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Geological Society of America Guide 6 2005

Basaltic volcanism of the central and western Snake : A guide to fi eld relations between Twin Falls and Home,

John W. Shervais Department of , State University, Logan, Utah 84322-4505, USA

John D. Kauffman Idaho Geological Survey, University of Idaho, Moscow, Idaho 83844-3014, USA

Virginia S. Gillerman Idaho Geological Survey, Boise State University, Boise, Idaho 83725-1535, USA

Kurt L. Othberg Idaho Geological Survey, University of Idaho, Moscow, Idaho 83844-3014, USA

Scott K. Vetter Department of Geology, Centenary College, Shreveport, 71134, USA

V. Ruth Hobson Meghan Zarnetske Department of Geology, Utah State University, Logan, Utah 84322-4505, USA

Matthew F. Cooke Scott H. Matthews Department of Geological Sciences, University of South Carolina, Columbia, South Carolina 29208, USA

Barry B. Hanan Department of Geological Sciences, San Diego State University, San Diego, 92182-1020, USA

ABSTRACT

Basaltic volcanism in the Plain of has long been associ- ated with the concept of a plume that was overridden by North America during the Neogene and now resides beneath the Yellowstone . This concept is consistent with the time-transgressive nature of volcanism in the plain, but the history of basaltic volcanism is more complex. In the eastern , erupted after the end of major silicic volcanism. The basalts typically erupt from small shield volcanoes that cover up to 680 km2 and may form elongate fl ows that extend 50–60 km from the central vent. The shields coalesce to form extensive of that mantle the entire width of the plain, with the thickest accumulations of basalt forming an axial

Shervais, J.W., Kauffman, J.D., Gillerman, V.S., Othberg, K.L., Vetter, S.K., Hobson, V.R., Zarnetske, M., Cooke, M.F., Matthews, S.H., and Hanan, B.B., 2005, Basaltic volcanism of the central and western Snake River Plain: A guide to fi eld relations between Twin Falls and Mountain Home, Idaho, in Pederson, J., and Dehler, C.M., eds., Interior Western : Geological Society of America Field Guide 6, p. xxx–xxx, doi: 10.1130/2005.fl d006(02). For permission to copy, contact [email protected]. © 2005 Geological Society of America 1 fl d006-02 page 2 of 26

2 J.W. Shervais et al.

high along the length of the plain. In contrast, basaltic volcanism in the western Snake River Plain formed in two episodes: the fi rst (ca. 7–9 Ma) immediately following the eruption of now exposed along the margins of the plain, and the second forming in the (≤2 Ma), long after active volcanism ceased in the adjacent eastern Snake River Plain. Pleistocene basalts of the western Snake River Plain are intercalated with, or overlie, lacustrine of Pliocene-Pleistocene Idaho, which fi lled the western Snake River Plain graben after the end of the fi rst episode of basaltic volcanism. The contrast in occurrence and chemistry of basalt in the eastern and western plains suggest the interpretation of volcanism in the Snake River Plain is more nuanced than simple models proposed to date.

Keywords: basaltic volcanism, basalt geochemistry, Snake River Plain, Bonneville fl ood.

