intra-vent peperites related to the phreatomagmatic 71 gulch volcano, western snake river plain...

Journal of Volcanology and Geothermal Research 183 (2009) 30–41

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Intra-vent peperites related to the phreatomagmatic 71 Gulch Volcano, westernSnake River Plain volcanic field, Idaho (USA)

Károly Németh a,⁎, Craig M. White b,1

a Volcanic Risk Solutions, Institute of Natural Resources, Massey University, PO Box 11 222, Palmerston North, New Zealandb Department of Geosciences, Boise State University, Boise ID-83725, USA

⁎ Corresponding author. Tel.: +64 6 356 9099 ext 73E-mail addresses: [email protected] (K. Néme

(C.M. White).1 Tel.: +1 208 426 1631; fax: +1 208 426 4061.

0377-0273/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.jvolgeores.2009.02.020

a b s t r a c t
a r t i c l e i n f o
Article history:Received 9 October 2008Accepted 23 February 2009Available online 16 March 2009

Keywords:peperitebasalttuff ringcraterventpyroclastic

The western Snake River Plain volcanic field in SW Idaho contains up to 400 basaltic vents and centers thatproduced lava shields, pahoehoe lava fields, scoria cones, and a great variety of phreatomagmatic volcanoesbetween late Miocene and middle Pleistocene time. Tephra deposits produced by phreatomagmaticeruptions are particularly well exposed in the walls of the Snake River canyon, where thick accumulations ofpyroclastic rocks indicate widespread phreatomagmatic eruptive events throughout most of the volcanichistory of the region. Previously, many of the phreatomagmatic deposits were considered to be the productsof subaqueous eruptions that took place on the floor of one or more large freshwater intra-continental lakes.Recent field based observations confirm the presence of widespread phreatomagmatic pyroclastic rocks;however, some that had been interpreted as being subaqueous exhibit textural features that are moreconsistent with subaerial depositional environments. Intrusive and extrusive magmatic bodies with featuresassociated with peperite formation have also been identified. Most of these peperites can be attributed tomagma–sediment mixing in intra-crater/conduit or vent settings, and therefore they can only be used aswidespread paleoenvironmental indicators with limitations to demonstrate magma and surface water (e.g.lake) non-explosive interaction. One of the studied sites (“71 Gulch Volcano”) was previously used toindicate the presence of a shallow lake. At this site there is clear field evidence that peperitic feeder dykescontacted muddy, sandy siliciclastic sediments forming globular peperite. The peperitic feeder dykestransition to pillowed, ponded lava up section. The ponded lavas are partially surrounded by a ~5-m-thickunit composed of gently dipping, dune bedded, volcanic glass shard-rich, unsorted, tuff and lapilli tuffcontaining abundant impact sags caused by volcanic lithics. We suggest that the 3D architecture of theerosional remnant of “71 Gulch Volcano” does not require the presence of a lake at the time of its formation;it is equally possible that that it represents a subaerial phreatomagmatic upper conduit — crater fillingsuccession. This interpretation opens up many questions about the Mio/Pliocene evolution of SW Idaho, thetiming of the volcanism, and its association with the evolution of the lacustrine systems in the region. Inaddition, re-evaluations of the volcanic features in SW Idaho have some general implications for the usage ofphreatomagmatic pyroclastic rocks for paleoenvironmental reconstruction.

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1. Introduction

Small-volume mostly basaltic volcanoes are the most commonvolcanic edifice type in the Earth and in the Solar system, and theyproduce awide range of eruption styles from effusive to magmatic gasdriven explosive to magma–water interaction-triggered phreatomag-matic (Wood, 1980a,b). These volcanoes are generally small andtheir eruption products usually are in the order of 1 km3 or less(Vespermann and Schmincke, 2000). Their architecture and theirresulting eruptive products, especially the pyroclastic ones, can be

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extremely complex (Houghton and Schmincke,1989). This complexityis even more pronounced in the craters, vents and conduits of suchvolcanoes; however, these settings can yield vital information aboutthe mechanisms controlling the intrusion of dykes and the fragmen-tation and eruption of magmas (Houghton et al., 1999; Valentine andKrogh, 2006; Valentine et al., 2006; Martin and Németh, 2006;Keating et al., 2007; Valentine et al., 2007). Although conceptualmodels for the shallow plumbing systems of small basaltic volcanoeshave been recently tested from field observations of strongly erodedscoria cones (Valentine et al., 2006; Keating et al., 2007), theseeruptions were dry, and purelymagmatic fragmentationwas involved.The situation is less well explored where magma encounters externalwater and the fragmentation is dominated by phreatomagmaticprocesses. Here we describe a strongly eroded and exceptionally well-exposed continental phreatomagmatic volcano, 71 Gulch Volcano,part of the late Miocene to middle Pleistocene western Snake River

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31K. Németh, C.M. White / Journal of Volcanology and Geothermal Research 183 (2009) 30–41

Plain volcanic field in Idaho, USA (Fig. 1). The volcano is inferred to havedeveloped along a fissure and to have evolved by amixture of magmaticand phreatomagmatic processes. The resulting volcanic sedimentaryfacies are complex and reflect the explosive fragmentation, transporta-tion and deposition of pyroclasts, as well as non-explosive mixing ofintruding magma into different types of host sediment, leading to theformation of peperite. These deposits each have their own set of char-acteristic sedimentary features, and the identification of the processesthat formed them is crucial to understanding the evolutionof this center.

