regional-scale assessment of a sequence-bounding paleosol...

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ABSTRACT

Recently developed sequence stratigraphic models for fl uvial fans suggest that sequence boundaries in these deposits are marked by laterally extensive paleosols; however, these models were based on paleosol correlations inferred between wells. To test this, we col-lected ~190 km of ground-penetrating radar (GPR) profi les on three fl uvial fans from the eastern San Joaquin Valley, California, to determine the lateral extent and character of a buried near-surface sequence-bound-ing paleosol. This paleosol, recognized on GPR by rapid shallow signal attenuation, extends across large areas on all three fl uvial fans. Limited areas of signifi cantly increased signal penetration were also identifi ed, and these zones are interpreted to indicate the absence of the paleosol. The zones where the paleosol is missing likely correspond to paleo-outwash channel activity on the fan surfaces that, when active, was able to partially or fully scour through the paleosol and deposit coarse-grained channel sediments in place of the sequence boundary. Erosional breaks are most common on the Kings River fan, while few breaks on the Tuolumne and Mer-ced River fans may indicate less paleochan-

nel activity on these fan surfaces during the last outwash event. Differences in channel activity between fans indicate that the Kings River migrated across its fan during the last outwash event, as evidenced by the large number of areas with increased GPR signal penetration and the presence of numerous channel deposits recorded on the soil surveys, while the Tuolumne and Merced Rivers only deposited fl oodplain fi nes, with the channels remaining inside a shallow incised valley, as evidenced by the relatively low number of areas with increased GPR signal penetration and the presence of primarily fi ne-grained material recorded on the soil surveys. Fac-tors controlling these differences may include variable valley subsidence rates and differ-ences in the San Joaquin Basin overall width at each fan location.

Keywords: fl uvial sequence stratigraphy, ground-penetrating radar, fl uvial fans, Qua-ternary, San Joaquin Valley.

INTRODUCTION

Fluvial and alluvial fan stratigraphy often appears to be complex and chaotic, thus com-plicating recognition of stratigraphic patterns. However, Wright and Alonso-Zarza (1990) and Alonso-Zarza et al. (1998) showed that paleo-sol distributions can help identify stratigraphic patterns in fan deposits. In recent work, Weiss-mann et al. (2002a) recognized that the Kings River fl uvial fan of the eastern San Joaquin Valley in California responded to Quaternary climate change through cycles of deposition and erosion, thus producing paleosol-bound stratigraphic sequences. Since deposits on the

eastern San Joaquin fans are dominated by fl u-vial processes (Janda, 1966; Huntington, 1971, 1980; Marchand, 1977; Marchand and All-wardt, 1981; Lettis, 1982, 1988; Harden, 1987; Weissmann et al., 2002a, 2005), we use the term “fl uvial fan” to distinguish these fans from allu-vial fans dominated by sheet-fl ood or debris-fl ow processes (Weissmann et al., 2005). On the Kings River fan, episodes of widespread fl uvial deposition occurred in response to signifi cant increases in sediment supply during periods of glacial outwash (Weissmann et al., 2002a). Con-versely, loss of glacial outwash sediment load during interglacial periods led to incision of the fans, leaving the upper fl uvial fan exposed to widespread soil development. Basin subsid-ence provided accommodation space, such that subsequent aggradational events buried the interglacial soils, thus forming stratigraphic sequences, where a sequence is defi ned as “…a relatively conformable succession of genet-ically related strata bounded at its top and base by unconformities, or their correlative confor-mities” (Mitchum, 1977, p. 210). The sequence-bounding unconformities of the Kings River fan are marked by relatively mature paleosols in the stratigraphic succession. This sequence stratigraphic approach provides an important framework for groundwater studies (e.g., Weiss-mann and Fogg, 1999; Weissmann et al., 2002a, 2002b, 2004; Burow et al., 2004).

Three main issues remain in the sequence stratigraphic model generated in previous stud-ies. We attempt to address these issues by inte-grating ground-penetrating radar (GPR) surveys with outcrop and borehole data on three fl uvial fans in the San Joaquin Valley: the Kings River, the Tuolumne River, and the Merced River fans (Fig. 1). The fi rst issue is that in previous work

Regional-scale assessment of a sequence-bounding paleosol on fl uvial fans using ground-penetrating radar, eastern San Joaquin Valley, California

George L. Bennett V†

Gary S. Weissmann‡

Department of Geological Sciences, Michigan State University, 206 Natural Sciences Building, East Lansing, Michigan 48824-0001, USA

Gregory S. Baker§

Department of Earth and Planetary Sciences, 1412 Circle Drive, University of Tennessee, Knoxville, Tennessee 37996-1410, USA

David W. Hyndman#

Department of Geological Sciences, Michigan State University, 206 Natural Sciences Building, East Lansing, Michigan 48824-0001, USA

†Present address: U.S. Geological Survey, Califor-nia Water Science Center, 6000 J Street, Placer Hall, Sacramento, California 95819-6129, USA; e-mail: [email protected].