INTRODUCTION 1992; Malde, 1991). The is currently located under the Yellowstone Plateau, which also forms the locus of a gigantic The Snake River Plain of southern Idaho is one of the most geoid anomaly that underlies much of western North America distinctive physiographic features of North America (Fig. 1). The (Smith and Braile, 1994; Pierce et al., 2002). topographic low that defi nes the plain cuts across the structural The hotspot or plume model for the Snake River Plain is grain of both the Idaho and the Basin and Range prov- supported by studies of tectonic uplift and collapse along the ince—even though formation of the plain coincided in time with plume track (Pierce and Morgan, 1992; and Sleep, 1992; Basin and Range extension. Evolution of the eastern Snake River Smith and Braile, 1994; Rodgers et al., 2002), the 1000-km-wide Plain has been associated with the passage of North America over geoid anomaly centered under Yellowstone (Smith and Braile, a mantle hotspot, forming a time-transgressive volcanic province 1994), seismic tomography of the underlying mantle (Saltzer that youngs from WSW to ENE (e.g., Morgan, 1972; Suppe et and Humphreys, 1997; Jordan et al., 2004), and helium isotopes al., 1975; et al., 1975). The onset of hotspot-related (Craig, 1997). Alternate models have been proposed, however, volcanism is marked by a series of overlapping com- such as localized asthenospheric upwelling associated with edge plexes, , and caldera-fi lling rhyolite lavas (Bonnich- effects of North American plate motion, and counter fl ow created sen, 1982a, 1982b; Bonnichsen and Kauffman, 1987; Pierce and by the descending Farallon slab (Humphreys et al., 2000; Chris- Morgan, 1992; Christiansen et al., 2002). The early rhyolite com- tiansen et al., 2002). plexes were followed by extensive eruptions of plains basalts, There are two aspects of Snake River Plain volcanism that which form a carapace on top of the earlier rhyolites (Malde and are not readily explained by the hotspot model. First, the erup- Powers, 1962; Greeley, 1982; Leeman, 1982; Kuntz et al., 1982, tion of basaltic lavas generally postdates passage of the hotspot in time, and may continue 2–3 m.y. after the onset of hotspot related volcanism farther to the NE. Second, the hotspot model does not explain the origin of volcanic rocks that do not lie on 45º 30’ the presumed hotspot track; in particular, basalts of the western Snake River Plain cannot be directly related to the passage of North America over the , although models W that link volcanism in the western Snake River Plain indirectly es ter to the hotspot have been proposed (Geist and Richards, 1993; n SRP Camp, 1995; Shervais et al., 2002; Camp and Ross, 2004). 2 Basalts of the , thought to represent the “head” MH 3 Eastern SRP of the Yellowstone plume, also lie well north of the presumed 1 hotspot track. Several models have been proposed to explain TF this anomaly, including defl ection of the plume by the Farallon plate (Geist and Richards, 1993), compression of the plume head 41º 59’ 42º 00’ 117º 02’ 111º 05’ by North American lithospheric mantle (Camp, 1995; Camp and Ross, 2004), or location of the plume head below northern Figure 1. Physiographic map of the northwestern United States show- Nevada (Pierce and Morgan, 1992; Pierce et al., 2002). ing the Snake River Plain (SRP) and related features. Note the strong topographic expression of the plume track and the absence of Basin The central Snake River Plain lies at the junction of the and Range extension across the axis of the plain. MH—Mountain physiographic western and eastern Snake River Plain and rep- Home; TF—Twin Falls. resents a critical link between the two volcanic provinces. The fl d006-02 page 3 of 26