Peperite results from interaction between magma and wetsediment and exhibits a range of complex textures (Brooks et al.,1982; Hanson and Schweickert, 1982; Kokelaar, 1982; Busby-Speraand White, 1987; Kano, 1989; White et al., 2000; Martin and White,

Fig. 1. A) Overview map of the extent of the Plio/Pleistocene western Snake River Plain volcaGulch Volcano. B) Overview of the 71 Gulch Volcano on a GoogleEarth image. Arrows point toC) Oblique view on a GoogleEarth image toward SE shows the characteristic elongated distVolcano. D) Close up view of the NW hillside of the 71 Gulch Volcano on a GoogleEarth imaforking dyke buds intruded into siliciclastic host sediment. Dotted black line limits the pyroccrater filling pyroclastic deposits intruded by feeder dykes. Along these dyke margins pepeexposed in the top of the hill. In the center upper level of the NW hill pillow (P) lava crops ouand tuff unit (Lt/t) inferred to be the result of base surges and phreatomagmatic fall near vepoints to the inferred vent zone (V). The vent zone is circled by black dotted line. White dottedrocks inferred to be deposited on the crater rim (Cr). Black arrows highlight the NW-SE-tre

2002; McClintock and White, 2002; Skilling et al., 2002; Martin andNémeth, 2005; White and Houghton, 2006; Martin and Németh,2007). Distinguishing peperite textures from the textures of volcani-clastic rocks formed by explosive pyroclast-forming eruptions isimportant for understanding the eruptive and sedimentary history ofa region influenced by volcanic events. In addition, the identificationof peperites is also important for interpreting the hydrologic en-vironment in which the volcanism took place, which has importantimplications for paleoenvironmental reconstructions. Peperite iscommon in a variety of geological settings, and peperite-formingprocesses in the sub-volcanic regions of phreatomagmatic volcanoeshave also been documented from several sites (White, 1991; Whiteand McClintock, 2001; Martin and White, 2002; McClintock and

nic field. The star in the south-eastern edge of the field represents the location of the 71a characteristic NW-SE oriented dyke. Volcanic rocks preserved in a NWhill and SE hill.ribution of pyroclastic rocks along the NW-SE-trending dyke, exposed in the 71 Gulchge. Arrows point to the NW-SE-trending dyke. White circles represent major upward-lastic breccia (Pb) (e.g. tuff breccia), lapilli tuff units inferred to be vent/upper conduit/rite formed. Continuous black line marks the extent of the scoriaceous lapilli units (S)t, marked by dashed black lines. White dotted line marks the contact between lapilli tuffnt along the crater rim. E) Oblique view of the NW hillside toward north. White arrowline represents the contact between pre-volcanic siliciclastic sediments and pyroclasticnding dyke.

32 K. Németh, C.M. White / Journal of Volcanology and Geothermal Research 183 (2009) 30–41

White, 2006; McClintock et al., 2008). Peperite has been commonlydescribed as being either blocky or globular. In some cases theproperties of the host sediment have been inferred to control thismorphology (Busby-Spera and White, 1987); however, other factorshave also been identified (Doyle and McPhie, 2000; Martin andNémeth, 2007).

In this paper the description of peperites at 71 Gulch Volcanodemonstrates some of the pitfalls that can occur when these rocks areused to draw broader conclusions about the paleoenvironment. Weemphasize that an understanding of the precise stratigraphic,volcanic–facies, and host–rock relationships is necessary to interpretaccurately the environment in which a particular peperite formed.Such work requires detailed volcanic facies analyses, which can onlybe performed in eroded volcanic landforms such as the subject of thiswork. However, even in eroded volcanoes it is not always an easymatter to distinguish pyroclastic successions as having formed withinthe conduit or vent, or as crater filling deposits, or as part of aconstructional edifice (in this report, the term “conduit” refers to thevolcanic and nonvolcanic debris filling the pipe-like structure of avolcano which connects through an aperture (vent) to the crater ofthe volcano). The problem is even more pronounced when peperite isidentified. Although peperites are commonly cited as evidence for awet eruptive environment, where magma invaded water-saturatedsediments, the presence of these textures does not necessarily prove

Fig. 2. A) Overview of the NW-hill looking to the north. Abbreviations: Ds — dyke intrudes inSc — scoriaceous lapilli capping unit; C — crater rim succession. B) Overview of the SE-hill, dC) The exposed volcanic facies of the NW-hill is consistent with a facies distribution of an eroits crater. The crater rim with phreatomagmatic pyroclastic succession (Ph) is partially presealso intrude into a volcanic upper conduit/vent/crater filling pyroclastic succession preservewater-filled crater of the former volcano (P). Pillowed contact of lava and sediment is markethe NW-SE-trending dyke in the SE-hill (in foreground). In the continuation of the NW-SE-trwell below the preserved upper conduit/vent/crater zone of the 71 Gulch Volcano. D refers tsedimentary host.

the presence of any standing water around the volcano or even thatthe region had an exceptionally high ground water level. Wet eruptiveenvironments are equally likely to represent locally water-saturatedzones in the sub-surface sediments, either siliciclastic or volcaniclas-tic, into which the magma intruded. Such a scenario is likely to becommon at phreatomagmatic volcanoes where the newly openedvolcanic conduit, vent and crater become filled with a slurry of water-saturated tephra.

2. Geological setting

The Snake River Plain (SRP) volcanic province extends from thesouthwestern corner of Oregon, northeastward across the state ofIdaho, to the Yellowstone Plateau in Wyoming (Fig. 1). The region hasbeen the site of voluminous bimodal volcanism during the last16 million years, and its origin has been attributed to the passage ofNorth America over the Yellowstone hotspot (Pierce and Morgan,1992). In contrast, the western SRP (WSRP) is a northwest trendingextensional rift basin that intersects the main SRP–Yellowstone trendnear the city of Twin Falls, Idaho. The basin is 300 km long and about70 km wide, and is filled with a 1–2 km thick sequence of fluvial andlacustrine siliciclastic sediments and lesser volumes of volcanic rocks(Wood, 1994). There is general agreement that subsidence in theWSRP began between 12 and 10 Ma and a rapid increase in the rate of

to siliciclastic sediment; Dpx — dyke intrudes into pyroclastic breccia; Pl — pillow lava;ominated by similar scoriaceous pyroclastic rock units forming the top of the NW-hill.ded phreatomagmatic volcano with scoria (Sc) cone construct and pillowed lava lake inrved. The NW-SE-trending dyke (Ds) intrudes into the host sediment (S). Feeder dykesd in the center part of the NW-hill (Dpx). Pillow lava inferred to be accumulated in thed by Ps. D) Overview of the 71Gulch Volcano toward the NW. Note the dyke buds alongending dyke, the dyke clearly intrudes into the pre-volcanic host sediment, and exposedo the dyke locations in volcanic host, while Ds refers to the locationwhere dyke intrude