‡Corresponding author present address: Depart-ment of Earth and Planetary Sciences, MSCO3–2040, 1 University of New Mexico, Albuquerque, New Mexico 87131-0001, USA; e-mail: [email protected].

§E-mail: [email protected].#E-mail: [email protected].

724

GSA Bulletin; May/June 2006; v. 118; no. 5/6; p. 724–732; doi: 10.1130/B25774.1; 6 fi gures.

SEQUENCE-BOUNDING PALEOSOLS ON FLUVIAL FANS

Geological Society of America Bulletin, May/June 2006 725

Kings River Fluvial Fan

Tuolumne and Merced River Fluvial Fans

CALIFORNIA

Figure 1. Maps of study area fans showing surface exposures of stratigraphic units, based on soil surveys by Arkley (1964), Huntington (1971), Arroues and Anderson (1976), McElhiney (1992), Ferrari and McEl-hiney (2002), and USDA/NRCS (2003). The Turlock Lake unit consists of deposits from two major glaciations in the subsurface, e.g., upper and lower Turlock Lake units (Weiss-mann et al., 2002a). The reported stratigra-phy is from Marchand and Allwardt (1981), with unit relationships to soil series based on Janda (1966), Helley (1966), Marchand (1977), Marchand and Allwardt (1981), Lettis (1982, 1988), Harden (1987), and Huntington (1980). Boxes outline the areas covered by maps of Figure 4.

BENNETT et al.

726 Geological Society of America Bulletin, May/June 2006

(Weissmann and Fogg, 1999; Weissmann et al., 2002a), buried paleosols of the Kings River fan have only been correlated between widely spaced wells; thus, uncertainty exists with respect to the validity of these correlations and whether the paleosols are truly laterally extensive and iden-tifi able on a regional scale. Second, Weissmann (1999) and Weissmann et al. (2002a, 2002b, 2004) suggested that discrete breaks or “holes” exist through the sequence-bounding paleosols in the San Joaquin Valley. They proposed that the breaks were formed by channel erosion that occurred when channels reoccupied the fl uvial fan surface during initial aggradation of the sequence. Evidence for the distribution of ero-sional breaks, however, was limited to interpreta-tions of paleochannel distribution observed from C-horizon textures reported in county soil sur-veys (Weissmann, 1999; Weissmann et al., 1999, 2002a, 2005). If such breaks exist, they create high conductivity pathways that may provide rapid groundwater recharge along with associ-ated contaminant transport into the San Joaquin Valley aquifer system (Weissmann, 1999; Weiss-mann et al., 2004). The third major issue is that if the sequence stratigraphic model is reasonable, sequence-bounding paleosols should be recog-nizable in the shallow subsurface of other fans in the eastern San Joaquin Valley.

Since its introduction, GPR has allowed for quick and effective imaging of the stratigraphy, internal structure, and, in some cases, the lithol-ogy of near-surface sands and gravels (e.g., Davis and Annan, 1989; Beres and Haeni, 1991; Jol and Smith, 1991; Smith and Jol, 1992; Hug-genberger, 1993; Beres et al., 1995; Jakobsen and Overgaard, 2002). GPR surveys produce continuous high-resolution profi les of the sub-surface that are similar to those produced using seismic-refl ection methods. Relatively short (~1.5 km) GPR surveys on the Kings River fl u-vial fan by Burow et al. (1997) indicated that the uppermost sequence-bounding paleosol could be tracked using GPR. In the survey by Burow et al. (1997), as well as surveys con-ducted for this study, the electrically conductive clay-rich paleosols rapidly attenuate the GPR signal, thereby forming a coherent unit, below which very little or no signal is detected. This study focuses on imaging the uppermost buried paleosol of the fl uvial fans (approximately mid-dle Quaternary in age). This paleosol is buried under late Quaternary fan deposits in the middle to distal portions of the fan, while toward the fan apex, it is exposed as a terrace above these deposits (Weissmann et al., 2002a). At the electromagnetic (EM) frequencies used for the GPR surveys (50 and 100 MHz), direct imaging of this paleosol was limited due to its shallow depth. The paleosol in most cases is the fi rst sig-

nifi cant refl ector encountered by the GPR signal; thus, in many profi les this shallow refl ector con-structively and destructively interferes with the direct EM wave, and interpretation is diffi cult. However, due to the highly attenuating behavior of the conductive clay-rich unit (where it exists), the lateral continuity of the uppermost paleosol is inferred via depth of penetration through use of extensive GPR surveys.