Basaltic volcanism of the central and western Snake River Plain 3 structural, geophysical, sedimentary, and volcanic features of Idaho batholith, whereas the eastern Snake River Plain transects these provinces are distinct and require different origins. In this older of the Basin and Range province (Malde, 1991). guidebook, we present a brief synopsis of geologic relations The eastern Snake River Plain is characterized by 1–2 km of within and between the central and western Snake River Plain basalt that overlies rhyolite and welded tuff (e.g., Leeman, 1982; and then examine the stratigraphy and of each prov- Kuntz et al., 1988, 1992; Greeley, 1982). Scientifi c drill holes at ince in the fi eld guide section. A compendium of recent papers on the Idaho National Laboratory (INL) site show that the basaltic the Snake River Plain has recently been published by the Idaho suite ranges from <100 m to over 1500 m thick, with rhyolite Geological Survey (Bonnichsen et al., 2002), and interested read- extending to depths in excess of 3000 m (Malde, 1991; ers are referred to that volume for more detailed information. Hughes et al., 1999, 2002). Sedimentary intercalations consist- ing of fl uvial sands, lacustrine muds, windblown sand, and loess THE SNAKE RIVER VOLCANIC PROVINCE: range from 2 m to ≈25 m thick. GEOLOGIC SETTING The rhyolite eruptive centers consist of overlapping caldera complexes and ignimbrites that represent the initial volcanic The central and western Snake River Plain comprise two dis- activity at any given location along the axis of the Snake River tinct provinces with different crustal structure, stratigraphy, and Plain, and are thought to mark the arrival of the hotspot (e.g., volcanic history. Both provinces are characterized by crust that Bonnichsen, 1982a; Christiansen, 1982; Draper, 1991; Morgan, is thicker (40–45 km) than crust in the adjacent Basin and Range 1992). Rhyolite eruptive centers become younger from SW to province (≈33 km; Mabey, 1978, 1982; Iyer, 1984; Malde, 1991). NE: the Bruneau-Jarbidge eruptive center (12.5–11.3 Ma), the The western Snake River Plain is also characterized by a positive Twin Falls eruptive center (10.0–8.6 Ma) the Picabo eruptive gravity anomaly that trends parallel to the axis of the plain, and center (10.2 Ma), the Heise eruptive center (6.7–4.3 Ma) and the magnetic anomalies that parallel its southern margin (Mabey, Island Park–Yellowstone eruptive center (1.8–0.6 Ma). The oldest 1982). In contrast, gravity and magnetic anomalies in the eastern basalts in the central and eastern Snake River Plain are slightly Snake River Plain are subdued and defi ne a NW-trending texture younger than the eruptive centers they mantle; the youngest that is normal to the trend of the eastern plain, but parallels struc- basalts erupted from a series of NW-trending volcanic zones tural trends in the adjacent Basin and Range province (Mabey, during the Holocene (e.g., the Shoshone fl ow, Craters of the 1982; Malde, 1991). These contrasts refl ect fundamental differ- , the Great Rift, Hells Half Acre, Wapi lava fl ow), with fl ows ences in the underlying structure and stratigraphy of the two ter- as young as 2000 yr B.P. reported (Kuntz et al., 1986). ranes: the western Snake River Plain is a true graben, whereas the Geophysical studies of the eastern Snake River Plain eastern Snake River Plain is structurally downwarped with little have documented differences in both the crustal structure and or no faulting along its margins (McQuarrie and Rodgers, 1998; lithosphere-asthenosphere boundary relative to both the adjacent Wood and Clemens, 2002; Rodgers et al., 2002). Basin and Range province and the Archean craton (Braile et al., 1982; Iyer, 1984; Dueker and Humphreys, 1990, Central and Eastern Snake River Plain 1993; Humphreys and Dueker, 1994; Peng and Humphreys, 1998; Saltzer and Humphreys, 1997; Humphreys et al., 2000). The ENE-trending central and eastern Snake River Plain The most signifi cant crustal feature observed in seismic profi les begins as a gentle structural on the Owyhee Plateau of the eastern Snake River Plain is a midcrustal “” ≈10 km that deepens to the NE and merges with the physiographic Snake thick and 90 km wide that underlies the entire width of the east- River Plain near Twin Falls. This structural depression continues ern Snake River Plain, with seismic velocities (≈6.5 km/s) inter- to the NE until it merges with the Yellowstone Plateau (Rodgers mediate between the mafi c lower crust and the more felsic inter- et al., 2002). The central Snake River Plain is defi ned loosely as mediate crust (Braile et al., 1982; Peng and Humphreys, 1998). that part of the eastern Snake River Plain trend that lies between This mafi c “sill” is inferred to represent basaltic melts that were the Owyhee Plateau and the Great Rift, or approximately between intruded into the crust and either fractionated to form the high 116°W and 114°W along the axis of the plain. temperature rhyolites of the eastern Snake River Plain (McCurry The increase in elevation of the eastern Snake River Plain and Hackett, 1999) or induced of the crust to form from SW to NE is thought to result from thermal buoyancy in these rhyolites (Leeman, 1982; Bonnichsen, 1982a, 1982b; Doe the under the hotspot (Dueker and Humphreys, et al., 1982). The crust is also underlain by a low velocity layer 1990; Pierce and Morgan, 1992; Smith and Braile, 1994). The that is thought to be partially molten (Peng and Humphreys, progressive decay of this thermal anomaly with time has resulted 1998). This low-velocity layer apparently thickens to the NE, in tectonic collapse in the wake of the “deformation parabola” toward Yellowstone, where it dominates the lower crustal section that emanates from the hotspot (Anders and Sleep, 1992; Pierce (Priestley and Orcutt, 1982). and Morgan, 1992; Smith and Braile, 1994). The elevation dif- Teleseismic tomography across the eastern Snake River ference between the Owyhee Plateau and the eastern Snake River Plain and adjacent domains show that the subcontinental litho- Plain probably results from differences in the underlying crust: sphere directly beneath the plain has been eroded to form a deep the Owyhee Plateau is underlain by the southern extension of the channel parallel to North American plate motion (Dueker and fl d006-02 page 4 of 26