widening was coincident with the shift from silicic to mafic volcanismin southwestern Idaho (Mabey, 1982; Bonnichsen and Godchaux,2002; Wood and Clemens, 2002). As it subsided, the WSRP wasoccupied by a succession of freshwater lakes, the youngest of which isknown as Lake Idaho (Malde and Powers, 1962). It has been suggestedthat as the North American continent migrated over the Yellowstonehotspot, the topographic divide shifted eastward, and the capturedrunoff raised the level of Lake Idaho, causing it to reach its spill pointaround 4 million years ago (Wood and Clemens, 2002). Down cuttingof the outlet progressively lowered lake levels during the next2 million years and it appears likely that the lake had largely dis-appeared by about 1.6 Ma (Othberg, 1994).

Sporadic episodes of basaltic volcanism produced a wide variety ofhydrovolcanic features in the WSRP during the last 10 million years.Hyaloclastites, pillow deltas, invasive flows, debris flows, tuff cones,tuff rings andmaars are all well represented in deposits and landformsassociated with the more than 400 volcanoes and eruptive points thathave been recognized in this region (Godchaux et al., 1992;Bonnichsen and Godchaux, 2002; Brand and White, 2007). Althougha general understanding of the WSRP lake systems is based largely onsedimentological and paleontological studies (Kimmel, 1982; Repen-ning et al., 1994), paleoenvironments in some areas have been inferredfrom the nature of phreatomagmatic deposits at specific volcaniccenters (Godchaux et al., 1992; Brand and White, 2007). Of particularinterest is the great variety of hydrovolcanic features produced by thebasalt volcanism that occurred between about 2.2 and 0.7 Ma, duringand following the draining of Lake Idaho. Because the nature andtiming of this transition from a basin-wide lacustrine environment tothe present high desert remains poorly understood, it is important tore-evaluate the use of peperites and related phreatomagmaticdeposits as regional paleoenvironmental indicators.

Fig. 3. A) Pyroclastic units top the pre-volcanic siliciclastic units in the NWedge of the NW-hoverlain by a base surge and phreatomagmatic fall unit (Bs/Pf) and capped by a lava spatter dcapping lava spatter-rich unit contains large (meter-size) irregular but fluidal shape lava sp

3. 71 Gulch Volcano and its tuff ring units

3.1. Description

The 71GulchVolcano is a kilometer longbyup to 200mwide zoneofpyroclastic and coherentmagmatic rocks traceable in 3 separate NW-SEelongate exposures (Figs. 1B, C, D and 2). The magmatic rocks containsmall phenocrysts of olivine and plagioclase, and chemical analysisindicates they are tholeiitic ferrobasalt. A single whole rock K–Aranalysis obtained for this study (at the Hungarian Academy of Sciences,Institute of Nuclear Research, Debrecen applying method described inBalogh andNémeth, 2005) yielded a date of 4.17±0.72Ma; however, allof the magmatic rocks contain small fragments of siliciclastic sedimentandadditional datingwill be requiredbefore theage of this center canbeestimated with confidence.

The area is dissected by a seasonal water course that runs NWtoward a tributary of the Snake River, and most of the volcanic rocksare exposed on the sides of hills that rise up to 70 m above this drywash (Fig. 2C). Neighbouring hills, some of which are higher than thehilltops hosting the volcanic exposures, consist entirely of the samesiliciclastic sediments mapped previously as undifferentiated IdahoGroup Sediments (Jenks et al., 1993, 1998) that underlie the volcanicrocks (Fig. 2A and B). At the NWend of the volcanic zone the exposedpyroclastic rocks consist of a diverse assemblage of tuffs and lapillituffs and are structurally complex (Figs. 1D, E and 2A, C). In the SE partof the zone, the volcanic rocks are dominated by units of coarse-grained scoriaceous lapilli (Fig. 2B and D). Dykes and plugs are presentthroughout the volcanic zone, commonly cross-cutting each other.

Exposures on theNWhill consist of a complex pyroclastic successionthat overlies siliciclastic sediments with what appears to be aconformable contact (Fig. 3A). The contact occurs about half way up

ill. The succession is inferred to be part of a crater rimwith a basal scoriaceous lapilli tuffominated unit. B) Close up view of the base surge and phreatomagmatic fall unit. C) Theatters. The hammer in each picture is 40 cm long.


the hillside (Figs. 1D, E and 2A, C) and bedding in both sequences dipsvery gently to the NW. Loading structures are present in a few placesalong the contact; however, no invasive sand or silt has been identifiedin the base of the pyroclastic units. The lowest unit in the pyroclasticsuccession is a coarse grained, normally graded sequence of scoriaceouslapilli tuff (Fig. 3A). It has a maximum exposed thickness of 20 m andappears to underlie the entire NE side of the hill (Fig. 2A and C). Itsthickness decreases to only about 7mwithin a hundredmeters from thecenter of the hilltop exposure (Fig. 3A). The texture of this lapilli tuffgrades from matrix-supported to framework-supported upwardthrough the unit (Fig. 3A). Although the juvenile clasts have varyingdegrees of vesicularity, the majority of them consist of moderately tohighly vesicular, angular black scoria. Ribbon-like bombs up to 25 cm indiameter are present throughout the unit. No accidental clasts wererecognized, but the matrix does contain small flakes of muscovite inaddition to abundant shards of palagonitized volcanic glass. As they aretraced farther away from the central part of the exposure area, thepyroclastic rocks tend to become finer grained and the irregularlyshapedfluidal bombs and blocks are less abundant. This unit is similar tothe scoriaceous tephras exposed in the SE hills.