Study Area

The Kings, Tuolumne, and Merced Rivers form large fl uvial fans where they enter the San Joaquin Valley from the Sierra Nevada in California (Fig. 1). The Kings River enters the San Joaquin Valley southeast of Fresno, and the Tuolumne and Merced Rivers enter the valley east and southeast of Modesto. Fluvial fans produced by these rivers have low gradients (~0.0013), with the drainage trending west to southwest on the Kings and Merced River fan, and west on the Tuolumne River fan (Weissmann et al., 2005). The Sierran drainage basins upstream of the Kings, Tuolumne, and Merced River fans cover areas of ~4385 km2, 3980 km2, and 2750 km2, respectively, while the fans cover areas within the San Joaquin Valley of ~2400 km2, 630 km2, and 840 km2, respectively. These fans are sites of ongoing hydrostratigraphic characterization and groundwater modeling studies (e.g., Burow et al., 1997, 2004; Gronberg et al., 1998; Weiss-mann and Fogg, 1999; Weissmann et al., 1999, 2002b, 2004), because increased urbanization of this predominately agricultural land has led to stresses on the groundwater resources in the area. Therefore, assessment of anthropogenic contaminant input and withdrawal demands on the system are required for protection of this increasingly valuable resource.

On the Kings River fan, as well as other fans in the eastern San Joaquin Valley, depositional cyclicity occurred with changes in the sediment supply to stream discharge ratio through the gla-cial-interglacial cycles during the Pleistocene (Wahrhaftig and Birman, 1965; Janda, 1966; Hun-tington, 1971, 1980; Marchand, 1977; Marchand and Allwardt, 1981; Lettis, 1982, 1988; Harden, 1987; Weissmann et al., 2002a, 2005). Fluvial processes dominated open-fan deposition (depo-sition across the entire fan surface), and aggrada-tion occurred during periods of signifi cant glacial outwash, with channel incision occurring during interglacial periods. Deep (~10–30 m) incised valleys formed across the length of the fans dur-ing interglacial incision, leaving widespread areas on the upper portions of the fans exposed to soil development (Weissmann et al., 2002a, 2005). Sequences in the upper and middle por-tions of the fl uvial fans consist of fl uvial open-fan and coarse-grained incised valley-fi ll deposits, bounded by the paleosols that developed during past interglacial periods (Fig. 2) (Weissmann et al., 2002a, 2005). These paleosols consist of red-dish, thick (~1 m) argillic (clay-rich) horizons that are readily recognized in cores and well logs and appear to be similar to modern soils that have had signifi cant time to develop (e.g., ~100,000 yr) (Weissmann et al., 2002a).

Multiple glacial episodes are represented by four Pleistocene-age stratigraphic units in the eastern San Joaquin Valley fans (Fig. 1), with several minor aggradational events recorded within each of these units (Marchand and All-wardt, 1981). The paleosol imaged in this study caps the Riverbank unit (Illinoisan-equivalent glacial stage). This paleosol was formed across the entire upper and middle fan surface (except for the location where a relatively deep incised valley was present) during the last interglacial

Figure 2. Schematic diagram showing the distribution of facies within sequences in the San Joaquin Valley fl uvial fans.



period (Huntington, 1980; Weissmann et al., 2002a). It was then buried over most of the study area by glacial outwash deposits of the Modesto unit (Wisconsin-equivalent gla-cial stage). Because this paleosol (and other sequence-bounding paleosols) was developed across the upper and middle fan surface during periods of fan incision, we expect the paleosol to be a widespread stratigraphic marker.

