We surveyed a scarp profile using the near horizontal barrier crest of Spit 2 as our datum. Vertical separation at the rupture trace is 3.0 ± 0.2 m. Considering that the natural variability in the height of constructional beach features is on the order of ± 3 m, it

30

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Contour plot of the Lake Lahontan basin showing measurement locations and deformation since the Sehoo highstand at about 12.7 ka. All elevations were measured on the crests of constructional highstand beach features. Contour interval is 3 meters.

Bathymetric contour plot of Lake Lahontan at Sehoo highstand. Contour interval is 30 m. Lake outline is taken to be 1330 m. Darker shades of gray indicate deeper water with black areas

signifying regions below 1180 m. Note that the largest water loads are in the Carson Sink, Pyramid Lake, Winnemucca Lake and Smoke Creek- Black Rock Desert areas.

Figure 10. Map of active faults in the Lake Lahontan region showing their relationship to the high shoreline outlined in blue. Faults are color coded according to their recency: Red- historic surface ruptures, Orange- Holocene offset, Yellow .. Holocene and/or Late Pleistocene offset, Green .. Late Pleistocene offset, Gray stippled .. undated young faults. Fault data is from Dohrenwend et al (1996).

32

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is often difficult to separate fault defonnation from natural variability when examining shoreline elevation patterns (Figure 8). This issue is still an area of active research.

Bruce Bills has modeled the results of the shoreline surveys in order to gain estimates of the effective elastic thickness of the crust and the viscosity of the upper mantle underlying this part of the Great Basin. The best fitting model has an elastic plate thickness of 22 Ian and a mantle viscosity of 1018 Pa s. The relaxation time for a model with this viscosity is only about 300 years. It is noteworthy that the short relaxation time requires that rates of isostatic rebound must have been on the order of centimeters per year soon after or during the desiccation of the Lake, an order of magnitude higher than tectonic rates of defonnation now occurring in the region.

After the stop turn vehicles around and retrace route back to the main range front road.

Intersection with main road. Turn left (south).

Well-developed high level gravel embankments at the base of the Stillwater Range are prominent from the Cox Benchmark Site for at least 5 miles south. High level gravel embankments were first described by Morrison (1964) along the northern front of the Desert Mountains which we will drive by later this afternoon. These types of deposits generally consist of thick accumulations ofshot:e gravel either eroded from adjacent cliffs and moved along shore or sourced from individual drainages. Not uncommonly, the shore gravels are intercalated with alluvial deposits. Their characteristic deeply-dissected or fluted appearance is due to the lowering of the lake, subaerial exposure and rapid erosion. Commonly, small Group 3 (Harvey and Wells, 1996, this volume) fans composed of beach gravel issue from the mouths of the gullies developed in the gravel embankments. The Holocene rupture along the range front is poorly preserved if at all, where it cuts the high level gravel embankments.

The prominent rocky headland in the mid ground at 9:00 is located on a paleoscarp that was probably accentuated by wave action. Holocene scarps can be traced from near the base of the paleoscarp to both the north and south of the rocky headland.

The overall morphology of the Stillwater Range once again changes as we continue south. From Grimes Canyon to this point near Dry Canyon, the range is primarily composed of Triassic/Jurassic sedimentary rocks and presenfs a rather steep linear range front. However, from about Dry Canyon south to West Lee Canyon (~ 6 mi), the dominant lithology of the range changes to Tertiary volcanic rocks ranging in composition from rhyolite to basalt (Wilden and Speed, 1974). The morphology of the range associated with the change in lithology is less precipitous at the range front, has larger embayments and is less linear than to the north. Group 1 fans (Harvey and Wells, 1996, this volume) issuing from the large embayments are truncated by the high shoreline of Lake Lahontan.

18.0 Turn left (east) on four wheel drive track, heading up the fan toward Lambing Canyon. This road is similar in roughness to those beading toward Grimes Canyon and the Cox Benchmark site. The turnoff is opposite a wind mill (Desert Well) and corral located to the right (west) of the main road.

35

19.2 Begin parking for Stop 2-5 at the mouth of Lambing Canyon where the road enters a dissected wash.

Stop 2-5. Harvey and Wells. Figure 6 in Appendix 6. (11:30' -1:00 PM). In this area is a complex of well-exposed lake and fan sediments. The highest shorelines can be traced cutting the older (group 1) fan sediments, and several rock headlands. At least two sets of older (group 1) fans can be seen forming remnant fan surfaces issuing from Lambing and West Job Canyons. The same sediments, capped by a buried soil, are overlain by lake sediments in the shoreline zone. As at the Cox Benchmark site, there appear to be two sets of lake sediments, an older cemented and fragmented set, overlain by younger non-cemented sediments.

The high level lake sediments form a beach ridge and flat complex south of one of the rock headlands, and are overlain on their landward side by thin intermediate (group 2) fans. The whole suite was then trenched as lake levels fell and younger (group 3) fans have prograded, issuing from the trenches.

A little to the north of Lambing Canyon is a suite of faulted older fan sediments and lake sediments, which again appear to be of two ages. Weare still working on these sections.

After stop, turn around and retrace route back down to the main road.

20.5 Intersection with main road. Turn left (south).

22.5 From about 9:00 through 1:00 the Stillwater Range front at about the elevation of the high shoreline is composed primarily of Tertiary basalt. Note the well developed shore terraces. From observations throughout the Basin it seems that terraces are best developed and preserved in mtermediate to basic volcanic bedrock. Group 3 fans (Harvey and Wells, this volume) issue from most of the gullies in this part of the range. Note their rubbly appearance.

There are several smaller roads intersecting the main road in this section. Stay on main road.

30.0 Intersection with road to Mountain Wells, La Plata and Dixie Valley. Continue straight on main road.

A soil profile on the high barrier in Mountain Wells Canyon shows poor development typical of Sehoo-age soils. We won't have time to stop at this soil pit today.

32.5 Stillwater Point is located to the southeast (left) at 9:00.

33.5 The Lahontan Mountains are directly in front of you in the mid ground from about 11 :00 through 12:30. The highest point on the left (east) side of this small range is Rainbow Mountain. In July and August of 1954 there were a series of earthquakes ranging from Mw 5.9 to 6.5 (Doser, 1986) that ruptured the ground surface along the eastern side of Rainbow Mountain. The two earthquakes on July 6 ruptured the surface from a point near the southern end of Rainbow Mountain north for about 18 km. Scarp heights ranged

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from 3 to 30 cm and motion was dominantly dip-slip although Caskey et al (1996) have documented right-lateral offsets (up to 1 m) of some small gullies along the rupture trace. The August earthquake (Mw= 6.5) re-ruptured areas of the July ruptures and extended ruptures 23 km further north into the Carson Sink (Bell, 1984). Scarp heights for the August event ranged up to 0.75 m and are generally more continuous than those of the July sequence.

The Lahontan Mountains and many of the place names therein were named by Roger Morrison in his landmark paper on Lake Lahontan stratigraphy (Morrison, 1964). Within the Lahontan Mountains are the type sections for the Eetza AF and the Churchill Geosol. Unfortunately, the type sections of both the Wyemaha and the Sehoo AFs were consumed by a gravel pit operation and are no longer in existence.

At the Sehoo highstand, the Lahontan Mountains were a series of islands and because of the large fetch to the north, shoreline development was excellent. Morrison (1964) mapped the highest shoreline features as dating from the Eetza lake cycle. However, a soil pit excavated on a high depositional beach feature displays soil characteristics typical ofa Toyeh soil (Appendix 2, Table 3, Site F-19). In addition, a 36Cl surface exposure age from the same soil pit indicates that the surface is no older than about 15 ka (Fred Phillips, written comm., 1995). Therefore, we interpret the age of the high shoreline in the Lahontan Mountains to date from the most recent or Sehoo lake cycle at about 12.7 ka.

In 1940, the husband and wife team of archeologists, S.M. and Georgia Wheeler excavated the mummified remains of a man from a rock shelter in the Lahontan Mountains known as Spirit Cave. They originally estimated that the remains were about 1500 to 2000 years old, but recognizing that dating technology was lacking at the time, preserved the remains in the Nevada State Museum for future researchers to study. In 1994, Amy Dansie and her colleagues from the Nevada State Museum radiocarbon dated the remains and various organic articles buried with the body and determined that the man died about 9,415 years ago making these the oldest mummified remains ever unearthed in North America.

Road bears to right. On the left (east) side of the road is a maintenance yard for the Stillwater Wildlife Refuge.

Pavement begins. Continue forward.

Town of Stillwater. No services. You are now on Nevada 116 West which will take you toward Fallon. This road is also called the Stillwater Road and has many twists and turns. Stay on the main road.

Intersection of Stillwater Road with U.S. Highway 50. Turn right (west) toward Fallon.

Entering Fallon, which is primarily a farm service community, but is also the oasis of Nevada and is home to the Fallon Naval Air Station and Top Gun jet pilot school.

37

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52.4

52.6

62.5

66.8

67.7

It will be about 51 miles to the next gas station in Schurz tomorrow, so gas up here if you are low. Also fill up on water, beer and food if necessary as tonight's campsite has none of these amenities.

Intersection of Highway 50 and Main street. Continue forward.

Intersection of Highway 50 and US 95 south. Turn left (south) towards Las Vegas. US 95 is also known as Taylor Street. There is a Safeway at the SW corner of this intersection.

The White Throne Mountains stretch out from about 10:00 to 11 :30 and the Desert Mountains are from about 11:30 to 2:00. From about 2:00 to 4:00 are the Dead Camel Mountains. High barriers on the north front of the Desert Mountains are at about 1334 m. Shore features along the north side of the Desert Mountains are particularly well-developed because of the large fetch to the north across the length of the Carson Sink. Morrison (1964; 1991) interprets the high shoreline in the southern Carson Desert to date from the Eetza lake cycle and asserts that the Sehoo highstand was about 3 m lower. However, several widely-spaced soil pits excavated on high barriers in this area display poorly-developed profiles more typical of soil development on Sehoo-age features (Appendix 2, Table 3, Sites F-9, F-7, F-6, and CC-4).

Also note the several large gravel operations along the northern front of the Desert Mountains. These operations are utilizing the thick deposits of shore gravel that are in the fonn of high level gravel embankments first described by Morrison (1964). As on the Stillwater Range front, these high level gravel embankments have a characteristically fluted appearance.

Here we cross the Wildcat Scarp, a semi-circular escarpment wrapping around the southern and eastern edges of the Carson Desert for some 25 km (Bell, 1984). At this location, the scarp is only one or two meters high but further to the east it ranges up to ten meters in height and offsets Bass Flats. Morrison (1964) was the first to describe the Wildcat Scarp and interpreted the escarpment to be a wave-modified fault scarp. Based on stratigraphic relationships of relatively shallow Holocene lakes, Morrison (1964) interpreted two faulting events in post-Sehoo time.

Tumofffor "Top Gun Drag strip". Continue forward.

68.9 First tumoffto the Russell Spit complex. The road parallels the highway for about 0.5 miles before heading up toward the Spit. Continue forward on Highway 95.

69.7 Russell Pass.

69.9 Russell Spit can be seen to the west (right) from about 3:00 to 4:00. I.C. Russell (1885) first described and mapped the geology of this area. Based on geomorphic relationships he interpreted the highest shoreline to date from the last lake cycle (Sehoo cycle). Morrison (1964) remapped the geology of the complex and interpreted the highest shoreline to date from the penultimate or Eetza lake cycle. Chadwick and Davis (1990) agreed with Morrison's (1964) interpretation and described the soils on the purportedly different aged features. Although the soil developed on the highest spit is better

38

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developed in terms of clay and silt accumulation than the soil developed on the next lower spit, and indeed better developed than the majority of what I interpret as Toyeh soil profiles, the high soil is not as well-developed as the buried soils at both the Jessup and Grimes Canyon localities. Therefore, the age of the high shoreline at Russell Spit remains enigmatic in my mind. In terms of the isostatic rebound research, the high shoreline which we measured (1334.7 m) is only about 1 m higher than the next lower spit. This is well within the stated precision of our measurements for Sehoo-age features and therefore does not affect the deformation modeling.

Earlier this year (1996), Russell Spit was in danger of becoming a landfill for the city of Fallon, Thanks to the concerted efforts of Marith Reheis, Steve Wesnousky, Roger Morrison, Pat Glancy, and many others too numerous to mention, the city of Fallon has withdrawn their application to use this historic area as a landfill. We hope that the Russell Spit complex will gain official recognition and protection from the BLM so that future workers and students will be able to appreciate and ponder the shore relationships so beautifully preserved at this site.

Turnoff to Lee Hot Springs to the east (left). The spring is located next to the lone tree about 1 mile off the highway.

At this point we are driving down toward Rawhide Flats which formed a small subbasin attached to the Carson Sink by a sill at about 1230 m. The bottom of Rawhide Flats is at about 1180 m, hence there is about 40 m of closure in this subbasin. It is likely that during lake transgression, overflow occurred from the Carson Sink into Rawhide Flats.

The Terrill Mountains are from about 10:00 to 1 :00. These mountains form the divide between the Walker Lake subbasin and Rawhide Flats and are also near the eastern margin of the Walker Lane Belt.

The accommodation of right-lateral relative plate motion between the Pacific and North American Plates is not limited to displacements along the San Andreas fault system. A significant component of right-lateral plate motion is also accommodated by a broad zone of strike-slip faulting that trends northward from the San Andreas system in the vicinity of the San Bernardino mountains through the Mojave desert and along the eastern boundary of the Sierra Nevada. That portion of the zone within the Mojave is termed the Eastern California Shear Zone (ECSZ). Further to the north, where the zone sits between the Sierra Nevada and the Great Basin, the zone offaulting is referred to as the Walker Lane Belt (Stewart, 1988). The Walker Lane is easily identified because the northwest­striking transcurrent faults which mark the zone have produced a distinct and siniilar striking structural grain that contrasts abruptly with the more northeasterly trending basin and range structure to the east. Displacement of Quaternary deposits by faults is evident along the entire length of both the Walker Lane and ECSZ. Geodetic work within the Mojave indicates that upwards of 12 mmlyr of displacement may be taken up by faults within the ECSZ (Saber,1994). Inception of movement within the ECSZ was likely 10-6 Ma ago and is responsible for about 65 km of cumulative right-lateral offset (Dokka and Travis, 1990), consistent with some 9% to 23 % of relative plate motion being aCCOl}l1nodated through this zone. Similar values of slip rate and cumulative displacement are reported within the Walker Lane. For example, Hardyman and Oldow (1991) report that faulting within the central Walker Lane began about 15 -20 Ma , and

39

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76.5

78.9

81.6

82.3

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has resulted in about 60 to 75 km of right-lateral offsets across this part of the zone (Oldow, 1992). Recent syntheses of geologic slip rates and space-based geodetic observations across the southern portion of the Walker Lane indicate that cumulative slip across this part of the zone is about 12 rnm/yr (Dixon, 1995; Reheis and Dixon, 1996). The ongoing displacement within the Walker Lane and ECSZ are underscored by the occurrence of the great M~8 Owens Valley earthquake of 1872 (Oakeshott et aI., 1972) and the June 28, 1992 Mw 7.5 Landers earthquake, respectively.

Entering the Walker River Paiute Indian Reservation.

Lyon County line.

Mineral County line.

Intersection with dirt road to Weber (pronounced Webber) Reservoir. Continue forward. We will be backtracking to this road after Stop 2-6.

Park in pullout on the right (west) side of the highway for Stop 2-6. Watch for traffic and be careful.

Stop 2-6. Reheis. US Highway 95 roadcut east of Weber Reservoir. (3:30 - 5:30 PM). From the area of the highway west to the Walker River at Weber Reservoir is a large area of pre-late Pleistocene lacustrine and terrestrial deposits. These are described in detail, including a geologic map and measured sections, in Reheis (Appendix 7, Figures 5 and 6). Exposed on both sides of the highway are late Miocene(?) to early Pleistocene(?) alluvial and lacustrine deposits that dip 25-30° west. Measured section 9 begins in Tertiary alluvium (unit Tt) east of the highway and ends at the top of a younger lacustrine unit (TI) west of the highway. A partially overlapping and longer section 10 was measured about 1 km northwest and extends up into the next younger terrestrial unit, QTt. All of these units contain tephra layers. The tephra in unit TI shows that these rocks are closely faulted but the faults have small displacements (west side of road cut). The only tephra that has yet been identified is in unit Tt near the bottom of section 10; it is a late Miocene or early Pliocene tephra erupted from the Snake River Plain (A. Sarna­Wojcicki, oral commun., 1996). At the top of the western cut, a remnant of much younger beach gravel of a former lake (units Qlo 1 or Ql02?) lies unconformably on both units Tl and QTt at an elevation of about 1433 m, the highest such outcrop yet found in the Walker Lake basin. Bone fragments occur locally as lag on these outcrops; please do NOT collect any and remember we are guests on the Walker River Paiute Reservation.

After Stop 2-6, turn vehicles around and proceed NORTH on US 95 back 1.2 miles to the to Weber Reservoir.

84.7 Turn left (west) on graded dirt road which travels down to Weber Reservoir.

There are a lot of small spur roads turning off the graded road to Weber Reservoir. Don't take any of them.

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89.0 There is a large flat area to your right (north) and immediately north of Weber Dam. This will be the main camping area for tonight's bonfire and business meeting. I suggest that large vehicles park in this flat area. If you want more privacy, there are many campsites situated along the shore of Weber Reservoir for at least 0.5 miles north of the dam. All camping is primitive, with only outhouses and garbage cans provided. There is also good camping below the Dam among the cottonwoods along the Walker River but these sites are further removed from the main gathering area.

The bonfire will begin when someone gets around to lighting it and the business meeting may commence at about 8:00 PM (give or take).

End of Day 2 road log.

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0.0 Reset odometer at northern abutment of Weber Dam adjacent to large flat camping area. Turn left (north) and retrace part of yesterday's route toward US 95.

0.4 Tum right (south) on dirt road.

0.6 Pass through fence line and bear left at fork in road, immediately past the fence line.

1.3 McGee Wash. Begin parking for Stop 3-7. The front of the caravan should drive across McGee Wash and up the other side before parking. Make sure that you are not blocking traffic. After parking, walk upstream (northeast) in McGee wash.

Those with four wheel drive vehicles and a certain degree of confidence may drive up the wash some distance before parking to avoid the crowded parking on the road. However, mileage for the road log was calculated by turning around at the wash.

Stop 3-7. Reheis. McGee Wash. (8:00 -10:30 AM) McGee Wash, named by Roger Morrison for W'H McGee of the Russell expedition, exposes deposits of four pre-late Pleistocene lacustrine units that lie above the Sehoo level (- 1330 m, 4360 feet). We will walk up this wash about 2 km to see the evidence for these lake rises (note that the youngest, Ql04, is not well exposed here). Much of the lower part of the wash is shown as a stratigraphic cross section in figure 2-7 of Morrison and Davis (1984). See Reheis (Appendix 7, Figures 5 and 6) for detailed descriptions, a geologic map, and measured sections of this area; the map overlaps Morrison and Davis' cross section at the western map edge. Terrestrial unit QTt is exposed at the base of much of the upper part of McGee Wash and underlies the four lacustrine units; unit QTt is equivalent to the Paiute Formation shown by Morrison and Davis (1984), which Morrison now equates to the older Lovelock Formation. Unit QTt is sequentially overlain by lacustrine units Qlol (only preserved in one small outcrop),Ql02, and Ql03, all separated by unconformities. Lacustrine deposits here consist mostly of well bedded sandy pebble gravel and siltstone, although the upper part of unit Ql02 also includes a thick lenticular bed of mudstone that may have accumulated behind a barrier bar. Unit Ql02 contains tephra, one layer of which has been correlated with the Bishop-Glass Mountain family of tephra. The highest outcrop of lacustrine deposits in this area is a curving berm-shaped feature composed of sandy beach gravel that rises to an elevation of 1400 m (4600 feet). Although faulting is common in this area, the coincidence of the elevation of this well-preserved berm with that of other shoreline remnants in unfaulted areas (Thorne Bar and Sunshine Amphitheater) indicates that faulting is not solely' responsible for the high elevation of this berm .

After Stop 3-7, retrace route back to graded road which heads toward Weber Dam.

45

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Passing through fence line, take the right fork of the road heading back toward Weber Dam road.

Intersection with Weber Dam road. Turn left (southwest) toward Weber Dam.

2.7 Northern abutment of Weber Dam. Proceed across Dam.

To your left (east) at 9:00 are some bluffs formed by the downcutting of Walker River. This view is featured on the cover of the 1991 GSA DNAG volume "Quaternary Nonglacial Geology: Conterminous U.S." edited by Roger Morrison. The deposits exposed in the bluffs represent over two million years of Lake Lahontan lacustrine and subaerial history.

After passing over the Dam, the road splits but both forks soon rejoin. The left fork has a more gentle gradient so buses and other large vehicles may want to bear to the left.

3.9 Turn hard left (soutb) on dirt road toward tbe fence line.

4.2

4.3

4.6

Pass through gate in fence line. Continue forward past several small gravel pits.

Bear right on small dirt road which heads down toward the playa. Drive to the south end of the playa and park. .

After parking, walk up onto the large spit and south the along crest until you arrive at the large gully.

Stop 3-8. Adams. Walker River Narrows. (11:30 -12:30 PM). Weare standing on the crest of a large spit complex that was built from south to north by waves coming from the vicinity of Walker Lake. In 1984, Roger Morrison and Jonathan Davis led a GSA field trip through this area and this was one of their stops which examined the relationships between Sehoo and Eetza shore features. They interpreted the highest shoreline here to date from the Eetza lake cycle and claimed that the highest Sehoo shoreline was 6 m lower residing at an elevation of 1324 m (Morrison and Davis, 1984) .

I interpret all of the surface ridges in this spit complex to date from the Sehoo lake cycle . The ridges lower than the highstand spit were formed during the regression from the Sehoo highstand. This interpretation is based on the fresh morphology of this complex as well as the poorly developed soil on the crest of the high spit (Appendix 2, Table 3, site F-29). The degree of soil development here is very similar to the degree of soil development on Sehoo age features in Jessup and elsewhere around the Basin (Appendix 2, Tables 1,2, and 3).

Figure 12. Air photo showing the Weber Dam and Walker River Narrows area. Note the locations of McGee Wash (Stop 3-7) and Stop 3-8. High constructional shorelines measured on opposite sides of the Walker River and interpreted to date from the Sehoo highstand are only about 0.5 m different in elevation. In contrast, the pre-Sehoo soils

47

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Figure 13. Map of Lake Lahontan showing net shore drift directions at the Sehoo highstand. Note that net shore drift is generally away from areas of large fetch. Also note area ofbi-directional shore drift in the Humboldt River Valley north of Jessup.

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that we have examined thus far are much better developed than the surface soil at this location. The elevation of this spit is 1332.0 m and the elevation of the high shoreline on the opposite side ofthe Walker River is 1332.5.

Morrison claims that winds rarely blew from the south or southeast during Sehoo time (Morrison, 1964,1991). However, I maintain that during the Sehoo highstand, winds at one time or another came from all directions. Figure 13 is a plot of net shore drift directions from around the Lahontan Basin. This figure is based on observations of spit formation directions, downdrift distributions of distinctive lithologies in beach gravels and other directivity indicators. My interpretation of figure 13 is that shore drift directions most commonly point away from areas of large fetch. Note that in the northern parts of the Basin, net shore drift was predominantly from south to north, but in the southern part ofthe Carson Sink, net shore drift was predominantly from north to south. In areas such as the Humboldt River Valley north of Lovelock, drift indicators support a bi-directional drift pattern, suggesting that these shores were strongly affected by waves coming from the south as well as from the north.

After Stop 3-8, retrace route back to the graded Weber Dam road.

Intersection with Weber Dam road. Turn left (southwest) toward Alternate US 95.

Crossing railroad tracks. Continue toward Alternate US 95.

Intersection with Alternate US 95. Turn left (south) toward Hawthorne.

For the next 35 miles or so, we will be traveling along the base of the Wassuk Range, which is one of the most active range fronts in the Lahontan basin in terms of both tectonics and fan deposition. Many Holocene fault scarps can be seen along this range front for the next 40 miles or so. Along much of the range front, the scarps lie below the upper limit of Sehoo shorelines which sometimes makes it difficult to differentiate between the two. I was not able to identify any constructional high shoreline features along this range front, thus I do not have any firsthand knowledge of the absolute deformation since the regression ofthe Sehoo lake. However, Demsey (1987) did an excellent job of documenting the tectonic geomorphology and amount of Holocene (post­Sehoo) deformation that has occurred here. She identified two Holocene ruptures with a total of six to seven meters of vertical separation at the scarps. To determine the absolute Holocene uplift of the Wassuk Range, Demsey (1987) surveyed the elevations of cemented beachrock zones and shoreline scarps cut into alluvium on the Wassuk Range and across the basin on the presumed stable Gillis Range front. Her results indicate that the Wassuk Range has risen by about 1.5 m in the Holocene, or by about 20 to 25% of the total displacement at the fault.

Schurz town limit sign.

Intersection of Alternate US 95 with US 95. Turn right (south) toward Hawthorne and Las Vegas. Gas is available at the truck stop on the east side of US 95 about 0.5 miles south of the intersection.

49

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15.5 To the east (left) of us from about 6:00 to 11 :00 is the present course of the Walker River. Walker Lake has dropped some 45 m since Russell first described it in 1885. The fall in lake level is primarily due to diversion and damming of the Walker River for agricultural purposes.

17.3

19.3

23.3

24.0

25.5

26.5

There is a fault scarp on the right (west) side of the road at about 9:00 which exposes ledgy, cemented lacustrine gravel. The light-colored layer near the top of the exposure is the Bishop tephra (-760 ka; Davis, 1978) which serves as a matrix for the coarse beach gravel.

For the next few miles or so, the Highway has been built on historic shorelines of Walker Lake. As we drive south toward Hawthorne the highway crosses the historic highstand shoreline several times. Apparently the highway department is counting on Walker Lake never again achieving its historic levels. The historic shorelines are distinctive in that they are less vegetated, appear "fresher", and have virtually no soil development when compared to Sehoo and older lake features.

From 9:30 to 11:00 are a group of nicely developed historic shorelines forming a cuspate barrier.

Walker Lake comes into view. The town ofHawthome and the Army Ammunitions Depot can be seen at the south end of Walker Lake.

Mt. Grant (11,240 ft; 3426 m), the highest peak in the Wassuk Range, comes into view. The relief between Mt. Grant and the surface of Walker Lake is over 7000 ft, attesting to the active uplift of this range.

Note the abundance of tufa and beachrock along this section ofrange front. Remnants of the old Walker Lake road can also be seen skirting the cliffs along this side of the lake.

27.6 Sign for "20 Mile Beach".

28.2 Copper Canyon, issuing from the Wassuk Range, is located at about 2:00. Copper Canyon has had an interesting recent history of severe flooding. There have been three major floods on this fan since 1975 with the most recent occurring during the summer of 1990 and extending across the highway to the lake (Yount et aI, 1993).

30.0 For the next 0.5 miles or so is a well-developed cemented shore gravel zone overlying wave-beveled bedrock.

Walker Lake is dying. It has receded 45 m since 1883 and continues to decline. Most of the water that once flowed down the Walker River from the Sierra Nevada is now diverted and stored for agricultural purposes. In fact, water rights allocations are such that the Walker River Basin needs 130% of normal precipitation simply to fulfill the legal water demands placed upon it. In years of normal precipitation as well as drought years, little if any water actually flows into the lake. Water did not flow into the lake for a seven year period during the drought years of the late 1980's and early 1990's. Consequently, the lake has grown increasingly more saline and the fish are in danger of dying off.

50

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Turnoff for Sportsman's Beach. Continue forward.

Town of Walker Lake. Watch for the abandoned highway patrol cars on the side of the road. Speed at your own risk. Parts of this small town are also built on historic shorelines. The townsfolk must also be banking on the notion that the lake will never again attain its historic highstand.

41.5 The town of Hawthorne again comes into view. Hawthorne lies just at about the level of the Sehoo highstand which is about 1329 m in this area.

45.1

46.0

Turn left (east) on Thorne Road opposite the main entrance to US Army Ammunition Depot. There is a flashing light at this intersection. Proceed easterly through the Ammunitions Depot.

The Gillis Range lies on the east side of Walker Lake and is located directly in front of you. This range is part of the central Walker Lane Belt.

46.6 Intersection with Bonanza Road. Continue forward on Thome Road.

48.0

50.5

54.8

55.0

55.6

56.7

Intersection with north 3rd St. Continue forward on Thome Road.

The main railroad crossing is at the Thome siding. Cross the railroad tracks and bear left (north) at the "Y" immediately after the tracks. Pavement ends. Stay on the main dirt road and continue north.

Intersection with side road which goes down to the railroad tracks and Walker Lake shore. Stay on main road and continue forward.

The near skyline in front of us is dominated by the bulk of the Thome Bar. This complex records at least four or five lake cycles which will be discussed at Stop 3-9.

Road bears to right and begins to climb the southern flank of the Thome Bar.

Park on or near the crest of the Thorne Bar for Stop 3-9.

Stop 3-9. Reheis and Adams. Thorne Bar. (3:00 - 4:30 PM) The Thome Bar is a large V -shaped or cuspate barrier complex of shore gravel built at the mouth of a large canyon draining the Gillis Range. See Reheis (Appendix 7) for a detailed discussion and figure 14 for a geologic map of the area. The barrier complex was previously described by King (1978), Mifflin and Wheat (1979), and Morrison (1991) as an outcrop of lacustrine gravel much higher than the Sehoo highstand shoreline; they believe that the higher elevated shore deposits are due to tectonic deformation. However, there appears to be no Quaternary deformation on this side of the Walker Lake basin (Demsey, 1987). The Thome Bar consists mainly of well­rounded gravel reworked from older fan deposits, with bedding ranging from horizontal to dips of 25° or more. The complex can be divided into four morphologic units. The lowest and youngest is the group of historic shorelines formed during the last 100 years and which reach an elevation of about 1250 m. The next higher group of shorelines are sharp, well-preserved barriers marked at the top by the Sehoo highstand shoreline at

51

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40'

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CONTOUR INTERVAL: 40 FEET

EXPLANATION

I Qfh I Fan deposits (Holocene); includes some Holocene lake deposits

I Qfy I Fan deposits (late Pleistocene)

lil19I,~W Deposits of late Pleistocene Jake

~ Fan deposits (middle ~ and early? Pleistocene)

F'd"'1 Nearshore deposits and beach gravel ". ,gle) of middle to early Pleistocene lakes

~ Volcanic and sedimentary rocks ~ (Pliocene and Miocene)

T"TT'T' Late Pleistocene shoreline

T TTT' Middle Pleistocene shoreline; dashed where inferred

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about 1330 m (ca. 4360 feet, see map in Appendix 7, Figure 2). Between 1330 m and about 1370 m (4500 feet), the complex consists of two nested, eroded V-shaped barriers; the lower barrier is better preserved than the upper. Between 13 70 and 1400 m (4600 feet) the complex has no preserved morphology, but deep arroyos expose well-bedded, tufa-cemented shore gravel, and beach pebbles locally occur as lag deposits on basalt outcrops. The degree of morphologic preservation and three reconnaissance soil pits strongly suggest the presence of at least three and probably more lacustrine units above the historic limit: the Sehoo-aged barrier, higher older barriers that reached elevations between 1335 and 1370 m, and at least one old bar that reached a minimum elevation of 1402 m.

This is the end of the main Friends trip. If you are leaving us here, the quickest way to almost anywhere is back the way we came to US 95 just north of Hawthorne. If you want to continue on to the optional stop make sure you are in a 4WD high clearance vehicle. The road was somewhat washed out in a few places this past winter. However, it is still passable.

Road log to optional Stop 3-10.

56.7 From the crest of the Thome Bar, continue north on main road.

59.1

59.6

60.1

60.2

Tum right (east) on 4WD road heading up toward Gillis Range front.

T -intersection with power line road. Tum left (north).

Bear right at fork in road that travels on east side of power line. The road then parallels the power line for about 200 m and then turns right (east) towards range front.

Park for optional Stop 3-10. Walk up gully to view exposure on north facing side of the wash.

Optional Stop 3-10. Reheis. North of Thorne Bar. (5:00 - 6:00 PM). An excellent small outcrop about 4 km north of the Thome bar (Appendix 7, fig. 4) exposes two sequences of high shore gravel separated by a tephra and a paleosol. Only continue to this outcrop with a high-clearance 4-wheel drive vehicle. See Reheis (Appendix 7, fig. 4) for a measured section and detailed description of this outcrop. The lower unit consists of a shoreface gravel that fines upward into silty sand containing a tephra and a paleosol. Based on chemical correlation and paleomagnetic measurements, the tephra is probably the 0.76-Ma Bishop ashbed. The paleosol is overlain by tufa­cemented, well-bedded pebble and cobble shore gravel that rises to an elevation of about 1395 m. Thus, the oldest and highest shoreline in this area is underlain by deposits of two different lakes: one that culminated at about the time of the eruption of the Bishop ash bed at 0.76 Ma and reached a minimum elevation of about 1355 m, and a second that postdated the Bishop ash and rose to a minimum elevation of about 1400 m.

End of the toad log. I hope everyone enjoyed the trip, and we'll see you next time. Drive safely.

53

..

References

Anderson, L.W., and Hawkins, F.F., 1984, Recurrent Holocene strike-slip faulting, Pyramid Lake fault zone, western Nevada: Geology, v. 12, p. 681-684.

Bell, lW., 1984, Quaternary fault map of Nevada, Reno sheet: NBMG Map 79.

Benson L.V., Kashgarian, M., and Rubin, M., 1995, Carbonate deposition, Pyramid Lake subbasin, Nevada: 2. Lake levels and polar jet stream positions reconstructed from radiocarbon ages and elevations of carbonates (tufas) deposited in the Lahontan basin: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 117, p. 1-30.

B~ggild, O.B., 1930, The shell structure of the mollusks: Kgl Danske Vidensk Selsk Skr, Naturvidensk Mat, afd 9, no. 2, p. 233-326.

Burch, lB., 1989, North American freshwater snails: Malacological Publications, Hamburg, Michigan, 365 p.