The lower unit exposed on the NW hill is overlain by a sequence ofmatrix supported, generally fine grained, unsorted,well bedded tuff andlapilli tuff in which cross-bedding is a characteristic feature (Fig. 3Aand B). Like the lower unit, this unit also thins away from the center ofthe NW hillside (Fig. 2C). Long wavelength (2 m) and low amplitude(dm-scale) dunes aswell as tabular cross-bedding aremost dominant inthe central part of this unit. Inter-beds of cm-to-dm thick, coarsergrained,matrix poor, scoriaceous, unsorted lapilli beds are common, andcan be traced for the entire length of the exposure (Fig. 3B). The unit is

Fig. 4. A) Pillow lava is exposed over siliciclastic sediments in the NW-hill. Hammer is 40 cmHammer in circle is 40 cm long. C) Pillow lava with small amount of hyaloclastitic matrix ipyroclastic breccia (arrow).

rich in blocky, non-to-moderately vesicular volcanic glass shards andcontains finely dispersed flakes of muscovite reflecting the Mesozoicgranitoid source of the sand. Larger lapilli (cm-size) are rare in the basaland middle part of this unit, but where present they form clast trains orlenses a few dm in length. Accretionary lapilli (rim-type) are moreabundant in the upper part of the unit, and thefine-grained tuff beds aremore commonly coupled with thin (up to 5 cm thick) layers of coarse-grained, unsorted scoria lapilli (Fig. 3C). Blocks and bombs become veryabundant at the top of the section. Most of these are vesicular and havefluidal shapes with chilled margins; they are commonly coated withmud and contain small inclusions of the siliciclastic sediment. Typically,the blocks donot appear to have produced impact sags in the underlyingfine-grained tuffs (Fig. 3C). The bombs and blocks increase in numberinward toward the central exposures at the top of the NW hill and theproportion of fine-grained bedded material decreases. The smoothrounded exposures that make up the highest areas on the NW hill aredominated by coarse-grained scoriaceous lapilli tuff similar to the lapillituff at the base of the pyroclastic section.

In the central part of the NW hillside at an elevation about 20 mbelow the scoriaceous lapilli-dominated hill tops is an exposure ofsub-horizontally bedded pillow lavas (Fig. 2A and C). The pillow lavasappear to be connected to a more coherent magmatic body thatextends downward at least as far as the contact between the basalsuccession of scoriaceous tephras and the underlying siliciclasticsediments. At the edge of its exposure area, about 50 m laterally fromthe tephra-sediment contact, the pillow lavas lie directly on thesiliciclastic sediments (Fig. 4A). At the contact, the lavas consist of apile of flattened pillows with siliciclastic sediment filling the spacesbetween the pillows (Fig. 4A). In the upper part of the approximately

long. B) Symmetric shape pillows with multiple rims in the center part of the NW-hill.n the SE-hill top section. D) Closely packed pillows in the top part of the SE-hills with


2-m-thick section of lavas, the pillows are more closely packed andmore cylindrical in shape (Fig. 4B). A similar section of pillowed lavasis exposed in the SE hills where it is closely associated with beds ofscoriaceous lapilli (Fig. 4C and D); however, the lower contact be-tween the pillow lavas and the underlying sediments is not exposed inthat area.

The NW hill also is the primary location where shallow intrusivebodies can be observed. Just below the pillowed lava units, and at least30mbelowtheprojected contact between the scoriaceous lapilli and thesiliciclastic sediment, coherent magmatic bodies form lobate intrusionsthat cut the pre-volcanic siliciclastic sedimentary units (Fig. 5A). How-ever, near the center of the hillside exposure, at about the sameelevation, dykes also intrude a vertically zoned complex of pyroclasticrocks (Fig. 5B). At least 2 pyroclastic units can be distinguished in thiszone. The outermost unit is a dark colored, scoriaceous lapilli rich tuffthat is similar to, but finer grained than, the tephras exposed at the baseof the pyroclastic succession (Px1 on Fig. 5B). Bedding in this unit isvariable, but generally dips inward toward the center of the hill. Thesecond unit has an irregular but steeply dipping contact with the firstunit and is composed of matrix-supported, poorly sorted, yellowishlapilli tuff and tuff breccia (Px2 on Fig. 5B). This unit is rich in bombs andblocks aswell as numerous chilled, blocky, angular basalt lapilli (Fig. 5B).Its matrix consists of volcanic glass shards as well as quartz grains andflakes of muscovite. Exposures of this unit along the side of the NW hillsuggest itfills a funnel shaped depression cut into the tephras of thefirstunit and the underlying sediments (Fig. 2A and C). Irregularly shaped

Fig. 5. A) Dyke bud in the NW-hill intruded into the host siliclastic deposits and formedglobular peperite. Hammer is in the circle 40 cm long. B) Feeder dykes in the center partof the NW-hill intruded into scoriaceous steeply inward dipping pyroclastic units (Px1).Feeder dykes with globular and blocky peperite intruded into pyroclastic breccia (Px2)toward the interior of the hill form chaotic mixture of coherent and clastic volcanicrocks. Unlabeled arrow points to a large sediment lump.

bodies of coherent basalt appear to “float” within this unit (Fig. 5B).These bodies have chilled borders and many of them enclose dm-sizedfragments of sandstone and siltstone similar to the pre-volcanic sedi-mentary rocks.