METHODS

We collected GPR data from the study areas using a Sensors and Software Inc. pulseEKKO 100TM system. Common-offset refl ection sur-veys (i.e., the standard collection method that maintains a fi xed distance between the trans-mitting and receiving antennas) were collected using this system in EH mode (q.v., Jordan and Baker, 2003; also known as parallel broadside or TE mode) with a 400 V transmitter pulser volt-age and interchangeable sets of 50 and 100 MHz center frequency antennas. We opted to use 50 MHz antennas for the large-scale surveys. This preference resulted in a decrease in near-surface GPR resolution yet added to the maxi-mum depth of signal penetration, which was important to the project objective. The system was towed on a fi berglass GPR cart one meter behind a car at ~5 km/h (Fig. 3). Each shot loca-tion in refl ection mode was stacked at least eight times within a time window of 800 ps (1600 in some cases). Stacking is a summation function that enhances the incoming refl ections collected

within the time window, thereby improving the signal to noise ratio in the presence of random noise by a factor of the square root of n, where n is the number of stacks. GPR soundings along the survey lines were collected every 0.5 m (step size), with each transmitted pulse triggered by an odometer wheel towed behind the cart. The common-offset distance (antenna separation) was 2 m. Profi les were processed and plotted with software developed specifi cally for GPR data presentation, WINEKKO version 1.0 and EKKOMAPPER version 2, both of which were developed for use with the Sensors and Soft-ware GPR hardware. Only minimal processing was required for the type of data interpretation required in this study. First, a low-frequency (“dewow”) fi lter was applied to remove any low-frequency energy generated from electri-cal inductive processes or as a result of possible dynamic range limitations of the unit. Second, automatic gain control (AGC) was uniformly applied to the data with consistent parameters. The AGC is designed to recover and reapply signal amplitude information potentially lost due to geometrical spreading of the transmitted waves or other attenuation factors. The reason for using an AGC gain rather than some other method (e.g., constant gain or spherical diver-gence correction) is that the preservation of rela-tive amplitude relationships among refl ections was not required. Instead, the objective was to examine depth of penetration as affected by the conductive paleosol; thus, enhancing all exist-ing refl ection information at depth was desired.

GPR profi les are typically displayed as either two-way traveltime (ns) of the transmit-ted signal or depth (after conversion) versus the distance (m) along the transect. In order to translate the two-way traveltime of the refl ected signal to depth below the surface (i.e., time-to-depth conversion) at the study area, an averaged near-surface velocity of 0.14 m/ns was used. This velocity was determined from ten common mid-point (CMP) surveys conducted at several sites on both the Kings and Tuolumne River fans (Bennett, 2003). The variance for this average was 1.4 × 10−4 m/ns. The CMP survey sites were selected to capture a variety of stratigraphic set-tings, including areas with a shallow Riverbank paleosol, areas with channel fi ll sands within a Riverbank paleosol break, and areas that are representative of the typical background facies encountered during the overall GPR survey.

The GPR surveys were designed to constrain the lateral extent of the paleosol that caps the Riverbank unit (e.g., the uppermost paleosol in the stratigraphic succession on these fans). The surveys were run along the preexisting grid of county roads on the fl uvial fan surfaces; the roads were chosen on the basis of the following criteria: they had (1) the upper fl uvial fan sur-face recorded as either Riverbank or Modesto deposits on the soil surveys, thus, we expected to observe the Riverbank-age paleosol in the shallow subsurface beneath the Modesto-age deposits, (2) a smooth surface, long distance, and relatively straight paths, (3) low traffi c vol-ume, and (4) they did not have sources of sig-nifi cant GPR signal interference (e.g., abundant overhead high-power lines and urban areas).

In total, ~120 km of GPR data were obtained on the Kings River fan, ~34 km on the Tuolumne River fan, and ~36 km on the Merced River fan (Fig. 4; for a more extensive presentation of the GPR data, see Bennett, 2003). The Tuolumne River fan survey focused on deposits south of the Tuolumne River, while the Merced River fan survey focused on deposits north of the Mer-ced River. Signifi cant urbanization of the area north of the Tuolumne River combined with limited time and funding prevented collection of GPR data across the entire surface of either the Tuolumne or Merced River fan. Instead, we performed an exploratory survey of the interfan area, which allows us to compare the GPR data from three separate eastern San Joaquin Valley fl uvial fans and discuss the observed differences between each region.

Data to Ground-Truth the Paleosol Location

Detailed lithologic core and well logs (Weiss-mann, 1999; Harter et al., 1999, 2005) were used

GPR Odometer Trigger Wheel

Transmitting 50 MHz Antenna

Receiving50 MHz Antenna

GPR Console, Laptop, and Power Supply in Car

Figure 3. The ground-penetrating radar (GPR) fi eld setup used for continuous-refl ection mode is shown with 50 MHz antenna and transmitters attached to a fi berglass cart. The odometer wheel trailing behind the cart triggered the system at regular distance intervals. A deep-cycle 12 V battery located in the trunk of the car powered the system, while fi ber optic cables connected the antenna to the systems console (in the back seat), which sent the data to a laptop computer in the passenger seat.