Caskey, S.J., Wesnousky, S.G., Zhang, P., and Slemmons, D.B., 1996, Surface faulting of the 1954 Fairview Peak (Ms 7.2) and Dixie Valley (Ms 6.8) earthquakes, central Nevada: BSSA, v. 86, no. 3, pp 761 - 787.

Chadwick, O.A., and Davis, 10., 1990, Soil-forming intervals caused by eolian sediment pulses in the Lahontan basin, northwestern Nevada: Geology, v. 18, p. 243-246.

Davis, lO., 1978, Quaternary tephrochronology of the Lake Lahontan area, Nevada and California: Nevada Archeological Survey Research Paper no. 7, 137 p.

Demsey, K., 1987, Holocene faulting arid tectonic geomorphology along the Was suck Range, west-central Nevada: unpublished M.S. Thesis, Univ. of Arizona, Tucson.

Dixon, T. H., 1995, Constraints on present-day Basin and Range deformation from space geodesy, Tectonics, 14, 755-772, .

Dohrenwend, lC., Schell, B.A., Menges, C.M., Moring, B.C., and McKittrick, M.A., 1996, Reconnaissance photogeologic map of young (Quaternary and late Tertiary) faults in Nevada, in D.A. Singer, ed., An Analysis of Nevada's Metal-Bearing Mineral Resources, Nevada Bureau of Mines and Geology Open-File Report 96-2.

Dokka, R. K., and C. 1. Travis, 1990, Late Cenozoic Strike-slip Faulting in the Mojave Desert, California: Tectonics, v. 9, p. 311-340.

Doser, D.l., 1986, Earthquake processes in the Rainbow Mountain-Fairview Peak-Dixie Valley, Nevada, region 1954-1959: Journal of Geophysical Research, v. 91, no. B12, p. 12,572-12,586. .

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Hardyman, R.F., and Oldow, J.S., 1991,. Tertiary tectonic framew~rk and Cenozoic hi~tory of the 1;I.j Central Walker Lane, Nevada zn Geology and Ore DepOSIts of the Great Basm: G.L. ..

54

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-------.,

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_ Raines, RE. Lisle, R.W. Schafer and W.H. Wilkinson, eds., Geological Society of Nevada, Reno, p. 279-30l.

tl King, G.Q., 1978, The late Quaternary history of Adrian Valley, Lyon County, Nevada [M.S. thesis]: Department of Geography, University of Utah, 88 p.

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Mifflin, M.D., and Wheat, M.M., 1979, Pluvial Lakes and estimated pluvial climates of Nevada: Nevada Bureau of Mines and Geology, Bulletin 94,57 p.

Morrison, R B., 1964, Lake Lahontan: geology of southern Carson Desert, Nevada,: U.S., Geological Survey Prof. Paper 401, 156 p.

Morrison, R B., 1991, Quaternary stratigraphic, hydrologic, and climatic history of the Great Basin, with emphasis on Lake Lahontan, Bonneville, and Tecopa, in Morrison, R.B., ed., Quaternary nonglacial geology: Conterminous U.S.: Boulder, Colorado, Geological Society of America, The Geology of North America, v. K-2, p. 283-320.

Morrison, RB., and Davis, 10., 1984, Quaternary stratigraphy and archeology of the Lake Lahontan area; A re-assessment, in Lintz, I, ed., Western geological excursions, v. 1, Reno, Mackay School of Mines, Geological Society of America 1984 annual meeting field trip l3 Guidebook, p. 252-281.

Oakeshott, G. B., R W. Greensfelder, and I E. Kahle, 1972, ... one hundred years later: California Geology, March, p. 55-62, .

Oldow, IS., 1992, Late Cenozoic displacement partitioning in the northwest Great Basin in Structure, Tectonics and Mineralization of the Walker Lane: S.D. Craig, ed., Geological Society of Nevada, Reno, p. 17-52.

Reheis, M. C., and T. H. Dixon, 1996, Kinematics of the Eastern California shear zone: Evidence for slip transfer from Owens and Saline Valley fault zones to Fish Lake Valley fault zone., Geology, 24, p. 339-342 .

Russell, I. C., 1885., Geological history of Lake Lahontan, a Quaternary lake in northwestern Nevada: U. S. Geological Survey Monograph 11,288 p.

Sauber, I, 1994, Geodetic slip rate for the eastern California shear zone and the recurrence time of Mojave Desert earthquakes: Nature, v. 367, p. 264-266.

Stewart, IH., 1988, Tectonics of the Walker Lane Belt, western Great Basin: Mesozoic and Cenozoic deformation in a zone of shear in Metamorphism and Crustal Evolution of the Western US: W.G. Ernst, ed., Englewood Cliffs, New Jersey, Prentice Hall, VII, p. 674-7l3.

Wilden, R, and Speed, R.C., 1974, Geology and mineral deposits of Churchill County, Nevada: Nevada Bureau of Mines and Geology, Bulletin 83, 95 p.

Wills, C. I, and Borchardt, G., 1993, Holocene slip rate and earthquake recurrence on the Honey Lake fault zone, northeastern California: Geology, v. 21, p. 853-856.

55

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Yount, J.C., Bell, J.W., dePolo, C.M., and Ramelli, A.R., 1993, Neotectonics of the Walker Lane, Pyramid Lake to Tonopah, Nevada--Part II, Road log in Lahren, M.M., Trexler, J.R., Jr. and Spinosa, c., Crustal Evolution of the Great Basin and Sierra Nevada: CordilleranIRocky Mountain Section, Geological Society of America Guidebook, Dept. of Geological Sciences, Univ. of Nevada, Reno, p. 383-408.

56

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Shoreline Processes and the Age and Elevation of the Lake Lahontan Highstand in the Jessup Embayment, NV.

Kenneth D. Adams Steven G. Wesnousky

Center for Neotectonic Studies and Department of Geological Sciences

University of Nevada Reno

Abstract Shoreline deposits and landfonns in the Jessup Embayment are used to refine the

timing and magnitude of the last highstand of pluvial Lake Lahontan. Modem analogs of coarse clastic barrier systems are used to interpret both fonn and process of coarse clastic paleo-beach barrier features. Shore features are broadly classified into erosional and constructional fonns and related to 1) preexisting morphology of the shoreline and lake bed 2) sources, physical properties and rates of sediment supply 3) climatic and wave energy environment and 4) the rate and direction of lake level change. Gastropod shells collected from an exposure along a progradational barrier complex near the head of the Embayment yielded AMS 14C ages of 13,280 ± 110 and 13,110 ± 110 yrs BP and defme the lake level during transgression when the surface of the Lake was at about 1326 and 1330 m, respectively. An AMS 14C age of 12,690 ± 60 yrs BP for a camel bone found behind a highstand barrier in a paleo-lagoon places a minimum limiting age on the highstand of Lake Lahontan which reached an elevation between 1338 and 1339 m in the Jessup Embayment. The maximum age of the highstand is constrained by the youngest age of the transgressive deposits (-13.1 ka) which predate the highstand. The elevation (-1338.5 m) and timing (-12.7 - 13.1 ka) of the highstand imply that its magnitude was larger and its age younger than previous estimates. During the regression from the highstand, the Lake fonned at least 28 barriers which are used to delineate the relative rate and character of the regression.

Introduction The shoreline deposits and 1andfonns of pluvial Lake Lahontan display both a

spectacular proxy record of climate change and an excellent sedimentary record of shoreline processes operating on coarse clastic systems. The highest shoreline of Lake Lahontan prescribes an extremely complex path of some 3000 km, dictated by Basin and Range topography, and has many embayments, headlands and islands (Figure 1). It is the preservation of the Lahontan shore record which supplies evidence for the number and timing oflake cycles, the levels attained by these various lake cycles through time, and the coastal processes and paleoenvironmental interpretations made from these deposits and landfonns.

The Jessup Embayment is a small bay located in the northwestern part of the Carson Sink (Figure 1) that was inundated during the last major highstand of Lake Lahontan. The Embayment opens to the southeast and is fronted by an arm of the Carson Sink playa which lies at an elevation of about 1185 m. The highest constructional shorelines in the Jessup Embayment are found at about 1340 m; therefore the shorelines record vertical water fluctuations of about 155 m. Shoreline features in the Embayment are particularly well-developed because of the large fetch (- 60 km) to the southeast (Figure 1) and provide a detailed record of fluctuations and shore processes operating during both transgressive and regressive stages of the last lake cycle.

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The purpose of this paper is to elucidate the lacustrine history of the Jessup Embayment utilizing geomorphic, sedimentologic, stratigraphic and age dating techniques. We present a new AMS radiocarbon date on a camel bone found in a lagoon behind a high barrier that provides a closely limiting age on the last highstand of Lake Lahontan. The observations and results of modem process studies of coarse clastic barrier systems are used as modem analogs to better interpret the landforms and deposits created by pluvial Lake Lahontan. Particular attention is paid to features and deposits of the last major lake cycle because they constitute the vast majority of surfaces and exposures.

Previous Work Israel Russell (1885) performed the seminal work on Lake Lahontan and to this

day his monograph remains the only comprehensive study which covers the entire Basin. From exposures within the major river canyons, Russell (1885) recognized the deposits of two major lake cycles separated by a package of subaerial sediments which he named the medial gravels. He termed the lacustrine deposits the upper and lower lacustral clays, which represent the last and penultimate lake cycles, respectively. Morrison (1964, 1991) redefmed Russell's stratigraphy and named the deposits of the last major lake cycle the Sehoo Alloformation (AF) and deposits of the penultimate lake cycle the Eetza AF. Subaerial and shallow lake sediments deposited between the Eetza and Sehoo lake cycles are now known as the Wyemaha AF. Morrison (1991) claims that the Eetza AF may represent as many as eight lake cycles which are thought to be broadly correlative with marine oxygen isotope stages 6, 8 and 10 (130-360 ka). However, age control on the Eetza AF is limited to two tephra exposures (Wadsworth tephra) along the Truckee River near Wadsworth, NY (Davis, 1978; Morrison, 1991) and several uranium-thorium dates ranging from 128 to > 300 ka on gastropod shells (Kaufman and Broecker, 1965), all of which are located within lacustrine deposits below an elevation of 1265 m. The age of the Wadsworth tephra is not agreed upon but probably lies between about 150 to 200 ka (Berger, 1991; Morrison, 1991). Shoreline deposits from the Eetza AF above 1265 m have never been directly dated and age assignments appear to be based on correlating coarse clastic shore gravels with fine grained inner basin deposits.

The age of the Sehoo AF is much better constrained than earlier lake cycles with literally hundreds of radiocarbon dates on tufa, gastropods, wood, bones and other carbon­bearing material (Broecker and Kaufman, 1958, 1965; Kaufman and Broecker, 1965; Born, 1972; Davis, 1978, 1982, 1983; Benson, 1978, 1981, 1991, 1993; Benson and Thompson, 1987a, 1987b; Benson et aI, 1990, 1992, 1995; Benson and Peterman, 1995; Thompson et aI, 1986, 1990; Lao and Benson, 1988; Dansie et aI, 1988; Dom et aI, 1990). In addition, Davis (1978) has identified no less than 11 individual tephra horizons within the Sehoo AF which have been instrumental in determining its age as well as correlating isolated stratigraphic sections. The beginning of the Sehoo period is debatable, but is commonly thought of as being correlative with oxygen isotope stage 4 (-70 ka) (Lao and Benson, 1988; Dansie et aI, 1988). However, Morrison (1991) has proposed that the base of the Sehoo be placed at the time of deposition of the Marble Bluff (Saint Helens C) tephra at about 35 ka. The end of the Sehoo period also varies between different workers but is commonly placed between 8 and 9 ka (Morrison, 1991; Benson et aI, 1992).

Lake level fluctuations within the Sehoo cycle have also received a great deal of attention through the years, particularly the timing of the Sehoo highstand. However, at present several chronologies exist all of which are based on some number of the radiocarbon dates referred to above (e.g. Thompson et aI, 1986; Morrison, 1991; Benson et ai, 1995) and which place the timing of the highstand between about 12.5 to 14 ka.

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An important point to keep in mind when discussing the lake level and depositional history (e.g. shoreline, deltaic and deep water deposits) of Lake Lahontan is that there is not a common history for all of the different subbasins in the system. Instead, each of the seven distinct subbasins have their own unique histories and shoreline records which are only shared with adjacent subbasins when lake level rose above the adjoining sill. Only when Lake Lahontan rose above its highest sill at Adrian Valley (-1311 m) did the entire Lake basin share a common shoreline record (Figure 1).

Controls on Shoreline Development There are several factors or conditions which control the type of landforms, features

and deposits found in coastal or shore settings. The factors have been delineated primarily from studies pertaining to ocean coasts, but we submit that these same controls are broadly applicable to large lacustrine systems such as Lake Lahontan. The main differences being that lakes experience negligible diurnal tidal fluctuations and generally have a restricted fetch. Regardless, there are several controlling factors in common which include 1) preexisting morphology of the shoreline and adjacent lake bed 2) sources, physical properties and rates of sediment supply 3) climatic and wave energy environment and 4) rate and direction oflake-Ievel change (Forbes and Syvitski, 1994). Each ofthese factors may act alone or in concert to produce unique combinations of shore features and deposits.

The bedrock geology of the Jessup Embayment has not been mapped in detail, but generally consists of Tertiary volcanic and sedimentary rocks resting on a basement of Triassic and/or Jurassic metasedimentary and metavolcanic rocks which are intruded by rhyolite plugs and dikes (Wilden and Speed, 1974). The distribution of the rhyolite units is later used to delineate shore drift directions within the Embayment.

The preexisting morphology of the Jessup Embayment was that of a fairly broad alluvial valley narrowing towards its head and which slopes southeastward at about 2-3 degrees along its central axis. The Embayment extends about 3 km from its mouth to the head of the bay and is bordered on the southwest and northeast by prominent headlands or islands depending on lake level (Figure 2). The central part of the Embayment has abundant sand and gravel deposits of unknown depth but extending to at least IO m in some locations. At the highstand the plan view of the shoreline was very irregular with many headlands and pockets formed by the superposition of a horizontal water plane on a landscape sculpted predominately by subaerial processes.

Most of the lacustrine sediment in the Jessup Embayment was probably derived from alluvial and/or lacustrine deposits existing within the Embayment prior to the Sehoo Lake cycle, produced by erosion of bedrock within the Embayment by wave action or introduced from areas upslope of the Embayment during the· Sehoo Lake cycle. This latter process likely had a relatively minor input because deltaic deposits grading to the highstand or any other lower lake level appear to be absent in the Jessup Embayment.

Most of the surficial sediment in the Embayment has been separated into two mapping units referred to as the beach gravel (Qsg) and the beach and offshore sand (Qss) units, both of which were deposited during the Sehoo Lake cycle (Figure 2). There are no appreciable fme-grained (silt and clay), deep water lacustrine units exposed within the Embayment However, the Jessup Embayment is fronted by extensive fme grained playa deposits of the Carson Sink (Qp!). The distribution of the Qsg and Qss units appear to be controlled by both local slope and longshore transport. The Qss unit is mostly found within the central part ofthe Embayment where the slope is relatively gentle (~ 20). The unit is also found below headlands and islands within the Embayment where the local slope shallows from relatively steep slopes (6-13 0) to relatively gentle slopes (-2-40). There are

3

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also isolated patches of the Qss unit which are found on gently sloping terrace treads formed on overall steeper slopes such as the southwest side of the east island (Figure 2). The dominant clast size in the Qss unit is medium to coarse sand with some well rounded pebbles and angular fragments of branching tufa.

The Qss unit is interpreted to be the result of offshore movement of sand due to wave action on a steep coast. Roy et al (1994) report that simulation modeling suggests that sand moves offshore on submarine slopes steeper than about 10 and onshore for slopes much less than 10. These trends operate despite the direction of relative sea-level movements. The mechanism which facilitates this offshore movement of sand is the suspension of sand size material by direct wave action (Roy et aI, 1994). This process therefore represents a winnowing effect on sediment in the wave affected zone and the resultant concentration of pebbles and larger clasts in many of the shoreline deposits and landforms within the Embayment. The barriers formed of unit Qss were built during slight stillstands in the overall regression from the Sehoo highstand.

The beach gravel (Qsg) unit is found throughout the Embayment on steep as well as gentle slopes. All of the highstand depositional features are composed of Qsg. Below the highstand, the unit is arranged into barrier ridges as well as sheet-like bodies on steeper slopes. The surficial sediments of the terraces are also composed of the Qsg unit. Clast size ranges from sand through pebbles, cobbles and occasional boulders depending on the local energy environment and sediment availability. Sand is generally a minor component in the unit and was probably winnowed and moved offshore through direct wave action as discussed above.

The distribution of a third lacustrine unit (Qbs) is limited to a spit-like feature in the southwestern part of the mapping area (Figure 2). The feature is known as the boulder spit and consists oflarge blocks (up to 1.5 m) of basalt arranged in a recurved fashion around a bedrock core. Some of the blocks exhibit rounding and the interior of the deposit is cemented by dense laminated tufa and the exterior is coated with branching tufa (terminology from Benson, 1994). The north side of the deposit slopes at steeper than the angle of repose but is essentially held up by the tufa cement. There are two terraces on the upper part of the deposit which imply that at least the upper surface has been affected by . waves. The top of the Qbs unit lies about 5 to 10m below the crests of adjacent highstand barriers and appears to lie stratigraphically beneath the Qsg and Qss units which surround the boulder spit. We interpret the boulder spit to be a wave formed deposit that probably predates the Sehoo Lake cycle.

The present climate of the Lahontan basin varies according to both latitude and elevation but can generally be characterized as semiarid to arid with hot summers and cool, relatively wet winters (Houghton et al, 1975). During the Sehoo Lake cycle, the climate was effectively much wetter, with inflow into the Basin exceeding evaporation (Mifflin and Wheat, 1979; Benson and Thompson, 1987b). Most attempts to model the pluvial climate in the Great Basin have called on some combination of cooler temperatures and increased precipitation (Mifflin and Wheat, 1979; Benson and Thompson, 1987b; Hostetler and Benson, 1990). For an excellent review of paleoclimate modeling efforts in the Great Basin see Benson and Thompson (1987b).

Although reasonable estimates have been made about the temperature, precipitation, amount of cloud cover and other paleoclimate indicators during the Sehoo Lake cycle, little is known about wind conditions during this time. Therefore, the wave energy environment that existed in the Lahontan basin during the Sehoo Lake cycle can only be described in . qualitative terms. Judging by the size, degree of development, and large clast sizes (up to

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50 cm) within many of the shore features, it is clear that Lake Lahontan was a place where large waves were generated by strong winds.

Prior to the Sehoo Lake cycle, the hydrologic condition of the Lake Lahontan basin was probably much as it is today with relatively small lakes occupying the lowest parts of the Basin (Morrison, 1964,1991). Therefore, the Sehoo Lake cycle can be viewed as a large scale transgression to the highstand and a similarly large scale regression back down to the Basin floor. There were certainly oscillations within these overall trends and the rates at which these major and minor fluctuations in water level occurred is not well known due to the relatively coarse resolution of existing lake level chronologies (Benson et ai, 1995). However, general rates of transgression and regression can be estimated using Benson et aI's (1995) lake level curve (Figure 3). The curve was developed for the western subbasins which were connected to the Carson Sink subbasin when lake level was above the sill at Fernley (-1265 m). Only when the lake was above the level of this sill did the two subbasins have a common history. With this constraint, the steep rise in the lake level curve between 1265 m and 1335 m during the time period from about 14.8 to 14.1 ka represents a lake level rise of approximately 100 mm/yr. Similarly, the drop from the highstand to the Fernley sill during the time period 13.6 to 13.2 ka represents a regressive rate of approximately 175 mm/yr (Figure 3). Both of these rates are extremely rapid, and imply that the shore features formed during both the transgressive and regressive stages were created rather rapidly, as the Lake probably did not stabilize at many of the shoreline levels for any length of time.

Shoreline Types Wave cut terraces

The most common type of shoreline in the Lahontan basin is the wave-formed terrace. Terraces are characterized by gently lakeward sloping platforms bordered at their upper margin by an eroded cliff (Gilbert, 1885). The juncture between the platform and upper cliff is known as the shoreline angle and the line formed by this juncture is almost always horizontal. The horizontal nature of terraces is what makes pluvial shorelines so distinctive when viewed from a distance.

The word terrace refers to the morphology of the feature and not to the material in which it is formed. ill the Lahontan basin, a spectrum exists where terraces are cut into bedrock as well as thick accumulations of surficial materiaL Even though terraces are usually considered erosional landforms, many terraces have a depositional component to them. Figure 4 shows a cross section through a cut-and-built terrace as defmed by Russell (1885). Note that there is a wedge of debris built out and extending the width of the terrace tread. Only the eroded bedrock should be considered a wave-cut platform. Oftentimes, the built part of the terrace is better developed than the cut or erosional part. However, an exposure through the platform is usually required to discern the relative importance of each of the components. When a terrace is excavated in surficial material it is very difficult to discern the built part of the terrace and hence, this discussion applies mainly to terraces formed on slopes with a thin depositional cover.

Cut-and-built terraces are the most characteristic form of terrace in the Lahontan basin (Russell, 1885) and are usually found on bedrock headlands and islands. ill the Jessup Embayment, cut-and-built terrace treads are commonly composed of cobbles and boulders up to 50 cm in diameter and are often cemented by both dense laminated tufa as well as branching tufa. Usually the branching tufa is found on the exterior of the terrace tread and lower riser with the dense laminated tufa forming a cementing matrix within the terrace tread cobbles and boulders. The boulders and cobbles concentrated on the terrace

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tread probably fonn as a lag deposit where the finer material is either moved directly offshore or along shore by wave action. These types of terrace treads may be likened to the o'I1ter boulder frames ofBluck (1967) and Carter et al (1990a, 1990b).

The most prominent of the cut-and-built terraces in the Embayment is found about 5 to 7 m below the crests of the highest depositional barriers. This terrace is best developed in the southwest comer of the Embayment and on the west side of the north island where it is cut into bedrock, ranges up to 40 m in width, slopes gently lakeward and usually has a well-developed cliff at its landward edge (Figure 2). The cliff on the west side of the Embayment as well as the cliffs on the islands may have been excavated during the formation of this prominent terrace and subsequently accentuated at later, slightly higher lake levels. In several places, the terrace appears to be composed of several closely spaced, vertically overlapping horizons of cemented beach gravel to boulders. As with lower cut­and-built terraces the interior of the deposit is cemented by dense laminated tufa and the exterior is coated by a thin layer of branching tufa. This terrace is probably broadly correlative with Russell's (1885) Lithoid Terrace and here is referred to as such.

Russell (1885) interpreted the Lithoid Terrace to represent the highstand of the penultimate lake cycle and due to its degree of development suggested that the penultimate highstand lasted comparatively longer than the most recent highstand. We have not made any observations contradicting this interpretation, and at this point can only state that the Lithoid Terrace predates the Sehoo highstand because small Sehoo highstand barriers are observed to lie on top of the Lithoid Terrace. If the Lithoid Terrace does date from the penultimate lake cycle, it is possible that it is correlative with the boulder spit (Qbs), as both lie at about the same elevation and are cemented by dense laminated tufa.

From observations within the Jessup Embayment and elsewhere throughout the Lahontan basin, it appears that the Lithoid Terrace and lower cut-and-built terraces are best developed and preserved on shores that are composed of intermediate to basic volcanic bedrock. Wallace (1977), in discussing the longevity offault scarps in different materials, postulates that fractured bedrock can maintain steep slopes for up to I Ma in the Great Basin. However, he does not distinguish between different types of bedrock, but our observations suggest that at least in terms of shoreline terrace development, intermediate to basic volcanic flow rocks display the best development and preservation. This may be because of the tendency of andesite and basalt to break into blocks and not disintegrate into small pieces. Because of their blocky weathering habit and relative resistance, there is probably a correspondingly low production rate,of colluvium on these types of shores which would act to mute the morphology of cut-and-built shorelines. Cementing of the terrace treads by dense laminated tufa also plays an important role in the longevity of cut­and-built terraces.

It is possible that the development and preservation of terraces is accentuated by the repeated effects of multiple lake cycles. The longevity of terraces formed in intermediate to basic volcanics may be such that the amount oftime it takes to completely mute these terraces by weathering, erosional and depositional processes is longer than the time between major lake cycles. Therefore, each successive lake cycle would tend to accentuate the fonn and development of the terraces. This concept is referred to as polycyclic shorelines and can also be extended to the rounding of beach gravel (polycyclic gravel) where the degree of rounding may be the result of more than one major lake cycle. The Lithoid Terrace may be an example of a polycyclic shoreline with its major development taking place during the penultimate or an earlier lake cycle and accentuation taking place during the Sehoo Lake cycle. However, the surficial sediments were mapped as Qsg because they were at least wave affected by the Sehoo Lake (Figure 2).

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The lower terraces do not fonn exclusive horizons separate and unique from the barriers in the Embayment but instead trend into barriers in at least two locations. In the southwest part of the Embayment is a cuspate barrier complex which has several erosional terraces on its southeastern flank (Figure 5) that trend directly into barriers at 1235 and 1227 m. As in all cases observed throughout the Lahontan basin, where erosional terraces trend into adjacent depositional features (spits, barriers, etc.), the crest of the depositional feature usually lies one to three meters above the shoreline angle fonned by the terrace tread and adjacent upper riser or cliff. This same phenomena was observed in the Lake Bonneville basin by Gilbert (1890, p. 122-125) and attributed to the idea that the shoreline angle approximates still water level of the lake whereas the crests of constructional features represent the effects of stonn deposition.

Constructional and beach barrier features Depositional or constructional shorelines are also prevalent in the Lahontan basin

and are commonly in the fonn of some sort of beach barrier. The essential distinction between coarse clastic barrier features and beaches is that all barrier types have identifiable crests and commonly have a well-defined backslope or back barrier depression (Carter and Orford, 1993). fu contrast, beaches always slope lakeward. For the purposes of this paper, depositional shorelines are further subdivided into spits, pocket barriers, cuspate barriers, loop barriers and other types according to the classification scheme outlined in figure 6. Although depositional shorelines can be broadly classified into drift or swash aligned features, most shorelines probably exhibit the effects of both processes at different stages of their development (Orford et aI., 1995). Excellent e),Camples of most of the above types of constructionallandfonns can be found in the Jessup Embayment. Prominent barrier ridges occur from an elevation of 1227 m up to the highstand at about 1340 m. Most of the larger and more continuous barrier ridges are located in the central part ofthe Embayment where the average slope is about 2 degrees (Figure 2).

The sequence of barriers from about 1328 m down to about 1227 m (Figures 2 and 5) are interpreted as recessional barriers and the surface on which they are fonned as a recessional strand plain (Roy et aI, 1994), even though the latter is on a smaller scale than its marine analog (Figures 2 and 5). Where the slope increases past about 5 degrees, as on the islands and headlands in the Embayment, it appears that cut-and-built terraces are the dominant shoreline type. This points to a slope control on the type of shoreline fonned in the lacustrine environment. However, other factors are probably involved which dictate the type of shoreline fonned including sediment character and availability (both locally derived and longshore drift derived) and irregularities in the shore that may act as sediment traps to longshore transport.

Sediment availability and the rate oflake level change (rising and falling) play important roles in the development and morphology of barrier ridges. For a stable or rising water level and limited sediment supply, swash aligned coarse clastic barriers tend to migrate landward (Carter and Orford, 1984; Orford et aI, 1991b; Orford et aI, 1995) through a process called barrier rollover. This is accomplished by overwash processes where material is moved from the lakeward side of the barrier, over the crest and deposited on the backside of the barrier (Carter and Orford, 1984; Orford et aI, 1991a, 199Ib). In contrast, overtopping is when the onrush of sediment and water just reaches the crest of the barrier, causing deposition at this location. Whereas overwashing leads to crestal breakdown and baJ:rier migration, overtopping leads to crestal buildup and increased stability (Orford and Carter, 1982; Orford et aI, 1991a, 1991b). Both of these processes lead to a net shoreward transfer of coarse clastic material. However, certain wave

7

!..-r ___ _

environments and the effects of gravity can move some sediment lakeward down the front of the barrier (Bluck, 1967). Another reason for the landward migration of coarse clastic barriers is that breaking waves probably generate larger tractive forces directed landwards than the backwash does directed lakeward. The difference in the magnitude of tractive forces can in part be explained by the lesser volume of the backwash due to infiltration of water into the barrier (Carter and Orford, 1993). The net effect of this difference results in the movement of coarse sediment in a landward direction.

The Progradational Barrier Complex A progradational barrier complex (PBC) is defmed as a shore feature where

multiple barrier ridges occur in close proximity to one another and at about the same elevation, which is indicative of a fairly stable water level and continued sediment supply (Orford et aI, 199Ia). A well developed PBC is located at the head of the Jessup Embayment about 6 to 8 ill below the elevation of adjacent highstand features (Figures 5 and 7). In the complex, the individual ridges range in elevation from about 1332 m to about 1334 m and can be seen to truncate one another in plan view (Figure 7). The highest ridge is in the center of the complex with lower ridges both lakeward and landward. We interpret this relationship to indicate that water level was fairly static or fluctuated around some small elevation range during the building of the complex. The highest ridge in the complex ( ~ 13 34; Lower Barrier 0 in figure 5) is not perfectly horizontal along its crest but instead has a noticeable trough about 50 em deep and 15 m wide oriented perpendicular to the crest. We interpret this trough to be a paleo-overwash channel and call attention to this feature to make the point that the crests of both swash and drift aligned features are commonly horizontal, but oftentimes have irregularities and sometimes slope toward their distal ends. The surface age of the PBCis < 15 ka based on 36CI exposure dating of clasts sampled from the upper two meters of the complex (Figure 8) (Fred Phillips, written comm., 1995).

An intermittent stream occupying the main wash of the Embayment has eroded the landward side of the PBC and provides a clear exposure along strike of the complex (Figure 7). The site is marked as figure 8 in figure 5 and a log of the exposure is provided in figure 8. The exposure reveals a complicated history of mUltiple drift directions and erosive events in an overall aggrading pile of coarse clastic beach sediments. The lowest part ofthe exposure displays horizontally bedded and well sorted sand (Unit 1) overlain by several packages (Units 2, 3, and 4) of coarse gravel tabular beds dipping to the south (15 to 250) which we interpret to be foresets indicating north to south spit building. At about the 55 m mark, the south dipping tabular foresets are overlain by north dipping tabular foresets (Unit 5), indicating a local reversal in net shore drift (Figure 8). Units 4 and 5 are in turn truncated by a coarse cobble gravel horizon (Unit 6) with a planar erosive base at about 1327.5 to 1328.5 m. Unit 6 fines upward into a well stratified pebble, sand and cobble zone (Unit 7) from about 1328.5 to 1329.5 m. On the log of the exposure (Figure 8), the apparent dip directions of Unit 7 are both to the north and to the south. However, these apparent dip directions are an artifact of the trend of the exposure. The true dip directions of this horizon and the horizons above are easterly and the apparent change in dip directions occurs because of the bend in section (Figure 8). The upper 20 to 40 cm of Unit 7 (~1330.5 m in the northern half of the exposure) is well cemented which we interpret to represent beachrock formed when this surface was the active beach. The surface of the beachrock is coated by a thin layer of branching tufa that coats only the upper parts of clasts embedded in the beachrock.

8

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In the southern half of the exposure a coarse cobble to boulder horizon (Unit 8) cuts out the beachrock and tufa layer. Clasts in Unit 8 range up to 30 cm and are generally well-rounded. Within Unit 8 are clasts that have only one side coated with the branching tufa indicating that these clasts were probably eroded from the underlying beachrock and tufa horizon (top of Unit 7). Unit 8 is also present in the northern half of the exposure, but lies on top of and has not eroded the beachrock and tufa horizon (Figure 8). Above Unit 8 lie a series of pebble to cobble gravel horizons (Units 9 through 12) that are mostly clast supported but some have a sandy matrix filling the interstices between clasts. Both shape and size sorting are well developed in these upper horizons.

The surface ridges of the progradational barrier complex are composed of Units 8 through 12 which are exposed in the long section (Figure 8). The ridges and upper few meters of the progradational barrier complex were deposited during the regressive phase of the Sehoo lake cycle. This interpretation is supported by crosscutting relationships where a regressive spit (feature 2b, Figure 7) to the west of and at about the same elevation as the PBC (feature 3, Figure 7) truncates the distal end ofa spit (feature 1, Figure 7) that began building during the highstand and was lengthened as lake level began to recede. The regressive spit in tum predates the barrier ridges of the complex because it is behind (landward of) the complex and so would be sheltered from strong wave action (Figure 7). Therefore, the barrier ridges also postdate the highstand. However, AMS radiocarbon dating suggests that the lower part (below about 1330 to 1331 m) of the PBC was deposited prior to the highstand. The contact between the transgressive (lower part of the exposure) and regressive (upper part of the exposure) deposits may be represented by the beachrock and branching tufa horizon.

Dating specific horizons within the PBC, combined with known elevations provide precise temporal and elevation estimates of lake level because the dated layers represent the shore of the Lake during the times of deposition. We collected tephra samples from three horizons within the lower part of the barrier complex exposure as well as gastropod shells from two of the same horizons. The tephra samples were taken from a horizontally bedded sand layer near the base of the exposure (Unit 1) (~1326 m), from a conspicuous fine grained layer (within Unit 2) sandwiched between the lower packages of tabular foresets (Units 2 and 4) (~1326 to 1326.5 m) and from just below the beachrock horizon about 15 m from the north end of the log (Unit 7) (~1330.5 m) (Figure 8). The three samples contained from 7 to 20 % volcanic glass shards so are not true ash layers, but rather ashy clastic sediments (Andrei Sarna-Wojcicki, written comm. 1995). The upper sample was collected from the matrix between coarse pebbles to cobbles and so has experienced some amount of reworking within a high energy beach environment. The middle and lower samples were probably also reworked because they too have low concentrations of glass shards.

The glass shards in all three of the ashy clastic sediment samples best correlate to one another and to a group of tephras known as the Walker Lake-N egit Island Causeway set of "proto" Mono Craters layers which are estimated to be between ~65 to ~80 ka in age (Andrei Sarna-Wojcicki, written comm. 1995). These correlations indicate that the tephra layer or layers were originally erupted in early Wisconsin time and not during the late Wisconsin or Sehoo time. However, it is clear from the sedimentology of the layers as well as the glass shard concentrations that all three of the ashy clastic sediment horizons have been reworked and do not represent original airfall. The question is, how much time elapsed "Qetween the original eruption of these tephras and their incorporation into the PBC?