3.2. Interpretation

The nature of the pyroclastic rocks and the 3D geometry of theirexposures indicate that they are part of an eroded phreatomagmatic tuffring complex. The conformable lower contacts displayed by thepyroclastic units at the NW end of the exposure area suggest theywere deposited directly on the syn-volcanic siliciclastic sedimentsurface. The scoria-rich lower unit in this succession is interpreted asthe basal part of a tephra/tuff ring rim. Because it is dominated byscoriaceous lapilli and contains a relatively small volume of fine ash, thisunit resulted from predominantly magmatic explosive eruptions. Thenormal grading suggests that the unit was deposited by pyroclastic falland accumulated on a sub-horizontal depositional surface. Thedominance of juvenile scoriaceous lapilli suggests that eruptions tookplace through an open conduit and were Strombolian in style. The up-section change in facies to a more matrix-supported, finer grained andvolcanic glass shard dominated tephra with dune beds and scour fillingstructures indicates a shift to phreatomagmatic explosive eruptions anda depositional environment dominated by base surges (Chough andSohn, 1990; White, 1991; Németh et al., 2001; Dellino et al., 2004;Vazquez and Ort, 2006). The non-systematic bedding noted at the baseof the upper unit suggests unstable conditions at the fragmentationsites, possibly the result of the sudden change from magmatic tophreatomagmatic fragmentation in the initial stages of this eruptivephase (Houghton and Schmincke, 1989; Houghton et al., 1999). Thelarger proportion of muscovite flakes, quartz grains and country rockfragments in this succession suggests that the vent was either partiallyclosed or that blocks of poorly consolidated, water-saturated sedimentcollapsed off the conduit wall into the fragmentation site. Alternatively,the eruption quarried downward and its locuswaswithin themuscovitebearing sand. The presence of more well bedded couplets of alternatingtuff and coarse lapilli at the top of this unit is consistent with morestabile process in the conduit/vent zone producing individual separateevents depositing bed texturally similar bed couplets (i.e. repeatedsimilar but separate events, each producing one couplet). The fine-grained beds are inferred to be deposited by base surges and the coarselapilli layers are the result of phreatomagmatic fall (e.g. Dellino and LaVolpe, 1995, 2000; Dellino, 2000). The large oxidized (e.g. red) spatter-like bombs and blocks with chilled rims, vesicular cores and fluidalshapes (Fig. 3C) suggest that small amount of magmawere fragmentedby magmatically. The general lack of bedding sags beneath these largeclasts suggests that theywere not deposited by direct ballistic impact onthe freshly deposited tephra nor were they originally part of thepyroclastic density current. Instead, it is likely that they fell into activelydepositing pyroclastic density currents and were then transported ashort distance from the impact site (Valentine et al., 2000).

The geometry of deposits in the central part of the pyroclasticsuccession in the NW hillside is consistent with it being an exhumedcrater deposit. The outer pyroclastic facies with steeply inwarddipping bedding surfaces is interpreted as fill deposited on the innerflank of the crater or upper conduit. The scoriaceous fragments inthese deposits suggest a possible facies relationship with the basaldeposits of the sub-horizontal pyroclastic sequence that accumulatedon the syn-eruptive surface. It is inferred that this facies is part of aninitial scoria cone that formed during the early stages of the eruption.

The yellow lapilli tuff and tuff breccia forming a crater/upperconduit-filling deposit that accumulated inside the growing crater of aphreatomagmatic volcano. These deposits were subsequently invadedby dykes (e.g. Valentine and Groves, 1996) that are interpreted to bethe feeders for the lavas that were extruded into the crater. The pillowforms produced by these lavas suggest there was water or muddy


slurry inside the crater, possibly a shallow crater lake that filled thecrater shortly after the onset of the eruption. The exposed contactbetween pillowed lava and the siliciclastic sediments indicates thatthe sediments were wet and may have been covered by shallowstanding water. The dominance of the scoriaceous successions in theSE hillsides suggests that they were produced by the same eruptionsthat formed the similar deposits exposed on the NW hill. The generallack of fine-grained phreatomagmatic tephras in the SE hillsidessuggests that they were either eroded away or were never depositedthere. If the latter interpretation is correct, then the best reconstruc-tion for the origin of these deposits would be that fissure eruptionsalong a NW-SE-trending dyke swarm initially formed a chain of smallscoria cones and spatter deposits. After the establishment of severalstable eruptive points along this fissure, the NW cones shifted to aphreatomagmatic eruption style and built a tuff ring on top of theinitial Strombolian deposits. This shift could have taken place due to adrop in the magma supply rate, allowing direct contact betweenmagma and water in the later stage of the eruption (Houghton et al.,1999). This would imply some access by free water to the vent site;however, the bedding features of the pyroclastic rocks indicate theywere deposited subaerially, indicating that if standing water werepresent during the eruption, it must have been confined to the vent/crater area.

Fig. 6. A) Globular peperite along a feeder dyke (D) intruded into the host siliciclastic units (the siliciclastic sediments (Sed) in the NW-hill. Pen is 15 cm long. C) Dispersed peperitic zoneis 40 cm long. D) Fluidization pipe (arrow) in the host sediment in the NW-hill, just abofluidization pipe structure. Pen is 15 cm long.

4. Intra-vent peperite

4.1. Description

Two types of intrusive contacts can be identified about half way upthe NW hill; (1) contacts between coherent magmatic bodies and thesub-horizontally bedded siliciclastic sediments that lie beneath theinferred tephra ring pyroclastic succession (Fig. 5A), and (2) contactsbetween coherent magmatic bodies and yellowish, massive, unsortedlapilli tuffs and tuff breccias that appear to fill a funnel shapeddepression in the central part of the hill (Fig. 5B). The intrusions cuttingthe siliciclastic units are located about one third of theway up the side ofthe NW hill, at a stratigraphic level that is well below the contact withthe pyroclastic units (Fig. 2A and C). The dykes have a pronounced NW-SE strike, and typical exposures are somewhat bud-like in appearance(Fig. 1B and C). Individual dyke buds are 5 to 10 m wide and havestrongly chilledmarginswithdm-scale protrusions (Fig. 6A). The tops ofthe buds fork into irregularly shaped, somewhatmore conformable, sill-like forms (Fig. 6A). Where it is in direct contact with the sediment, thesurface of the chilled basalt has a globular geometry on a scale of cm tomm (Fig. 6B). Bedding structures in the host siliciclastic sedimentsdisplay various degrees of disruption in these exposures. Where theywere pervasively invaded by the basalt, all of the bedding has been