4 J.W. Shervais et al.

Humphreys, 1990; Humphreys and Dueker, 1994; Saltzer and White et al., 2002; Shervais et al., 2002). The older (7–9 Ma) Humphreys, 1997; Humphreys et al., 2000). The channel con- lavas and late Miocene to Pliocene sediments comprise the Idaho sists of low velocity, partially molten asthenosphere buttressed by Group of Malde and Powers (1962). Young volcanic activity of high-velocity, melt-depleted mantle. The depth of the (<2.2 Ma) in the western Snake River Plain consists of: (1) pla- lithosphere-asthenosphere boundary beneath the plain is roughly teau forming eruptions of tholeiitic basalt that form the volcanic constant parallel to its axis (Saltzer and Humphreys, 1997). Since uplands north and south of the Snake River, (2) tholeiitic shield the subcontinental mantle lithosphere is thicker toward the NE, volcanoes that sit on top of these uplands, and (3) young shield the sublithospheric channel must be more deeply eroded into the and vents of alkaline to transitional alkaline basalt lithosphere to the NE. (Shervais et al., 2002; White et al., 2002). The younger lavas Tomographic images of the Yellowstone plume show that it comprise the Snake River Group of Malde and Powers (1962) dips steeply to the north from Yellowstone and extends to a depth and correlate with the more abundant young volcanic rocks of of 400–600 km (Montelli et al., 2003; Jordan et al., 2004; Dueker the eastern Snake River Plain (Leeman, 1982). Volcanic rocks et al., 2004). This is consistent with recent models of deep-seated of similar age and character are also found in the mantle plumes, which show that they may tilt signifi cantly from drainage 40 km north of Mountain Home (Howard and Shervais, vertical and follow complex fl ow lines within the mantle (e.g., 1973; Howard et al., 1982; Vetter and Shervais, 1992). King, 2004; Farnetani and Samuel, 2004). The physiographic junction between the western and eastern portions of the plain is located west of Twin Falls, Idaho (Fig. 1). Western Snake River Plain Structurally, boundary faults of the western Snake River Plain graben extend SE into the eastern Snake River Plain plume track In contrast to the eastern Snake River Plain, the western as far as Hagerman, and linears extend as far as Buhl, near Twin Snake River Plain is a NW-trending structural graben, ≈70 km Falls. The axial gravity high of the western Snake River wide and 300 km long, bounded by en echelon normal faults Plain also extends into the plume track, and is defl ected eastward and fi lled with up to 2–3 km of late Miocene through Pliocene south of the Mount Bennett Hills (Mabey, 1976, 1982). The (Wood and Clemens, 2002; Shervais et al., 2002). positive magnetic anomaly along the SW margin of the western Structural relief, based on deep drill holes that intercept base- Snake River Plain curves to the NE where it intersects the plume ment, ranges from 2900 m near Mountain Home to over 3500 m track (Mabey, 1982), possibly outlining the southern margin of on the southwest side of the (Malde, 1991). Sedimentary the Bruneau-Jarbidge eruptive center (Bonnichsen, 1982a). The deposits in the western Snake River Plain range in age from NW-trending gravity and magnetic anomalies, which character- Miocene through Quaternary; these deposits are dominantly ize the eastern Snake River Plain, are absent in this area. lacustrine, with subordinate fl uviatile and phreatomagmatic deposits (Wood and Clemens, 2002; Godchaux and Bonnichsen, BASALTIC VOLCANISM: FIELD RELATIONS AND 2002; Shervais et al., 2002). Miocene sediments were deposited GEOCHEMISTRY in small, interconnected , precursors to the large Pliocene “Lake Idaho” (Kimmel, 1982; Smith et al., 1982; Jenks and Bon- Central Snake River Plain nichsen, 1989; Malde 1991; Godchaux et al., 1992; Jenks et al., 1993; Wood and Clemens, 2002). The western Snake River Plain Late Neogene basalts of the central Snake River Plain north is topographically lower than the eastern Snake River Plain, with of Twin Falls form large shield volcanoes (Fig. 2) clustered along elevations ranging from 670 m to 1100 m (Malde, 1991), but the axis of the plain, which overlie rhyolite of the Twin Falls recent data suggest that an ancestral Snake River system fl owed eruptive center (Bonnichsen and Godchaux, 2002). This area southwards during the Miocene, implying higher elevations to has been mapped in detail during the past decade by the Idaho the NW (Smith and Stearley, 1999; Link and Fanning, 1999). Geological Survey, by graduate students from the University of Paleontological evidence similarly suggests southward drainage South Carolina and Utah State University, and by undergradu- in the late Miocene (Repenning et al., 1995). ates from Centenary College. The results of this work have been Volcanic activity in the western Snake River Plain began compiled by the Idaho Geological Survey and are being prepared with the eruption of the silicic volcanic rocks along the northern for publication as the Twin Falls 30′ × 60′ quadrangle geological and southern margins of the basin between ca. 11.8 and 9.2 Ma map. Readers are referred to the preliminary copy of that map for (Clemens and Wood, 1993; Wood and Clemens, 2002). Major the names, and extent of volcanic units in the central Snake River basaltic activity in the western Snake River Plain occurred in two Plain and their relation to sedimentary units. time periods: 9–7 Ma, and <2.2 Ma (White et al., 2002; Bon- All of the hills seen on this part of the plain are constructive nichsen and Godchaux, 2002). The earlier episode is represented volcanic vents. The shield volcanoes of the oldest vents have sub- by basalt fl ows and phreatomagmatic vents intercalated with late dued topography and radial drainage and a thick cover of loess Miocene sediments and by the acoustic basement that underlies with well-developed soils, and they are typically farmed almost much of the western graben (Malde and Powers, 1962, 1972; all of the way to the vent summits (e.g., Top , Johnson Amini et al., 1984; Malde, 1991; Wood and Clemens, 2002; Butte). Many of the older vents appear smaller than the largest fl d006-02 page 5 of 26