9

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Data bearing on this question are in the fonu of AMS radiocarbon dates from gastropod shells collected from the ashy clastic sediment layers. We collected Vorticifex (Parapholyx) solida shells (Burch, 1989) from both the upper and middle ashy clastic layers for radiocarbon dating and additional shells from throughout the section for X-ray diffraction studies. The dates were provided by the Swiss Federal Institute of Technology in Zurich. Shells from the upper ashy clastic sediment layer date from 13, 110 ± 110 yr BP, while shells from the middle ashy clastic sediment layer date from 13,280 ± 110 yr . BP (Figure 7) (Irka Hajdas, written comm., 1994). Although these radiocarbon estimates are in stratigraphic order, they certainly do not agree with the age estimates provided by the tephra correlations.

This situation implies one of two possibilities. Either 1) the shells and glass shards were deposited with the beach gravel sometime between 60 and 85 ka and then at some later date the shells were recrystallized, thereby incorporating young carbon and providing anomalously young ages or 2) the beach gravel, shells and glass shards were all deposited about 13.1 to 13.2 ka, which implies that the glass shards were derived from a deposit in the area. To ascertain which of these scenarios is most likely correct, we used X-ray diffraction to examine the composition of shells from each of the ashy clastic sediment layers, shells from the upper part of the sequence which we interpret to post date the highstand (:s 12 .7 ka) and shells from the shore of modern Pyramid Lake which we interpret to represent recently living examples of Vorticifex. According to B~ggild (1930), freshwater pulmonate gastropods which includes Vorticifex are composed of aragonite when living. All of the shell samples that we examined were also composed of aragonite and none were composed of calcite implying that the shells from the ashy clastic sediment layers have not been recrystallized and that their radiocarbon ages represent the age of the lower or transgressive part of the progradational barrier complex.

Natural Variability in the Height of Depositional Shorelines The highstand constructional shorelines in the Jessup Embayment are a mixture of

both drift and swash aligned features including spits, pocket barriers, and loop barriers (Figures 2 and 5). These features are the clearest examples of transgressive depositional features within the Embayment simply because of their location at the top of the stack of shorelines. Figure 5 shows the results of detailed surveys of the elevations of constructional highstand features. The shorelines were measured using a Total Station surveying instrument which combines an electronic distance measuring (EDM) device with a theodolite. We used local benchmarks for elevation control. The accuracy of the instrument is within a centimeter on shots of up to a few kilometers, but because the benchmark elevations are reported to the nearest 0.1 foot, the precision of the measurements is assumed to be well within ± 0.1 m. Even though there is as much as 2.6 m of difference in the 10 highstand shoreline measurements (Figure 5), we submit that all of these features were built during the last highstand and the differences reflect natural variability in the height at which the crest of a depositional shoreline fonus above a still­water plane. Natural variability in the height of shorelines is controlled by the size of waves reaching a particular shore which in turn is controlled by fetch, lake bottom configuration, geometry of the shoreline (e.g. embayment vs. headland) and the presence or absence of offshore obstructions (i.e .. islands or shoals) (King, 1972). Shores that tend to have the highest elevated barriers, relative to still water level generally have areas oflarge (10's of kilometers), unobstructed fetch and moderately steep slopes approaching the shore.

The measured highstand shorelines can be further separated into swash or drift aligned features. Overall, the average height of the swash aligned features is 1339.8 m,

10

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while the average height of the drift aligned features is 1339.0 m or about 0.8 m less. However, the lowest measurement was taken on a swash aligned pocket barrier (Shoreline F, figure 5) located to the north of the progradational barrier complex at the head of the Embayment. For reasons explained in the section above, we interpret that there was a smaller barrier complex at the same site as the progradational barrier complex prior to the highstand when Shoreline F was formed. The smaller barrier complex may have acted as an offshore obstruction and therefore dissipated wave energy at the location of Shoreline F during the highstand. The relatively poor development of Shoreline F also supports the hypothesis that wave energy was not as vigorous at this location as compared to the rest of the highstand shorelines.

Longshore Drift Directions and Their Determination The net longshore drift direction near the head of the Embayment is clockwise or

from southeast to northwest to northeast (Figure 2). This pattern was determined by several techniques including mapping of a distinctive lithology in beach sediments, interpretation oflandforms, stratigraphy and tracing the size and sorting of clasts in the barrier deposits. A distinctive flow-banded rhyolite outcrops and is found in the sediments in the western part of the mapping area (Figure 2). The distribution of this rock type in the sediments was mapped near the head of the Embayment and is denoted by a coarse dotted line in figure 2. Several observations can be made about the distribution of the flow-banded rhyolite. First, the furthest east that it is found in alluvial sediments is the western part of the main drainage upstream from the progradational barrier complex (Figure 2). This rock type was not found in any of the alluvial sediments upstream from the high shorelines in the drainages to the east of the main drainage. However, the banded rhyolite can be traced in the recessional barrier ridges to the northwest side of the north island. Banded rhyolite is found in all of the high shorelines from Shoreline A through Shoreline E (Figure 5). To the north and east of these high shorelines the upper limit of distribution of the banded rhyolite descends to Recessional Barriers 3 and 4 at about 1320 and 1317 m, respectively. The high pocket barriers on the east side of the main wash (Shorelines G through J, Figure 5) do not have any banded rhyolite within them, but instead all of the sediment within each pocket . barrier was most likely derived from local sources (Figure 5). The distribution of the banded rhyolite in both the high barriers and recessional barriers demonstrates that this distinctive lithology was progressively spread to the east in successively lower barriers during the recession from the highstand.

Another method used to interpret net shore drift directions was noting the directions that spits were built. Shorelines B and D (Figure 5) are both spits built during the highstand from SSE to NNW which is consistent with a clockwise shore drift pattern at the head of the Embayment. As discussed above, after the development of the regressive spit (feature 2b, Figure 7), the ridges of the PBC (feature 3, Figure 7) were emplaced, effectively sealing the head of the Embayment. The spit and crest of the PBC are at about the same elevation, indicating that at this location continuing sediment influx via longshore drift caused a change in the character of shorelines from being dominantly drift-aligned to being dominantly swash-aligned. However, the banded rhyolite was spread to the northeast (clockwise) in the PBC and lower barriers which is evidence that the apparently swash­aligned barriers also exhibit characteristics of drift-aligned features.

Lower Barrier 4 is generally composed of coarse gravel on the surface, but at a depth of about 130 em changes to a well sorted sand. We interpret this lower sand as the' beach and offshore sand unit (Qss) and further maintain that the upper gravel part of the barrier was drifted in from the west over the top of the Qss unit. Our interpretation that unit

11

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Qss was formed by the offshore movement of sand when the lake was at relatively high levels is strengthened because the recessional gravel barrier stratigraphically overlies unit Qss. '

The last piece of evidence we present for a clockwise drift direction at the head of the Embayment is the eastward fining of sediment in Lower Barriers :3 and 4. We have not performed particle size analyses of sediment along the crests of these barriers, but observations in the field indicate that the mean particle size decreases while the percentage of sand increases. All of the above observations and interpretations point to a dominant clockwise drift pattern at the head ofthe Embayment, although the morphology and orientation of some of the recessional barriers may also be used to suggest that they were dominantly swash-aligned features.

The clockwise drift direction in the Jessup Embayment and the fact that it faces south to southeast implies that wind and waves which built the highstand barriers and lower, regressive barriers were primarily coming from the south or southeast. This observation is at odds with those of Morrison (1964, 1991) who states that during Sehoo time, strong storm winds never came from the south or southeast. However, judging by the caliber ofSehoo beach sediments (up to 30 cm) and excellent development of shore features in the Jessup Embayment, strong storm waves did come from the south and southeast during Sehoo time.

Shoreline Processes at the Highstand A small highstand pocket barrier with an enclosed playette behind it, herein referred

to as the Jessup playette, is located in the northwestern part of the Embayment (Figure 2 and Shoreline C in figure 5).We excavated a 5 m deep trench perpendicular to the barrier and into the playette. The exposure enables a more detailed understanding of shoreline processes at the highstand and an estimate of its timing. Figure 9 shows a detailed topographic map of the trench site and locations of the trench and adjacent soil pit. The . crest of the predominately swash-aligned barrier lies at an elevation of 1339.9 m and the surface of the playette is about 20 cllliower. The closed depression which was subsequently filled by the playette sediments was created by the emplacement of the barrier across this small wash. The extent of the drainage basin available to fill the closed depression is limited to the small hills surrounding the playette which have a combined area of about 6000 m2 (Figure 9). It is possible that the relatively large drainage along the southwest part of the map (Figure 9) supplied sediment into the playette prior to the breaching of the barrier by the drainage, but does not seem likely.

The sediment deposited in the wash and the associated fan located to the south and east of the playette (Figure 2) consists of coarse cobbles to boulders of the banded rhyolite as well as intermediate to mafic volcanics. Even though this coarse sediment is post-pluvial in age, we fmd it unlikely that the caliber of the material moving down this drainage has changed very much since the highstand. The slope between the axis of the drainage and the top of the lagoonal sands near the bottom of the trench (measured in a straight line) is about 5°. This is approximately equal to the slope of the drainage adjacent to the playette. lfthe drainage spilled into the closed depression behind the barrier, there should be stratigraphic or sedimentologic evidence. However, there are no cobbles or boulders within the playette­fill sequence as described below. The coarsest material present in the playette-fill are rare clasts ranging in size to a few centimeters which appear to be sourced from the surrounding hillsides.

Figure lOis a log of the trench exposure showing the relationships between the different packages of sediments used to interpret the history and timing of the highstand at

12

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this location. We intetpret the sediments in the exposure to record a single lake cycle at this elevation. The different packages of sediments will be discussed beginning with the oldest and ending with the youngest.

The oldest package of sediment exposed in the trench is located at the base of the exposure in the northwestern half of the trench (Figure 10). This massive, well­consolidated unit consists of poorly sorted, angular mafic volcanic clasts (::; 10 cm) supported by a matrix of medium sand to gravel. Clasts tend to be somewhat concentrated at the unit's upper surface. Within the unit there are common fine to very fine root casts that are iron stained. Overall, the unit appears weathered but there is not an identifiable paleosol developed on its upper surface. This unit is intetpreted to be alluvium which was deposited at some time prior to the Sehoo lake cycle and is probably relatively thin, although the base was not reached. The contact with the next overlying unit is shatp and slopes gently to the southeast.

The next younger unit is referred to as the lagoonal sands and consists of a wedge­shaped package thickening to the southeast to a maximum of about 50 cm (Figure 10). The base of the unit is poorly sorted, matrix supported cobbles and gravel grading upward to well-sorted fme to medium sand at the top of the unit. Gravel and cobbles within the unit are angular, hematite-stained volcanics and increase in abundance to the northwest. The upper sandy part has a greenish cast, possibly indicating that this unit is reduced. There is also limonite-hematite staining along common fine root casts and precipitated along horizontal bands. Cross-bedding (amplitude ~4-5 cm) is present in the upper part of the unit with apparent local transport directions both to the southeast and northwest.

The lagoonal sands are intetpreted to represent sediment deposited in a back-barrier lagoon during the highstand. The contemporaneity of the lagoonal sands and the highstand is demonstrated by the way in which the barrier gravels interfinger with the lagoonal sands (Figure 10). Cross-bedding within the lagoonal sands is intetpreted to represent in part, the effects of overwash where a volume of sediment and water was washed over the crest of the barrier and into the lagoon generating local currents. Sedimentation continued in the back barrier lagoon after the time when the last waves washed gravel over the crest of the barrier and into the lagoon and is represented by the thickness oflagoonal sands (~ 12 cm) above the tail of barrier gravel (Figure 10). We intetpret this relationship to mean that the lake did not recede immediately upon depositing the overwashed barrier gravels in the lagoon, but probably maintained a level above ~ 1336.5 m. A relatively high lake level and the high permeability of this type of coarse clastic barrier may have provided the mechanism by which water could seep from the lake into the lagoon, thereby maintaining a high water level in the lagoon (Carter and Orford, 1993).

The barrier gravels are contemporaneous with the lagoonal sands, as stated above, and tend to be well rounded, well stratified and well sorted within strata. There is also a . tendency for the gravels to be sorted according to shape which is a common characteristic of beach deposits (Bluck, 1967; Carr, 1971; Orford and Carter, 1982; Orford et aI, 1991a). Shape sorting is particularly evident in the southeastern part of the trench where there is a popUlation of disc shaped clasts that range up to about 25 em and are oriented parallel to the ground surface (dipping ~ 11 0 SE) (Figure 10). Although the barrier gravels are comprised of lithologies ranging from mafic to felsic volcanics and metasedimentary rocks, the lithology of the coarse disc population is limited to the banded rhyolite. Because the banded rhyolite tends to break into platy clasts, it was preferentially sorted to the exclusion of other more equidimensional shaped clasts due to hydrodynamic conditions within the surf zone (Orford and Carter, 1982; Orford et aI, 1991a). The package of coarse disc shaped cobbles can be seen to truncate the fmer gravel layers below them and, hence, probably represents

13

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the last phase of barrier sedimentation at this site. In cross section, the barrier is arranged into somewhat tabular beds that dip lakeward (8-100 ) on the lakeward side of the barrier, flatten near the crest of the barrier and dip steeply (33-340 ) landward on the back of the barrier. This arrangement of dip directions can be likened to foresets, !opsets and backsets according to the position within the barrier.

The steeply dipping (33-340 ) backsets are tabular in their central part and smoothly grade into a horizontal surface near their distal end in an asymptotic relationship (Figure 10). The tabular nature is in part defined by the alignment of platy clasts. Individual sand layers within the horizontal portion of the barrier tail can be seen to ramp up into the steeply dipping backsets. We interpret the sedimentologic relationships to demonstrate progressive accretion and migration of the backsets toward the northwest. Because the barrier gravel interfmgers with the lagoonal sands, it appears that the entire barrier migrated to the NW probably through barrier rollover (Figure 10)

The hinge point where the topsets steepen into the backsets occurs at a position about 25 meters from the southeastern end of the trench (Figure 10). The steepness of the backsets (33-340), tabular arrangement and lack of erosional surfaces within and at the base of the package all suggest that these beds were deposited in a standing body of water and are analogous to Gilbert-type forsets in a deltaic environment (Gilbert, 1885). If water level in the lagoon was relatively low « 1336 m) and the package of steeply dipping backsets was deposited on the dry, landward dipping part of the barrier, erosion, channeling and truncation of depositional surfaces would be expected within this package and at its basal contact with the lagoonal sands. However, no evidence of erosion or channeling was observed, but instead the beds are regular and relatively continuous thereby supporting the hypothesis that these sediments were deposited in a standing body of water. The water level in the back barrier lagoon during deposition of the gravels can be approximated by the elevation of the crest of the hinge point which is at about 1338.8 m (Figure 10).

Water may have entered the lagoon by several ways including direct precipitation, runofffrom the surrounding hills, waves washing over the crest of the barrier or by water infiltrating through the barrier from the Lake. Because the drainage basin for the lagoon is quite small (-6000 m2), direct precipitation and runoff from the surrounding hills was probably small compared to the amount of input of water from the Lake. Direct overwash of waves probably contributed significantly to raising water levels in the lagoon during storm periods.

The volume of water moving through the barrier depends on the hydraulic conductivity of the gravel and the hydraulic head defmed by the difference in water height between the lake and lagoon (Carter et aI, 1989). Observations of the barrier gravel in the trench down to an elevation of about 1338 m indicate that the hydraulic conductivity is probably quite high due to the clast supported nature of the gravel and the relative lack of· fine grained matrix. During storm surges, the absolute lake level at the shore is increased which would cause a concomitant increase in the hydraulic head and a possible increase in the water level ofthe lagoon, depending on the rate of infiltration through the barrier gravels. The rise in water level at the shore produced by the storm surge also increases the probability of overtopping and overwashing by stann waves which would also lead to increased water levels in the lagoon.

Based on the interpretation that water level in the lagoon was at a minimum of about 1338.5 m and the majority of this water was due to percolation through the barrier gravels from the lake, we estimate that the still water level at the highstand of Lake Lahontan at this location was somewhere between 1338 and 1339 m. This estimate

14

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suggests that this and other highstand barriers in the Embayment rose from 0.2 to 2.8 m above still water level (Figure 5).

The youngest package of sediments exposed in the trench were deposited subsequent to lowering of the Lake. We refer to the sediments as playette-fill deposits and they consist of alternating layers of sandy alluvial wash or fluvial deposits separated by layers of fine sand, silt and clay (Figure 10). The alluvial layers tend to be less continuous across the exposure than the fine grained layers and consist of fine to coarse sand with rare concentrations of granules to several centimeters. The beds are generally tabular but several wedge-shaped beds which pinch out to the southeast are observed in the northwestern half of the playette exposure. Many of the sandy layers have erosive bases and common crossbedding. The amplitude of cross-bedding ranges from 3 to 7 cm and, in most instances, local transport direction is from NW to SE. Less common cross bedding indicating SE to NW transport is also present. The thickness of the beds range from several cm to about 10 cm. In some cases their upper contact with the finer grained layers is abrupt but in others the contact is gradational indicating a fming upward sequence. We interpret the sandy layers to represent alluvial or fluvial wash layers generated by local rainfall events and sourced from the surrounding hillsides.

Finer grained layers interbedded with the alluvial layers are generally massive with no apparent sedimentary structures. The particle size distributions of two of the more prominent fme grained layers were examined and found to consist primarily of silt, fme sand and clay in order of decreasing abundance (Table 1). The thickness of the fine layers range from a few centimeters up to about 8 cm for two of the thickest layers near the southeastern part of the playette exposure (Figure 10). Almost all of the fme layers have common, fme vesicular pores and vertical cracks. Many of the fine layers also have common fine to coarse vertical root casts. Although there is evidence for subaerial exposure and drying, there is no evidence for soil horizonation below - 1339 m other than the pores, cracks and root casts.

The fine grained layers tend to be more continuous than the coarse, sandy alluvial layers and many can be traced across the entire playette exposure (Figure 10). Most of the fine layers are relatively horizontal, but several dip gently to the SE and have as much as 20 to 30 cm of relief. The layers also tend to be less continuous near the top of the exposure. Based on the particle size distribution (Table 1), massive nature, vesicular pores, and

Table 1. Particle size distributions for two of the more prominent fme grained layers in the playette-fill sediments.

Sample # Jesp95-7 Jesp95-3

Sand (wt %)

13.03 15.10

Silt (wt %)

79.72 74.00

Clay (wt %)

7.24 10.90

vertical cracks, we interpret the fme grained layers to represent eolian dust deposits that were either deposited directly on the playette surface as it aggraded or settled out of columns of muddy water during ephemeral flooding of the playette surface. This latter explanation would provide a mechanism for concentrating the dust that had fallen on the surrounding hills above the playette (Young and Evans, 1986).

At the top of the playette-fill sediments, above about 1339 m is a zone of soil development delineated on the log (Figure 10) and characterized by strong platy structure with intersecting vertical cracks causing the soil to break into strong prisms. The soil is

15

dominated by silt sized material with lesser amounts of both fine sand and clay. The interbedded, sandy alluvial units present lower down in the stack do not appear to be present in the soil but may be masked by bio- or pedoturbation. The soil is also characterized by strong effervescence, indicating an accumulation of carbonate most likely derived from atmospheric sources (Reheis et aI, 1989, 1995; Chadwick and Davis, 1990). The zone of carbonate accumulation appears to be roughly coincident with the zone of soil structure. In contrast to the upper part of the fill, all of the playette-fill sediments below about 1339 m do not effervesce, indicating that there is no accumulated carbonate in these sediments. We do not think that carbonate was leached from these layers after deposition. because evidence is lacking of leaching and reprecipitation lower down in the stack.

Papke (1976) reported that CaC03 is common in Nevada playas and Chadwick and Davis (1990) proposed that the introduction of carbonate-bearing eolian material derived from wind erosion of playas such as the Carson Sink was a major factor in the formation of soils in the Lahontan basin. The presence of CaC03 in the upper part of the trench exposure coincident with the depth of soil formation fits the model of Chadwick and Davis (1990) where much of the soil formation was the result of the introduction of carbonate rich dust into the surface sediments of the playette. However, if we are correct in our interpretation that the fine grained layers in the lower playette-fill package also represent dust accumulation, it is perplexing that these dust layers do not contain CaC03. Therefore, either the carbonate has been removed, which we fmd unlikely as explained above, or there was not carbonate in the dust when it was originally deposited. Because the age of the playette-fill sediments is known to be younger than 12.7 ka, the deposition of the dust layers within the playette may record dust deposition through the Holocene.

The lack of carbonate in the lower layers of dust implies that the source of dust may have changed through the Holocene. If this is true, during most of the filling of the playette, the dust that was present in the area and actively aggrading on the surface of the playette probably did not come from the Carson Sink playa. Instead, the dust may have traveled from further removed sources, but its origin is uncertain.

Timing and Magnitude of the Sehoo Highstand A new minimum age constraint on the timing of the Sehoo highstand is provided

by an AMS radiocarbon age estimate of 12,690 ± 60 yr BP on a camel bone found near the bottom of the trench (Figure 10). The distal end of a radioulna (fused front fore limb) and a metacarpal (foot bone) from a Camelops hesternus were found at the contact between the lagoonal sands and the playette-fill sediments (Figure 11). Amy Dansie from the Nevada State Museum identified the bones (Personal comm., 1995) and the age estimate was provided by Thomas Stafford from the University of Colorado, Boulder (Written comm., 1996). The metacarpal was found in front of and adjacent to the radioulna suggesting that these two bones were connected by soft tissue when deposited and buried. We interpret that the bones were deposited shortly after the death of the animal, probably within a matter of months to years, not decades to millennia. Hence, the bones were deposited and buried immediately upon the recession from the highstand, coincident with the change in sedimentation rate and style.

The bones were found at the contact between the lagoonal sands and the playette-fill sediments which we interpret to represent a major change in depositional processes and sediment sources (Figure 11). Whereas the lagoonal sands represent subaqueous deposition in a standing body of water, the playette-fill sediments represent periods of fluvial and/or alluvial deposition separated by periods of stability and dust accretion either by direct subaerial deposition or settling out of shallow, ephemeral water bodies. The

16

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contact between the lagoonal sands and the playette-fill sediments is abrupt and marks the time of recession of Lake Lahontan from the highstand. There is no evidence of weathering or soil formation at this contact and so we interpret that the Sehoo highstand occurred immediately prior to 12,690 ± 60 yr BP, the age of the camel bones.

When combining the age of the camel bone with the two AMS radiocarbon dates in the PBC, we may place new constraints on the timing and magnitude of the Sehoo highstand. An advantage of these dates is that they all pinpoint a shoreline elevation for a specific time or range of time. The two shell dates (13,280 ± 110 and 13,110 ± 110 yrs BP) were taken from beach deposits and therefore place elevational as well as temporal constraints on the location of the lake shore leading up to the highstand (Figure 8). The minimum limiting date (12,690 ± 60 yr BP) on the camel bone combined with the elevation of water at the highstand (1338 to 1339) provides a minimum constraint on the age and elevation of the highstand in the Jessup Embayment (Figure 12). A maximum time constraint on the highstand is provided by the age of the upper shell date (13,110 yr BP) (Figures 8 and 12) because the deposits from which the shells were sampled from predate the highstand. Therefore, the timing of the last Lake Lahontan highstand was between about 13.1 and 12.7 ka, but was probably closer to 12.7 ka because we interpret this to be a closely limiting age constraint. The duration of the highstand is unknown but was probably on the order of decades to maybe one hundred years. This interpretation is based on the relative development of constructional barrier features around the Basin and the relative lack of erosional terraces fonned in bedrock near the elevation of the highstand. This observation implies that the highstand was of long enough duration to fonn well developed spits and barriers but not long enough to fonn a well developed terrace.

Comparison of our data with Benson et aI's (1995) most recent lake level curve implies the timing of the highstand was younger and the elevation higher than previously proposed (Figure 12). The differences between the two curves may in part be explained by the different materials used to estimate both thetiming and magnitude of the highstand. Benson et al (1995) primarily used radiocarbon dates from inorganic carbonate (tufa) to constrain both the timing and magnitude of the highstand. However, the water depth at which tufa forms is not known so the upper limit of tufa growth is a minimum estimate oflake level. From field observations of over two hundred high shoreline localities throughout the Basin (Adams and Wesnousky, 1994, 1995), we have never observed tufa present at the high shoreline level. Commonly, tufa is found about 5 to 7 m lower. The lack of tufa observed on high shorelines may reflect our bias of examining constructional highstand features. Tufa is most commonly found on stable substrates in places that received high wave energy. However, we have not observed tufa on steep bedrock slopes or cliffs adjacent to and at the same elevation as highstand constructional shorelines. These observations may suggest that approaching the time of the highstand, lake level rise was so abrupt and the duration of the highstand so short that tufa did not have time to precipitate. Alternatively, the chemistry of the lake water may have changed enough during the brief rise to the highstand that geochemical conditions were not conducive to tufa precipitation. Both of these possibilities imply that Lake Lahontan received a sudden influx of water that caused a steep rise in lake level for a relatively brief amount oftime, which is reflected in both Benson et aI's (1995) curve as well as the data from this study.

17

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Regression from the Sehoo Highstand The physical record of regression from the Sehoo highstand is well displayed in the

Jessup Embayment. There is a series of no less than 28 distinct barrier ridges formed as lake level dropped back down to the floor of the Carson Sink (Figures 2 and 5). The elevations of 12 of the more prominent ridges are shown in figure 5. We earlier used the lake level curve of Benson et al (1995) to estimate an overall regression rate of 175 mm/yr (Figure 3). By using the ages and elevations of packrat middens from the Winnemucca dry lake (WDL) subbasin (Thompson et aI, 1986), the age ofthe highstand (~12.7 ka) from this study and assuming that the Carson Sink and WDL subbasins had similar dessication histories below the level of the Fernley sill, we can estimate that the lake fell from about 1338 m to about 1230 m in 630 years which corresponds to a rate of about 170 mm/yr. This estimate is essentially identical to the one using the curve from Benson et al (1995). Within the overall regression there were several elevations where the lake paused for a long enough period of time to build several ridges at about the same elevation. The highest of these is the PBC which is discussed above and lies about 6 to 8 m below adjacent highstand features. The next lowest group of ridges occurs at an elevation of about 1235 m and is located between lower barriers 10 and 11 in figure 5. A distinctive characteristic of these ridges is that they are completely covered with branching tufa and have abundant tufa heads and tufa towers (~ 3 m) growing from them. The tufa covered barriers are designated Qss/t on figure 2.

The tufa appears to grow from a particular horizon as viewed in stream cuts through the barriers. Below the horizon, the beach gravel and sand is well cemented but there is little tufa present. Tufa heads are rooted in the horizon and clasts there are coated with branching tufa. The tufa is generally associated with the Qss unit, but in places also coats the Qsg unit. The surface width of the tufa coated barriers is about 250 m in the central part of the Embayment. However, near the headlands to the NE and SW (Figure 2), the tufa dies out only to reappear at the same elevation in other embayments to the north and south of Jessup. The common characteristics of these tufa bearing zones is their elevations and their locations near the mouths of broad alluvial valleys. We interpret the tufa-bearing ridges to represent a temporary stillstand in the overall regression where ground water was moving down the broad alluvial valleys and forced to come to the surface near the shore-lake interface. As the ground water moved through the lacustrine deposits in the upper parts of the embayments, it may have dissolved and moved calcium and/or carbonate in solution which was then precipitated when the ground water came in contact with the lake water. The reason that tufa did not precipitate downslope from rocky headlands is that there was not sufficient water moving through the thin sediments as there was in the broad alluvial filled valleys. The absolute age of the tufa coated barriers is not " known.

After the formation of the tufa coated barriers the lake further regressed to some unknown elevation, but then rose again almost to the same level. Lower barrier 11 is at an elevation of 1234.6 m and stratigraphically overlies the tufa coated barriers (Figures 2 and 5). Whereas the surface of the tufa-coated barriers is completely covered with fragments of branching tufa, the surface and interior oflower barrier 11 is clean, well-washed beach gravel with only occasional fragments of tufa. After the lake built lower barrier 11, it regressed to about 1227m where it built 3 barriers at about the same elevation (Figure 5). The highest of these is designated lower barrier 12. An interesting feature to note is that downslope from the lowest barriers is a sand sheet designated as Qss in figure 2. The deposit probably represents the offshore movement of sand during this minor stillstand, similar in respects to the thick deposits of sand in the upper, central part of the

18

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Embayment. There are no lower barriers recognized between about 1227 m down to floor of the Carson Sink at about 1185 m, but this zone is dominantly covered by alluvium. Work is currently in progress to define the ages of the lower barriers.

Summary This paper has delineated the lacustrine history of the Jessup Embayment of Lake

Lahontan using a combination of geomorphic, sedimentologic, stratigraphic and age dating techniques. Modern process studies of coarse clastic barrier systems were also used to provide a framework within which the paleo-shorelines ofthe Embayment could be interpreted. The results of this study have confirmed and further elucidated the operation of similar shore processes in the ancient lacustrine record.

Other significant results of this study are 1) documentation of probable pre-Sehoo shorelines located about 7 to 10m below the Sehoo highstand, 2) characterization and distribution of constructional shorelines and terraces in relation to sediment availability and local slope, 3) a classification scheme for constructional shorelines in pluvial lake basins is introduced, 4) the natural variability at which the crests of constructional shorelines form above a water plane was determined to be about 3 m and depends on fetch, lake bottom configuration, geometry of the shoreline and the presence or absence of offshore obstructions, 5) the net longshore drift pattern in the Embayment was clockwise indicating that large waves came from the south and southeast, 6) during the transgression to the highstand, lake level was at about 1326.5 m by 13.2 ka and at about 1330.5 m by 13.1 ka, 7) highstand constructional features in the Embayment average about 1339 m and date from the Sehoo highstand which reached an elevation between 1338 and 1339 m, 8) an AMS radiocarbon date on a camel bone from a lagoon enclosed behind a high pocket barrier indicates that the Sehoo highstand occurred at about 12.7 ka, 9) comparisons with the lake level curve of Benson et al (1995) indicate that both the magnitude of the highstand was higher and the timing younger than previous estimates, 10) discrepancies in the curves may be explained by the observation that tufa is not observed on highstand constructional features, but is found some meters lower, 11) the presence of carbonate-bearing dust at the surface of the playette enclosed behind the high pocket barrier and lack of carbonate dust at depth in the playette-fill sediments may indicate that dust sources have changed through the Holocene, 12) during the overall regression from the highstand, the lake formed at least 28 distinct barriers in the Jessup Embayment, 13) lake level paused long enough to form multiple barrier ridges at three different elevations which are at about 1332, 1235 and 1227 m, respectively, 14) tufa-coated barriers at about 1235 m probably reflect ground water interactions with lake water during a pause in the regression, and 15) after the formation of the tufa coated baniers, the lake receded but retransgressed to about 1234.6 m and then regressed to the floor of the Carson Sink.

19

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References

Adams, K.D., and Wesnousky, S.G, 1994, Isostatic rebound of the pluvial Lake Lahontan basin, Nevada and California: Progress report: EOS Transactions, AGU, v.75, no. 44, p. 581. .

Adams, K.D., and Wesnousky, S.G, 1995, The age and synchroneity of the highest Lake Lahontan shoreline features, northwestern Nevada and northeastern California: GSA Abstracts with Programs, v. 27, no. 4, p. 32.

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Benson, L.V., 1978, Fluctuations in the level of pluvial Lake Lahontan during the last 40,000 years: Quaternary Research, v. 9, p. 300-318.

____ -'. 1981, Paleoclimatic significance oflake-leve1 fluctuations in the Lahontan Basin: Quaternary Research, v. 16, p. 390-403.

____ -', 1991, Timing of the last highstand of Lake Lahontan: Journal of paleolimnology, v. 5, p. 115-126.

____ --', 1993, Factors affecting 14C ages oflacustrine carbonates: Timing and duration of the last highstand lake in the Lahontan Basin: Quaternary Research, v. 39, p. 163-174.

____ -', 1994, Carbonate deposition, Pyramid Lake subbasin, Nevada: 1. Sequence of fonnation and elevational distribution of carbonate deposits (Tufas): Palaeogeography, Palaeoclimatology, Palaeoecology, v. 109, 55-87.

Benson, L.V., and Thompson, R.S., 1987a, Lake-level variation in the Lahontan Basin for the past 50,000 years: Quaternary Research, v. 28, p. 69-85.

Benson, L.V., and Thompson, R.S., 1987b, The physical record oflakes in the Great Basin in Ruddiman, W.P., and Wright, H.E., Jr., eds., North America and adjacent oceans during the last deglaciation: Boulder, Colorado, Geological Society of America, The Geology of North America, v. K-3, p. 241-260.

Benson, L.V., Currey, D.R., Dorn, R.I., Lajoie, K.R., Oviatt, CG., Robinson, S.W., Smith, G.I., and Stine, S., 1990, Chronology of expansion and contraction of four Great Basin lake systems during the past 35,000 years: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 78, p. 241-286.

Benson, L.V., Currey, D., Lao, Y, and Hostetler, S., 1992, Lake-size variations in the Lahontan and Bonneville basins between 13,000 and 9000 14C yr B.P.: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 95, p. 19-32.

Benson L.V., Kashgarian, M., and Rubin, M., 1995, Carbonate deposition, Pyramid Lake subbasin, Nevada: 2. Lake levels and polar jet stream positions reconstructed from

20

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radiocarbon ages and elevations of carbonates (tufas) deposited in the Lahontan basin: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 117, p. 1-30.

Benson, L.V., and Peterman, Z., 1995, Carbonate deposition, Pyramid Lake subbasin, Nevada: 3. The use of 87Sr values in carbonate deposits (tufas) to determine the hydrologic state of paleo lake systems: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 119, p. 201-213.

Berger, G.W., 1991, The use of glass for dating volcanic ash by thermoluminescence: Journal of Geophysical Research, 96, p. 19705-19720.

Bluck, B.l, 1967, Sedimentation of beach gravels: examples from South Wales: Journal of Sedimentary Petrology, v. 37, p. 128-156.