S) in the NW-hill. B) Globular margin of the strongly chilled dyke margin intruded intowith fluidized zones (arrow) of the host siliciclastic sediments in the NW-hill. Hammer

ve a dyke bud. Note the dispersed peperite (black angular lava fragments) below the


destroyed. Larger screens of sediment between dykes preserve theiroriginal sub-horizontal bedding texture to some degree; however, thesedomains are irregular in shape and the transitions between areas ofcompletely homogenized sediment and areas of preserved bedding aregradual. In many places micas in the siliciclastic sediments appear to bealigned parallel to the contactswith the intrusions. Globular and angularshaped fragments of coherent basalt up to few dm in size are concen-trated next to many of the contacts, giving the impression that clots ofmagma had detached from the main part of the intrusions (Fig. 6C).These fragments become smaller at greater distances from the contactswhere fine lapilli or ash sized angular basalt clasts are commonlymixedwith the siliciclastic sediment. Pipe-like structures occur within thesediment overlying the dyke buds at some locations. These pipes aregenerally dm tometers in length and contain clast trains similar to thosein elutriation pipes caused by fluidization (Fig. 6D). The pipes are morestrongly cemented than the surrounding sediment and are thereforemore resistant to erosion.

Fig. 7. A) Dyke intrudes into coarse-grained tuff breccia in the NW-hill. The dyke margin is ifluidization textures results from its mobilisation by the intruding dyke. B) Along the dykcommonly mantled with thin (mm–cm thick) sand/mud coats suggesting sediment movemPencil is in the circle 15 cm long. C) Blocky peperite in the host tuff breccia of the NW-hill. Tmud and sand (arrow). Hammer in the circle is 40 cm long.

The contacts between the coherent magmatic bodies and the lapillituff and tuff breccia typically have irregular geometries (Fig. 7A),although the exact location of a contact can be hard to define becausethe large number of angular basalt lapilli in the tuff breccia are verysimilar in appearance to fragments derived from the disruptedsurfaces of the intrusive bodies. However, within a few dm of thecontacts the matrix of the breccia appears more homogenized and it isslightly lighter in color, due to the mixing in of grains carried up alongthe margins of the dykes from the siliciclastic sediments just below.The chilled surfaces of the dykes are commonly coated with a film-likelayer of fine mud, and both the dykes and the larger magmatic bodiesin the tuff breccia commonly contain enclaves of homogenizedsiliciclastic sediments (Fig. 7B and C). In addition, protrusions andsheets of the homogenized fine-grained matrix of the yellow tuffbreccia cut through the fragmented zones of magmatic bodies in amanner similar to clastic dykes. The basalt dykes split into fork-likeapophyses as they are traced upward through the tuff breccia. The

rregular. In the right hand side of the dyke, a large siliciclastic sediment fragment withe/tuff breccia margin in the NW-hill, larger chilled coherent magmatic fragments areent along the intruding dyke due to the boiling effect of the dyke on the host sediments.he closely packed blocky peperite commonly encloses large clasts of strongly fluidized


main branches of the dykes are traceable upward into the stack ofpillow lavas at the top of the central zone of the NW hillside.

The complex mix of peperites and tuff breccias described abovewas not found in the exposures on the SE hill. However, the SE hilldoes contain outcrops at a few locations where basalt dykes intrude ayellowish lapilli tuff/tuff breccia similar to the unit that forms thefunnel shaped deposit on the NW hill. The dykes in the SE haveglobular margins (locally truly pillow-like) and commonly containcm-to-dm-sized enclaves of white siliciclastic sediment. The pyro-clastic deposits are homogenized in the zones adjacent to the dykecontacts and display a weak reddish discoloration.

4.2. Interpretation

The textures along the contact zones between the basaltic dykes andtheir host rocks indicate they are peperites formed by the in situfragmentation of coherent magmatic bodies that were emplaced intowet sediments (White et al., 2000; Skilling et al., 2002). At each locationthe dykes can be identified as sub-vertical coherent bodies of magmathat invaded and homogenized the host sediment. The majority of thepeperite has a globular texture, which occurs at scales ranging fromdecimeters to millimeters. At the meter scale, however, basalt clasts inthe contact zones have a more blocky texture. This is especially truewhere the intrusions cut the coarser-grained lapilli tuff/tuff breccia.

The fragmentation of the magma and its mixing with the hostsediments are interpreted to be the result of the tearing apart of magmaat the intrusion–sediment interface by shear stress and its shaping intoglobular, pillow-like bodies (Doyle, 2000). Blocky peperite is inferred toform due to phreatomagmatic reactions and/or quench fragmentationin the course of the breakdown of an insulating vapor film at thesediment–magma interface (White,1996). Globular peperite is believedto represent styles of coarse mixing during fuel-coolant-interactions(FCIs) between the magma and the sediment-laden impure coolants inphreatomagmatic settings (White, 1996; Hooten and Ort, 2002). Otherstudies have shown that globular textures predominate when the hostsediment is fine grained (e.g. Busby-Spera and White, 1987). At the 71Gulch exposures, globular textured peperite is indeed more common atevery domain dimension where the dykes intrude the siliciclasticsediment than where they intrude the pyroclastic breccias. Thisdifference is probably a direct result of the fact that the sands and siltsof thepre-volcanic sedimentary rocks arefiner grained andbetter sortedthan the matrix of the pyroclastic breccias.