Basaltic volcanism of the central and western Snake River Plain 5

young vents because their fl anks have been partially buried by of the western Snake River Plain near Mountain Home (Fig. 3). younger lava fl ows (e.g., Skeleton Butte, Bacon Butte, Lincoln The wide range in K, P, and K/P ratios at constant MgO implies a Butte). The surfaces of fl ows from younger vents are character- range of parent derived from a similar source by variable

ized by extremely rugged, chaotic topography, with infl ated fl ow degrees of partial melting. Fe8 values (≈13) imply deep melting

fronts, collapsed fl ow interiors, ridges, and collapse pits (e.g., or a source higher in FeO than MORB asthenosphere, while Na8 Owinza Butte, Rocky Butte, Notch Butte, Wilson Butte). These values (2.4–3.2) imply moderate but variable degrees of partial surfaces lack established drainages but thin loess is present in melting. Partial melting models based on 18 incompatible trace local depressions. The surfaces of fl ows from vents of intermedi- elements indicate 5%–10% melting of a spinel lherzolite source ate age have nearly continuous loess mantles of variable thickness similar in composition to the E-MORB source. Garnet-bearing and well-developed soils, but lack well defi ned surface drainage sources are ruled out by the slope of the rare element pat- and are rarely farmed (e.g., Crater Butte, Dietrich Butte). Where terns, implying pressures less than 20–25 Kb, i.e., within the the Bonneville fl owed overland, loess and soil were com- sublithospheric channel that has been imaged seismically. monly stripped from basalt surfaces yielding surface morphology Most of the chemical variation within fl ows from single that appears younger then it should, which complicates identifi - vents can be explained by low-pressure fractionation of the cation of basalt units based on surface characteristics. observed phenocrysts ( + ). Line scans of Shield sizes vary and are diffi cult to constrain with con- olivine phenocrysts show no reversals in composition or other fi dence because the onlapping of younger lava fl ows often evidence of mixing. Line scans of plagioclase pheno- conceals the true size of the older shields. Even so, the largest crysts show minor reversals that indicate fl uctuations in magma are commonly 15–20 km across the main shield edifi ce—not chamber vapor pressures (Cooke, 1999; Matthews, 2000). The counting elongate fl ows that may extend 50–60 km from the occurrence of cumulate xenoliths (ol+cpx+plg+oxide) in vent. Perhaps the most extensive shield in the is the Sid Butte vent is consistent with high-pressure fractionation Ridge Crater, located at the eastern edge of the central plain at midcrustal levels, within the “basaltic sill” imaged seismically (113.96°W), which is ≈32 km across and covers ≈680 km2—all beneath the eastern plain (Matthews, 2000). The incompat- or parts of eight 7.5′ quadrangles. ible element ratios K/P and K/Ti decrease with mg# ( = molar Shield volcanoes with extremely long fl ows include Black 100*Mg/[Mg+Fe]), ruling out extensive assimilation of older, Ridge Crater, Notch Butte, Wilson Butte, and Rocky Butte. The Black Ridge Crater vent has a major sinuous lobe that fl owed over 40 km from the vent, terminating just south of Notch Butte. Rocky Butte and Wilson Butte both fed extensive fl ow fi elds. Flows from Notch Butte extend west as far as Hagerman, a dis- A tance of ~42 km. A few radiometric dates are available for basalts in the cen- tral Snake River Plain (Armstrong et al., 1975; Tauxe et al., 2004; Idaho Geological Survey, 2005, unpublished data). These dates, along with stratigraphic relations, indicate that most of the shield volcanoes north of the Snake River are Quaternary, whereas those south of the river are early Quaternary or Tertiary. Older volcanoes north of the river, such as Lincoln Butte, Johnson Butte, and Sonnickson Butte, are surrounded and nearly buried by younger basalt fl ows, and typically have subdued morphology and radial drainage patterns. Remanent magnetic polarity of most B of the Quaternary basalts north of the river is normal and within the Bruhnes polarity chron (<0.78 Ma); an exception is Sid Butte, which has reverse polarity, but because of its geomorphic charac- teristics it is thought to be early Quaternary and in the later part of the Matuyama chron. The older basalt units south of the river have both normal and reverse polarity and represent Tertiary to early Quaternary polarity events. Basalts of the central Snake River Plain have MgO similar to mid-ocean ridge basalt (MORB; 5%–10%) but with higher FeO* (12%–15%), TiO (1.6%–4.3%), P O (0.4%–1.2%), and Al O Figure 2. Field photographs contrasting the stratigraphy and topogra- 2 2 5 2 3 phy of the central and western Snake River Plain. (A) Shield volcanoes (14%–17%). They are also higher in FeO* than similar basalts of of the central Snake River Plain north of Twin Falls. (B) Lacustrine the eastern Snake River Plain (e.g., Idaho National Engineering sediments of the Glenns Ferry Formation near Mountain Home, over- Laboratory or INEL site) but less Fe-rich than typical ferrobasalts lain by plateau-forming Pleistocene basalts. fl d006-02 page 6 of 26