B~ggild, O.B., 1930, The shell structure of the mollusks: Kg! Danske Vidensk Selsk Skr, Naturvidensk Mat, afd 9, no. 2, p. 233-326.

Born, S.M., 1972, Late Quaternary history, deltaic sedimentation, and mud-lump formation at Pyramid Lake, Nevada: Reno, Center for Water Resources Research, Desert Research Institute, University of Nevada, 97 p.

Broecker, W.S., and Orr, P.C., 1958, Radiocarbon chronology of Lake Lahontan and Lake Bonneville: GSA Bulletin, v. 69, p. 1009-1032.

Broecker, W.S., and Kaufman, A., 1965, Radiocarbon chronology of Lake Lahontan and Lake Bonneville II, Great Basin: GSA Bulletin, v. 76, p. 537-566.

Burch, J.B., 1989, North American freshwater snails: Malacological Publications, Hamburg, Michigan, 365 p.

Carr, A.P., 1971, Experiments on longshore transport and sorting of pebbles: Journal of Sedimentary Petrology, v. 41, p. 1084-1104.

Carter, RW.G., and Orford, J.D., 1984, Coarse clastic barrier beaches: A discussion of the distinctive dynamic and morpho sedimentary characteristics: Marine Geology, v. 60, p. 377-389.

Carter, RW.G., Forbes, D.L., Jennings, S.C., Orford, J.D., Shaw, J., and Taylor, R.R, 1989, Barrier and lagoon coast evolution under differing relative sea-level regimes: Examples from Ireland and Nova Scotia: Marine Geology, v. 88, p. 221-242.

Carter, RW.G., Orford, J.D., Forbes, D.L., and Taylor, R.B., 1990, Morphosedimentary development of drumlin-flank barriers with rapidly rising sea level, Story Head, Nova Scotia: Sedimentary Geology, v. 69, p. 117-138.

Carter, R.W.G., Jennings, S.C., and Orford, J.D., 1990, Headland erosion by waves: Journal of Coastal· Research, v. 6, no. 3, p. 517-529.

21

~-

Carter, R.W.G., and Orford, J.D., 1993, The morphodynamics of coarse clastic beaches' and barriers: A short- and long-term perspective: Journal of Coastal Research, Special Issue 15, p. 158-179.

Chadwick, O.A., and Davis, J.O., 1990, Soil-forming intervals caused.by eolian sediment pulses in the Lahontan basin, northwestern Nevada: Geology, v. 18, p. 243-246.

Dorn, R.I., Jull, A.lT., Donahue, DJ., Linick, T.W., and Toolin, L.J., 1990, Latest Pleistocene lake shorelines and glacial chronology in the western Basin and Range province, U.S.A.: insights from AMS radiocarbon dating ofrock varnish and paleoclimatic implications: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 78, p. 315-331.

Dansie, AJ., Davis, lO., and Stafford, T.W., Jr., 1988, The Wizards Beach recession: Fanndalian (25,500 yr RP.) vertebrate fossils co-occur with early Holocene artifacts in Willig, lA., Aikens, C.M., and Fagan, lL., eds., Early human occupation in far western North America: The Clovis-Archaic interface: Carson City, Nevada, Nevada State Museum, Anthropological Papers no. 21, p. 153-200.

Davis, lO., 1978, Quaternary tephrochronology of the Lake Lahontan area, Nevada and California: Nevada Archeological Survey Research Paper no. 7, 137 p.

___ --', 1982, Bits and pieces; The last 35,000 years in the Lake Lahontan area, in Madsen, D.B., and O'Connell, IF., eds., Man and environment in the Great Basin: Society for American Archeology Papers, no. 2, p. 53-75.

___ --', 1983, Level of Lake Lahontan during deposition of the Trego Hot Springs Tephra about 23,400 years ago: Quaternary Research, v. 19, p. 312-324.

Duffy, W., Belknap, D.F., and Kelley, IT., 1989, Morphology and stratigraphy of small barrier-lagoon systems in Maine: Marine Geology, v. 88, p. 243-262.

Forbes, D.L., and Syvitski, lP.M., 1994, Paraglacial coasts in Carter, R.W.G., and Woodroffe, C.D., eds., Coastal evolution: Late Quaternary shoreline morphodynamics: Cambridge, Great Britain, Cambridge University Press, p. 373-424.

Gilbert, G.K., 1885, The topographic features oflake shores: U.S. Geological Survey Fifth Annual Report, p. 69-123.

___ --',1890, Lake Bonneville: U.S. Geological Survey Monograph 1,438 p.

Hostetler, S., and Benson, L.V., 1990, Paleoclimate implications of the high stand of Lake Lahontan derived from models of evaporation and lake level: Climate Dynamics, v. 4, p. 207-217.

Houghton, lG., Skamoto, C.M., and Gifford, R.O., 1975, Nevada's weather and climate: Nevada Bureau of Mines and Geology, Special Publication 2, 78 p.

22

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Kaufman, A., and Broecker, W.S., 1965, Comparison of Th230 and C14 ages for carbonate materials from Lakes Lahontan and Bonneville: Journal of Geophysical Research, v. 70, no. 16, p. 4039-4054 .

King, C.A.M., 1972, Beaches and coasts, 2nd edition: London, Edward Arnold, 570 p.

Lao, Y., and Benson, L.v., 1988, Uranium-series age estimates and paleoclimate significance of Pleistocene tufas from the Lahontan basin, California and Nevada: Quaternary Research, v. 30, p. 165-176.

Mifflin, M.D., and Wheat, M.M., 1979, Pluvial Lakes and estimated pluvial climates of Nevada: Nevada Bureau of Mines and Geology, Bulletin 94, 57 p.

Morrison, R B., 1964, Lake Lahontan: geology of southern Carson Desert, Nevada,: U.S., Geological Survey Prof. Paper 401, 156 p.

Morrison, R R, 1991, Quaternary stratigraphic, hydrologic, and climatic history of the Great Basin, with emphasis on Lake Lahontan, Bonneville, and Tecopa, in Morrison, R.B., ed., Quaternary nonglacial geology: Conterminous U.S.: Boulder, Colorado, Geological Society of America, The Geology of North America, v. K-2, p. 283-320.

Orford, lD., and Carter, R.W.G., 1982, Crestal overtop and washover sedimentation on a fringing sandy gravel barrier coast, Camsore Point, southeast Ireland: Journal of Sedimentary Petrology, v. 52, p. 265-278.

Orford, lD., Carter, RW.G., and Jennings, S.c., 1991a, Coarse clastic barrier environments: Evolution and implications for Quaternary sea level interpretation: Quaternary International, v 9, p. 87-104.

Orford, J.D., Carter, R.W.G., and Forbes, D.L., 1991b, Gravel barrier migration and sea level rise: Some observations from Story Head, Nova Scotia, Canada: Journal of Coastal Research, v. 7, no. 2, p. 477-488.

Orford, J.D., Carter, R.W.G., Jennings, S.C., and Hinton, A.C., 1995, Processes and timescales by which a coastal gravel-dominated barrier responds geomorphologically to sea-level rise: Story Head barrier, Nova Scotia: Earth Surface Processes and Landforms, v . 20, p. 21-37.

Papke, K.G., 1976, Evaporites and brines in Nevada playas: Nevada Bureau of Mines and Geology Bulletin 87,35 p.

Reheis, M.C., Harden, J.W., McFadden, L.D., and Shroba, R.R, 1989, Development rates oflate Quaternary soils, Silver Lake Playa, California: Soil Sci. Soc. Am. Journal, v. 53, p. 1127-1140.

Reheis, M.C., Goodmacher, lC., Harden, J.W., McFadden, L.D., Rockwell, T.K., Shroba, R.R., Sowers, lM., and Taylor, E.M., 1995, Quaternary soils and dust deposition in southern Nevada and California: GSA Bulletin, v. 107, no. 9, p. 1003-1022.

23

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Roy, P.S., Cowell, PJ., Ferland, M.A., and Thorn, B.G., 1994, Wave-dominated coasts in Carter, RW.G., and Woodroffe, C.D., eds., Coastal evolution: Late Quaternary shoreline morphodynamics: Cambridge, Great Britain, Cambridge University Press, p. 121-186.

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Thompson, R.S., Benson, L.V., and Hattori, E.M., 1986, A revised chronology for the last Pleistocene lake cycle in the central Lahontan basin: Quaternary Research, v. 25, p. 1-9.

Thompson, R.S., Toolin, L.l, Forester, R.M., and Spencer, R.l, Accelerator-mass spectrometer (AMS) radiocarbon dating of Pleistocene lake sediments in the Great Basin: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 78, p. 301-313.

Wallace, R.E., 1977, Profiles and ages of young fault scarps, north-central Nevada: GSA Bulletin, v. 88, p. 1267-1281.

Wilden, R, and Speed, RC., 1974, Geology and mineral deposits of Churchill County, Nevada: Nevada Bureau of Mines and Geology, Bulletin 83, 95 p.

Young, lA., and Evans, R.A., 1986, Erosion and deposition offme sediments from playas: Journal of Arid Environments, v. 10, p. 103-115.

24

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Figure 1. Map of Lake Lahontan as it appeared during the Sehoo highstand. The locations of the Jessup Embayment and other geographic features are also shown.

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Figure 2. Geologic and geomorphic map of the Jessup Embayment showing the distribution of deposits and landform elements within the Embayment. Qa: alluvium of modem washes and fans. Qp2: playette deposits. Qpl: playa deposits of the Carson Sink. Qpo: buried playette deposits predating Sehoo highstand. Qsg: beach gravel of Sehoo Lake cycle. Qss: Beach and offshore sand of Sehoo Lake cycle (may contain-areas of Qsg). Qss/t: Surficial coating of branching tufa and occurrence of small tufa domes in unit Qss. May in places be associated with unit Qsg. Qsglc: beach gravel of Sehoo Lake cycle mixed with lesser amounts of colluvium. Qc/sg: colluvium mixed with lesser amounts of beach gravel. Qbs: boulder spit ofpre-Sehoo (?) age. Bx: Triassic and/or Jurassic metasedimentary rocks intruded by Tertiary rhyolitic plugs and dikes.

The northward limit of banded rhyolite in surficial sediments is denoted by a coarse dotted line near the head of the Embayment.

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~ Lithoid Terrace (?) of Russell (1885).

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-175 mm/yr

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1265 m

Figure 3. Lake level curve of Benson et al (1995) showing the estimated rates of lake level rise and fall above 1265 m when the Pyramid Lake and Carson Sink subbasins were connected .

Figure 4. Idealized cross section through a cut-and-built terrace showing the relationship between erosional and depositional elements in this type of feature. After Russell (1885).

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Explanation

~ Beach barrier or depositional feature; dashed where discontinuous, dotted where partially buried.

~ Erosional or cut-and-built terrace.

~ Lithoid Terrace (?) of Russell (1885).

~ Beach cliff or erosional escarpment.

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Elevations (m) of Lower Barriers

@1334.4 (1)1328.5 (2)1327.7 (3)1320.4 @131 @1311

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Figure 5. Location map showing shoreline elevations and figure locations. Swash-aligned highstand barriers and elevations are designated by shaded letters and drift-aligned highstand barriers and elevations are designated by unshaded letters. Recessional barriers are designated by numbers. We interpret all <?f the high shorelines to date from the Sehoo highstand. The difference in elevations (-2.6 m) is attributed to natural variability in the height of formation above a given water plane.

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Figure 6. Shoreline classification scheme for pluvial features found in the Lahontan basin. After L.H.laHJl~Jl and Duffv et al (1989).

•••

~.HJ.auJ.~J. (1992), King (1972)

l

Figure 7. Aerial interpretation of barrier features at the head of Jessup Embayment (see figure 5 for location) showing cross-cutting relationships demonstrating surface ridges of progradational barrier complex (PBC) postdate the Sehoo Symbols are as in figures 2 and 5.

Feature 1 is a spit that began building during the highstand and continued to elongate to the north as lake level began to recede. Feature 2a is an erosional scarp that truncates the highstand spit and is probably contemporaneous with Feature 2b, which is a spit built from south to north at a level about 8 meters lower than adjacent highstand features. Feature 3 comprises the surface ridges of the PBC which also lie about 6 to 8 meters below adjacent highstand features.

interpret these shorelines to indicate that first the highstand spit was built, and second, the end of the highstand spit was truncated and the recessional spit built. The ridges of the PBC were then emplaced across the head of the Embayment through continuing longshore drift and a relatively stable lake level. The lower recessional barriers were then formed as lake level continued its overall decline.

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Long Section of Progradational Barrier Complex at the Head of the Jessup Embayment NE

50 90 Distance (meters)

Figure 8. The progradational barrier complex (PBC) at the head of the Jessup Embayment showing main structural and sedimentary features exposed in a stream cut. AMS radiocarbon dates (13,280 +/-110 yr BP and 13,110 +/- 110 yr BP) on gastropod shells from the lower part of the section indicate that the package below the tufa and beachrock horizon predates the highstand at about 12.7 ka. Cross-cutting relationships of the surface barrier ridges (Figure 7) are interpreted to indicate that the sediment above the beachrock and tufa horizon post-dates the highstand. The numbered horizons are discussed in the text.

I

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I _____________ ~50 m

Contour interval is ) u em

/ Barrier crest

L Outline of Jessup

Figure 9. Topographic of the Jessup Playette trench site showing the location of the trench and soil pit in relation to the highstand barrier and its associated playette. The enclosing barrier marks the highstand of pluvial Lake Lahontan.

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Zone of bioturbation and soil development

Jessup Playette Trench

Hinge point

Crest of barrier I Edge of playette on surface

EXPLANATION

e3 Playelte-fill deposits. Interbedded alluvial wash and dust layers witb dust layers represented by black lines.

o Barrier gravels of Sehoo lake cycle.

Lagoonal sands ofSehoo lake cycle.

~ Older alluvium pre-dating Schoo Lake cycle.

<€) Krotovina

~ Soil development

Radioulna and metacarpal

•••

NW

Came/ops hesternus (12,690 ± 60 yr B.P.) I I I I

o 10 20 30 40 50 60

Distance (m)

Figure 1 Log of the Jessup Playette trench showing the location of the air-fall tephra camel bones in of the barrier, lagoon sands and playette-fill deposits. An AMS 14C date on collagen extracted from the an age of 12,690 + 60 yr BP, thus providing a minimum limiting age on the highstand. location of the from northeast wall onto the southeast wall shown in this log.

V.l.UUV.u. to the sediments bone yielded

bones was projected

Figure 11. Photograph and drawing of situ camel bone (12,690 + 60 yr BP) in the Jessup playette trench at contact between the lagoonal sands and the playette-fill package.

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11 12 13 14 15

Age (103 yr B.P.)

Figure 12. Figure showing Benson et aI's (1995) Lahontan lake level curve compared with data and interpretations from this study. A) Simplified lake level curve from Benson et al (1995). B) Lake level curve constructed from the data from this study superimposed on the enlarged lake level curve of Benson et al (1995). Inverted black triangles designate the radiocarbon ages and elevations from this study. Hollow circles represent radiocarbon ages and elevations of tufa samples from Benson et al (1995). Right-side up hollow triangle represents radiocarbon age and elevation of organic material in rock varnish on high terrace from northern Pyramid Lake basin (Dam et aI, 1990).

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Appendix 2

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Soil Development, Spatial Variability and the age of the Highest Late Pleistocene Lake Lahontan Shorelines, Northwestern Nevada and Northeastern California.

Introduction

Kenneth D. Adams Steven G. Wesnousky

Center for Neotectonic Studies and Department of Geological Sciences

University of Nevada, Reno.

Over the last 1 Ma, the Lake Lahontan basin has been the locus of at least five major lake cycles (Morrison, 1991; Reheis, 1996, this volume). Although many researchers have studied the deposits and lacustrine landforms of the Basin, there has been little agreement as to the age of the highest shoreline features. The debate centers around whether the high shoreline dates from the most recent cycle (Sehoo period) at about 12.7 ka or from the penultimate lake cycle (Eetza period) at about 130 - 350 ka.

LC. Russell (1885), who accomplished the seminal work on Lake Lahontan, maintained that the uppermost shoreline, which he termed the Lahontan Beach, dated from the most recent lake cycle. Jones (1925) and Antevs (1925) agreed with this interpretation. However, Morrison (1964; 1991) claims that the highest shoreline in the southern Carson Desert area dates from Eetza time. More specifically, Morrison (1991) supports that the high shoreline there dates from the middle Eetza highstand which he estimates to be about 280 ka. This is in contrast to the Pyramid Lake subbasin where Benson (1993) has interpreted features related to the high shoreline to date from -13.5 ka, or the Sehoo highstand. Based on key soil exposures in the northern subbasins, Mifflin and Wheat (1971, 1979) postulated that the age of the highest shoreline in the northern part of the Basin dated from Sehoo-time whereas the age of the highest shoreline in the southern part of the Basin dated from Eetza-time. They called upon regional, down to the north tilting during the Eetza-Sehoo interpluvial period to explain this relationship.

Determination of the age of the highest shoreline from throughout the Lahontan Basin is important in light of our current effort to determine the isostatic rebound of the Basin resulting from the desiccation of the most recent highstand lake (Adams and Wesnousky, 1994). In order to do this, we needed to be reasonably sure that we were measuring the elevations of the highest constructional shore features dating from the Sehoo lake cycle throughout the Basin. Therefore, we adopted a somewhat simple strategy for determining the age of the high shoreline. We have employed both numerical and relative age-dating techniques to date particular constructional high shoreline features and their associated soils. We then correlated the soil development of these features to other undated high shoreline localities (Adams and Wesnousky, 1995; 1996). For comparison of relative soil development, we have also described a number of paleosols related to pre-Sehoo lake cycles to further test our conclusions about the age of the high shoreline.

Pre-Sehoo pluvial lake deposits and landforms residing well above the Sehoo limit have long been recognized in the Walker Lake subbasin (e.g. Russell, 1885; King, 1993). Marith Reheis is currently studying these features and deposits (see Reheis, 1996, this volume) and has identified some super-elevated lacustrine deposits in the northern subbasins of Lake Lahontan. We recognize that the existence of earlier Pleistocene lacustrine deposits.above the late Pleistocene limit complicates our task somewhat and muddles our definition of "highstand". However, we emphasize that the late Pleistocene deposits and landforms are readily distinguishable both in the field and on aerial

p

photographs in terms of better development, continuity and preservation. Therefore, when we use the term "highstand" we are speaking of the readily identifiable upper limit of prominent shorelines found throughout the Basin which were likely formed in the last (Sehoo period) or penultimate (Eetza period) lake cycles.

Methods The relative development of27 soil profiles located on high shorelines from

throughout the Basin were compared in order to test the hypothesis that the high shoreline of Lake Lahontan dates from more than one lake cycle (Figure 1). Seven additional profiles developed on regressive barriers post-dating the Sehoo highstand at 12.7 ka but older than about 11 ka were also used for comparison. To assess the degree of development of demonstrably pre-Sehoo soils and to compare these to the highstand soils we described and sampled seven more profiles located from descriptions in the literature (i.e. Morrison, 1991; Morrison and Davis, 1984) and from our own travels in the Basin (Figure 1).

Field descriptions of soil profiles included color, texture, structure, consistence, reaction to dilute HCL, root distribution, and the presence and character of clay films and pores. The surficial geology of each site was also described in terms of 1) the type of beach feature in which the soil developed, 2) lithology, rounding and sorting of clasts on surface and at depth, 3) development of desert pavement and rock varnish, 4) degree of dissection or other surficial modifications, 5) aspect, 6) slope, 7) vegetation, 8) amplitude of beach feature and, 9) direction of net shore drift. In general, soil pits were excavated on the flat or gently sloping « 1 0) crests of constructional beach features such as spits, barriers and tombolos. Constructional beach features are ideal locations to examine soil development due to their relative stability resulting from their positive relief with respect to the surrounding landscape.

The paleosols used in this study for comparison with the highstand soils were formed on deposits of diverse sedimentary environments (i.e. multiple parent materials) . and may not date from the same period. The two paleosol profiles at Wadsworth Amphitheater and Rye Patch Dam (Figure 1) were located from the literature and are developed in fluvial deposits of the Wyemaha Alloformation (AF) which post dates the Eetza AF, but predates the Sehoo AF (Morrison, 1991; Morrison and Davis, 1984). Considering that the last highstand of the Eetza lake cycle was at about 130 ka (Morrison, 1991) and that the Sehoo lake did not begin to rise until approximately 30 ka (Benson et aI, 1995), sediments mapped as the Wyemaha AF in different areas may have been deposited over the span of as much as 100,000 years. Consequently, soils developed on these deposits may differ by tens of thousands of years. In addition, the Wyemaha AF in the Wadsworth Amphitheater consists of coarse sand and gravel, whereas the Wyemaha AF at Rye Patch Darn predominately consists of well-sorted fine to medium sand. Thus, the variation in parent materials may also have contributed to the differences in soil development at this site.

The two pre-Sehoo sUlface soils described at the Thome Bar on the southeast side of Walker Lake (Figure 1) are developed in coarse clastic beach deposits, but their absolute age is unknown. The paleosol at Jessup is developed in coarse clastic beach deposits and is overlain by Sehoo shore deposits. The two remaining paleosols, in Quinn River Valley and at Grimes Canyon (Figure 1) are also both developed in coarse clastic beach deposits and are buried by 2 and 9 m of highstand beach deposits, respectively. These last two paleosols are the best developed of all the soils described in this study.

The ages of thirteen of the profiles used in this comparison are known or can be closely approximated. The highstand barrier that fronts the Jessup Playette dates from

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about 12.7 ka (Adams and Wesnousky, 1996a, this volume). We described and analyzed five separate profiles across the barrier and into the playette as well as two additional profiles from a separate soil pit on the crest of the barrier (Figure 2). The three profiles described in the trench across the crest of the Jessup Playette barrier serve as a micro­catena with which to assess the influence of topographic position on soil development on a single-age surface. Seven additional soil profiles were described on four regressive barriers which postdate the Sehoo highstand but are probably older than about 11 ka (Figure 3) (Curry, 1988; Benson et aI, 1992). Soils were described on these regressive barriers to determine whether or not there was a systematic change in the degree of soil development on progressively younger and lower barriers. We also obtained a 36Cl surface exposure age of ~ 15 ka (Fred Phillips, 1995, written comm.) and soils data for a highstand feature in the Lahontan Mountains (Figure 1 and Table 3, Site F-19) that Morrison (1964) had mapped as part of the Eetza AF.

In addition to field descriptions of all 41 soil profiles compared in this study, particle size analyses were conducted for eight highstand profiles, seven regressive Sehoo profiles, two Churchill profiles and two profiles developed on the Holocene-age surface of Jessup playette. Particle size analyses for the remaining twenty eight profiles are still pending.

Results The results section is organized in the following manner: first, descriptions and

particle size analyses for soils from the Jessup Playette and its associated barrier are presented in Table 1. Next, soils data for regressive barriers in the Jessup Embayment are presented in Table 2. Third, soils data for highstand soils from around the Lahontan basin are presented in Table 3 and last, soils data for pre-Sehoo soils are presented in Table 4. Interpretations and correlations are presented in the Discussion section.

Discussion Due to natural variations in soil development factors, each soil profile in this study

is different. However, in a gross sense we submit that the soils described in this study can be separated into two main groups. The first group encompasses all of the soils developed on highstand features throughout the Basin and the soils developed on regressive Sehoo barriers in the Jessup Embayment (Figure 1; Tables 1,2 and 3). We interpret these soils to have developed on highstand and regressive features fonned during the Sehoo Lake cycle which reached its highstand at about 12.7 ka (Adams and Wesnousky, 1996a, this volume). The second group encompasses all of the soils that are demonstrably older than the Sehoo Lake cycle. As stated in the methods section these older soils are not necessarily the same age.

There is a certain amount of variability in tenns of soil properties within each major group of soils. Considering that Jenny (1941) defmed five factors which influence soil development, variability between profiles is to be expected even if they are the same age, because time is just one of the five soil fonning factors. The other four factors are topography or relief, parent material, organisms (both plant and animal) and climate (Jenny, 1941). Each of these five factors can significantly influence soil development and will be discussed in tenns of how they might be responsible for the spatial variability observed in the Sehoo-age soils.

In this study, soil profiles from the late Pleistocene high shoreline were all examined on the crests of constructional features such as spits, barriers and tombolos. Hence, all of the sites tend to be well-drained and have deep ground water. Most of the

features are composed of coarse clastic beach material, but some are composed primarily of sand with minor amounts of gravel. The difference in size and sorting of parent material can influence depth of wetting and water retention which in turn can influence the type and density of vegetation found on a particular landform.

Coarse clastic beach features are different than adjacent contemporaneous alluvial fans in terms of initial character and particle size distribution, hence soil development also differs. Alluvial fans are commonly poorly sorted with grain sizes ranging from clay through boulders (Blair and McPherson, 1994). However, in the Lahontan basin, beach features tend to be composed of well-sorted, clast-supported, coarse clastic sediment with more or less sand forming a matrix between the larger clasts. Tables 1 and 2 show the particle size distributions for eleven C horizons from barriers in the Jessup Embayment that appear to be dominated by sand. However the particle size distributions only reflect the < 2 mm size fraction (fine earth fraction). In actuality, the majority ofC horizons ( excluding Lower barrier 4 and the two Playette profiles) are composed of greater than 90% gravel and cobbles, with the fine earth fraction accounting for <10% of the total volume of material. When considering the silt and clay sized fractions in comparison to the total particle size distributions of the C horizons, only a very small percentage « 1) is comprised of clay and silt (Tables 1 and 2).

It has long been recognized that the addition of eolian dust significantly influences soil development in many different climatic regimes (Yaalon and Ganor, 1973; Peterson., 1980; Machette, 1985; McFadden and Weldon, 1987). ill semiarid and arid areas, eolian dust influx constitutes a major soil forming process (Reheis et aI, 1995). The late Pleistocene soils developed on Sehoo-age features are no exception.

The A and B horizons of the highstand profiles as well as the regressive Jessup profiles contain considerable amounts offme sand, silt and clay (Tables 1,2 and 3). There is little evidence of clast weathering in these profiles, therefore we concur with the conclusions of Chadwick and Davis (1990) that virtually all of the fme earth fraction contained in the vesicular A horizons, and most in the underlying B horizons, came 'from atmospheric sources. Additional eviqence in support of an eolian source for the fines includes the common, discontinuous loess blankets that are found on many beach features, especially around the bases of bushes. Chadwick and Davis (1990) introduced the idea that rapid soil formation resulted from temporally limited eolian pulses that they associated with desiccation of the Lake. They also postulated that the degree of soil development has a positive correlation with the amount of upwind playa surface. This idea is exemplified by observations within the Carson Sink where huge plumes of dust are blown north from the surface during spring wind storms. Soils developed on the north side of the Carson Sink are better developed than soils on the south (downwind) side (Chadwick and Davis, 1990).

The amount of calcium carbonate accumulation in soils is commonly used as a relative age indicator (Gile et aI, 1966; Machette, 1985). This approach assumes that most of the carbonate is introduced by the addition of calcareous dust. The commonly calcareous A v horizons in the Lahontan basin support this idea. However, when examining soils developed in Lahontan beach gravel, the amount of carbonate present is not a reliable indicator of age. The waters of Lake Lahontan contained a great deal of dissolved carbonate, as evidenced by the amount of tufa and cemented beach rock within the Basin. In stream cuts and artificial exposures, tufa or carbonate coated clasts often extend many meters into the deposit. In the soil forming zone, carbonate is often preferentially concentrated on the undersides of clasts, indicating that the carbonate is affected by soil forming processes. Because much of the carbonate was already present in the parent

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material (beach gravel) and not due to the slow addition of calcareous dust, the amount of carbonate in a soil profile should not be used to estimate the age of the soil.

Soil development on highstand and regressive barriers in the Lahontan basin is greatly influenced by bioturbation, primarily in the form of rodent burrowing. This effect is readily seen in the Jessup Playette trench (Figure 2) where soil development across the crest of the barrier seems to closely track the depth of rodent burrowing. Near the southeast end of the trench there is a zone of coarse (~25 cm) disc-shaped cobbles that appear to have limited the depth of rodent burrowing. Consequently the profile developed in this area is relatively thin.

The location and density of plants also influences soil development, at least indirectly. Surface vegetation acts as a surface roughness element by trapping eolian material around the bases of plants. Rodents often burrow near the bases of bushes thereby increasing the amount of [mes mixed into the profile beneath the bushes. Salt brush is a common constituent of vegetation communities growing on beach features. Peterson (1980) reports that sodium-influenced soils can rapidly develop Bt and even argillic horizons. The concentration of sodium in the leaves of these salt bushes may influence the rate of clay translocation directly beneath the plant. As the plant continues to grow and drop leaves on the ground beneath it, the sodium in these leaves may be incorporated back into the soil causing a local increase in the rate of clay translocation. Evidence for this process is seen where Bt horizons locally thicken beneath individual bushes. As discussed above, variation in the thickness of the Bt horizon may also be due to rodent burrowing.

The high shoreline of Lake Lahontan extends through about 3° of both latitude and longitude. As a result, there are climatic gradients within the Basin which have probably affected soil development. However, we do not yet have a clear understanding of how these gradients have changed through time or what their influence has been on soil development.

Pre-Sehoo Soils The demonstrably older than Sehoo soils described in this study are all better

developed than the Sehoo surface soils. The two best developed profiles are those at Grimes Canyon and in Quinn River Valley (Figure 1, Table 4). These soils are developed in coarse clastic barrier gravels, much like the younger surface soils. However, their thickness, amount of clay accumulation, structural grade, consistency and color all indicate that these soils represent development over a much longer period of time than do the surface soils. If these older soils are developed on Eetza deposits then they may be as old as 140 ka or 280 ka (Morrison, 1991). Considering the Ubiquitous influence of dust on the younger surface soils, it is not unreasonable to consider that the older soils were also greatly influenced by the introduction of dust. However, once the dust was incorporated into the older profiles it may have had time to chemically weather and dramatically change the character of the soils.

The two pre-Sehoo surface profiles at the Thome Bar (Figure 1) are apparently not as well-developed as some of the other paleosols (Table 4). However, it is possible that these soils have been somewhat stripped. The original morphology of the landforms is still present, but appears somewhat muted.

Conclusions The soil correlations made in this study imply that the highstand barriers found

throughout the Lake Lahontan basin date from the Sehoo lake cycle. This is in contrast to the conclusions of Morrison (1964; 1991) regarding the age ofthe highstand in the

southern Carson Sink:. Soil formation on Sehoo-age beach features is largely a product of the introduction of dust into generally coarse clastic deposits. Spatial variability in soil development appears to be influenced by bioturbation and also the distribution of vegetation. Spatial variability due to the proximity of the profile to dust sources (i.e. playas) is still under investigation.

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References

Adams, K.D., and Wesnousky, S.G, 1994, Isostatic rebound of the pluvial Lake Lahontan basin, Nevada and California: Progress report: EOS Transactions, AGU, v.75, no. 44, p. 581.

Adams, ICD., and Wesnousky, S.G, 1995, The age and synchroneity of the highest Lake Lahontan shoreline features, northwestern Nevada and northeastern California: GSA Abstracts with Programs, v. 27, no. 4, p. 32.

Adams, K.D., and Wesnousky, S.G, 1996, Age relationships and soil development of beach barrier features in the Jessup Embayment of the Lake Lahontan basin, NV and CA: GSA Abstracts with Programs, v. 28, no. 5, March 1996.

Antevs, E., 1925, On the Pleistocene history of the Great Basin: in Quaternary Climates: Carnegie Institute, Washington, Pub. 35, p. 51-144.

Benson, L.V., 1978, Fluctuation in the level of pluvial Lake Lahontan during the last 40,000 years: Quaternary Research, v. 9, p. 300-318.

_____ :, 1991, Timing of the last highstand of Lake Lahontan: Journal of Paleolimnology, v. 5, p. 115-126.

____ -', 1993, Factors affecting 14C ages of lacustrine carbonates: Timing and duration of the last highstand lake in the Lahontan Basin: Quaternary Research, v. 39, p. 163-174.

Benson, L.V., Currey, D., Lao, Y, and Hostetler, S., 1992, Lake-size variations in the Lahontan and Bonneville basins between 13,000 and 9000 14C yr B.P.: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 95, p. 19-32.

Benson L.V., Kashgarian, M., and Rubin, M., 1995, Carbonate deposition, Pyramid Lake subbasin, Nevada: 2. Lake levels and polar jet stream positions reconstructed from radiocarbon ages and elevations of carbonates (tufas) deposited in the Lahontan basin: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 117, p. 1-30.

Blair, T.C., and McPherson, lG., Alluvial fans and their natural distinction from rivers' based on morphology, hydraulic processes, sedimentary processes, and facies assemblages: Journal of Sedimentary Research, v. A64, no. 3, p. 450-489.

Chadwick, O.A., and Davis, lO., 1990, Soil-forming intervals caused by eolian sediment pulses in the Lahontan Basin, northwestern Nevada: Geology, v. 18, p. 243-246.

Currey, D.R., 1988, Isochronism offinal Pleistocene shallow lakes in the Great Salt Lake and Carson Desert regions of the Great Basin: AMQUA Prog. Abstracts, Tenth Biennial Meeting, p. 117.

r-

L

Davis, 10., 1978, Quaternary tephrochronology of the lake Lahontan area, Nevada and California: Nevada Archeological Survey Research Paper 7, 70 p.

___ --', 1982, Bits and pieces: environmental history of the western Great Basin during the last 35,000 years: in D. Madsen and J. O'Connell ~ds., Man and environment in the Great Basin: Society for American Archeology Paper No.2, p.53-75.

Gile, L.R., Peterson, F.F., and Grossman, R.B., 1966, Morphological and genetic sequences of carbonate accumulation in desert soils: Soil Science, v. 101, p. 347 -360.

Jenny, H., 1941, Factors of soil formation: New York, McGraw-Hill, 281 p.

Jones, IC., 1925, The geologic history of Lake Lahontan: in Quaternary Climates: Carnegie Institute, Washington, Pub. 35, p. 1-50.

King, G.Q., 1993, Late Quaternary history of the lower Walker river and its implications for the Lahontanpaleolake system: Physical Geography, v. 14, p. 81-96.

Machette, M.H., 1985, Calcic soils and calcretes of the southwestern United States, in Weide, D.L., ed., Soils and Quaternary geology of the southwestern United States: GSA Special Paper 203, p. 1-21.