It is generally understood that FCIs involve the sudden contactbetween a cold, vaporizable liquid (coolant) and a hot liquid (fuel) (e.g.Wohletz, 1986). This interaction begins by converting some liquidcoolant along the contact to vapor, producing a stable and isolatingvapor-film which serves to insulate the fuel from the surroundingcoolant (White, 1996). In magma–water systems, this allows themagma, at least initially, to develop complex globular forms withoutundergoing brittle quench fragmentation, commonly referred as “bulkinteraction phase” (Wohletz, 1986; Zimanowski et al., 1995; White,1996; Zimanowski et al., 1998). Busby-Spera and White (1987)suggested that the absence of fluidal peperite in coarse-grainedsediment in their study area was due partly to the relatively highpermeability of the host, which allowed rapid escape of heated porefluids and thereby inhibited formation of stable, insulating vapor filmsaround intruding magma bodies. Such direct grain-size effects wereinterpreted to be the main control on formation of blocky vs. fluidal(globular) peperite from a single intrusion, reflecting the fact thatcoarser-grained sedimentary particles cannot be entrained in thestreaming vapor films surrounding intrusive bodies (Busby-Spera andWhite, 1987; White, 1996). This process was demonstrated to be morecomplex where magma intrudes host sediments that have stronglyheterogeneous textures, such as many pyroclastic rocks (Martin andNémeth, 2007). Volcanic sediments, especially in vent/conduit-fillingsettings, are usually very poorly sorted and commonly have a great

variety of clast sizes and shapes in their larger grain-size fractions.Moreover, the different densities and thermodynamic properties of thedifferent types of clasts in these deposits can further expand the range ofinteractions that can take place between intrusions and the hostpyroclastic sediments.Magma fragmentation andmixingwith sedimentare generally believed to be related to complex intra-vent/conduitprocesses especially in small-volume phreatomagmatic volcanoes(Hooten and Ort, 2002; Martin and Németh, 2007). Dynamic processessuch as (1) tearing apart of the magma and shaping of the magma/sediment interface into globular geometries (e.g. fluidization orliquefaction) (Zimanowski and Buttner, 2002), (2) mechanical force ofintrusion of the magma, and non-dynamic characteristics of the hostsuch as (1) framework clast-matrix ratio of the host sediment and (2)water content of the host-sediment together are responsible for theformation of complex peperite in the intra-vent/conduit zone of aphreatomagmatic volcano.

Blocky peperite results from quench fragmentation of the magmaand/or mild phreatomagmatic reactions due to the break-down of theinsulating vapor films at themagma/wet-sediment interface. However,the texture may form upon contact betweenmagma and host sedimentwhen there is a sudden or gradual change in magma viscosity and/orflux rate during the eruption, as a non-steady process (Zimanowski andButtner, 2002). Low-viscosity magma, such as basalt, may favor theformation of globular peperite, butwhen their viscosity increases due tocooling or crystallization, it may instead form blocky peperite uponcooling across the brittle/ductile transition.

Considering the small 71 Gulch Volcano, the possibility that sub-stantial chemical or physical changes took place in the magma duringits intrusion can be ruled out for the following reasons: (1) the 3Ddistribution of the coherent intrusive bodies is very small (e.g. theyprobably represent the same eruptive event); 2) the basalt of theintrusions is nearly identical in its micro and macro texturalcharacteristics; and 3) from the field relations, the timeframe of theeruption can be inferred to be very short (days to weeks). Therefore,we attribute the textural differences observed in the peperites at 71Gulch Volcano to be a direct result of the textural differences of thehost sediments and the degree to which they were saturated withwater.

Pipe-like features in the fine-grained siliciclastic host sedimentsabove the major dyke buds indicate a strong fluidization effect due tothe heat the dyke produced (Skilling et al., 2002). The heat probablyboiled the pore water in the host sediment which effectively fluidizedor liquefied the sediment in a manner similar to the elutriation pro-cesses described from classic sedimentology (Massari et al., 2001) andvolcanology (Ross and White, 2005). Boiled pore water movedupward toward the surface carrying small, commonly angularfragments from the disaggregating dykes. Other evidence for strongfluidization in the host siliciclastic sediments includes the completelyhomogenized halo-like cm-to-dmwide zone along the dykemargins afeature which has been documented elsewhere (Martin and Németh,2004). Perhaps the sediment protrusions deep into the fragmentedmargin of the dykes are further evidence of mobility of the fluidizedsediment.

5. Peperitic dykes outside of the vent zone

5.1. Description

About 300 m from the NW hill, a road cut exposes a 1–2 m widemafic dyke which intrudes the pre-volcanic siliciclastic sediments(Fig. 8). The siliciclastic sediments have the same texture, compositionand bedding characteristics as the sediments cut by the funnel shapeddeposits on the NW-hill and which lie beneath the pyroclastic rocks atthat location. Also like those sediments, the white to light grey, wellbedded, fossiliferous sands and silts in the road cut are only semi-consolidated. The dyke is black in color and more vesicular in its

Fig. 8. Irregular bulbous margin of the NW-SE-trending dyke's NW segment. The dykecuts through the siliciclastic host sediment. This site represents a region about 20 mbelow the other dykes exposed in the NW-hill. Dotted white lines mark the stronglyhomogenized zone (e.g. no original bedding preserved) of the host sediment.


center than it is at its glassymargin. Flow foliation is apparent near thecontacts but diminishes toward the center of the dyke (Fig. 8). At thislocation, the dyke has a general E–W strike and a nearly vertical dip.The contact surface of the dyke is undulatory, with cm-to-dmprotrusions that intrude into the host sediment (Fig. 8). At the mm-to-cm scale, the dyke margin is globular or bulbous, and detached

Fig. 9. A diagrammatic representation of the original volcanic facies architecture of the 71 Gacross the volcano indicates the potential water cover during and after the eruption.

fragments of glassy basalt are dispersed through the homogenized,fluidized sediment host within about 30 cm of the contact. Thedispersed magmatic fragments are angular and entirely glassy, withonly limited alteration of their outer surface. The original bedding ofthe host sediment is completely disrupted for a distance of about ameter away from the dyke margin. Micas and other types of platyfragments in the sediment adjacent to the dyke are oriented parallel tothe contact.

5.2. Interpretation

The location, orientation and appearance of the dyke exposed inthe road cut indicate that it is part of themain NW-SE-trendingmasterdyke system that cuts through the 71 Gulch Volcano andwas probablythe feeder for the eruptions that took place there. The exposed sectionis at about the same stratigraphic level as the section of pre-volcanicsediment intruded by dykes at the NW hill. The wide chilled zonearound the dyke margin indicates sudden cooling upon contact withthe host sediment. The destroyed original texture of the host sedimentindicates it was poorly consolidated and water saturated when thedykes were emplaced; however, the generally coherent nature of thedyke and the limited production and distribution of detachedfragments suggests the sediment was probably not completelyunconsolidated at that time. The globular texture of the peperitealong the contact indicates ductile deformation of the dyke andmixing of the basalt with the adjacent sediments. However, theangularity of the dispersed peperite zone's glassy volcanic fragmentssuggests brittle deformation also took place, possibly due to themechanical stress the dyke emplacement caused through the semi-consolidated and moderately water-saturated host sediment.