BENNETT et al.


to compare interpretations of GPR data to identi-fi ed presence or absence of the shallow near-sur-face paleosol. Small-scale surveys were collected at the Kearney Agricultural Center, an extension of the University of California, located on the Kings River fan (Fig. 5A). This relatively small-scale (~1 ha) study area was formerly a nectarine orchard plot that has had its trees removed and is now the site of ongoing vadose zone hydrostrati-graphic investigations (Harter et al., 1999, 2005). Numerous continuous sediment cores were drilled down to a depth of 15.8 m along east-west and north-south cross sections within the 1 ha orchard plot. GPR data were collected on a grid matching the geometry of the former nectarine tree rows, resulting in fourteen profi les that pass as close as possible to borehole locations used in the Harter et al. (2005) study. Figure 5A shows the well log descriptions superimposed on the GPR profi les produced using the 50 MHz antenna. This GPR profi le shows that the signal becomes attenuated just below the paleosol, supporting the hypothesis that the presence of the paleosol rapidly attenu-ates GPR signal and thus signifi cantly reduces the depth of signal penetration.

Additionally, a section located along Del Rey Road (Fig. 5B) also shows attenuation of the GPR signal beneath the shallow paleosol, where lithologic information from nearby well logs and cores were used for ground truth of the GPR interpretations. Conversely, when the paleosol is absent, the GPR signal penetrates to signifi cantly greater depths (Fig. 5C). Thus, shallow GPR signal attenuation helps us to read-ily identify areas where the paleosol is present.

RESULTS

Our regional-scale GPR survey effectively identifi ed the distribution of the paleosol cap-ping the Riverbank unit on all three fl uvial fans (Figs. 4, 5, and 6) by identifying regions with rapid attenuation of GPR signal caused by the clay-rich paleosol unit. As previously noted, the interpretation that the paleosol caused the observed GPR signal attenuation was verifi ed by comparing nearby core and driller’s well logs to the GPR results at several locations on the Kings River fan (Fig. 5). Presence or absence of the paleosol was confi rmed in these well data,

and the depth of the paleosol, where it was pres-ent, was compared to the depth noted in core and well logs. The lines could not be supported by ground data at all locations, especially on the Tuolumne and Merced River fans, because of the cost limitations imposed by the regional scale of the study. However, surveys on these fans appear to have similar character to those of the Kings River fan; thus, we infer the presence of a shallow paleosol where the signal is attenu-ated in the near-surface (~1–4 m). Soil surveys and drillers’ logs from water wells in the area support this interpretation (Arkley, 1964; McEl-hiney, 1992; Burow et al., 2004).

Shallow paleosols are present in all sur-veys conducted on the fans, indicating that the paleosols are widespread stratigraphic mark-ers (Fig. 4). Initially, we expected to see the Modesto unit signifi cantly thicken basinward, with the Riverbank paleosol dipping deeper into the subsurface below a thickening Modesto unit. However, these GPR surveys indicate that the Modesto unit is a relatively thin (~1–4-m-thick) aggradational unit over the surveyed areas of the three fans.

GPR PROFILES

A Kings River Fluvial Fan

SangerSanger

FowlerFowler

Hwy 99

PaleosolDeep IncisionShallow IncisionCorrupt ProfileQuestionable

SandGravel

Overbank

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ModestoModesto

Hwy 99

TurlockTurlock

B Tuolumne and Merced River Fluvial Fans

Tuolumne River

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Figure 4. Interpreted ground-penetrating radar (GPR) lines on (A) the Kings River fan and (B) the Tuolumne and Merced River fans showing the distribution of the Riverbank paleosol and locations of breaks through this paleosol. The background map shows locations of channel and overbank deposits interpreted from soil textures (Weissmann et al., 2002a). Numbers show locations of GPR profi les in Figures 5 and 6.



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Figure 5. (A) Raw data and interpreted ground-penetrating radar (GPR) profi les (50 MHz antennas) from the Kearney Agricultural Center showing signal attenuation below the Riverbank paleosol (marked by the dashed line). Lithology from core provides ground truth for paleosol depth and presence. (B) A typical GPR profi le (50 MHz antennas) from the Kings River fan showing signal attenuation at the River-bank paleosol (marked by the dashed line on the interpreted profi le). Presence and depth of the paleosol is supported by lithologic logs from nearby wells. The prominent diffrac-tions (hyperbolas) observed at 285, 505, 600, 670, 1100, and 1190 along the profi le represent refl ections caused by passing under overhead power lines or over irrigation canals. (C) GPR profi le (50 MHz antennas) from the Kings River fan showing an interpreted paleochannel that has eroded through the River-bank paleosol. A paleochannel is interpreted to exist between 110 and 400 m along this tran-sect. Absence of the paleosol in this area is supported by litho-logic logs from nearby wells.