McFadden, L.D., Wells, S.G., Brown,W.I, and Enzel, Y., 1992, Soil genesis on beach ridges of pluvial Lake Mojave: hnplications for Holocene lacustrine and eolian events in the Mojave Desert, southern California: Catena, v. 19, p. 77-92.

McFadden, L.D., and Weldon, R.I, 1987, Rates and processes of soil development on Quaternary terraces in Cajon Pass, California: GSA Bulletin, v. 98, p. 280-293.

Mifflin, M.D., and Wheat, M.M., 1971, Isostatic rebound in the Lahontan basin, northwestern Great Basin: Geological Society of America Abstracts with Programs, p. 647.

Mifflin, M.D., and Wheat, M.M., 1979, Pluvial lakes and estimated pluvial climates of Nevada: Nevada Bureau of Mines and Geology, Bulletin 94, 57 p.

Morrison, R.B., 1964, Lake Lahontan: Geology of southern Carson Desert, Nevada: U.S. Geological Survey Professional Paper 401, 156 p.

,

_____ , 1991, Quaternary stratigraphic, hydrologic, and climatic history of the Great Basin, with emphasis on Lakes Lahontan, Bonneville, and Tecopa: in Morrison, R.B., ed. Quaternary Nonglacial Geology; Conterminous U.S.: Boulder, Colorado, Geological Society of America, The Geology of North America, v. K-2, p. 283-320.

i

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Reheis, M.C., Harden, lW., McFadden, L.D., and Shroba, R. R., 1989, Development rates of late Quaternary soils, Silver Lake Playa, California: Soil Sci. Soc. Journal, v. 53, p. 1127-1140.

Reheis, M.C., and seven others, 1995, Quaternary soils and dust deposition in southern Nevada and California: GSA Bulletin, v. 107, no. 9, p. 1003-1022.

Russell, I.C., 1885, Geological history of Lake Lahontan, a Quaternary lake of northwestern Nevada: U.S. Geological Survey Monograph 11,288 p.

Wells; S.O" McFadden, L.D., and Dohrenwend, le., 1987, Influence oflate Quaternary climatic changes on geomorphic and pedogenic processes on a desert piedmont, eastern Mojave Desert, California: Quaternary Research, v. 27, p. 130-146.

Yaaion, D.H., and Ganor, E., 1973, The influence of dust on soils in the Quaternary: Soil Science, v. 116, p. 146-155.

Table 1. Soil Data From the Jessup Playette and High Barrier. Horizon DeI1th (em; Color' __ Texture2 Size (% wI) Strueture3 CQn~i~1!ln!<X4 __ CaC03. ·-Pores6 Roots7 Lower

Top Base Dry Moist Sand Silt Clay Dry Moist Wet efferveseenee5 BoundaryS {matrix, clasts}

Trench Profiles

JPT Profile I Av 0 5 IOYR 7/3 IOYR 5/4 SCL 32.3 47.6 20.1 ICPR,2MGR lo,sh fi vs, p es 2vf,fv If aw 2Bw 5 15 IOYR 6/4 IOYR4/4 GSL 54.8 35.5 9.7 1,2MCR,SBK 10,sh fr so, ps 0, tde 0 2vf ew 3Bk 15 80 IOYR 6/4 10YR 4/4 VGS 53.5 42.7 3.8 0 10 10 so, po es, tde 0 If ew 3Ck 80 150+ N.D. G,C and F 85.5 11.2 3.3 N.A. NA tde N.A. 0 N.D.

JPT Profile 2 Av 0 7 IOYR 7/3 IOYR 5/3 SCL 36.0 50.5 13.5 2CPR sh to h fr ss, ps es 3f,mv If aw 2Bw 7 20 IOYR 6/3 IOYR 5/4 GLS 52.5 32.6 15.0 o to IMCR 10 to so 10 so, po e, tde 0 2vfto f cw 3Bk 20 46 IOYR 7/3 IOYR 6/4 VGS 80.9 14.6 4.5 0 10 10 ss, po es, tde 0 I to 2f ew 3Ck 46 150+ N.D. G 82.5 12.5 5.0 N.A. N.A. tde N.A. 0 N.o.

JPT Profile 3 Av 0 15 IOYR 7/3 10YR 5/3 SL 48.5 38.7 12.8 2CPR, lCPL so to sh fr ss, ps e 2,3 fv If aw 2Bw 15 33 IOYR 6/4 IOYR4/4 GSL 60.2 31.1 8.7 o to IMCR 10 to so 10 so, ps o to e, tde 0 2vfto f cw 2Bk 33 60 IOYR 7/3 10YR 5/4 GSL 61.2 30.8 8.0 o to IMSBK 10 to so 10 so, po es, tde 0 2f ew 2B'w 60 95 IOYR 6/4 IOYR4/4 GTOVGSL 55.1.35.5 9.4 o to IMSBK 10 to so 10 so, ps e, tde 0 If gw 3Ck 95 200+ N.D. G 81.8 12.6 5.7 N.A. NA tde N.A. 0 N.D.

JPT Profile 4 A 0 II IOYR 712 IOYR4/3 SL 44.4 47.3 8.3 2CPR,IMPL 10 to so 10 so,po e Ifv If aw 2Av I 35 IOYR 7/3 10YR 5/3 SCL 34.4 52.5 13.1 2CPR to 2MPL so to sh 10 ss, ps es 2fv If ew 3Bw 35 65 IOYR 6/4 IOYR 4/4 SCL 37.6 47.2 15.2 o to 1M, CSBK so to sh 10 sO,ps o to e 2fv If gw 3C 65 150+ N.D. N.D. 38.9 54.5 6.6 N.A. NA 0 NA 0 N.D.

JPT Profile 5 Av 0 28 JOYR 8/3 10YR 5/4 SC 5.8 62.3 31.9 2CPR to 3CPL so to sh fi ss, p es 3fv 2vfto f cw 2BC 28 48 IOYR 6/4 10YR 5/4 SCL 37.2 51.0 11.7 o to IMCR 10 to so 10 ss, ps 0 0 If ew 2C 48 170+ IOYR 6/4 IOYR4/4 SeL 33.3 58.8 8.0 N.A. NA 0 NA 0 N.D.

Soil Pit Profiles

JPBP Profile I Av 0 8 IOYR 712 10YR 412 SCL 40.1 49.7 10.2 2CPR to 2F,MPL so to sh fr ss, ps o to e 3fv I to 2vf aw 2Bw 8 46 IOYR 6/4 10YR4/4 VGLS 50.2 40.9 9.0 o to IFSBK 10 to so 10 to vfr so, po 010 e, tde 0 2vf ew 2Ck 46 77 IOYR 8/3 10YR 6/4 EGS 68.1 22.9 9.0 N.A. 10 10 so, po ev, tde 0 2vf ew 2C 77 170+ N.D. G 94.9 3.1 2.0 NA NA tde NA 0 N.D.

JPBP Profile 2 Av 0 6 IOYR 7/2 IOYR 5/3 SCL 29.3 55.8 14.9 3CPR,2MPL so to sh fr ss, ps 010 e 3 fv Ivf as 2Bt 6 15 IOYR 5/3 IOYR 4/4 GSCL 44.0 42.8 13.2 010 VF,FCR 10 fr ss, ps 0 0 2vfto f aw 2Bk 15 25 IOYR 7/3 IOYR 5/4 VGS 63.9 31.0 5.1 o to IVFCR 10 10 so, po ev, tde 0 2vfto f as matrix free zone 25 34 N.A. G No fine fraction NA NA Ide NA Ivf as 4Bk 34 57 IOYR 7/3 IOYR 5/4 VGS 57.7 41.0 1.3 0 10 10 so, po ev, Ide 0 Ivf ew 4Ck 57 200 N.D. G 94.7 2.8 2.5 N.A. N.A. Ide N.A. 0 N.D.

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Table 2. Soil Data From Regressive Barriers in the Jessup Embayment. Horizon Dellth (em} !:O!QI' -- Texture2 Size (% wI} Strueture3 !::Qn~i§!~!l1<:t4 __

Top Base Dry Moist Sand Silt Clay Dry Moist Wet

Lower Barrier Profiles

!:I-Jt! EXIlQ£ur~ Avk 0 12 2.5YR 6/3 IOYR3/3 SL 57.6 36.8 5.6 IFPL to IFSBK 10 to so 10 so,ps 2Bwk 12 21 10YR 6/3 10YR 4/3 vaSCL 25.9 61.7 12.4 2MSBK sh fr so,ps 2Bk 21 41 10YR 6/4 10YR4/3 VaSL 38.4 S3.7 7.9 o to IFSBK 10 10 so,po 2BC 41 SI IOYR 7/3 IOYR4/4 BaS 49.0 41.7 9.3 0 10 10 sO,po 3Ck 51 150+ N.D. a N.D. NA NA

LQwe[ Barri~[ ~ LB-3 Profile I Av 0 14 IOYR 7/4 10YR 5/3 S 68.5 2S.3 6.2 3VCCPR, lCPL 10 1010 vfr so,po 2Bk 14 39 IOYR 7/3 10YRS/4 vas 71.4 IS.7 13.0 o to IFSBK 1010 so 10 so,po 2Ck 39 55 N.D. vas 92.5 4.9 2.6 NA NA 3Ck 55 150+ N.D. a 95.5 3.4 1.1 NA NA

LB-3 Profile 2 Av 0 9 10YR 712 10YR 4/2 aSCL 47.0 45.4 7.7 2CPR 10 to sl! 10 sO,ps 2Bk 9 16 IOYR 7/3 10YR 5/3 aLS 73.1 13.4 13.4 o to IMCR 10 to so 10 so,po 2Ck 16 30 N.D. a 87.7 4.8 7.6 NA NA 3Ck 30 150+ N.D. a 97.6 2.6 0.8 NA NA

LQ:!lCer Blll[ri~r 4! LB-4 Profile I Av 0 10 IOYR 712 10YR 5/3 SCL 48.9 42.3 8.9 2CPR so to sh 10 sO,po 2Btk 10 23 10YR 6/2 10YR 5/3 VaSCL 66.7 11.8 21.5 o to IMCR 1010 so fr SO,ps 2Ck 23 105 N.D. a 92.2 4.8 3.0 NA NA 3C lOS 132 N.D. as 91.1 6.8 2.1 NA NA 4C 132 200+ N.D. S 91.8 8.2 0.0 NA N.A.

LB-4 Profile 2 Av 0 10 10YR 7/3 IOYR4/3 SL 49.5 44.0 6.4 o to ICPR so to sl! 10 so,po 2Bw 10 30 10YR 6/3 10YR 5/4 BaS 77.3 14.6 8.2 o to IMCR 10 to so 10 so,po 2Ck 30 120 N.D. a 89.1 10.9 0.0 NA NA 3C 120 140 N.D. as 91.1 6.8 2.1 NA NA 4C 140 200+ N.D. S 91.8 8.2 0.0 NA NA

L!l:!lC~[ narri~r 11 LB-II Profile I Av 0 12 10YR 6/2 10YR 5/3 SCL 39.3 42.0 18.7 2CPR to 2MPL so to sl! fr so,ps 2Bk 12 40 10YR 7/3 IOYR 5/3 va,cs 79.2 20.2 0.6 o to IMCR 10 10 so,po 2Ck 40 200+ N.D. a 92.8 6.1 1.2 NA NA

LB-II Profile 2 Av 0 12 IOYR 7/3 10YR 4/3 SL 47.9 34.8 17.3 o to 2CPR 10 to so 10 so,po 2Btk 12 24 10YR 7/3 10YR 5/3 asc 39.2 34.6 26.2 o to 2M,CSBK so to sh fr ss,ps 2Ck 24 200+ N.D. a 95.0 3.4 1.6 NA NA

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CaC03, Pores6 -Roots 1 Lower efferveseenee5 Boundary8 (matrix, clasts)

es 3fv 1,2f aw e 10 es, tde 2vfir 2vf aw

es, Ide lfirto 0 I to 2vf aw ev, Ide 0 2vf to f aw

Ide NA 0 N.D.

o to es 3fv Ivf .aw es, Ide ° Ivf,f ew 0, tde NA 0 aw 0, Ide NA 0 N.D.

es 3fv 0 aw es, Ide 0 2f cw 0, Ide NA 0 aw 0, Ide NA 0 N.D.

o to e 3fv If ew es, Ide 0 1,2f gw 0, Ide NA 1,2f;lm as

0,0 NA 0 as 0,0 NA 0 N.D.

e to es 3fv If es o to e, Ide ° 1,2f aw

0, Ide NA 1,2f as 0,0 NA 0 as 0,0 NA 0 N.D.

es 3fv If aw es, tde 0 J,2f ew s, Ide N.A. a N.D.

es 2,3fv If ew es, tde 2fv If gw 0, Ide NA 0 N.D.

1

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Table 3. Soil Data From Highstand Barriers in the Lake Lahontan Basin. Horizon Oegth (em) ~olQrl Texture2 -- Size (°61 wt) Strueture3 ~Qn§il!!!:ne:t4 __ CaC03, - Pores6 Roots' Lower

Top Base Ory Moist Sand Silt Clay Ory Moist Wet efferveseenee5 Boundary8 (matrix, clasts}

0-28 A 0 S 2.SY 6/3 2.SY 4/3 VGSL ND o to lFSBK 10, sh NO so, po 0 0 2f as Av S 14 2.SY 6/3 2.SY 4/3 SCL ND 2FSBK so, sh NO SS, ps 0 2fv 2f as 2Bw 14 25 IOYR 6/2 10YR 4/2 VGSCL NO o to lFSBK 10, so NO ss, ps es, tde 0 2f cw 2Ck 25 100+ NA EGS ND NA NA es, tde 0 2f580 ND

EM-33 Av 0 3 2.5Y 6/3 2.5Y S/3 L ND IFPL,IFCR lo,sh fr so, ps e 3vf,fv lvf aw 2Bw 3 14 2.5Y 7/3 2.5Y 5/4 VGSL NO IFSBK 10, sh fr so, po e 2vfv 2f cw 2Bk 14 26 lOYR 7/4 IOYR 6/4 EGS,LS NO o to IFCR 10 10 sO,po es, tde 0 Ivf aw 2C 26 100+ NA EGS ND NA NA tde 0 0 NO

OM-IO Av 0 4 2.5Y 6/3 10YR 3/3 SL 47.3 44.4 8.3 ICPR,IFPL 10, so NO so, ps 0 2vf,fv 2vf,f aw 2Bw 4 30 IOYR 7/3 10YR 4/3 VGSL 39.1 SO.S 10.4 o to IMCR 10, so fr so, ps 0 2vf,f 2vf,f gw 2Bk 30 41 2.5Y 6/3 2.5Y 4/3 EGSL 4S.4 44.6 10.0 o to IMCR 10, so 10, fr ss, ps es, tde 2vf,fv 2,3vf,f gw 2C 41 100+ NA EGS ND NA NA es, patchy 0 0 ND

EM-26N Avk 0 17 2.SY 6/3 2.SY 4/3 GSL ND 2FPL,IMSBK so, sh NO so, ps es 2fv 1,2vf cw 2Bk 17 37 2.SY 7/3 2.SY S/3 VGSL ND o to lFSBK 10, so ND sO,ps ev, tde 0 Ivf gw 2Ck 37 100+ NA EGS ND NA NA ev, t,the 0 0 ND

HRC-l Avk 0 8 2.SY 6/3 2.5Y 4/3 GSCL ND IFPL,IFSBK so, sh NO ss, po e, patchy 2fv Ivf as 2Bw 8 28 !OYR4/3 IOYR 5/4 EGSCL ND o to lFCR 10, so NO s, p 0 0 2vf,f es 3Ck 28 100+ NA GCO ND NA NA e, tde 0 2vf570 ND

G-IS Av 0 6 2.SY 6/3 2.5Y 4/3 S ND o to IMeR 10 NO sO,po 0 2fv 0 cw 2Bw 6 19 2.5Y 7/3 2.5Y 6/4 SCL ND IMCR,IMSBK 10, so NO sO,po e 2fv If cw 2Bk 19 25 2.SY 7/3 2.SY 4/4 SCL ND 2MCR so, sh NO ss, p es 2fv If cw 2C 25 100+ NA S ND NA NA 0 0 0 ND

KP-16 A 0 3 IOYR 5/3 NO S ND 0 10 NO so, po 0 0 0 aw Bwl 3 28 2.SY 6/3 IOYR4/3 LS NO IMGR \0, so NO ss, ps 0 0 1m aw Bw2 28 41 10YR 5/4 ND S NO 0 10 NO so, po 0 0 1m gw C 41 100+ NA GS ND NA NA 0 0 0 ND

CS-l1h Av 0 8 10YR 712 2.SY 5/3 SC ND 2MPR,2FPL sh, h fr ss, ps ev, tde 2vfv Ivf aw 2Bt 8 23 2.SY S/4 2.5Y 4/4 SCL ND IFCR s, sh 10 so, p es, tde 0 Ivf ew 2Bw 23 44 2.SY 5/4 2.5Y 4/4 GS ND o to IFCR 10 10 so, po es, tde 0 0 ew 2Ck 44 100+ NA VGS NO NA NA es, thee 0 0 NO

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a • '.,lC~ a.ai a H H. H,~~ H H H :~' --------- ,

.. I'.'d ml~ q .. Table 3 continued. Soil Data From Highstand Barriers in the Lake Lahontan Basin.

Horizon Oepth (cm) Color l Texture2 Size (% wtl Structure3 Consisien-;0 __ --Top Base Ory Moist Sand Silt Clay Ory Moist Wet

L-4 Av 0 9 10YR 712 NO SCL ND IMPR,IFPL lo,so NO NO 2Bw 9 55 2.5YR 6/3 NO VGSL ND IMGR lo,so NO NO 2C 55 100+ ND ECS NO NA NA

KP-7 A 0 9 10YR 5/3 NO SL NO IMPR so, sh NO NO 2BI 9 36 IOYR4/4 NO VGSCL NO 2MSBK sO,sh NO ND 2Ck 36 80+ NO C ND NA NA

L-26 Av 0 10 IOYR 7/2 NO CL ND IMCR lo,so ND NO 2Bw 10 40 10YR 6/4 NO VGSL ND IMGR toO 10 NO NO 2Ck 40 100+ NO EGS NO NA NA

FCM-7 Av 0 2 10YR 7/2 NO SCL ND IMPR h,vh NO ND 2Bw 2 50 2.5YR6/3 NO GLS ND 1,2FCR lo,so NO NO 2Ck 50 100+ ND VGS ND NA NA

R-24 Av 0 5 10YR 6/3 NO GCL ND IMPR sh, h NO NO 2Bt 5 II IOYR4/3 NO VGSCL NO IMGR 10, so NO ND 2Btk II 40 IOYR4/3 NO EGSCL NO 2MSBK sO,sh NO ND 2Ck 40 100+ NO EGS ND NA NA

CS-41 Av 0 8 10YR 7/3 ND VGSL ND IMCR lo,so ND ND 2Bw 8 30 IOYR 6/3 NO VGSL ND IMGR 10 NO ND 2C 30 250+ NO EGS NO NA NA

CS-27 Av 0 6 10YR 7/2 NO SCL ND 2MPL sO,sh NO NO 2Bw 6 26 IOYR 6/4 NO GLS NO IMCR 10 NO ND 2Ck 26 100+ NO EGS ND NA NA

F-6 Gravelly sand 0 4 IOYR6/3 ND GS ND 0 10 NO NO 2Bw 4 25 IOYR6/3 NO GS ND OlolMGR 10 NO NO 2Ck 25 100+ NO CS ND NA NA

F-19 Av 0 6 10YR 6/3 IOYR 312 GCL 36.9 35.7 27.4 2FPL,2FSBK so, sh fr ss, ps 2Bw 6 15 2.5Y 7/3 2.5Y 4/4 VGL 39.1 47.7 13.2 O,IFSBK 10, so \0, fr ss, ps 2Bk 15 22 2.5Y 7/3 2.5Y 4/4 VGL 40.5 49.1 10.4 O,IFCR 10, so 10 so, ps 2Ck 22 50 NA G ND NA NA

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CaC03: - Pores6 R~01s7 Lower effervescence5 Boundary8 (matrix, clasts}

es 2vfv NO cw es NA 1,2f dw

NO NA 0 ND

0 Ivfv If cs 0 NA If gw

es, Ide NA NA NO

es 2fv ND aw es, Ide NA ND ew

Idc NA ND ND

es 2mv ND ew es, Ide NA ND gw

Ide NA ND NO

0 Imv ND aw 0, Ide NA ND aw 0, Ide NA NO ew

Ide NA ND ND

es 2mv ND aw c, Ide NA ND ew

Ide NA ND NO

es 2mv ND cw es, Ide NA ND gw

Ide NA ND ND

0 0 ND aw 0 NA ND cw

0, Ide NA ND ND

e 2,3v[v If aw e 1vfv 2vf,f cw

es, Ide Ivfv 2vf,f cw Ide NA 0 NO

Table 3 continued. Soil Data From Highstand Barriers in the Lake Lahontan Basin. Horizon Oel!th (em) Colorl __ Texture2 Size C'l] wt) Strueture3 !::QD~is!~nl<:£4 __ CaC03, Pores6 Roots7 Lower

Top Base Ory Moist Sand Silt Clay Ory Moist Wet efferveseenee5 Boundary8 (matrix, c1asts2

F-9 Av 0 4 2.5Y 7/3 2.5Y 4/4 GSL 35.7 51.1 13.2 2FSBK,IFPL so, sh fr ss, ps e 2fv lvf aw 2Bw 4 22 IOYR 5/4 IOYR 3/3 EGL 35.8 43.1 21.1 o to IFSBK 10, so 10, fr so, ps 0 0 Ivf,f cw 2Bk 22 32 10YR 5/3 10YR 3/3 EGL 48.0 44.0 8.0 o to IFSBK 10 10 so, ps e, tde 0 l,2vf,f cw 2Ck 32 100+ NA G NO NA NA tde 0 0 NO

F-7 Av 0 5 IOYR 6/3 10YR 4/3 SL NO o to IFPL 10, so fr so, po 0 2,3vfv Ivf cw 2Bw 5 17 10YR 6/4 IOYR 4/4 VGSL NO o to IMSBK 10, so 10, fr so, po 0 Ivfv 1,2vf cw 2Bk 17 34 IOYR 7/4 iOYR 4/4 VGLS NO o to IMSBK 10, so 10 so, po O,e,tde 0 2,3vf,f gw 2C 34 100+ NA G NO NA NA 0 0 0 NO

CC-4 Av 0 9 IOYR 6/3 IOYR4/3 VGSL NO O,IF,MSBK 10, so NO so, ps 0 2,3fv 1,2vf cw 2Bw 9 22 IOYR6/3 IOYR4/3 EGSL NO o to IFSBK 10 NO so, ps tde 0 3vf,f gw 2Bk 22 40 10YR 5/3 IOYR4/3 EGLS NO o to IFSBK 10 NO so, ps es, thde 0 3vf,f gw 2Ck 40 100+ NA G NO NA NA thde 0 0 NO

F-29 Av 0 8 10YR 6/3 10YR 4/4 SL NO IFSBK 10, so NO so, ps 0 3fv If as 2Bw 8 22 10YR 5/4 10YR 4/4 GS NO OlolVFSBK 10 NO so, po 0 0 2f cw 2C 22 100+ NA GS NO NA NA 0, Ide 0 0 NO

WL-Sc sand 0 3 2.5Y 8/3 2.5Y 5/4 S NO IFCR 10, so NO so, po 0 NO NO aw Avk 3 9 2.5Y 8/2 2.5Y 5/4 GSL NO 3FSBK,IMPL sh NO ss, ps es 0 NO cw 2Btjk 9 20 2.5Y 8/2 2.5Y 7/4 VGSCL NO 2F,MSBK sh NO s, ps es, tde 0 NO aw 2Ckq 20 90+ NA EGS NO 0 NA e 0 NO NO

I '· 1- I····· \' ' ' ~ " R Fl H A H~ tl 8' ~. .. .,..". EWe I1 JI:1.,. ..IlL~ , .... ~~.

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Table 4. Soil Data From Pre-Sehoo Age Lacustrine and Related Deposits in the Lake Lahontan Basin. Horizon Depth (cm) __ --'C"'o"'lolUr i __ Texture2

Top Base Dry Moist

Wadsworth Amp. Btklb 0 19 10YR 513 IOYR4/3 L

Btk2b 19 52 10YR 6/4 10YR 5/4 SCL

Clb 52 90 NA SL

C2b 90+ NA S

Rye Patch Dam Btkb 0 36 10YR 6/4 10YR 5/4 SC Bkb 36 60 IOYR 6/4 10YR 5/4 SC

Cb 60+ NA ND

Beach Denosits

Quinn River Valley (OM-to) Btb 0 47 10YR 6/4 2.5Y 5/4 VOSL

Btk1b 47 100 10YR 5/6 iQYR 5/8 VOCL

Btk2b 100 175+ IOYR 6/4 IOYR4/6 VOSCL

Jessup paleosol Bt1b 0 15 2.5Y 6/3 2.5Y 5/3 SC

Bt2b 15 60 10YR 6/4 lOYR 5/4 OSC

Ckb 60 250+ NA as

Grimes Canyon Btlb 0 57 10YR 5/3 nOYR 5/4 EGSC

Bt2b 57 155 10YR 6/3 IOYR 5/3 EGSC

Bt3b 155 192 IOYR 5/3 JOYR4/3 EGSC Bedrock NA NA NA

Thorne Sar- mid sand 0 3 2.5Y 6/2 IOYR4/4 S Avk 3 9 IOYR 7/3 10YR 5/4 SL 2Btjk 9 16 2.5Y 7/3 IOYR 5/4 L 2Bkl 16 30 IOYR 7/3 IOYR 5/4 LS 2Bk2 30 45 IOYR 7/3 IOYR 6/4 S 3Bqk 45 64 IOYR 6/4 IOYR 5/4 S 3Cqk 64 100+ IOYR 6/3 IOYR 513 S

Thorue Bar-upper sand 0 3 2.5Y 7/3 10YR 5/4 S Av 3 8 2.5Y 8/3 IOYR 6/3 SL BI 8 16 IOYR 8/3 IOYR 6/4 L 2Btjk 16 31 IOYR 7/3 10YR 6/4 VOSL 2Bky 31 55 lOYR 7/3 10YR 6/4 EGLS 3Bkq. 55 95 IOYR 6/3 10YR 6/4 EGS 4Ckq 95 110+ 2.5Y 8/2 2.5Y 7/3 EGS

-..... !: I!r II

Size (% wI) Structure3 Consistency4 __ CaC03, effcrvescence5

(matrix, clasts)

Pores6 Roots 7 Sand Silt Clay Dry

43.2 59.1 66.7 86.9

58.6 33.6 54.7

I:,

.' 'I :5, $M. "'" -,

32.0 18.8 24.8 9.5

ND ND ND

26.8 30.2 19.9

ND ND ND

ND ND ND NA

ND ND ND ND ND ND ND

ND ND ND ND ND ND ND

11'\ i~,' §}'.' ~ ~I

24.8 2MPR,2FSBK 10, sh 22.1 2FSBK to 2FPL sh, h 8.5 NA 3.6 NA

3FPR,3FSBK sh, h 2FPR,2F,MSBK sh

NA

17.4 2FSBK sh, h 36.2 3VF,FSBK h,vh 25.4 2FSBK to 2FPL sh,h

3MSBK,3FPL h,vh 2MPR, 3MSBK sh, vh

NA

2FSBK sh,h 3F,MSBK vh, eh 3MSBK eh

NA

0 so 2MPL,2MCR sh

IMCR so o to IMCR 10

0 10 o to IMCR 10, sh

0 10

0 10 2FPL,3FCR so

IFPL,2MSBK sh 1,2F,MSBK so

IFSBK so 0 10 0 10

•' '; .21.~: , ~0 := ... 8 1

. lm '.;

, -

Moist Wet

ND s, p e, es 0 Ifvf,f ND ss, p e, es 0 If NA 0 0 0 NA 0 0 0

ND s, p es 0 0 ND s, p es 0 0 NA 0 0 0

Ii s, p es, patchy ND 0 Ii s, p es, tdc ND 0

fr to Ii s,p es, tdc ND 0

ND s, p e, patchy 0 0 ND s, p es, patchy 0 0 NA es, patchy 0 0

ND s, p O,e, patchy 0 0 ND s, p e,es, patchy 0 0 ND s, p es 0 0 NA NA NA NA

ND so, po e 0 ND ND ss, po es 0 ND ND ss, ps es, ev 0 ND ND so, po ev 0 NO ND so, po ev 0 ND ND so, po ev 0 NO ND so, po e, es 0 ND

ND so, po 0 0 0 ND so, po e 3vf, fv lr,m ND s, ps es 3vf 2f,3vf ND s, ps es 0 3vf,l,f ND s, po e 0 3vf, If ND so, po e 0 If ND so, po e 0 0

<+ ; i' 8 ' -• " .' II' II II .., ~ ;til - ;;;;~ .- Wit " ,., - :

Lower Boundary8

cw gw gw ND

cw gw ND

cw dw ND

aw gw ND

cs cs cs

mantle?

aw aw cw cw dw dw ND

aw cw cw cw as as ND

I iii II II II II " Ii '< '¥ II ill &I II U iM all .. ~ Note: Descriptions and abbreviations follow criteria in Soil Survey Divisions Staff (1993), except: Av = vesicular A horizon. 1) From Munsell Soil Color Charts (1990). 2) G, gravelly or gravel; VG, very gravelly; EG, extremely gravelly; Co, cobbly or cobbles; ECo, extremely cobbly; F, flaggy or flagstones; S, sand; LS, loamy sand; SL, sandy loam; SCL, sandy clay loam;

CL, clay loam; SC, sandy clay; SL, silt loam. 3) 0, single grained; 1, weak; 2, moderate; 3, strong; VF, very fine (very thin); F, fine (thin); M, medium; C, coarse (thick); VC, very coarse (very thick); GR, granular; CR, crumb; PI, platy; PR, prismatic;

CPR, columnar; ABK, angular blocky; SBK, subangular blocky. 4) Dry: 10, loose; so, soft; sh, slightly hard; h, hard; vh, very hard; eh, extremely hard. Moist: 10, loose; vfr, very friable; fr, friable; fi, firm. Wet: so, non-sticky; ss, slightly sticky; s, sticky; po, non-plastic;

ps, slightly plastic; p, plastic. 5) Matrix: 0, not effervescent; slightly effervescent; es, strongly effervescent; ev, violently effervescent. Clasts: (dc, thin discontinuous carbonate coatings; th, thick coatings; cc, continuous coatings on

undersides. 6) vr, very fine; f, fine; m, medium; 1, few; 2, common; 3, many; ir, irregular; v, vesicular. 7) vr, very fine; f, fine; m, medium; I, few; 2, common; 3, many. 8) a, abrupt; c, clear; g, gradual; d, diffuse; s, smooth; w, wavy; i, irregular.

41 0

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Northern Desert Mountains

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Figure Location map of the Lake Lahontan basin at its last highstand at about 12.7 ka. Black dots are locations of existing soil pits and field reconnaissance descriptions. Circled black dots are locations of paleosols associated with beach deposits and younger soils. Circled inverted triangles are locations of paleosols not associated with beach deposits. Boxed labels are soil profiles that have been fully described, sampled and analyzed in the lab.

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1341 E 1340 ~ 1339 .2 1338 ~ 1337 ~ 1336 w 1335

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Jessup Playette Trench

EX PLANA TlON

o Barrier gravels of Schoo lake cycle.

Lagoonal sands ofSehoo lake cycle.

~ Older alluvium pre-<ialing Schoo Lake cycle.

t€) Krolovina

TTIT Soil development

Crest of barrier

1 Profile 2

Edge ofl playette on surface

1 Profile 4

Radioulna and metacarpal from Camelops hesternus (12,690 + 60 yr B.P.)

I , I - I

o 10 20 30 40 50 60

Distance (m)

Figure 2. Simplified log of the Jessup Playette trench showing the location of the described soil profiles features related to soil development. This view is looking to the southwest, but all soils were described on the northeasr of the trench and their locations projected to the southwest wall. The location of the camel bones was also projected from the northeast to the southwest

No vertical exageration.

lli a

,

~

~ ~

Explanation Beach bamer or depositional feature; dasbed where discontinuous, dotted where partially buried.

Erosiooal or cut-and-built tenace.

Lithoid Terrace (1) of Russell (1885).

~ Beach cliff or erosiooal escaxpment.

N

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( , Elevations (m) of

regressive barriers

)1 ,,/ " o ~ km})

@1334.4 (1)1328.5 ('2)1327.7 (3)1320.4 @1317.2 ®1311.3

jl ./

3. Sketch map of the Jessup Embayment showing the locations of soil profiles with respect to beach barrier features.

,

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Appendix 3

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LAKE LAHONTAN GEOLOGY OF THE FERNLEY SINK, NEVADA: PRELIMINARY RESUL~S FROM LAKE SHORELINE PROFILES

Kurt Cupp, Graduate Student at UNR & The Desert Research Institute.

Introduction

The Fernley Sink is a sub-basin of paleo Lak~ Lahontan that is

located some 56 km east of Reno, Nevada, in Lyon, Washoe, and

Churchill Counties. Past studies have included the Fernley Sink as a

small part of the adjoining Carson Sink sub-basin, however, it is a

true isolation sub-basin of Lake Lahontan. The sink has an area of

approximately 200 square km and is bounded by the Truckee Range, Hot

Spring Mountains, and Virginia Range.

To the southeast, in the pass between the Hot Springs Mountains

and the Virginia Range, is located the Darwin Pass Sill. This sill g.~.~.'."\7f¥"

has an elevation of 1238 m and it separates the Fernley Sink from the

I~II Carson Sink sub-basin. To the southwest, in the pass between the

~ ~

tGI"'" " •. ~{,' 14: ,;.,v

U"

~.:i ~

II

--

Truckee Range and Virginia Range, is the Fernley Pass Sill. The

Fernley Pass Sill separates the Fernley Sink from the Pyramid Lake

sub-ba.sin.