6. Discussion

The diverse suite of volcanic rocks identified in the 71 GulchVolcano was produced by both explosive and non-explosive fragmen-tation of the rising magma as well as pure effusion through feederdykes. The basal scoriaceous tephras that occur throughout the NW-SE trending zone are interpreted to be part of a flat lying pyroclasticpile produced by the initial eruptions along an approximately 4 km

ulch Volcano. Dashed-dotted line represents the present day surface. Dashed fine line


long fissure (Fig. 9). Fragmentation that produced these tephras wasmagmatic; however, the eruption then switched to a dominantlyphreatomagmatic process driven by the interaction of the magmawith external water (Fig. 9). The phreatomagmatic phase producedthe flat lying base surge deposits and the phreatomagmatic falldominated units exposed in the NW hill (Fig. 2). We interpret thecircular geometry of the breccias exposed in the central part of this hillas indicating that these deposits formedwithin the upper conduit andlower crater of the volcano (Fig. 9). Dykes invaded these crater fillingdeposits as well as the siliciclastic sediments of the conduit walls. The“master” feeder dyke eventually fed effusive eruptions that producedthe stack of pillow lavas that accumulated on the floor of the crater.The volcanism followed a NW-SE-trending fissure along individualvent sites developed, each invaded by dykes and sills interacted withthe host sediment as well as the crater/conduit-filling slurry. Thelateral migration of vent sites is inferred to be the result of the water-saturated soft-substrate the feeder dykes encountered along a fissuresimilar to those described from Korea (Sohn and Park, 2005) orHungary (Auer et al., 2007).

The enclaves of fluidized siliciclastic sediment within the complexof large detached dykes that intrudes the pyroclastic crater fill attest todynamic mixing within the active vent/conduit system at thisvolcano. The textures of the rocks in these exposures provide strongevidence that both the siliciclastic sediments and the volcanic brecciaswithin and adjacent to the vent/conduit were at least partiallysaturated with water (e.g., a slurry) at the time the dykes wereintruded (“slurry dyke” was previously used as mapping term todescribe such dykes in the WSRPVF; Godchaux, 2007 pers. com.). Thepresence of pillowed lavas above the peperitic domains indicates thatsome free water also had to be present on the crater floor.

The textural characteristics of the tephra ring facies are consistentwith subaerial deposition. The areal distribution and 3D geometry ofthese deposits suggest that standing water, if present at all, must havebeen shallow enough to be pierced by the eruption columns, and thatthe accumulating tephra pile quickly emerged above the surface.However, the water content of the pre-volcanic sediments must havebeen great enough to cause the non-explosive fragmentation of thedykes and the formation of themostly globular peperite (Fig. 9). At thesame time, phreatomagmatic explosions would have gradually filledthe expanding crater with wet pyroclastic debris composed of amixture of juvenile particles and recycled, water-saturated siliciclasticsediment (e.g. Valentine and Groves, 1996) (Fig. 9). This sequence ofevents inferred for the 71 Gulch Volcano may have importantimplications for interpreting peperite facies at other eruptive centerswithin the western Snake River Plain. In eroded volcanoes where thecrater floor to upper conduit zones are exposed, the identification ofpeperite may not necessarily mean that the eruptive environment wassubaqueous. Instead, some phreatomagmatic volcanoes may create anextremely localized, water saturated, sedimentary environmentwithin their crater and upper conduit which mimics the conditionsof a more widespread subaqueous environment. Ongoing volcano-seismic events also can enhance free water separation from the craterfilling slurry. The crater may therefore even host small bodies ofstanding water capable of producing pillow forms in lavas; therefore,the presence of these features on a local scale would not necessarilyindicate the presence of a shallow lake beyond the crater itself.

7. Conclusion

In this paper we reconstructed the volcanic architecture of astrongly eroded small-volume basaltic volcano from the westernSnake River Plain. Previous workers (Godchaux et al., 1992) suggestedthe volcano erupted beneath a shallow lake. Here we gave sedimen-tological evidence that the peperitic textures observed at this siterepresent interactions between magma and wet sediments thatincluded both pre-volcanic siliciclastics as well as crater/vent and

conduit-filling pyroclastics. Although the eruptive environment of the71 Gulch Volcano must have been wet, the peperitic textures do notrequire that standing water was present prior to the eruption. Thecrater/vent/upper conduit zone of the volcano must have accumu-lated water-saturated debris and probably some free water, but theaffected area would have been very small and did not necessarilyreflect the more general environment outside the crater. Othersedimentary features in the preserved pyroclastic units are moreconsistent with a subaerial magmatic and phreatomagmatic eruption.In this paper we highlighted the potential pitfalls of using peperitetextures to establish the eruptive environment of a whole volcano(Németh et al., 2007). A sedimentological facies analysis is absolutelynecessary to accurately reconstruct the paleoenvironments associatedwith phreatomagmatic volcanism.

Acknowledgements

This research was supported by a Fulbright Foundation VisitingResearcher Fellowship to KN, which helped fund the field work Idahohosted by Boise State University. Various aspects of this research werealso supported in part by the Hungarian Science Foundation (OTKA F043346) and the New Zealand FRST Post-doctoral research grant (KN)(MAUX0405). Helpful reviews by Greg Valentine (SUNNY Buffalo,NY), Martha Godchaux (Idaho Geol. Surv., Moscow, ID), James D.L.White (Otago University, New Zealand) and Gerardo J. Aguirre-Diaz(UNAM, Mexico) are also thankfully acknowledged.

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intra-vent peperites related to the phreatomagmatic 71 gulch volcano, western snake river plain...

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