BENNETT et al.


Each of the three fl uvial fans described is unique in its basinal position and geomorphic character (Weissmann et al., 2005). The follow-ing sections describe specifi c results from each fl uvial fan, which are then followed by a discus-sion of how these factors may be related to fan development.

Kings River Fan Results

Paleosols on the Kings River fan appear to have been dissected by paleochannels, as shown by zones of relatively deep signal penetration on the GPR profi les (Figs. 4 and 5C). Areas along the profi les that showed notable increases in the depth of signal penetration are assumed to be areas where the paleosol is missing (i.e., break in continuity). The GPR refl ections in some of these breaks show refl ectors consistent with channel scour shapes (Fig. 5C). These apparent channel incisions varied in character from deep (>10 m), relatively narrow incisions to shallow and wide lateral scour zones that removed the paleosol but rarely incised deeper than 6 m (Fig. 5C). In every case, the paleosol was observed to exist between discrete channel incisions.

In most instances, paleosol breaks identifi ed in the GPR profi les on the Kings River fan are directly correlated to paleochannel locations observed on county soil surveys, where chan-nels are shown by sandy C-horizon textures of the soil series (Fig. 4; Weissmann et al., 2002a). This observation, along with the chan-nel shape evaluated from the GPR surveys, sug-gests that Modesto-age paleochannels eroded

sections of the laterally continuous Riverbank paleosol. The areas where the GPR surveys indicate paleosol breaks exist but channel sedi-ments are not observed on the soil survey have a number of possible explanations, including (1) a paleochannel may exist but does not “out-crop” on the soil surveys, (2) the paleosol was removed for agriculture (e.g., ripping to increase drainage), or (3) a shallow paleosol was exca-vated for road construction.

Tuolumne and Merced River Fan Results

On the Tuolumne River fan, the Riverbank paleosol was again identifi ed as a widespread stratigraphic marker, yet it lacks the abundant paleochannel incisions observed on the Kings River fan, thus indicating greater lateral con-tinuity of the paleosol (Figs. 4 and 6A). Addi-tionally, overlying Modesto deposits appear to be thinner on most of the Tuolumne River fan (~1–2 m) than those observed on the Kings River fan (~1–4 m). Finally, soil surveys across the Tuolumne and Merced fans show that fi ne-grained facies dominate the Modesto deposits (e.g., fl oodplain sediments) (Fig. 4).

Several small breaks were identifi ed in the Mer-ced River fan paleosol based on GPR data (Figs. 4 and 6B); however, more recent eolian deposits cover the surface of this fan. Therefore, we could not interpret these breaks as Modesto channel incisions. The overlying Modesto deposits appear to be ~1–2 m thick on the Merced River fan.

The presence of the Riverbank paleosol dip-ping basinward at a low angle (<1°) below the

Modesto deposits on both the Tuolumne and Merced River fans is consistent with similar sequences on the Kings River fan. In the case of the Tuolumne River fan, the lack of recorded channel breaks through the paleosol appears to indicate that either the Modesto (Wisconsin-equivalent age) Tuolumne River never migrated across the fan surface or our limited surveys in this region did not cross a Modesto-age channel position. If the Tuolumne River never migrated across the fan surface, the Modesto incised val-ley fi ll would lie beneath the present Tuolumne River channel. The depth of the modern incised valley (30 m) and the similar thickness of past incised valley fi ll deposits observed on the Tuolumne River fan (Weissmann et al., 2005) indicate that partial fi lling of the Modesto incised valley must have occurred. Without par-tial fi lling, fl oods during Modesto deposition could not have spilled onto the fan surface.

Since the Riverbank paleosol is present in the Tuolumne and Merced River fan subsurface, we expect to fi nd other sequence-bounding paleo-sols in the deeper subsurface of these fans (e.g., paleosols capping the Turlock Lake and Laguna units). Recent drilling in the Modesto area indi-cates that these sequences are indeed present in both fans (Burow et al., 2004).