The Fernley Pass Sill is an important sill in the control of Lake

Lahontan paleohydrology. The elevation of the Fernley Pass Sill is

reported as 1265 m (Benson, 1994; Morrison, 1964). The sill divides (

Lake Lahontan into two large independent sets of lake sub-basins. The

northwestern set is comprised of the Black Rock - Smoke Creek Desert,

Pyramid Lake, Winnemucca Lake, and Honey Lake sub-basins. The

southeastern set is comprised of the Fernley Sink, Carson Sink,

Humboldt Valley, .and Buena Vista Valley sub-basins. The only sub-

_ basin excluded from these two sets is the Walker Lake sub-basin which

til

1

doesn't coalesce with other Lake Lahontan sub-basins until an

elevation of 1308 m (Adrian Valley Sill).

Both the northwestern and southeastern sets of sub-basins are

each supplied with lake waters from several major river drainages.

The northwestern sub-basins receive water from the Truckee, Quinn, and

Susan Rivers. The southeastern sub-basins receive water from the

Humboldt and Carson Rivers. Also, the southeastern sub-basins

sometimes receive water directly from the Walker River (King, 1993;

Benson, 1994).

Fernley Sink Lake Lahontan. Shoreline Profiles

Seven detailed lake shoreline profiles were reconstructed from

vertical sets of erosional and constructional platforms located

throughout the Fernley Sink (profiles, pp. 9-12). The erosional

shoreline platforms are .eroded in the relatively steep Tertiary basalt

foothills of the sink's bounding mountain ranges. The profile

elevations were measured using a total station and anchored to

existing bench marks. Profile locations include the southern and

northern Truckee Range, southern and northern Hot Springs Mountains,

and an un-named bedrock island in the north-central Fernley Sink (map,

fig. 1, p. 8). Reported shoreline profile elevations have a

calculated accuracy of + 0.5 m.

Fernley Sink Isostatic Rebound and Deformation

In the Fernley Sink the "Russell Lithoid Terrace" (Russell, 1885;

Morrison, 1964) is a well developed shoreline platform that is quite

distinct from all other platforms. The Russell Lithoid Terrace is

coated and cemented with lithoid tufa and is the highest erosional

shoreline in all but Profile #1. The age of the Russell Lithoid

.,

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Terrace is currently unknown. Russell (1885) concluded that the

feature was formed by maximum Eetza Lake Lahontan, however, Morrison

(1964) concluded that the higher elevation "Lahontan Beach" features

were likely formed by maximum EetzaLake Lahontan.

The identification of the distinct Russell Lithoid Terrace in the

Fernley Sink shoreline profiles enables the estimation of post~Sehoo

isostatic rebound and deformation rates. Post-Sehoo deformation was

found to be minimal as the Russell Lithoid Terrace has a relatively

uniform elevation throughout the Fernley Sink. The elevation of the

terrace ranges from a minimum of 1331.0 m in Profile #5 to a maximum

of 1332.5 m in Profile #2. All other terrace elevations fall within

this relatively small 1.5 m interval. In addition, Morrison (1964)

reports that the highest elevation for the Russell Lithoid Terrace in

the adjoining southern Carson Sink is 1327.5 m or 3.5 m lower than its

minimum elevation in the Fernley Sink (1331.0 m). This estimate of

isostatic rebound correlates with research by Ken Adams utilizing the

elevation of Lahontan Beach features (this guidebook).

Lake Lahontan Paleohydrology and the Fernley Pass Sill

The Fernley Pass Sill (1265 m) separates the Fernley Sink from

the Pyramid Lake sub-basin. Benson (1994) documented the presence of

prominent lacustrine features in the Pyramid Lake sub-basin at the

approximate elevation of 1265 m. The features include the top of

Needles Rocks, dramatic changes in tufa lithologies, and prominent

lake shorelines. Benson (1994) concludes that the control for these

prominent 1265 m lacustrine features is the Fernley Pass Sill which

corresponds to this same elevation.

Benson (1994) data indicates that the filling of the northwestern

sub-basins, to the elevation of the Fernley Pass Sill, was likely

3

completed well before the filling of the southeastern sub-basins.

This produced an extended still-stand of water in the northwestern

sub-basins as water spills over the Fernley Pass Sill into the

southeastern sub-basins. Only after all Lake Lahontan sub-basins

reached a lake level of 1265 m could lake levels resume their rise.

In establishing a scenario to clarify the existence of these

prominent 1265 m lacustrine features, Benson and Peterman (1995) used

strontium isotope ratios to trace the likely discharge path of the

paleo Humboldt River. In addition, they"dated the duration of

discharge diversion using tufa age estimates. Benson and Peterman

(1995) conclude that from about 25,000 to 15,000 years B.P., during

the vast majority of Sehoo Lake Lahontan filling, the Humboldt River

did not flow to the Carson Sink but, rather, it was diverted to the

Black Rock - Smoke Creek Desert sub-basin. This long term diversion

would have greatly decreased southeastern sub-basin water supplies

while greatly increasing river discharge to the northwestern sub-

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~ basins. Therefore, little or' no opportunity existed during Sehoo Lake

Lahontan for lake waters to extensively still-stand at 1265 m in the I:~ southeastern sub-basins.

Parallel to Benson's (1994) findings, the seven Fernley Sink

shoreline profiles reveal that the most prominent shoreline platforms

in the Fernley Sink have an elevation of approximately 1265 ru (1264.4

- 1265.8 ru). The 1265 m platform averages 7 m in width, compared to a

Fernley Sink overall platform development average of 1.75 m. In

addition, profile site #1 preserves a large Lake Lahontan beach gravel

deposit at approximately 1265 m elevation (~1262 - 1267 m).

One hypothesis that can explain the physical evidence described

above is a "physical control hypothesis". If the prominent 1265 m

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~----------------

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=I

lacustrine features in the Fernley Sink are the result of the physical

control of lake levels by the Fernley Pass Sill, then the features

represent a paleohydrologic event when the southeastern sub-basins

filled more rapidly then the northwestern sub-basins. This would

create a still-stand in lake levels in the southeastern sub-basins for

an extended period of time as water flowed over the Fernley Pass Sill

into the northwestern sub-basins.

One way to accommodate this physical control hypothesis would be

1"11 to return the discharge of the Humboldt River back to the Carson Sink.

In addition, its effect could be amplified by diverting the flow of

the Walker River through the Adrian Valley directly into the Carson

Sink. Because Benson and Peterman (1995) conclude that there is no

opportunity for this scenario to occur during Sehoo Lake Lahontan, the

hydrologic event and resultant shorelines would be pre-Sehoo in age.

A second hypothesis that can explain the physical evidence is a

~_ "climate control hypothesis". Several Lake Lahontan researchers have

speculated that the Fernley Pass Sill may not be a bedrock controlled

sill but, rather, a delta of the Truckee River (personal

- communication). The Fernley Pass Sill has an overall shape similar to

a delta, it is in close proximity to the Truckee River, and - stratigraphy in the Wadsworth area suggests that these adjoining

I:JI sediments could be deltaic (Smoot, 1993). However, no one has yet

researched the origin or lithology of the Fernley Pass Sill.

II

-If' the Fernley Pass Sill is a soft sediment feature it could not

be a candidate for any extended physical control of lake levels in

either the Pyramid Lake or Fernley Sink sub-basins. If this is the

case, then the 1265 m lacustrine features in both sub-basins date from

the same hydrologic event and the Fernley Pass "Delta" is likely

5

I i I

,.-I

graded to this elevation by that event. If you eliminate the major

physical control of the Fernley Pass Sill, then the 1265 m features

likely represent a climatic control when infiltratio~f evaporation and

precipitation were balanced in Lake Lahontan for an extended period of

time. Additionally, the Chocolate Sill, at approximately 1265 m

elevation (after rebound adjustment) in the relatively small and

distant Buena Vista Valley sub-basin, could possibly extend and

intensify any climate controlled still-stand duration in the Fernley

Sink area.

The most probable hypothesis at this point in the research

appears to be that of climate control. The climate control hypothesis

is currently supported by three observations. First, the western

leading edge of the Fernley Pass Sill has been deeply dissected by a

drainage from the Virginia Range for a distance of about 1.5 km. This

deep dissection has not exposed any bedrock.

Second, the most likely ~ocation for bedrock control within the

Fernley Pass Sill is at a bedrock prominence that extends into the

sill from the Truckee Range approximately 5 km from the sill's western

leading edge. However, at this point in the Fernley Pass Sill

elevations drop to a minimum of approximately 1250 m.

Third, the large 1265 m Lake Lahontan beach deposit at profile

site #1 is likely middle to early Sehoo in age. This is because the

lacustrine deposits above and in contact with the unit have been AMS

radiocarbon shell dated in three locations and are late Sehoo in age

(12,800 ±60 - 13,130 ±60 yr. B.P.). Work is currently underway to

directly AMS radiocarbon date lacustrine shells from this deposit.

"~~C~~~'"""'"~ ________ _

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References Cited

Benson, L., 1994. Carbonate deposition, Pyramid Lake subbasin, Nevada 1, Sequence of formation and elevational distribution of carbonate deposits (tufas): Palaeogeography, Palaeoclimatology, Palaeoecology, vol. 109, no. 1, p. 55 - 87.

Benson, L., and Peterman, Z., 1995. Carbonate deposition, Pyramid Lake subbasin, Nevada 3, The use of (87)Sr values in carbonate deposits (tufas) to determine the hydrologic state of paleolake systems: Palaeogeography, Palaeoclimatology, Palaeoecology, vol. 11 9, . no . 4, p. 201 - 21 3 .

King, G., 1993. Late Quaternary history of the lower Walker River and its implications for the Lahontan Paleolake system: Physical Geography, vol. 14, no. 1, p. 81-96.

Morrison, R., 1964. Lake Lahontan: Geology of'southern Carson Desert, Nevada: United States Geological Survey Professional Paper, paper #401, 156pp.

Russell, I., 1885. Geological history of Lake Lahontan, a Quaternary lake of northwestern Nevada: U.S. Geol. Survey, Mon. 11.

Smoot, J., 1993. Field Trip guide: Quaternary-Holocene lacustrine sediments of Lake Lahontan, Truckee River Canyon, north of Wadsworth, Nevada: U.S.G.S. Open-File Report 93-689, 35 pp.

7

-- --

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ley Sink Sub-basin

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The Alluvial Fan Stratigraphy of Buena Vista Valley, North Central Nevada: Implications for a Synchronous Geomorphic Response on Alluvial Fans In a Semiarid Climate

Introduction

by

John B. Ritter and Christopher Coonfare Department of Geology Wittenberg University

P.O. Box 720 Springfield, OH 45501

and

Jerry R. Miller and Jen Husek Quaternary Sciences Center

Desert Research Institute P.O. Box 60220

Reno, NV 89506

Tectonism and climate are the primary variables considered in conceptual models of Quaternary alluvial fan evolution; however, their roles and relative importance with respect to periods of fan aggradation and entrenchment are far from resolved (Ritter and others, 1995). Early studies of Quaternary alluvial fans were conducted in tectonically active areas (e.g., Eckis, 1928) and emphasized the role of faulting in initiating source area erosion and fan aggradation (e.g., Davis, 1905; Blissenbach, 1954). Later studies related tectonism to fan morphology, including segmented radial fan profiles (Beaty, 1961; Bull, 1964), fan-head incision (Denny, 1967; Hooke, 1967) and the development oftan complexes (Denny, 1967). Alternatively, Lustig (1965) argued that climate changes would induce changes in stream regimen and vegetation density of the source area sufficient to affect sediment supply. Such a geomorphic response would necessarily be widespread and characterized by synchronous periods of aggradation and entrenchment for all fans in a region. This has been demonstrated in more recent fan stUdies (e.g., Pierce and Scott, 1982; Wells et aI., 1987; Nemec and Postma, 1993). If a causative link indeed exists between either extrinsic control, climate or tectonics, and Quaternary alluvial fan activity, it has important implications for using alluvial fans in reconnaissance studies as a first-order indicator for either tectonic activity or climate change.

Understanding the role extrinsic controls such as climate or tectonics play in Quaternary alluvial fan development is dependent on (1) delineating stratigraphic relations between the alluvial fan and climatic and tectonic features or deposits and (2) providing a temporal framework, locally constrained by radiometric dates. The objectives of this study include

• delineating the surficial stratigraphy of alluvial fan deposits in Buena Vista Valley, north-central Nevada;

• relating the alluvial fan stratigraphy to geomorphic features and deposits associated with paleoclimate and tectonism;

• comparing the alluvial fan stratigraphy with those developed locally by Hawley and Wilson (1965) for the Winnemucca area and regionally in the western U.S.; and,

• examining the driving forces of aggradation and entrenchment on alluvial fans in the Buena Vista Valley. .

J

Description of the Study Region

Buena Vista Valley is a north-northwest trending basin in the northern Great Basin, bordered on the west by the Humboldt Range and on the east and southeast by the East and Stillwater Ranges, respectively (Figure 1). Major external controls that might affect alluvial fan development include source area lithology, tectonism, climate and climate change, and base level. Alluvial fans sourced in the Humboldt Range are composed of sediments derived from the Triassic Prida Formation, a limestone and dolomite unit, to·the north and the Rochester Rhyolite to the south (Wallace and others, 1969, Tatlock and others, 1977). Alluvial fans sourced in the East Range are dominated by sediments derived from the Ordovician Valmy Formation, composed of argillite, chert, greenstone, and vitreous quartzite, and the Triassic Grass Valley and Raspberry Formations, weakly metamorphosed detrital rock units (Tatlock and others, 1977). Proximal alluvial fan deposits are faulted along both eastern and western margins of the basin, suggesting the area has been tectonically active during at least the middle Pleistocene. Drainage basin source areas and alluvial fans in Buena Vista Valley have experienced variable climates during the Quaternary Period. Past climates and climate changes are represented by the rise and fall of Lake Lahontan and its sub-basins. BuenaVista Valley is presently a closed basin; however, during the Pleistocene, it was a major subbasin of pluvial Lake Lahontan, which occupied the southern part of the basin during its late Pleistocene highstand (Figure 2). Base level for alluvial fans in the study basin has varied between the elevations of the maximum highstand of Lake Lahontan (1330 m; Figure 2) and the present playa surface (1230 m). Because elevations of alluvial fans vary south to north in the basin, increasing in elevation, each fan has been exposed to a unique base level history (Figure 3).

Methodology

Alluvial fans in the Buena Vista Valley are comprised of 3-4 time-stratigraphic units, each represented by a distinct geomorphic surface. The stratigraphy is defined on the basis of topography, stratigraphic relationships, soil profile development, and surface morphology. Fan deposits were initially delineated from aerial photographs in the laboratory on the basis of tonal an~ textural differences and supplemented by relief between deposits in the field. Relative ages of deposits were SUbstantiated by qualitative descriptions of soil profile development and surface morphology. Soil profiles were described for each fan and glacial deposit in soil pits .. and bank exposures according to methods and nomenclature outlined by the Soil Conservation Service (1981) and Birkeland (1984). Laboratory analysis of soil particle size distribution was performed using the wet sieve/pipette techniques of Singer and Janitzky (1986). Surface morphologic properties include frequency of exposed clasts and relief of original depositional features, such as longitudinal bars on the fan surfaces. Klondike Canyon alluvial fan, along the eastern margin of the basin, had exceptionally well-developed desert pavements. Relative-age methods described by McFadden and others (1989), including descriptions of desert varnish and rubification of surface clasts, were employed to characterize fan deposits on this fan.

R~sults

Alluvial fans in Buena Vista Valley are composed of four fan-stratigraphic units: Of1, 0f2, Of3, and Of4, from oldest to youngest. Geologic maps of American Canyon and Klondike Canyon alluvial fans illustrate the distribution of alluvial fan units and their relation to lacustrine features and deposits (Figure 4).

Unit Qf1 Unit Of1 is not present on all fans in the study area, but where present it is preserved in proximal

fan areas, generally within the mountainfront. Where associated with faulting, it is preserved on the upthrown block. Soils developed in Of1 deposits are characterized by a truncated K horizon, the rubble of which is scattered across its surface.

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Valley

Figure 1. Location and physiography of Buena Vista Valley. north central Nevada.

"'UH'"

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16 1320 iD

...J

~ ~ .8 ~ ctJ ~ Gi ::I

.E

1300

1280 - • • •

Chocolat. Butte Sill 1280 ,..,. -. - - ~" _A .- - .-

1240 .. .. • - •

'~_~_1

.g- 1220 • i i i i i i ;

::J ... +J 4(

§ 1: ::J tJ)

G) ~ tU

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1200

1180 ~_._._.""1I1~/ 1160

Exphllnatton

o MIrtM~i1I 'Ollilled ~ hike ~ • MlBtwllale toni ed $1:10l1li 1Gk. lewf T

ctwon~

1140~"LI ____ L-__ -i ____ ~ __ ~ ____ ~ __ ~ ____ ~ ____ ~ __ ~

o 5 10 15 20 25 30 35 40 45 Tim e, In Thousands of Veers Before Present

from ~ lind l1'Iom pson. 19f1f1

Figure 2. Late Pleistocene levels of Lake Lahontan, JIIU5rram

Chocolate Butte sm which connects Buena Vleta Lahontan.

the relation with with Lake

III ..

1

(m) (It)

1550

1500

1450

c:: 0 ;: 1400 ~ I).

m 1350

1300

1250

5200 North

4800

4600

4400 . Maximum lake lavel1330 m (4364 It) ~~~~~~~~~-~~~~~~~~~~~~~

4200 Range of llBnllol Base level FaJl

4000 Playa Elevation 1230 m (4035 It)

o 2ml

o 2 4 !em

South

Figure3. Radial fan profiles of alluvial fans in Buena Vista Valley illustrating their variable relation to the maximum highstand of Lake Lahontan and their relation to present base level, the playa floOr.

..

=-• Unit Qf2

Unit Qf2 is the dominant surficial alluvial deposit in Buena Vista Valley. Where exposed, Q12 deposits are dominated by sheetflood deposits of well-sorted sands and gravels, but poorly-sorted debris flow deposits are also present. Soils are characterized by a Bt or Bw horizon overlying a Stage III-IV K horizon (Figure 5). In medial and distal fan areas, Q12 is truncated by wave-cut benches and beach ridges. The ridges are constructional features with 1-2 m of relief, consisting of well-sorted pebble- and granule-size gravels. locally they bury the 8t horizon of the 012 soil. The ridges tend to project from the zenith point of fans (point of maximum downfan deflection of fan contour), especially on those that protrude into the basin or occur at bends in the basin. Radiometric ages (AMS) from shells incorporated in ridge sediments range in age from 11,730+/-60 to 13560+/-60 yrs B.P. (Table 1).

Unit Qf3 Unit Of3 is inset into Q12 in the proximal fan area and forms secondary fans in medial and distal

fan areas. In proximal areas, Qf3 consists of both debris flow and sheetflood deposits, whereas in medial and distal areas, it is dominated by stratified, well-sorted sheetflood deposits. Soils developed in Of3 are weakly developed, consisting of an A over 8k or Cox horizonation with Stage I carbonate morphology (Figure 6). In medial and distal areas, Qf3 abruptly truncates wave-cut benches. Where fan channels cut across constructional beach ridge deposits, Qf3 sediments are ponded above the ridge, locally burying the ridge, and radially splay at a pOint immediately downfan from the ridge (Figure 7). Where present, this pattern is typically repeated downfan over a series of 3 or 4 beach ridges. In distal fan areas, Of3 and Of4 deposits form distinct secondary fans. Mazama ash preserved near the top of proximal Qf3 deposits of two fans, indicates that Qf3 aggradation was well underway by 6,800 yrs B.P. (Table 1). On at least one with an exceptionally well-exposed proximal section fan, Willow Creek alluvial fan (Figure 1), Qf3 deposits consist of internal cut and fill structures. A radiometric age on charcoal of 1,250+/- 50 yrs B.P. indicates that proximal Qf3 deposits were entrenched and that entrenched channels were filled during the late Holocene (Table 1). (

Unit Qf4 Unit Qf4 consists of the fan deposits associated with present fan channels. These channels are

confined in proximal and medial fan areas and unconfined in distal areas.

Discussion

Alluvial fans in Buena Vista Valley formed during synchronous periods of aggradation, stability, and entrenchment during the late Quaternary. The similarity of particle size distribution and horizonation of soil profiles between fans for Q12 and Of3 deposits (Figures 5 and 6) suggest the respective units have been stable for similar periods of time. Because the Q12 deposits are clearly truncated by beach erosional and constructional features, which in turn are truncated by Qf3 deposits, the timing of units Q12 and Qf3 relative to the filling and high stands of lake lahontan are also similar. The degree of soil development in Q12 deposits, including those buried by beach ridge deposits (truncating a Bt horizon), indicate that Q12 aggradation occurred during the early late Pleistocene, followed by a substantial period of fan stability. During this period, some minor fan entrenchment probably took place given that some beach ridges are inflected along incisions.

The abrupt truncation of wave-cut benches and beach ridges by Qf3 channels and deposits, the ponded and splayed morphology of Qf3 deposits where associated with ridges, and the apparent lack of reworking of Qf3 deposits by shoreline processes, indicate entrenchment of Q12 fan deposits and aggradation of Qf3 deposits occurred during or following lake recession. It follows that discharge from drainage basins in the Buena Vista Valley probably did not contribute Significantly to Lake lahontan; rather, the highstands of Lake lahontan in Buena Vista Valley resulted from backfilling as the level of Lake Lahontan rose beyond the sill level separating it from this subbasin (Figure 2). On the basis of our correlation of Qf2 and Of3 fan segments between fans around the basin and the synchronous nature and timing of aggradation, stability, and entrenchment, we infer that climate has controlled fan evolution in the Buena Vista Valley. Extrinsic variables such as tectonism, base level, and basin lithology vary along the

1 0- i i o - :g

...... a -

kl 7i7

lej:j:

I i ° 1_

Iwjw

1 a

i ....

.. - !

.........

5 40 "-""

o

••••••••••

American Canyon

AVk2

ABk

Blk

o

40 60 80 100

Clay

Coyote Creek

AVk

ABk

Bwk

Bk

K (partlcte size not measured)

20 40 60 80

Relative UI~.",~ra .. ~

I~ _ U I Slit

Figure 5. Particle size distribution and horizonation of soil profiles Canyon, Coyote Creek. and Klondike Canyon alluvial

": ':Cl

." 1'il: lI"l;id ~,11,~~,I'T " 1M

o 20 60 80 100

o

20

...-. E 40 U ......... .c .... 60 a.. Q) c

80

100

o 20 40 80

o

Coyote Creek Klondike

o 20 40 .60

20 40 60 80 100

elative Percentage

o Slit Sand

horizonation of soil profiles developed in unit Qf3 on American Klondike Canyon alluvial fans.

"

=­=-

• I. • • • • • • •

Figure 7. Schematic representation of Qf3 deposits ponded above and radially splaying below constructional beach ridges on the medial fan area and truncating beach ridges and forming secondary fans on the distal fan area.

perimeter of the basin; however, their impact on fan evolution appears negligible as the Qf.2 and Qf3 stratigraphy and their stratigraphic relation to beach features is consistent between fans.

The timing of fan aggradation, relative to the backfilling of Lake Lahontan into the Buena Vista subbasin, suggests that it occurs during and following climate change from cool, wet to warm, dry climates. Critical to our argument for climatically-driven fan aggradation and entrenchment is the regional synchroneity of fan aggradation and entrenchment. Tentative correlation with fans regionally (those studied by Hawley and Wilson, 1965) on the basis of segment morphology and surface characteristics, such as bar and swale topography and varnish cover, suggest the response was regional. Comparison of the alluvial fan stratigraphy in this region with the stratigraphies of other fan studies, conducted in both semiarid and arid climates, in the western U.S. (Figure 8) yields some interesting results. The alluvial fan stratigraphies from these studies are correlated with that developed in the Buena Vista Valley; the results are summarized in Figure 9.

The timing of fan aggradation in Buena Vista Valley is essentially contemporaneous with fan aggradation in other arid climates, including southeastern California (Wells and others, 1987; Ritter, 1987) and east central California and southwestern Nevada (Reheis and others, 1994 and in review). Wells and others (1987) provided a process-response model for this sequence of fan aggradation and entrenchment. They proposed that increased rates of eolian deposition from pluvial lake desiccation influenced pedogenesis and infiltration-runoff relations: Increased runoff from impermeable hills lopes, coupled with increased sediment availability due to a probable decrease in vegetation density, resulted in fan aggradation in spite of reduced effective precipitation. Regional correlation of their alluvial fan stratigraphy by McFadden and others (1989) indicates that such a response was widespread and occurred in basins underlain by different lithologies as well (Ritter, 1987).

In contrast to fans in the arid climates of Nevada and California, which aggraded in response to climate change, fans in the semiarid climates of Montana (Ritter and others, 1993, 1995), Idaho (Funk, 1976 and Pierce and Scott, 1982), and New Mexico (Pazzaglia and Wells, 1990) aggraded during glacial climates. In glaciated basins in southeastern Idaho and southwestern Montana, geomorphic surfaces underlain by fan gravels can be traced to moraines of Pinedale and Bull Lake age (Funk, 1976; Pierce and Scott, 1982; Ritter and others, 1993, 1995). More significantly, correlative surfaces in unglaciated basins in Idaho indicate that a glaciated source area was not necessary for fan aggradation to occur and suggest that maximum sediment and water discharge were associated with annual snowmelt during full glacial conditions (Pierce and Scott, 1982). In the northern Rio Grande rift, Pazzaglia and Wells (1990), have mapped and correlated Quaternary deposits in four major physiographic landscapes, one of which was composed of alluvial fans. The fans are comprised of three deposits, each temporally correlative to major periods of glaciation, including the youngest which was correlated to neoglaciation during the middle Holocene (Figure 9). Pazzaglia and Wells (1990) concluded that while the various physiographic landscapes and their associated landforms owed their morphologic expression to tectonism, deposition within each landscape was influenced primarily by climatic fluctuations associated with glaCial and interglacial events on the basis of regional correlation of Quaternary deposits between landscapes.

Conclusions

• The surficial stratigraphy of alluvial fan deposits in Buena Vista Valley consists of four fan-stratigraphic units: Qf1, Of2, Qf3, and Of4, from oldest to youngest.

• While Qf1 may be as old as early middle Pleistocene, Qf2 is truncated by late Pleistocene beach deposits which are in turn truncated by Qf3 fan deposits. Qf4 is inset into late Holocene deposits and form the modern channel deposit.

• Units Qf1, Qf2, and Qf3 correlate with Qoa, Qpl, and Qya, respectively, from the alluvial fan stratigraphy developed by Hawley and Wilson (1965). In addition, the units correlate with fan units

• • •'.: .• '"0""

iWW.': • iiJI.. ~

Id It1 ~. ~

-=

It1 ~

Ib

t" !J;" ,I ~\ ,<>,'; C ,,:,- " ,::, ~, { ;~ "'" !"~ "'t ' i; '';;l , ' ~,,; ; .'1' 'f: ' 'I"" ~~""'I I .. ··' I~"· 11'" I·'" 11"" I';' 11-·"·' D--I' ;' " < :~" {' \" t, ' l" ,q l'~ / 'r ,;{ \\~ :.;. :, \ }i

Field Sites

1 Madison River Valley, Ritter and others (1993,1995) and Birch Creek Valley, Funk (1976) and Pierce and Scott, 1982

2 Northern Rio Grande Rift, Pazzaglla and Wells (1990)

3 Eastern Mojave Desert, Sliver lake, Wells and others (1987) and Salt Spring Hills, Ritter (1987)

4 Fish ~ake Valley, Rehels and others (1994 and In review)

5 Winnemucca Aru of Humbolt River Basin, Hawley and Wilson (1965)

6 Buena Vista Valley, this study

figure 8. Locations and references of alluvial studies in the western U.S. correlated in this study.

U "II '~' ... AI

Southwestern Montana and

Central Idaho i

Southeastern East Central

California and West Central Nevada4

North Central Nevada5

Buena Vleta Valley North Central

Nevada

11,730 t 60 11,820 t 70 11,880 t 50 12,220 t 60 12,390 t 70 12,9OO t 60 13,1oot 70 13,560 t 60 14,540t 60

1 Madison Riwf Ritter and others (1993, 1995) and Birch Creek Valley, Funk (1976) and Plarce and Scott, 1982. 2 Northern Rio Grande Rift. pazzaglla and Wells (1990). 3 Eastern MojavQ Desert, Sliver Lake, WQlIs and others (1967) and Salt Spring Hilla, Ritter (1987). 4 Fish Lake Valley, Reheie and others (1994 and In 5 Winnemuooa. Area of Humbolt River Basin, Hawley and WUson (1965). 6 Lake Chronology is based on Morrison (1991). 7 Quaternary age assignments adopted from Rlohmond (1986) and Imbrie and others (1984).

lake lahontan

Chronologv6

Age Ka7

Time Dlvlalon

r

J: o

3: ~ ~

m

r I» -CD "'0 <i i 0 n (D ::l ('I)

r

3: 0: Q.

CD "'0

3: CD in -g CD ::l CD

m

Figure 9. diagram Valley,

studies in the western U.S. for comparison with the fan stratigraphy of

III " " II" ••

-------------~

=­• =­=-=­=-•

from other studies conducted on arid alluvial fans; however, they do not directly correlate with fan units from studies conducted on alluvial fans in semiarid climates.

• Stratigraphic relations with beach features and deposits, the relative timing of the fan units, and their correlation between fans in the study area as well as regionally suggest that climate change from wetter to drier is the driving force behind fan aggradation. Entrenchment occurs in response to decreasing sediment supply in either a wet or dry climate.

Acknowledgements

This work was supported by a cooperative agreement with DOE (DE-DCOB-93NV11417); however, such support does not constitute an endorsement by DOE of the views expressed in this article.

References

Beaty, C. B., 1961, Topographic effects of faulting: Death Valley, California: Annals, Association of American Geographers, v. 51, p. 234-240.

Blair, T. C. and Bilodeau, W. L., 198B, Development of tectonic cyclothems in rift, pull-apart, and foreland basins: Sedimentary response to episodic tectonism: Geology, v. 16, p. 517-520.

Blissenbach, E., 1954, Geology of alluvial fans in semiarid regions: Geological Society of America Bulletin, v. 65, p. 175-190.

Bull, W. B., 1964, Geomorphology of segmented alluvial fans in western Fresno County, California: U.S.Geological Survey Professional Paper 352-E, p. 89-129.

Davis, W. M., 1905, The geographical cycle in an arid climate: Journal of Geology, v. 13, p. 381-407. Denny, C. S., 1967, Fans and pediments: American Journal of Science, v. 265, p. 81-105. Eckis, R, 1928, Alluvial fans in the Cucamonga district, southern California: Journal of Geology, v. 36, p.

111-141. Funk, J. M., 1976, Climatic and tectonic effects on alluvial fan systems, Birch Creek Valley, east central

Idaho: Ph.D. thesis: Lawrence, University of Kansas, 246 p. Hawley, J.W. and Wilson, W.E., 1965, Quaternary geology of the Winnemucca area, Nevada: Desert

Research Institute Technical Report No.5, Reno, Nevada, p. 1-66. Heward, A. P., 1978, Alluvial fan sequence and megasequence models: With examples from Westphalian

D - Stephanian B coalfields, northern Spain: Sedimentology, v. 25, p. 451-488. Hooke, R L., 1967, Processes in arid-region alluvial fans: Journal of Geology, v. 75, p. 438-460. Lustig, L. K, 1965, Clastic sedimentation in Deep Springs Valley California: U.S. Geological Survey

Professional Paper 352-F, 192 p . Nemec, W., and Postma, G., 1993, Quaternary alluvial fans in southwestern Crete: Sedimentation

processes and geomorphic evolution, in Marzo, M., and Puigdefabregas, C., eds., Alluvial sedimentation: International Association of Sedimentologists Special Publication 17, p. 235-276.

Pierce, K L., 1979, History and dynamics of glaciation in the northern Yellowstone National Park area: U.S. Geological Survey Professional Paper 729-F, 90 p.

Pierce, KL., and Scott, W. E., 1982, Pleistocene episodes of alluvial-gravel deposition, southeastern Idaho, in Fonnichsen, B., and Breckenridge, R. M., eds., Cenozoic geology of Idaho: Idaho Bureau of Mines and Geology Bulletin, v. 26, p. 685-702.

Ritter, J. B., and 16 others, 1993, Quaternary evolution of Cedar Creek alluvial fan, Montana: Geomorphology, v. 8, p. 287-304.

Ritter, J.B., Miller, J.R, Enzel, Y., and Wells, S.G., 1995, Reconciling the roles of tectonism and climate in Quaternary alluvial fan evolution: Geology, v. 23, p. 245-248.

Singer, M.J. and Janitzky, P., 1986, Field and laboratory procedures used in a soil chronosequence study: U.S. Geological Survey Bulletin 1648.

Tatlock, D.B., JohnsOn, M.G., Burke, D.B., and Stewart, J.H., 1977, Geologic Map of Pershing County, Nevada: in, Geology and mineral deposits of Pershing County, Nevada, M.G. Johnson, Nevada Bureau of Mines and Geology Bulletin 89.

i I I"

Wallace R.E., Tatlock, D.B., Silberling, N.J., and Irwin, W.P., 1969, Geologic map of the Unionville Quadrangle, Pershing County, Nevada: U.S. Geological Survey Map GQ-820.

Wells, S. G., McFadden, L. D., and Dohrenwend, J. C., 1987, Influence of late Quaternary climatic changes on geomorphic and pedogenic processes on a desert piedmont, ~astern Mojave Desert, California: Quaternary Research, v. 27, p. 130-146.

Table 1. Summary of dateable samples and their associated absolute age and fan stratigraphic relation .

Sample Alluvial Dated Material Age Stratigraphic

Fan Relation

Klondike Shells 13,100+/-70 Beach ridge deposit burying

Canyon Qf2 and truncated by Qf3

Klondike Shells 12,390+/-70 Beach ridge deposit burying

Canyon Qf2 and truncated by Qf3

Klondike Shells 11,880+/-50 Beach ridge deposit burying

Canyon Qf2 and truncated by Qf3

ACF-UR1 American Shells 12,900+/-60 Beach ridge deposit burying

Canyon Qf2 and truncated by Qf3

ACF-UR2 American Shells 13,560+/-60 Beach ridge deposit burying

Canyon Qf2 and truncated by Qf3

ACF-BR1 American Shells 11,880+/-50 Beach ridge deposit burying

Canyon Qf2 and truncated by Qf3

ACF-BR2 American Shells 12,220+/-60 Beach ridge deposit burying

Canyon Qf2 and truncated by Qf3

BS-BR4 Buffalo Shells "11,730+/-60 Beach ridge deposit burying

Springs Qf2

BS-BRS Buffalo Shells 11,820+/-70 Beach ridge deposit burying

Springs Qf2

WS-1 Willow Creek Charcoal 1,250+/-50 Upper beds of Qf3

ACF-Ash1 American Ash 6,800+/-50 Interbed in upper Qf3

Canyon

WS-Ash1 Willow Creek Ash 6,800+/-50 Interbed in Qf3

--- ---

.. j' "' ..

II ,c;,'l,

"'

-= II II

•

• II

~

~

~

=t

II

---til

=­at (II

Appendix 5

==-...... 7." ..•. -~ ,V',' ,_'"

,

~ .• r.

I·

,

~ ~ ~

~ ~

1

EOLIAN DEPOSITS AND LANDFORMS OF THE CARSON SINK.

Nicholas Lancaster and Kurt Cupp, Desert Research Institute, UCCSN.

:Introduction

Eolian deposits and landforms of Holocene age are widespread in the Lahontan

Basin, with the greatest areal extent in the Carson Sink. They include dunefields

(active and vegetation-stabilized) I sand sheets and sand ramps, and eolian dust

(loess) mantles on alluvial fan and shoreline deposits. Despite their extent, these

deposits and landforms have received little detailed study. The major eolian units

in this area were recognized, but not fully mapped, by Morrison (1964), who

subdivided them into the Turupah and Fallon Formations, separated by the Toyeh Soil.

Dollarhide (1975) mapped the extent of eolian d~posits in the Fallon-Fernley area in

greater detail and these soil maps provide valuable information on the nature of

eolian deposits and landforms in the region. Local studies of dunes are documented

by Katzer (1988) and Lancaster (1993). The geomorphology and sediments of Sand

Mountain were briefly described by Snyder (1984), while the sources and transport

pathways for sand were discussed by Eissmann (1990).

stratigraphy and sedimentology of Holocene eolian deposits in the carson Sink area

Two major eolian allostratigraphic units were recognized by (Morrison, 1964).

Eolian sand is the characteristic and most extensive deposit of the Turupah

Formation which disconformably overlies lacustrine deposits of Sehoo age. The

Turupah Formation is capped by the Toyeh Soil, which is disconformably overlain by

the Fallon Formation, comprised of eolian sand, alluvium, and the deposits of

shallow lakes. Within the Turupah Formation is a thin and discontinuous volcanic

ash layer. The Toyeh soil is described by (Morr~son, 1964) as moderately developed

with a vesicular A horizon, an oxidized B and a weakly developed Cca horizon.

The eolian sand of both Formations occurs as sand sheets, sand ramps,

isolated dunes and dunefields with a thickness of up to 15 m. The distribution of

the major areas of eolian sand deposits is shown in Figure 1. There are five main

areas of dunes: (I) The lunette dunes of the north and east Carson Sink playa margin

(described below); (II) An extensive area of parabolic and linear dunes that extends

from Hazen northeast to the Upsal Hogback and to the area north of the Stillwater

Marshes; (III) An area of parabolic and crescentic dunes on the southeast side of

the Stillwater marshes, northwest of the Lahontan Mountains; (IV) Sand sheets and

crescent dunes on the east side of the Dead Camel Mountains, south of the Sheckler

Reservoir; and (V) Sand sheets, sand ramps, parabolic and crescentic dunes that

cover the area of the Blow Sand Mountains, extending north east to the Salt Wells

flats, with a possible connection to Sand Mountain (Eissmann, 1990).

In all areas, linear and parabolic dunes are the most common types, crescentic

dunes are found in some areas/ and nebkhas (vegetation-anchored dunes) <;I.re

widespread. Sand Mountain is a complex of star dunes and smaller crescentic ridges.

Turupah Formation sands are fine to medium in size, whereas those of the Fallon

Formation are fine to fine-medium. Both formations also contain deposits of coarse,

bimodal sand. The source of the sand is believed to be sandy members of the Sehoo

and Wymeha Formations, which have been extensively deflated. Morrison (1964)

describes deflation basins 1 - 5 kID across and 6 to 12 m deep cut into Sehoo and

Wymeha deposits and terms the Carson Sink a "huge deflation plain". He estimated

that 2 kID3 of sediment had been removed from the area of the Carson Sink by wind

erosion. Much of this material was deposited as 2 to 30 em thick "loess" that

blankets piedmont areas on the eastern side of the Carson Sink. This silty material

is a major component of the soils in this area, as discussed by Chadwick and Davis

l ______ _

-=

• II

--= II

--~ -= ~

It

II:

~ rti ti

~ (1990), who suggested that periods of enhanced eo~ian dust influx to soils gave rise

to episodes of rapid soil formation, the "soi 1 forming episodes U described by

Morrison.

A preliminary analysis of the distribution of dune areas described· above

suggests that fluvial and deltaic deposits of the Carson and Walker Rivers may be an

important source of sediment for dunefields in the Carson Sink, rather than pre-

existing lacustrine and fluvial sediments. In most cases, sand dune areas can be

traced back to well-defined source areas. Dune areas II and III appear to be

sourced from the lower Carson River and its deltaic distributaries in the Fallon -

Carson lake area. The area of dunes on the east side of the Dead Camel Mountains

(area IV above) was probably derived from the Carson River in the area of the

Churchill Valley. Dunes of area V were probably derived from the Walker River in

the area of Mason Valley, as suggested by Eissmann (1990). Investigation of

~.'.".' . .... relations between the Holocene history of fluvial and eolian deposits in the region

is needed to understand the dynamics of this extensive eolian sediment transport

system.

The age of the Turupah and Fallon eolian deposits is debated. Morrison (1964)

believed the Turupah was formed during the altithermal period of Antevs (7,500 -

4,000 B.P.), with the Fallon Formation eolian deposits being laid down in the dry

intervals of the past 4,000 yr. I especially during the recession from the "First

Lake" period. Davis (1978), however, questions this interpretation. The tephra

I~II layer described from the base of the Turupah Formation by Morrison (1964) is the

Turupah Flat Bed which is assigned an age of less than 2,000 yr. (Davis, 1978). As a

result, Davis argues that much of the Turupah Formation should be mapped as the

Fallon Formation, although early- to mid-Holocene eolian deposits capped by the

Toyeh Soil do occur "in the area as a distinct stratigraphic unit.

Lunette dunes and yardangs in the north-east Carson Sink (Field trip stop)

A large complex of lunette dunes consisting of two and locally three ridges up

to 40 m high occurs on the northeast margin of the Carson Sink playa (Figs. 1 and

2). The lunette dunes are up to 1000 m wide and extend for 35 - 40 km around the

east and northeast playa margins at an elevation of 1190 to 1210 m. The southwest

slope of the dunes has an angle of 3 to 6°, whereas the north-facing slope is up to

30°, or close to the angle of repose. The dune ridges locally act as dams for

ephemeral streams draining toward the Carson Sink from the Stillwater, West

Humboldt, and Buena Vista ranges, but some of the larger streams (e.g. Packard Wash)

~ , ~ ...,

have cut or maintained their courses through the dunes. C~_I

The general form of these I~~

lunette dunes is very similar to those described from Australia (Bowler, 1983) and

southern Africa (Lancaster, 1978; Thomas et al., 1993). Although no true clay dunes

occur in this area of the Carson Sink, they have been described from the Stillwater

area (Katzer, 1988).

The stratigraphy of the dunes is shown in Fig. 3. The outermost, or north-

II'· ,.<)"1.

"'

-=~ .. Y ••

~ ."j I'. ".1.,

~ • easterly, lunette ridge consists of a core of poorly sorted medium and fine sand IItI partially cemented by saline clay, up to 8% gypsum, and silt (Unit 1) with avalanche

face cross-beds dipping to the north-east at 25 - 30°, as well as planar sets of

wind ripple laminae with dips to both the northeast and west at 2 - 5°. The modal

size of this material is 2.0 phi, with phi standard deviations ranging between 0.86

and 1.04. A weakly developed paleosol is found near the middle of this unit in some

localities. Overlying this unit on the crest and lee side of the ridge is 2 - 5 m

of unconsolidated poorly sorted (phi standard deviation 0.84 - 1.00), very fine and

coarse sand (mean grain size 1.92 to 2.38 phi) that forms an active avalanche face

up to 25 m high on the east side of the ridge (Unit 3). The inner, or south-

westerly, ridge (Unit 2) is up to 5 m high and consists of both partially indurated

II

lit

AI and mobile sand with lee-face laminae dipping to the north-east at 20 - 30°. Mean

W.,I,;',"-' •

-,,,,,' .,.

ale' f>

IZ ~;;

grain size of this unit is 1.77 to 2.36 phi.

At a number of localities, the indurated core of the larger dune ridge is

carved into yardangs, Or streamlined small hills with a lemniscate shape that result

from wind erosion of homogenous sediments. The yardangs here are 10 - 20 m long

with a bluff "bow" and a sharply tapering "stern", and increase in height from 1 ~ 2

m on the windward slopes of the dune to as much 4 m at the crest. They are similar

in form to yardangs at Rogers Lake, CA (Ward and Greeley, 1984) and appear to be

actively forming today.

The formation of playa margin or lunette dunes requires a restricted range of

environmental conditions. Work in Australia suggests that sandy lunettes form

during periods of high lake levels with a relatively low salinity, whereas clay-rich

lunettes form in low lake level, high salinity times, when clay pellets are deflated

from a zone of efflorescence where the capillary fringe of a shallow groundwater

table intersects the playa surface (Bowler, 1983). The Buena Vista dunes represent

an example of the former conditions, with the dunes being fed by deflation of sands

from an adjacent beach. The poor sorting of the eolian sands suggests a low energy

lacustrine wave regime and probably local sediment sources in nearby washes and

alluvial fan systems.

The dunes overlie, with an erosional contact, Late Pleistocene saline

lacustrine clays of paleolake Lahontan. They represent at least two episodes of

mid- to late-Holocene deflation of sediments from beaches of the Carson Sink playa.

Erosion of the dunes and yardang formation suggests: (1) termination of sediment

supply from the playa as a result of reduced sediment supply and runoff from the

Carson River, (2) cementation of the dunes by clay and silt accumulation, and (3)

modern eolian erosion through flow acceleration on dune windward slopes.

5

If'"" M

References Cited

Bowler, J. M., 1983, Lunettes as indices of hydrogeologic change: a review of

Australian evidence: Proceedings of the Royal Society of Victoria, v. 95, no.

3, p. 147-168.

Chadwick, o. A., and Davis, J. 0., 1990, Soil-forming intervals caused by eolian

sediment pulses in the Lahontan Basin, northwestern Nevada: Geology, v. 18,

no. 3, p. 243-246.

Davis, J. 0., 1978, Quaternary Tephrochronology of the Lake Lahontan Area, Nevada

and California: Nevada Archaeological Survey, Research Paper, v. 7, p. 137 pp.

i

~ III

~

Dollarhide, W. E., 1975, Soil survey of Fallon-Fernley area, Nevada, USDA Soil ~

Survey, USDA Soil Conservation Service, 112 p.

Eissmann, L. J., 1990, Eolian sand transport in western Nevada [MS thesis]:

University of Nevada, Reno.

Katzer, K. L., 1988, Age and paleoenvironmental significance of a clay dune field in

stillwater Marsh, Carson desert, western Nevada: Geological Scoiety of

America, Abstracts with Programs, v. 20, p. 172.

Lancaster, N., 1978, Composition and formation of southern Kalahari pan margin

dunes: Zeitschrift fur Geomorphologie, v. 22, p. 148-169.

Lancaster, N., 1993, Lunette dunes and yardangs of the Carson Desert, Nevada:

Implications for Holocene eolian acti vi ty in the northern Great Basin:

Geological Society of America, Abstracts with Programs, v. 25, p. 166.

Morrison, R. B., 1964, Lake Lahontan: Geology of southern Carson Desert, Nevada:

United States Geological Survey Professional Paper, v. 401, p. 156 pp.

Snyder, C. T., 1984, Sand Mountain, Western Geological Excursions, Geological

Society of America, Department of Geological Sciences, Mackay School of Mines,

University of Nevada, Reno, p. 137-139.

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It1

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Id

•

7

=- Thomas, D. S. G., Nash, D. J., Shaw, P. A., and Van der Post, C., 1993, Present day

lunette cycling at Witpan in the arid southwestern Kalahari Desert: Catena, v.

20, p. 515-527.

Ward, A. W., and GreeleYt R., 1984, Evolution of the yardangs at Rogers Lake,

California: Geological Society of America Bulletin, v. 95, p. 829-837.

J

Pyramid Lake

" Carson Sink

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1 Sketch map of the Carson Sink area, showing major areas of dunes and sand

sheets mentioned in the text. Map compiled from 1: 80 1000 scale aerial

photographs.

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Lunette Dunes

2. Lunette dunes at the northern end of the Carson Sink, showing their relations

to beach ridges and washes.

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Relations between alluvial fans and Lake Lahontan shorelines: stillwater mountain front, Nevada.

A M Harvey, (Dept of Geography, University of Liverpool, UK) S G Wells, (Desert Research Institute, Reno, Nevada).

Introduction

This short paper reports preliminary work on the geomorphology of the alluvial fans on the western mountain front of the stillwater Range, Nevada. It is concerned with the general relationships between the alluvial fans and the shorelines of Pleistocene Lake Lahontan, and examines some of the morphometric characteristics of the fans in relation to the shoreline sequence. It is based on field and air photo mapping of the fans and shoreline features (Fig I), and on field and map derived morphometric parameters.

Fans and shorelines

Three groups of alluvial fans and fan sediments can be recognised on the western mountain front of the stillwater Range, in relation to the high shorelines of late Pleistocene Lake Lahontan.

These are:-

Group 1. Alluvial fan sediments older than the last high stand of the Lahontan stage of Lake Lahontan (pre £ 13000 BP) •

Group 2. Alluvial fan sediments more or less contemporaneous with the last high stand of Lake Lahontan.

Group 3. Alluvial fan sediments which post-date the last high stand of Lake Lahontan.

Groups 1 and 3 can be further subdivided, Group 1 on the basis of field relationships and which may relate to previous (pre­Lahontan) stages of the lake, and Group 3 on the basis of field relationships and possibly in relation to the last fluctuations of lake level and the lowest shorelines.·

The alluvial fans and shoreline features have been mapped along the western stillwater mountain front between the Table Mountain fans and Cox (Fig 1), as an initial stage in the investigation of fan/shoreline relationships. Much work, especially on soils and sedimentary sequences still needs to be done. Preliminary differentiation between the groups of alluvial fans has been made on the basis of their field relationships, their spatial relationships. to lake sediments, stratigraphic relationships exposed in sections, and soil and pavement development on the fan surfaces.

The older (group 1) fans are cut by the highest shorelines, and form remnants of several stages of fans, landwards of those

j

shorelines. A broad differentiation between two sub-groups of the older fans can be made on the basis of field relationships and post depositional dissection of the fan surfaces. Older fan deposits also occur, buried by lake sediments, in the shoreline zone. The old fan surfaces preserve well developed desert pavements over well developed soil profiles (with pale thin Av horizons; Bt horizons of thicknesses >30cm, and 7.5 YR coloration; and Bk horizons with at least stage II CaC03 accumulation). Further work on the pavements and soils is planned, in order to assess whether the several stages of the older fans can be differentiated. Further work is also planned on the sediment sequences which show relationships between fan and lake sediments. Particularly important is whether there are Eetza-age lake sediments (Morrison, 1991) buried within the shoreline zone, and if so their relationships to the sub­groups of the older fans.

The intermediate (group 2) fans post-date the highest lake shorelines, but pre-date the deep dissection of the mountain front following the rapid late Pleistocene / early Holocene lowering of lake level. These small fan remnants occur near the mountain front, usually below and cutting the highest shoreline, but have themselves been cut by younger shorel ines and are locally buried by lake sediments. They pre-date the deep dissection of the shoreface which followed lake recession. soil development is much less mature on these fan surfaces than on the older fans.

The younger (group 3) fans post-date lake level lowering, and prograde beyond the shoreline zone. The are usually well below the high lake levels and accumulated after the dissection which followed lake level lowering. Where group 2 fans occur, group 3 fans are inset well below that fan group. At least two stages can be differentiated on the basis of fan morphology, and relationships to the lowest (Holocene?) shoreline features.

Fan morphometry

Simple morphometric measures have been derived for the fans and their feeder drainage basins, from map and air photo analyses and from field measurements.

For the young fans, fan gradient varies in relation to drainage area in a manner similar to that identified in much of the literature for other arid zones (Harvey, 1989). This relationship (Fig 2) can be expressed by:-

Gyf

= 0.087 Ad -0.128 (n=24, R = 0.74, SE = 0.0789)

where Gyf is fan gradient and Ad is drainage area (km2). omitted from the analysis (but shown on Fig 2) are small confined fans within the mountains and small fans on the shoreface formed solely from dissection of the lake sediments.

iW·'· .. ~ .. I ~.~'. • ~ .. ~ ~

-= ~ • -=

This relationship for the old fans (also shown on Fig 2) is similar to that for the young fans, suggesting that the controls ~ over the depositional slopes of both sets are the same. In that

-=

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-

case:-

Gof = 0.091 Ad -0.127 (n=24, R = 0.85, SE = 0.0606)

(Gof is fan gradient).

For the same population of young fans, fan area varies in relation to drainage area, again in a manner similar to that previously identified in other areas (Harvey, 1989). This relationship (Fig 3) can be expressed by:-

~f = 0.281 Ad 0.686 (n,,;;:;24, R = 0.90, SE - 0.220)

where ~f is fan area (kro2).

The relationship for the total fan areas for those fans where old fan seg-ments are preserved I and taking into account all fan segments (also shown on Fig 3), differs from that for the young fans, only in the regression constant. In that case:-

Atf = 0.366 Ad 0.696 (n=24, R = 0.94, SE = 0.182)

(Au is fan area).

Examining the residuals from the young fans regressions against drainage area (Fig 4), gives some, indication of the factors controlling alluvial fan sedimentation since Lake Lahontan time. Four groups of fans can be recognised. on the basis of their residuals from the two regressions.

Group I with relatively high fan gradients are the mostly small fans fed by the bas.al t catchments of Table Mountain. Most of these fans comprise coarse bouldery debris flow deposits. They accumulated on the steep shoreface on the west of Table Mountain. Group II are also small fans which accumulated on a steep shoreface, between Cox Canyon and Cox in the north of the the area. Their catchments are mainly on Triassic sedimentary rocks, but also include deep dissection through Lahontan lake sediments. Group III, with moderate fan gradients and moderate fan areas per drainage area, includes the large fans in the central part of the area, which drain the older volcanic rocks and issue onto a low gradient shoreface. Group IV, with very large relative fan areas includes the large fans in the northern part of the area issuing onto steep shoreface slopes and prograding beyond the shoreface zone.

Summary

The geology of the feeder catchments, together with the geometry of the shoreface zone seem to have influenced fan morphometry. Tectonics appears to have had only a minor and local influence on fan development here, in contrast to the Dixie Valley on the east side of the stillwaters (Bell and Katzer, 1987). The several fan stages appear to primarily reflect climatically controlled pulses of excess sediment generation.

The old fan surfaces pre-date the late Wisconsin high stand of

j

Lake Lahentan, and may date frem earlier than wiscensin time. If that is the case, this appears to' indicate a different majer fan aggradatien peried here in nerthern Nevada than in the Mejave Desert further seuth (Harvey and Wells, 1994; Wells et ala 1987), presumably ,reflecting 'different Pleistecene climates. The intermediate fans appear to' have been depesited during the Late Pleistecene to' Helecene transitien. They are much smaller in extent in cemparisen to' fans depesited at the same time further seuth in the Mejave Desert (Harvey and Wells, 1994; Wells et ale 1987), again perhaps reflecting (Late Pleistecene/Helecene) climatic centrasts between nerthern and seuthern Basin and Range. The yeung fan surfaces are clearly Helecene in age, and can be subdivided en merphelegical greunds into' several greups. Cerrelatien and dating ef these must await further field werk.

Acknewledgments

We are grateful to' Jehn Bell fer lending his valuable air phetegraphs. AMH is grateful ,to' the Fulbright cemmissien fe~ the award ef a Fulbright Schelarship enabling him to' be based at the DR! and to' werk with SGW.

Field steps

Two. field steps are planned to' examine the relatienships between fan and shereline features; at Cex headland and in the Lambing Canyen zenes.

step 1: Cex. (Fig 5)

A cemplex ef lake sediments is seen here. We de nO.t understand' the relatienships, but expesed in a trench nerth ef the headland are what appear to' be two. sets ef lake sediments, the elder set partially cemented and apparently uncenfermably everlain be the yeunger nen-cemented set. Furthermere, expesed en the nerth side ef the trench, fan and lake sediments are interbedded. The yeunger lake sediments at highest lake levels ferm a bar and flat cemplex linked to' the reck headland. These are everlain by a thin veneer ef intermediate (greup 2) fan depesits. The whele set is faulted by a small arcuate nermal fault. Abeve the highest shereline are pre-lake surfaces ef uncertain erigin. The lake sediments are deeply trenched, and yeung fans (greup 3) issue frem the trenches.

step 2: Lambing Canyen. (Fig 6)

In this area is a cemplex ef wellexpesed lake and fan sediments. The highest sherelines can be traced cutting the elder (greup 1) fan sediments, and several reck headlands. At least two. sets ef elder (greup 1) fan sediments, can be seen ferming remnant fan surfaces issuing frem Lambing and West Jeb Canyens. The same sediments, capped by a buried seil are everlain by lake sediments, in the shereline zene. As at Cex, there appear to' be two. sets ef lake sediments, an elder cemented and fragmented set, everlain

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by younger non-cemented sediments.

The high level lake sediments form a beach ridge and flat complex south of one of the rock headlands, and are overlain on their landward side by thin intermediate (group 2) fans. The whole suite was then trenched as lake levels fell and younger (group 3) fans have prograded, issuing from the trenches.

A little to the north of Lambing Canyon is a suite of faulted older fan sediments and lake sediments, which again appear to be of two ages. We are still working on these sections.

References

Bell J.W. and Katzer, T. 1987, Surficial Geology, Hydrology, and Late Quaternary Tectonics of the IXL Canyon Area, Nevada. Nevada Bureau of Mines and Geology, Bulletin 102.

Harvey A.M. 1989. The occurrence and role of arid-zone alluvial fans. in Thomas D. S. G. Arid Zone Geomorphology, Bellhaven, London, 136-158 .

Harvey A.M. and Wells S.G. 1994, Late Pleistocene and Holocene changes in hillslope sediment supply to alluvial fan systems: Zzyzx, California. in Millington A.C. and Pye K. (eds) , Environmental Change in Drylands: Biogeographical and Geomorphological Perspectives. Wiley, Chichester, 66-84.

Morrison R.B. 1991, Quaternary stratigraphic, hydrologic, and climatic history of the Great Basin, with emphasis on Lakes Lahontan, Bonneville, and Tecopa. in The Geology of North America: Quaternary Nonglacial Geology: conterminous u.s., The Geological society of America, Vol K-2, 283-320.

Wells S.G., McFadden L.D. and Dohrenwend J.C., 1987, Influence of late Quaternary climatic change on geomorphic and pedogenic processes on a desert piedmont, eastern Mojave Desert, California. Quaternary Research, 27, 130-146.

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USGS-OFR-96-514

to the Guidebo,ok. Trip,

by

Marith C. Reheis 1

Open-File Report 96-514

This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey·editorial standards (or with the North American Stratigraphic Code)

1996

1U.S. Geological Survey, Denver, Colorado

1lllI ___ _

i ·1

2 ~ 121 0 1190

"170 43°~~------~----~------~ ____ ~

41

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SEVERAL LAKES NOT

SHOWN

OR

CA I NV

~I "

Introduction

To Owyhee River

L. Columbus

4/ t1 III;

Figure 1. Regional map showing Lahontan basin and late Pleistocene areas of Lake Lahontan (light shading) and other pluvial lakes (dark shading). Dots, previously known sites of pre-late Pleistocene lacustrine sediment; triangles, locations from this study. BR, Black Rock Desert; SC, Smoke Creek Desert; R.Fj Rye Patch Dam; WD, Weber Dam; WL, Walker Lake.

During highstands of the late middle and late Pleistocene, pluvial Lake Lahontan covered more than 21,000 km2 and extended more than 350 km from northern Nevada to near the southern end of Walker Lake (fig. 1). Deposits and elevations of shorelines of this age have been studied for over a century (e.g. Russell, 1885; Morrison, 1991). However, relatively little information was available on older deposits, and shorelines of older lakes in the Lahontan basin reported in the present paper have not been previously identified. In 1995, new mapping was undertaken to investigate possible older extensions of Lake Lahontan to the south of Walker Lake and to investigate Mifflin's (1984) hypothesis oflong-term northward tilting of northwestern Nevada. Astonishingly abundant sedimentologic and geomorphic evidence of lakes much older,

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and shorelines much higher, than the late Pleistocene lakes in western Nevada (Lakes Lahontan, Columbus, and Russell) is preserved in many places. Shoreline elevations at sites in the Lahontan basin seem to indicate that northward tilting has not occurred, although the shorelines probably have been locally displaced as much as 20 m by faulting.

The purpose of this paper is to present preliminary information on the sedimentology, stratigraphy, and estimated shoreline elevations of pluvial lakes in the Lahontan basin prior to the late middle Pleistocene (the past ca. 200 ka, fig. 2). Discussion will focus on several sites in the Walker Lake basin which preserve abundant evidence of old lake cycles, in particular two areas (McGee Wash and Thorne Bar, fig. 3) which will be visited on the field trip. Similar thick sedimentary sequences have not yet been found in the main Lahontan basin north of the Desert Mountains; however, a few areas (triangles in fig. 1) exhibit beach pebbles or gravel at anomalously high elevations that correspond to shoreline elevations inferred from the Walker Lake area. Although previous workers have restricted the use of "Lake Lahontan" to the Eetza and Sehoo Fonnations ofthe Lahontan Valley Group, in this paper it is extended to all Pleistocene deep-lake deposits in the Lahontan basin.

TIME years ALLOSTRATIGRAPHIC AND x 103

10

20

100

200

(J)

a: q;

~ 300 l1.. 0 (J)

0

~ 400 (J) ~ 0 :I: l-

?:: 500 w :::;: i=

600

700

800

'" ... ttl > ttl ... " ttl c: e>

CHURCHILL GEOSOL AND WYEMAHA ALLOFORMATION

(sub a erial! s ha IIow-lake)

EETZA ALLOFORMA TlON (sediments of three deep­

lake cycles)

--------1------~

PAIUTE ALLOFORMATION AND COON GEOSOL COMPLEX (alluvium and eolian sand

with many paleosols)

--------~?------~. YE PATCH ALLOFORMA TION 11/!:t':z;S-;;2

(deep lake sediments) 1---

LOVELOQK ALLOFORMATION AND PEDOCOMPLEX

alluvium with many paleosol

? ?------~ DEEP-LAKE/DEL TAlC

SEDIMENTS

Figure 2. Principal exposed Quaternary units in the Lahontan basin, tephra layers, and inferred fluctuations in lake level (from Morrison, 1991). In the lake-level graph, solid indicates lacustrine episodes; hatching indicates soil­forming episodes. Letters in ash-bed column denote tephra layers of known age: T, Turupah Flat, 1.2 ka; M, Mazama, 6.8 ka; S, Mt. Saint Helens W-Marble Bluff, 35 ka; Wa, Wadsworth, -150 ka; Ro, Rockland, 400 ka; D, Dibekulewe, 610 ka; L, Lava Creek B, 620 ka; Rp, Rye Patch Darn, 630 ka; B, Bishop, 760 ka (see references in Morrison, 1991, for tephra ages).

118030' F' ~ M f i \,,~" ,cJ, _, 'II») .39°15' 19ure-,. apo

5 10 L-l

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EXPLANATION 'J2J Area of , late Pleistocene. L .', ' lake wIth 1330-m shorelme

r-":"\'~;B Area of middle Pleistocene t.....::..::;..; lake with 1400-m shoreline r::/~ . Probable additional area of 2',~1 middle Pleistocene lake

accounting for sedimentation

(/l

c. ...,::

TB Locality of pre-late Pleistocene A lacustrine sediment or shoreline

x Tephra locality in non-lacustrine sediment

;::> ~

-;::)

I.P (\)

Q -' " " -' <Jl

~ ~

-:> \9-

(\)

39°00'

38°45'

Walker Lake basin and southernmost Carson Desert (Raw­hide Flats) showing study sites and areas of lake at different times. Area of lake at 1400 m is drawn using modem topo­graphy, recognizing this is an approxima­tion of topography in the middle Pleistocene due to faulting, erosion, and sedimentation. SA, Sunshine Amphitheater; CA, Campbell Amphitheater; WR, Weber Reservoir; MW, McGee Wash; TB, Thome Bar,

This project was initially conceived and funded as a Gilbert Fellowship by the U.S.

4

Geological Survey, and I thank my co-proposer and behind-the-scenes director, Marty Mifflin, for all his inspiration and vast store of Lahontan knowledge. Roger Morri.son reviewed early versions of the proposal and kindly introduced me to several areas where pre-late Pleistocene Lahontan deposits were known. Ken Adams occasionally raised his eyes above the Sehoo and Eetza levels to observe and feed me interesting sites to study; it's been a great pleasure bouncing hypotheses off him. Jim Honey, Chuck Repenning, and Bob Miller kindly identified vertebrate fossils and Platt Bradbury identified diatom samples. Rich Reynolds and Joe Rosenbaum provided laboratory facilities, taught me how to analyze paleomagnetic samples, and advised on interpretation; Becky G"agliardi helped with analyses. Andrei Sama-Wojcicki and Charlie Meyer, in the face of great obstacles and turmoil in the U.S.G.S., have again worked miracles to provide a chronological framework from numerous tephra samples-what would we do without you? Finally, I thank my volunteer field assistants, who worked very hard in a lot of bad weather and put up with my camping habits: Andrea Lonner, Nadine Calis, and Cassie Fenton.

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Previous Work

Mifflin and Wheat (1979) and Benson (1978) observed that the Eetza (ca. 180-130 ka, but possibly as old as 300 ka; Morrison, 1991) and Sehoo (ca. 35-12 ka; Benson et al., 1990) shorelines in unfaulted positions throughout the Lahontan basin lie at about 1330 m. Isostatic rebound has caused shoreline elevations to range from about 1318 m at the northeastern margin of the lake to as much as 1343 m in the Carson Desert (Mifflin, 1984). The highest faulting rates of late Pleistocene shorelines are along the Wassuk fault, which bounds the west side of the Walker Lake basin (fig. 3). Leveling surveys by Demsey (1987) showed that elevations of the Sehoo shoreline along the Wassuk Range front range from 1325 m to 1333 m. She found no evidence of Quaternary faulting or deformation along the Gillis Range east of Walker Lake, where the average altitude of the Sehoo shoreline is 1329.5 ± 0.9 m.

Morrison (1991) summarized the known ages and elevations of early middle (ca. 780-610 ka; Richmond and Fullerton, 1986) and early Pleistocene (>780 ka) deposits of Lake Lahontan (fig. 2). Five sites were in outcrops below the Sehoo shoreline where incision exposed older deposits buried by Sehoo and Eetza sediment. The sixth site, Thome Bar, is described below. Of the five, the Wyemaha Valley locality has been destroyed by gravel-pit operations, and lateral erosion by the Truckee River below Wadsworth has revealed the old deltaic gravel at the base of the canyon to be a fallen block of much younger gravel (field observations of the author). The two pre-Eetza lake cycles identified by Morrison (1991) are the Rye Patch Formation, about 630-610 ka, and unnamed deposits containing ~l,OOO-ka Glass Mountain tephra. He believed that the 760-ka Bishop ash bed was deposited during a long period of non-lacustrine sedimentation (the Lovelock Formation) with no deep lakes in the Lahontan basin. However, the Bishop ash bed is contained within or near the top of lacustrine deposits in basins to the south, including pluvial lakes Rennie (Fish Lake Valley; Reheis et al., 1993), Owens (Smith et al., 1993), Tecopa (Sarna­Wojcicki et al., 1987), and Manley (Death Valley; Knott et al., 1996).

Only in the Walker Lake basin are there previously known outcrops of undisputed beach and shallow-water gravel at an elevation higher than the Sehoo and Eetza shorelines. W.J. McGee observed that the eastern end of the "crest of the gravel embankment separating Walker Lake Valley from the '" Walker River ... is now fully 200 feet above its original position ... " [that is, the Sehoo shoreline] (Russell, 1885, p. 142 and plate 28). This locality appears to be the area at the head ofa drainage informally named McGee Wash east of Weber Dam (fig. 3; field trip stop 4 of day 2). Curiously, it seems never to have been resurveyed until the present study. The Thorne Bar, at the southeast edge of Walker Lake (figs. 1 and 3) was first reported by King (1978) and Mifflin and Wheat (1979) to have pre-Eetza "shore gravel with layers oflake­deposited carbonate, preserved in a much-eroded V-bar complex ... up to 1443 m altitude" (Morrison, 1991, p. 296). All of these authors attributed the anomalously high shore gravel to tectonics. At the McGee Wash site, Russell (1885) thought the gravels had been folded into an anticline; recent mapping (see fig. 5) and that of Morrison and Davis (1984) confirms the presence of numerous faults and tilted strata. The Thorne Bar was attributed either to faulting (King, 1978, p. 67-68) or to northward tilting of the entire Lahontan basin (Mifflin and Wheat, 1979; Mifflin, 1984; Morrison, 1991). However, this site is across the Walker Lake basin from the tectonically active Wassuk fault, and detailed leveling surveys by Demsey (1987, p. 39-40) and air-photo interpretation for the present study reveal no evidence of Quaternary faulting or deformation either at the Thorne Bar or northward along the Gillis Range.