Observed Differences Between Fans

Although similar sequences have been observed and described on the Tuolumne, Mer-ced, and Kings River fans, notable differences exist between them. These include differences

A Tuolumne River Fan

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Figure 6. (A) A typical ground-penetrating radar (GPR) pro-fi le (50 MHz antennas) from the Tuolumne River fan showing signal attenuation at the River-bank paleosol (marked with an arrow). The prominent diffrac-tions (hyperbolas) observed along this line represent refl ec-tions caused by passing under overhead power lines or over irrigation canals. (B) Typical GPR profi le (50 MHz anten-nas) from the Merced River fan showing signal attenuation at the Riverbank paleosol. Deeper signal penetration on the south-ern end of this profi le shows a possible deep channel incision.



in (1) the apparent volume of Modesto-aged sediment preserved in each fan, (2) the number of observed erosional breaks in the underlying Riverbank paleosol, and (3) the overall size of each of the fans. Thus, these results aid in interpreting controls on sequence development within the San Joaquin Basin.

Variable subsidence rates between the north-ern and southern parts of the San Joaquin Valley may have infl uenced the observed morphology and size of these fl uvial fans (Weissmann et al., 2005). Lettis and Unruh (1991) calculated the subsidence rate of the San Joaquin Basin using the Corcoran clay, a regionally extensive 615 ka lacustrine unit, and showed that the southern valley is subsiding faster then the northern val-ley (0.4 m/k.y. and 0.2 m/k.y., respectively). Thus, the Tuolumne and Merced River fans have subsided at roughly half the rate of the Kings River fan to the south. The differing subsidence rates may therefore be a factor con-trolling differences in the observed Modesto unit thickness and lithologies, with the lower subsidence rate in the northern valley provid-ing less accommodation space for sediments on the Tuolumne and Merced River fans (Weiss-mann et al., 2005). This is refl ected in Modesto deposits on the Tuolumne and Merced Rivers fans that are approximately half the thickness of those on the Kings River fan. Additionally, the greater accommodation space on the Kings River fan may have provided room for Modesto channels to reoccupy the Kings River fan sur-face, leading to paleosol breaks on the Kings River fan in contrast to the limited breaks and fi ne-grained nature of Modesto deposits on the Tuolumne and Merced River fans.

The overall width of the San Joaquin Valley at each of the fan locations may also control the differences in fl uvial fan size (Weissmann et al., 2005). The Tuolumne and Merced River fans are signifi cantly smaller than the Kings River fan, yet all the fans have drainage basins that were glaciated during the Quaternary and probably experienced similar glacial outwash sediment loads. The northern San Joaquin Basin (near the Tuolumne and Merced River fans), however, is signifi cantly narrower than the southern basin near the Kings River fan (see Fig. 1). Thus, overall accommodation space appears to be lim-ited by basin width.

It could be argued that sediment supply to discharge ratio differences between these riv-ers could also have caused some differences in sequence thickness between the fans. However, all three fans have similar drainage basin charac-teristics (size and connection to glaciated portions of the Sierra Nevada; Weissmann et al., 2005). Therefore, we expect that similar sediment sup-ply to discharge ratio changes occurred on these

fans and do not believe this to be a major control on differences observed between fans.

Finally, the variable number of breaks observed in this study between these fl uvial fans could be related to the amount of data col-lected on each fan. Fewer GPR survey lines on the Tuolumne and Merced River fans may have resulted in a biased sampling, thus showing fewer observed breaks.

DISCUSSION

Paleosol Character and Distribution

The paleosol that caps the Riverbank sequence was constrained using GPR surveys across all three study area fans; however, numerous and potentially signifi cant breaks were observed on the Kings River fan, and possible minor breaks were observed on the Merced River fan. Breaks in the Kings River fan paleosol identifi ed on GPR surveys generally correlated with loca-tions of Modesto paleochannels observed on county soil surveys (Fig. 4). However, the chan-nel widths observed on the GPR profi les tended to be narrower then those observed on the soil surveys. This difference could be due to the cross-sectional shape of the channels, whereby the paleosol breaks exist at the narrower base of paleochannels where scouring took place, while the soil survey shows the full width of past chan-nel tracts. This research indicates that geophysi-cal surveys may provide a better estimation of break geometries than estimates based on sur-face mapping. This difference in break width may be signifi cant when incorporating facies geometries into groundwater fl ow models that attempt to capture fl uvial fan heterogeneities.

Since these breaks through the paleosols likely provide high-conductivity pathways for ground-water fl ow and contaminant transport (Weiss-mann, 1999; Weissmann et al., 2002b, 2004), dif-ferences in large-scale groundwater movement are expected between the three study area fans. These paleosol breaks may also have an effect on local ecology due to different levels of interaction between groundwater and surface water in these three systems. Further studies that image deeper sequence-bounding paleosols are needed in order to understand whether similar breaks exist through other sequence-bounding paleosols.

In addition to the paleosol breaks, the GPR surveys indicate that topography exists on the paleosol surfaces, where the top surface of the paleosol is uneven or slightly undulating rather than strictly planar (Figs. 5 and 6). This topogra-phy most likely refl ects the interglacial surface character of the fan prior to open-fan aggradation during the glacial outwash event. This topogra-phy is important to understand in the context of

groundwater resources, as low regions in the paleosol surface may provide recharge points through the vadose zone by focusing and forcing water through the paleosol layer. The approach presented in this study shows promise for identi-fi cation of high-conductivity pathways through the paleosol layer(s). Imaging such areas of potential focused recharge in these fl uvial sys-tems may lead to an improved understanding of fl uid/contaminant pathways into the regional aquifers of the San Joaquin Valley fl uvial fans.

CONCLUSIONS

Imaging the presence or absence of the near-surface Riverbank paleosols on the Kings, Tuolumne, and Merced River fans is feasible using a towed GPR cart. Identifi cation of the shallow sequence-bounding paleosols on the GPR surveys is made possible due to the clay-rich nature of these sediments. The penetration depth of the radar signal is limited by rapid signal attenuation associated with the highly electrically conductive paleosol, therefore, these conductive zones can be “mapped” by correlat-ing signal penetration with the existence of the paleosol. We identifi ed discrete erosional breaks through the paleosols on the Kings River fan, and hypothesize that they are paleochannels that incised into, fi lled, and then migrated across the upper fl uvial fan surface during glacial peri-ods (aggradational periods). Obvious increases in the depth of GPR signal penetration effec-tively highlight the location of erosional breaks through or incisions into the paleosol. Generally, breaks identifi ed using GPR correlate with loca-tions of mapped paleochannels observed on the county soil surveys (shown by sandy C-horizon textures). These erosional breaks in the clay-rich paleosols are believed to be hydrostratigraphi-cally signifi cant, since they may act as fast paths (zones of high conductivity) into the aquifers. The Tuolumne River fan lacked evidence for similar breaks, and only limited breaks were observed on the Merced River fan.

The differences in both the number of ero-sional breaks through the paleosol between these fl uvial fans and in the thickness of Modesto deposits may indicate different historical con-trols on aggradational cycles between the north-ern fl uvial fans (Tuolumne and Merced River fans) and the southern fl uvial fans (Kings River fan in particular). The Kings River fan exhib-its more erosional breaks and thicker Modesto deposits than the Tuolumne and Merced River fans, possibly indicating that the Kings River was more apt to aggrade and switch its channel location than the Tuolumne and Merced Rivers. Factors that controlled channel aggradation (or accommodation space) during glacial periods

BENNETT et al.


may have included differing valley subsidence rates (more in the south, less in the north) and differences in the San Joaquin Basin’s overall width (Weissmann et al., 2005).

Identifi cation and characterization of the sequence-bounding paleosols, along with breaks in these paleosols, may provide an important framework for future groundwater management and modeling in the San Joaquin Valley. Future work should build on this framework in order to better understand the complex hydrodynamic system of these fl uvial fan aquifers.

Through application of GPR surveys, we show that sequence-bounding paleosols form laterally extensive stratigraphic markers in the fl uvial fans of the San Joaquin Basin. We hypothesize that similar paleosols may be observed in other fl uvial fans, thus providing improved stratigraphic char-acterization of this complex depositional system type. Additionally, this work indicates that one should expect differences in paleosol form and distribution, and thus sequence geometry, related to fan position within a basin. These geophysi-cal surveys provide a regional-scale approach to determining sequence geometry in fl uvial fans, thus offering a relatively low-cost tool for strati-graphic evaluation of fl uvial fans.

ACKNOWLEDGMENTS

Acknowledgment is given to the Donors of the American Chemical Society Petroleum Research Fund for support of this research (PRF#37731-G8). We also thank Dietz Warnke and the Department of Geological Sciences at California State University, Hayward, for the use of their ground-penetrating radar (GPR) equip-ment (purchased under the National Science Founda-tion grant DUE-9980881). Discussions with and com-ments provided by the Geophysics graduate group at State University of New York Buffalo are sincerely appreciated. The manuscript also benefi ted greatly from comments by three anonymous reviewers.

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