~--~----------------

Evidence for Old, Very High Pluvial Lake Levels

The evidence for high stands of pre-Eetza pluvial lakes and shoreline elevations is primarily sedimentologic. The classic shoreline morphologies, such as wave-cut cliffs and beach

6

.. ~

Ij

berms, are commonly not detectable after half a million years of geomorphic change, although 1IIIli.l.' two such berms and one dissected V-bar are preserved in the Walker Lake basin. The twofold • basic approach was developed in combination with investigations of known (Thome Bar) or suspected (Campbell and Sunshine Amphitheaters, first noticed by Ken Adams) sites: (1) comb ... geologic maps for areas shown as late Tertiary lacustrine units or well stratified deposits (bedrock I mappers commonly assumed that all lacustrine deposits above the Sehoo shoreline must be Tertiary in age); and (2) based on the elevation of shore gravel at the Thome Bar, search ~ topographic maps for bench-and-riser morphology between 1330 and 1400 m. ..

. Outcrops ofpre-Eetza lacustrine sequences in the Walker Lake basin range from well c.

bedded mudstone, siltstone, and sandstone to steeply dipping coarse gravel conglomerate. The Ie highest elevation of pebble- or cobble-sized beach gravel is taken as a minimum shoreline -elevation. Locally, well-rounded beach pebbles of various lithologies lie as scattered lag on ~ bedrock outcrops and also yield minimum shoreline elevations. Fine-grained beds locally contain .. tephra layers; these are being identified by chemical correlation with known tephra by the U.S. Geological Survey Tephrochronology Laboratory in Menlo Park. Ages of the units are further .'. \ constrained by magnetostratigraphy and by identification of vertebrate fossils (Jim Honey and Il Chuck Repenning, U.S. Geological Survey). This work is ongoing and ages given in this report are preliminary. A unified stratigraphic sequence for the Walker Lake area has not been ~ ... constructed because unit ages are preliminary or unknown. Thus, units with the same names It among the individual stratigraphic sequences may not be the same age (see fig. 8 beloW).

Walker Lake Basin

The Walker River drains the Sierra Nevada (fig. 1) and presently feeds Walker Lake; in the past, however, the river has sometimes drained north into the Carson Desert via Adrian Valley (fig. 3; King, 1993). The Walker Lake basin mainly trends north-south and is bounded on the west by the Wassuk Range and an active fault and on the east by the Gillis Range. At the north end of the Gillis Range, active right-lateral faults of the Walker Lane intersect the basin and cause a northwest shift in the trend of the basin (fig. 3). The Thome Bar and a nearby site lie in a tectonically quiescent area of the basin (Demsey, 1987). Sites north of Schurz have been faulted and tilted.

Thorne Bar area

The Thome Bar is a large V-shaped bar of shore gravel built at the mouth of a large canyon draining the Gillis Range (fig. 4). The bar consists mainly of pebble to cobble gravel, locally tufa-cemented, with bedding ranging from horizontal to angles of 25° or more. The bar can be divided into three morphologic units: the lowest in elevation and youngest is a sharp, well preserved V-bar marked at the top by the Sehoo shoreline at 1330 m (ca. 4360 feet, fig. 4). Between 1330 m and about 1370 m (4500 feet), the bar consists oftwo nested, eroded V-shaped berms; the lower berm is better preserved than the upper. Between about 1370 and 1402 m (4600 feet) the bar has no preserved morphology, but deep arroyos expose well bedded, tufa-cemented shore gravel,and beach pebbles locally occur as lag on basalt outcrops (fig. 4). No stratigraphic relations have been deciphered within these gravel deposits. However, the degree of morphologic preservation of the nested bars and reconnaissance soil" pits strongly suggest the presence of at least four and probably more lacustrine units: the Sehoo-aged bar, two older bars that reached

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Fan deposits (Holocene); includes some Holocene lake deposits

Fan deposits (late Pleistocene)

Deposits of late Pleistocene lake

EOfm=1, Fan deposits (middle Pleistocene)

o I o

EXPLANATION

~S~a

TTTT"

North of Thorne Bar Section 1

Pebbles lagged on bedrock slope up to elevation - 4585 feet (1397 m)

EXPLANATION

7

Sandstone and silty sandstone, locally conglo­merate beds

23 m omitted

cement

: ... : EL21-WL (Bishop ash)

Matrix-poor

5

(J)

a; ill E

0

~

~

~ EL21-WL

lTTT

Sandy pebble ' conglomerate, well rounded clasts (near­shore deposits)

Sandy cobble conglomerate, well rounded clasts (beach deposits)

Boulder conglo­merate, poorly bedded (shore­face deposit)

Volcanic rocks

Steep dips (foreset beds)

Calcareous cement, locally tufa

Tephra layer and sample number

Buried soil

2 KILOMETERS I

I 1 MILE

CONTOUR INTERVAL: 40 FEET; 200 FI;ET IN MOUNTAINS

Fan deposits (middle and early? Pleistocene)

Nearshore deposits and beach gravel of middle to early Pleistocene lakes

Volcanic and sedimentary rocks (Pliocene and Miocene)

Late Pleistocene shoreline

Middle Pleistocene shoreline; dashed where inferred

Figure 4. Map and measured section of area around Thome Bar (see fig. 3 for location). Most contacts were mapped using air-photo interpretation. Triangle in northern map area is location of measured section.

8 I elevations between 1330 and 1370 m, and at least one old bar that reached a minimum elevation of 1402 m. King (1978) reported shore gravel as high as 1443 m; I found subrounded fan gravel but no obvious beach clasts above 1402 ·m.

An excellent small outcrop about 4 km north (measured section in fig. 4) of the Thome Bar exposes two sequences of shore gravel, separated by a paleosol, that overlie bedrock; both sequences are well above the Sehoo shoreline. The lower unit, about 7 m thick, consists of a basal shoreface gravel that fines upward into silty sand containing tephra, in tum overlain by a pebbly poorly bedded stratum with a paleosol (BtlBk horizons). The tephra (EL-21-WL) is chemically correlative with the Bishop-Glass Mountain family of tephra erupted from Long Valley, California, and paleomagnetic measurements on the silty beds indicate normal polarity; thus this tephra is probably the 760-ka Bishop ash bed. The paleosol is abruptly overlain by about 38 m of tufa-cemented pebble and cobble shore gravel, commonly steeply bedded, that rises to an elevation of about 1393 m. Beach pebbles continue upward as lag on bedrock to an elevation of about 1395 m. Because the outcrops here and of the oldest unit at the Thome Bar are similar in cementation and preservation and rise to about the same elevation, I infer that they are equivalent. Thus, the oldest and highest shoreline in this area is underlain by deposits of two different lakes: one that culminated at about the time of the eruption of the Bishop ash bed at 760 ka and reached a minimum elevation of about 1355 m, and a second that postdated the Bishop ash and rose to a minimum elevation of about 1400 m.

McGee Wash

The study site at McGee Wash extends from Weber Reservoir on the Walker River eastward to U.S. Highway 95 (figs. 3 and 5). The good outcrops in this area exhibit numerous faults and tilted sediments (Morrison and Davis, 1984); only a few of the larger faults are shown on figure 5. Dips are generally greatest in the east near the highway and decrease to the west. Younger units are progressively less deformed. The overall structure appears to be that of a block or horst tilted progressively westward with time. This block was periodically partly or entirely covered by lakes that prograded eastward and by alluvial and other terrestrial deposits that prograded westward. The alluvial deposits are mostly distal fan deposits, locally interbedded with eolian and colluvial deposits.

The oldest units are exposed near the highway and consist of a conformable sequence (measured sections 9 and 10, figs. 5 and 6) of alluvial deposits (unit Tt) overlain by about 50 rn of well-bedded lacustrine mudstone, siltstone, and sandstone capped by beach gravel (unit TI), in tum overlain by a very thick (unmeasured) sequence of alluvium and eolian deposits (unit QTt) containing numerous carbonate-enriched paleosols. Units Tt and TI both contain tephra layers but only one, EL-19-WD in unit Tt, has yet been identified; it is tentatively correlated with an upper Miocene to lower Pliocene tephra erupted from the Snake River Plain. A few vertebrate bone fragments are scattered on the surface of lacustrine unit TI, but none have yet been identified. In the south-central part of the map area, erosion between the time of deposition of units Tt and QTt has removed unit Tl.

Terrestrial unit QTt apparently extends westward to underlie younger lacustrine units around McGee Wash (fig. 5), where it was mapped by Morrison and Davis (1984) as the Paiute Formation (fig. 2). Morrison (oral commun., 1995) now equates this to the older Lovelock Formation. Three tephra layers crop out in unit QTt in McGee Wash (EL-5-, -6-, and -57-WD), but none have yet been correlated with tephra of known age. EL-5-WD has normal polarity.

Four pre-Sehoo lacustrine units overlie unit QTt in this area and are separated by unconformities. The oldest, Ql01, is a thin (~2 m) bed of pebble gravel grading up into silty sand. It appears to be preserved in only one place too small to show at the scale offigure 5: it is

c~ ____________________________ __

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• • • • • • • • • • • I

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Ql04

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1 MILE

CONTOUR INTERVAL: 40 FEET

~------------~--------------~------~------~----------~39°01'

EXPLANATION

E Eolian sand (Holocene)

Fan deposits (Holocene and late

Pleistocene)

Lacustrine and fan deposits, undifferen­

tiated (Holocene and late Pleistocene)

Colluvial and eolian deposits

(Holocene and late Pleistocene)

Deposits of early late (?)

Pleistocene lake

Fan deposits (middle Pleistocene)

Deposits of middle Pleistocene

lake; pattern shows beach gravel

Colluvial deposits (middle Pleistocene)

~ Deposits of middle? and early Pleistocene lakes;

QIO,,'21o

; d. pattern shows possibly correlative beach gravel

8 Alluvial, eolian, and colluvial deposits QT~

(early Pleistocene and Pliocene)

~ Deposits of Pliocene? lake

1!!!IIT~IIII!1 Alluvial deposits (Pliocene and Miocene)

~ Volcanic and sedimentary rocks

Tvs / / (Pliocene and Miocene)

Fault; bar and ball on downthrown side; dashed

where inferred, dotted where concealed

Figure 5. Geologic map of area around McGee Wash, east of Weber Dam. See figure 3 for location. Triangles with codes (e.g. 5, 6-WD) are tephra localities. Hachures show shorelines.

9

If

East of Weber

Section 9

(near highway 95)

Section 10

o (/) L.. Q) ....

McGee Wash Section 8b

:::: I partly : : :: covered

25 morritted

o

\ \

\ - --=1 Smectite- and gypsum-nch; - =-=l weathers reddish-brown \

\ 'E:~';J' \ ~,:.<.~ '\, Unit TI 1I .. ",,,,.h ... \,, .•.••. ~ .steo south'

,o·r~~< dip ~reases Unit Tt (terre"hi~1 : ~~o·~ ~~ \

~~i~{'~ \\ ,·~.··o·; \ : '0°0': (' \ o .. ,? ~O.:

~ ~~'~':' ~ '\ ".0'0' \

\

~--~ Interval below

\ \

\

measured to east

\ \

\

Q)

/ l) ° l)

/ '0°0°0 E

/ 0°0°0

5 / 't) 0 00 01 Bedding not exposed

/ ° 0

'" / ,°0 0 00

" / <)00 poorly expo~d / [) 0 000

lOmomitted '\ Section 8a / Unit Ql03 '[)OOOO

'" / (lacustrine) ~":O: 0

\ \

':'~'.~:":'.

'::".~~'

.:..~.:-::..:.~ ..

, " 0 0 , ) ° 0 , 0 0 0

" ) 0 0 , l> 0 [)

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'" -":'-"'-: ~--.. F3-WD - ~ ~

Locally a moderate buried soil; to east, mudstone is overlain by well bedded sand and silt beneath

'" '.~ " .. ~ - - _ ~ Unit Ql02 ==--== '" ::: :::: Unit Ql01 (lacustrine) ==-===

, • <>' •• 0 (I'" t' )\ -- --.' ..... " acus nne __ _ :f.~~.(;::.{ \ ·oo .• ~+. \

~:~ \ ~Sit: \

Unit an (terrestrial) ;~:i<'~:; \ Unit TI (Iacustrin~) ~y.~~~y~ \

\ . ° .Q. ° \ \ ;;:~~:;: \

bedded' well \ ·:k~: \ I (.O~'o~(

rounded pebbles \ ·: ... ~:Loo: ../' F2-WD \ \ v.~~04':~' \

\ ~r~1~:~' \ \~o:.:~<~.: Unit QTt \ \ 0-1,:+.' (terrestrial) \ \ ·~···o· .~:.

~·o 0.6<:'0 ( \

Step south

same unconformity

Green, smectite-rich, well bedded; not saline; weathers brown

Gypsum-rich but well bedded

\ .~~:.:~<~'. tan to brown; weak \

\ \\~\~: buried soils; fossil \ _ .. _ .. _ Locally truncated by

\ .~ 0:: .~ 0: ../' \ :.:.:::-.::::-...::: between 0103 and 0102

\ o?· Q~ bone fragments; .:.:::-=;:::::- / foreset gravel beds of \ ~~:'. many small faults \ ~.~~~. unit 0103 or by colluvium

\ 0';· /:;.; ~ F1-WD \ .-::-::==: ______ (section steps southeast)

\ ,:~>t~\ EL7-WD \ .~~~~~: EL20-WD (Bishop ash bed 25·~~~ted Partly

\ .~ ~.: :.0. \ ...... ;~. or Glass Mountain tephra) \ ~~; \::::::: \ J5~:~o.; .:::.~:.~': ~- Angular unconformity

~'2 •. ~~~~ covered

~ EL19-WD (upper Miocene or \\

early Pliocene, Snake R. Plain?)

\ "'x,1 EL6-WD '~o:;~';~o~ \ \

EXPLANATION

~:-~~ Mudstone

is±~ Interbedded mudstone ;=:-;-.. -:~. and sandstone

r= ~~~l Siltstone and sandy ... -.. siltstone

Fl U m~]'1' • 0 ••• ••. :~; .. ::~. ••• G •••

filij.<.? 0." .(). 0 <V.

",;. · ... u··'v·.·' • 0 • •. ·0 . ').fJ I 0 .....

[3

TnT

Sandstone and silty sandstone, locally conglomerate beds

Sandy pebble conglomerate, well rounded clasts (nearshore deposits)

Sandy cobble conglomerate:' well rounded clasts (beach deposits~

Sandstone and con­glomerate, poorly to moderately sorted

Fanglomerate, poorly sorted and bedded

Calcareous cement; tufa in lacustrine beds

Buried soil

F3-WD Fossil bone locality

[-- -] Tephra layer and !< x x EL-16-WD sample number

Figure 6. Measured sections of lacustrine and terrestrial deposits in the area around McGee Wash. See figure 5 for lUl,.,aUUU;).

scale difference between sections 9 and near U.S. Highway 95 and sections 8a and 8b in McGee Wash. Note

If II II. .~ • . ' • )-1

o

• • •

----=-til

till

=­II II

II II

II

11

exposed beneath unit Ql02 near the top of measured section 8a (fig. 6), but is cut offby a fault and removed by erosion west of the fault prior to deposition of unit Ql02. Unit Ql02 is locally much thicker and crops out around the head of McGee Wash (fig. 5) to a maximum elevation of about 1390 m (4560 feet). This unit consists of a basal bed of sand and pebble gravel that grades up into well bedded silt and sand containing at least one tephra layer (EL-20-WD, EL-58-WD). Sample EL-20-WD near the base of measured section 8b (figs. 5 and 6) has been identified as one of the Bishop-Glass Mountain tephra layers; paleomagnetic analyses have not yet been done. A beach-pebble facies of unit Ql02, if deposited, has mostly been removed by erosion, but remnants of beach gravel that appear to be older than that of unit Ql03 are locally preserved capping units QTl and QTt near the highway at an elevation of about 1430 m. Unit Ql03 unconformably overlies silt and sand of unit Ql02 around the head of McGee Wash, and consists predominantly ofloose sand and beach gravel (figs. 5 and 6). A paleosol (Bw or weak Bt and/or Bk horizon) is locally preserved at the unconformity, and in one place a reddish colluvial unit appears to be preserved between units Ql02 and Ql03. Unit Ql03 is well exposed only at the south edge of the map area. In aerial photographs this unit has the morphology of a shoreline berm. The berm rises to an elevation of 1402 m ( 4600 feet), the same elevation as the highest shore gravel at the Thome Bar. The youngest of the four lacustrine sequences around McGee Wash, unit Ql04, is poorly exposed beach sand and gravel inset into all older units. This unit rises to an elevation of about 1365 m at the south edge of the map area.

Other sites in the Walker Lake basin

Campbell and Sunshine Amphitheaters are northwest of Weber Dam and lie on opposite sides of the Walker River near the point where the river bends in a U-turn (fig. 3). Both of these sites were first identified by Ken Adams as potentially preserving sediment of pre-Eetza lake cycles above the Sehoo shoreline.

Outcrops in Campbell Amphitheater are complicated by a network of north- and northwest-striking strike-slip faults of the Walker Lane. In addition, it is difficult at this site to associate areas of beach gravel that mark shorelines with exposed finer-grained sediment. Three lacustrine units are preserved (fig. 7). The oldest, Qlol, laps onto subjacent basalt where it consists of shoreface conglomerate that grades rapidly upward into fine-grained beds. A tephra layer, EL-15-CA, drapes this facies change from shoreface into deeper-water deposits. The tephra is correlated with the Bishop-Glass Mountain family of tephra and preliminary paleomagnetic results indicate normal polarity; thus, the tephra is probably the Bishop ash bed. Unit Qlol is unconformably overlain by beach gravel of unit Ql02; a paleosol (Bt/Bk) and colluvium are locally preserved at the contact. Unit Ql02 rises to at least 1378 m, and beach pebbles that may represent this unit are lagged on basalt up to an elevation of 13 90 m. The pebbles stop just below an abrupt change in slope that appears to be a preserved shoreline angle. Beach gravel of unit Ql03 is apparently inset into the older units, and forms a berm at an elevation of 1356 m.

Sunshine Amphitheater is a very large exposure of pre-late Pleistocene lacustrine deposits offour different ages. It is remarkable for the abundance and preservation of vertebrate fossils; thus far, identified remains include those of giant sloth 0. Honey, oral commun., 1995), horse, camel, bird (C. Repenning, oral commun., 1996), and cutthroat trout (R. Miller, written commun., 1996). Faults are exposed but displacement appears to be minor, in contrast to the McGee Wash and Campbell Amphitheatre sites. The three oldest lacustrine units are relatively fine-grained in part, and have been sampled for paleomagnetic interpretation (fig. 7). Briefly, the oldest unit QTlo 1 consists mainly. of reversely polarized, brown laminated mudstone capped by pebbly sandstone. It is overlain by unit QTt, alluvial deposits containing carbonate-enriched paleosols. Unit QTt is conformably overlain by well bedded pebbly sandstone, siltstone, and mudstone of unit Ql02, which also has reversed polarity. A distinctive horse bone from this unit appears to

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constrain the age to between 1.7 Ma and 1 Ma (C. Repenning, written commun., 1996), which is consistent with paleomagnetic data. An unconformity, locally marked by thin colluvium and a paleosol (Bw horizon), separates units Ql02 and Q103. Many fossils are associated with this stratigraphic interval, including a distinctive sloth bone identified (J. Honey, oral commun., 1995) as post-Blancan in age «2.5 Ma; Berggren et al., 1995). Unit Ql03 consjsts of poorly indurated siltstone and sandstone that coarsen upward into beach gravel extending to an elevation of 1396 m. Well rounded beach pebbles mixed with fan gravel extend to an elevation of 1414 m; the pebbles may represent yet another lacustrine unit at this site. Paleomagnetic measurements from the siltstone beds indicate that this unit has reversed polarity except for an interval about 2 to 8 meters above the base, which has nonnal polarity. Combined with the horse bone beneath the unconformity, these data indicate that the nonnal interval probably correlates with the Jaramillo Normal Subchron at about 1 Ma. Thus, the three oldest lacustrine units at Sunshine Amphitheater lie within the Matuyama Reversed Chron, unit Ql02 was deposited between 1.7 and 1.0 Ma, and unit Ql03 was deposited at about 1 Ma (ages of Berggren et al., 1995). Unit Ql04 consists of sandy beach gravel inset into the older units, and rises to an elevation of about 1378 m. Its paleomagnetic polarity is unknown.

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Another site about halfway between Sunshine Amphitheatre and McGee Wash records '~/'l shorelines cut into two basalt hills. Subdued breaks in slope on the west sides of these hills are • the remains of shoreline angles. From a distance these slope breaks resemble flow contacts, but rounded beach pebbles are associated with the most prominent and consistent slope break at an~c. elevation of 1365 m, and they are identical to pebbles that abundantly mark the Sehoo shoreline I. cut on the same hills at 1335 m. Additional small remnants of slope breaks may also record shoreline angles at 1353 m and 1396 m. A high shoreline is further supported by an outcrop on~c,,¥ the east side of one of the hills. Here, well rounded gravel of rhyolite and basalt overlie a rhyolite .. flow weathering into angular clasts; the rounded gravel crops out as high as 1390 m.

Basins North of Walker Lake

In the late Pleistocene, the main,area of Lake Lahontan was connected to the Walker Lake .. basin only by a narrow neck through Adrian Valley (figs. 1 and 3; Davis, 1982). A lake that rises to 1400 m in the Walker Lake basin also spills to Lake Lahontan through Rawhide Flats over a pair of sills at 13 70 m (fig. 3). However, it is possible that in the early Pleistocene, Adrian Valley did not exist (King, 1993) and the sills leading into Rawhide Flats could have been higher in elevation. Hence, I sought evidence of high shorelines north of the Desert Mountains to establish that the very high shorelines found in the Walker Lake basin apply to Lake Lahontan as a whole. No such shorelines had been previously reported; however, Marty Mifflin had seen some "suspicious-looking" benches above ~he Sehoo shoreline during his field work in the 1960's and suggested a search in the Truckee River canyon east of Reno and in the Smoke Creek Desert north of Reno (fig. 1). Work in the Truckee River canyon has revealed some interesting geomorphic features with rounded cobbles at about 1400 m elevation, and one roadcut exposes well bedded sediment that may be lacustrine or deltaic at about 1360 m. However, these features could also be fluvial in origin. Indisputable evidence of high, old shorelines exists at three sites in the Smoke Creek Desert and potentially at one site in the northern Black Rock Desert (fig. 1). Unfortunately, no stratigraphic sequences containing tephra or other datable material of probable Quaternary age have yet been found north of the Desert Mountains.

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The southern site in the Smoke Creek Desert is west of a Quaternary fault that bounds the western edge of the basin but apparently has not displaced late Pleistocene deposits (Dohrenwend et aI., 1991). Two subtle benches and risers cut on basalt and rhyolite flows are defined by topographic contours. The upper bench terminates upward at a slope break at 1400 m and well rounded small pebbles of several lithologies are scattered over the bench between 13 95 and 1400 -=

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m. The pebbles are not derived from alluvial beds between the flows. The lower bench is at about 1375 rn and is blanketed with abundant well rounded pebbles and some pieces of lacustrine tufa (definitely not pedogenic CaC03). A few kilometers to the north, another site also has abundant well rounded pebbles at about 1390 m.

The eastern site (fig. 1) has benches and risers cut on basalt. The benches and risers are covered with basalt clasts, locally closely fitted together in pavements interspersed with rougher surfaces and on steeper slopes with matrix-poor stone stripes trending downslope. Many of the clasts on both the benches and the risers are well rounded and polished rather than being angular or subangular as is typical of clasts in colluvial block fields. The rounding and polishing require vigorous transport by water. Alluvial transport can be ruled out because the benches are laterally continuous and the topography does not pennit stream erosion, even far in the past. The upper bench at this site rises gradually from about 1370 rn to a subtle slope break at about 1400 m; the lower bench rises from a flat surface at 1360 m to a distinct slope break at about 1365 m.

A borrow pit in the northern Black Rock Desert exposes very well sorted, well bedded loose sand at an elevation of about 1410 m. The sand is capped by a well developed soil with an argillic horizon extending down about 1 rn, and the sand overlies a well developed indurated argillic paleosol. The sand unit is too well bedded and sorted to be fan alluvium, despite its proximity to range fronts on both sides, and could represent a beach sand. The site needs to be re-excavated with a backhoe.

Summary and Implications

Several good outcrops in the Walker Lake basin document numerous high stands of a lake that rose far above the Sehoo and Eetza shorelines (fig. 3) during the early and middle Pleistocene. Stratigraphic relations among four sites (fig. 7) permit a conservative correlation of the youngest units; these units always crop out as beach gravel inset into all older units. The maximum elevations of these younger units range from 1356 to 1378 m at the three tectonically deformed sites. Unit Ql03 at the Thorne Bar has not been deformed and its maximum elevation is about 1370 m, the same as that of the shoreline angle cut on basalt hills north of Weber Reservoir (fig. 3). Thus, I infer that these youngest units represent one or more high stands at about 1370 m, shoreline deposits and remnants of which have since been deformed. The presence of the Bishop ash bed in lacustrine deposits at the Thorne Bar and Campbell Amphitheater permits correlation of the Qlo 1 units at these sites (fig. 7). Unit Ql02 in McGee Wash contains a Bishop­like ash and may also be correlative; however, if this tephra should prove to be paleomagnetically reversed, it would be ca. 1-Ma Glass Mountain tephra and would tie this unit to that discovered by Morrison (1991; Morrison and Davis, 1984) west of Weber Reservoir. The lacustrine units overlying units containing tephra at Thorne Bar, McGee Wash, and Campbell Amphitheatre are also those that rise to the highest elevations at each site: about 1400 at the first two and about 13 90 m at the latter. These data suggest, but do not prove, that these units represent the same high stand at about 1400 m elevation. Paleomagnetic data from the Sunshine Amphitheater show that the three oldest lacustrine units there are older than those at the Thorne Bar and Campbell Amphitheater, but they could be in part correlative to older units near McGee Wash. A conservative correlation among all the sites known in the Walker Lake basin (fig. 7) yields a total of six lacustrine units (other correlations would increase this number) whose shorelines exceeded those of the Sehoo and Eetza lakes; two of the units postdate the 760-ka Bishop ash bed (one of these probably correlates with the Rye Patch Formation, fig. 2), one contains the ash bed, and three predate the ash bed but are younger than 2.5 Ma.

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Figure 7. Preliminary stratigraphic correlations in the Walker Lake basin. Units and symbols are the same as those used in figures 4,5, and 6, except Nand R are nonnal and reversed polarity, respectively. Note that units with the same names in different areas may not be correlative.

Shoreline evidence from the sites in the Smoke Creek Desert indicates high stands at elevations of about 1370 and 1400 m.' No age control yet exists for these sites, but the correspondence of these two shoreline elevations with those at the Thome Bar and elsewhere in the Walker Lake basin strongly argues that they represent a continuous Lake Lahontan at two different times in the middle Pleistocene. The implications of a 1400-m shoreline are staggering (fig. 8): for example, such a lake would inundate Granite Springs Valley, would back up the Truckee and Carson Rivers to submerge all or part of present-day Reno and Carson City, would back up the Humboldt River at least to present-day Battle Mountain, and would extend 60 km southeast from Walker Lake to Rhodes Salt Marsh. If allowances are made for local tectonics and sedimentation in the past half-million years, the lake would have flooded into Dixie and Fairview Valleys and inundated many other flat-lying valleys whose floors are now only a few tens of meters above the 1400-m level. These high lake stands may prove key to understanding the distribution of native populations of fish and amphibians in the Great Basin (e.g. Hubbs and Miller, 1948; Hubbs et aI., 1974).

Lake Lahontan is not the only pluvial lake in western Nevada that exhibits evidence of very high, old shorelines. Field reconnaissance has located several sites around the margin of pluvial Lake Columbus (figs. 1 and 8) that demonstrate shorelines as much as 70 m higher than the late Pleistocene level and show that this lake was once contiguous with an early to middle Pleistocene Lake Rennie in Fish Lake Valley (Reheis et aI., 1993). Very high shorelines are also preserved on the eastern side of Mono Lake; during this high stand, Mono Lake may have drained north into the Walker River rather than south into the Owens River (fig. 8). Together, these findings suggest that western Nevada was much wetter during pluvial periods of the early and middle Pleistocene than during those of the late middle to late Pleistocene. The reasons for

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Figure 8. Regional map showing Lahontan basin, selected shoreline elevations in meters,. and areas of middle (lightest shading) late \l..U.C;Ul.u..Ll.L

dark shading) Pleistocene lakes. Queried arrows on drainages ShOVl

possible links between basins during highest lake stands; dashed lines show possible enlarged area of Lahontan basin. BR, Black Rock Desert; W, Winnemucca; BM, Battle Mountain; GS, Granite Springs Valley; R, Reno; CC, Carson City; RM, Rhodes Salt Marsh.

this may include: (1) increasing rain shadow through time due to uplift of the Sierra Nevada; (2) changes in the position of the jet stream; (3) drainage changes that have diminished the size of the Lahontan drainage basin through time (fig. 8); (4) more moisture crossing the Sierra Nevada in the early and middle Pleistocene due to the presence of Lake Clyde east of the range (Sarna­Wojcicki, 1995) .

REFERENCES CITED

Benson, L. V., 1978, Fluctuation in the level of pluvial Lake Lahontan during the last 40,000 years: Quaternary Research, v. 9, p. 300-318.

16

Benson, L. v., Currey, D. R., Dorn, R. 1., Lajoie, K. R., Oviatt, C. G., Robinson, S. W., Smith, G. 1., and Stine, S., 1990, Chronology of expansion and contraction of four Great Basin lake systems during the past 35,000 years: Palaeogeography, Palaeoclimatology, Palaeocology, v. 78, p. 241-286.

Berggren, W. A, Hilgen, F. J., Langereis, C. G., Kent, D. v., Obradovich, J. D., Raffi, 1., Raymo, M. E., and Shackleton, N. J., 1995, Late Neogene chronology: New perspectives in high-resolution stratigraphy: Geological Society of America Bulletin, v. 107, p. 1272-1287.

Davis, J. 0., 1982, Bits and pieces: The last 35,000 years in the Lahontan area, in Madsen, D. B., and O'Connell, J. F., eds., Man and Environment in the Great Basin: Society for American Archeology, No.2, p. 53-75.

Demsey, K., 1987, Holocene faulting and tectonic geomorphology along the Wassuk Range, west-central Nevada [MS thesis]: University of Arizona, Tucson, 64 p.

Dohrenwend, J.C., McKittrick, M. A, and Moring, B. C., 1991, Reconnaissance photogeologic map of young faults in the Lovelock 1° by 2° quadrangle, Nevada and California: U.S. Geological Survey Miscellaneous Field Studies Map MF-2178, scale 1:250,000.

Hubbs, C. L., and Miller, R. R., 1948, The Great Basin. II. The zoological evidence: University of Utah Bulletin, v. 38, p. 17-166.

Hubbs, C. L., Miller, R. R., and Hubbs, L. C., 1974, Hydrographic history and relict fishes of the north-central Great Basin: California Academy of Sciences Memoir, v. 7,259 p. .

King, G. Q., 1978, The late Quaternary history of Adrian Valley, Lyon County, Nevada [MS thesis]: University of Utah, Salt Lake City, 88 p.

King, G. Q., 1993, Late Quaternary history of the lower Walker River and its implications for the Lahontan paleolake system: Physical Geography, v. 14, p. 81-96.

Knott, J. R., Sarna-Wojcicki, A M., Meyer, C. E., Tinsley, J. C. 1. 1. 1., Wan, E., and Wells, S. G., 1996, Late Neogene stratigraphy of the Black Mountains piedmont, eastern California: Implications for the geomorphic and neotectonic evolution of Death Valley: Geo!. Society of America Abstracts with Programs, v. 28, no. 5, p. 82.

Mifflin, M., 1984, Paleohydrology of the Lahontan Basin, in Lintz, J., Jr., ed., Western Geological Excursions: Guidebook for the 1984 Annual Meeting: Reno, Nevada, Geological Society of America, v. 3, p. 134-137.

Mifflin, M. D., and Wheat, M. M., 1979, Pluvial lakes and estimated pluvial climates of Nevada: Nevada Bureau of Mines and Geology Bulletin 94, 57 p.

Morrison, R. B., 1991, Quaternary stratigraphic, hydrologic, and climatic history of the Great Basin, with emphasis on Lakes Lahontan, Bonneville, and Tecopa, in Morrison, R. B., ed., Quaternary Nonglacial Geology: Contenninous U.S: Boulder, Colorado, Geological Society of America, v. K-2, p. 283-320.

Morrison, R. B., and Davis, J. 0., 1984, Quaternary stratigraphy and archeology of the Lake Lahontan area: a re­assessment: Supplementary Guidebook for Field Trip 13, Geological Society of America 1984 Annual Meeting, Desert Research Institute Social Sciences Center Technical Report No. 41, 50 p.

Reheis, M. C., Slate, J. L., Sarna-Wojcicki, A M., and Meyer, C. E., 1993, A late Pliocene to middle Pleistocene pluvial lake in Fish Lake Valley, Nevada and California: Geo!. Society of America Bulletin, v. 105, p. 953-967.

Richmond, G. M., and Fullerton, D. S., 1986, Introduction to Quaternary glaciation in the United States of America, in Richmond, G. M. and Fullerton, D. S., eds., Quaternary glaciations in the United States of America: Quaternary Science Reviews, v. 5 (Quaternary glaciations in the Northern Hemisphere), p. 3-10.

Russell, I. C., 1885, Geological history of Lake Lahontan, a Quaternary lake of northwestern Nevada: U.S. Geological Survey Monograph, v. 11, 288 p.

Sarna-Wojcicki, A. M., 1995, Age, areal extent, and paleoclimatic effects of "Lake Clyde", a mid-Pleistocene lake that formed the Corcoran Clay, Great Valley, California: abstract for Glacial History of the Sierra Nevada, California-a symposium in memorial to Clyde Wahrhaftig, Sept. 20-22, 1995, White Mountain Research Station, Bishop, California, lOp.

Sarna-Wojcicki, A M., Morrison, S. D., Meyer, C. E., and Hillhouse, J. W., 1987, Correlation of upper Cenozoic tephra layers between sediments of the western United States and eastern Pacific Ocean, and comparison with biostratigraphic and magnetostratigraphic age data: Geological Society of America Bulletin, v. 98, p. 207-223.

Smith, G. 1., and Bischoff, J. L., eds., 1993, Core OL-92 from Owens Lake, southeast California: United States Geological Survey Open-file Report, v. 93-683, 397 p.

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