6 J.W. Shervais et al. Shoshone Mtn Home Tholeiites Mtn Home Alkali

4.5 4.0 3.5 3.0 2.5 2.0

17 16 15 14 13 12

17 16 15 14 13 12 11

12 11 10 9 8

3.5

3.0

2.5

1.0

.5

1.5

1.0

.5

350

300

250

60 50 40 30

400 350 300 250 200 150

10 9 8 7 6 5 MgO Figure 3. MgO variation diagrams for basalts of the central and western Snake River Plain; data from Cooke (1999), Matthews (2000), and unpublished results (Shervais). The tholeiitic basalts show extensive Fe and Ti enrichment not seen in the alkali series basalts, which are much higher in K. Variations in alumina at low MgO result from plagioclase accumulation or removal. fl d006-02 page 7 of 26

Basaltic volcanism of the central and western Snake River Plain 7 felsic crust; these trends may be due to assimilation of previously may be related to reactivated partial melting of plume-modifi ed injected gabbroic dikes at midcrustal depths. We infer that these lithosphere, in response to the onset of Basin and Range exten- basalts represent a mixed asthenospheric-lithospheric source that sion (Wood and Clemens, 2002; Shervais et al., 2002). formed in response to the Yellowstone melting anomaly; these Basaltic lavas of the western Snake River Plain are generally melts evolved by a combination of high-pressure and low-pres- distinct from lavas of the eastern Snake River Plain trend, and are sure crystal fractionation, with possible assimilation of previ- commonly ferrobasalts with up to 17% FeO* (Fig. 3; Shervais et ously intruded midcrustal ferrogabbros (Shervais et al., 2004). al., 2002; White et al., 2002). This is clearly seen in the Bruneau- Jarbidge eruptive center, where basalts of the eastern Snake River Western Snake River Plain Plain trend are overlain by younger ferrobasalts of Salmon Falls Butte, which represents the SE extent of western Snake River The western Snake River Plain graben formed over a short Plain (Vetter and Shervais, 1997). In general, the time period ca. 10–12 Ma, coincident with the eruption of the oldest (pre–Lake Idaho) basalts are the most Fe-rich; younger extensive rhyolites that now form the fl anks of this structure basalts include ca. 2.0–0.7 Ma Fe-Ti basalts (equivalent to the (Wood and Clemens, 2002). Late Miocene basalts (9–7 Ma) Boise River Group 1 of Vetter and Shervais, 1992 and group M2 underlie the central part of the graben, as shown by geophysical of White et al., 2002) and the <0.7 Ma alkali basalts which are surveys and deep drilling results (Wood, 1994; Wood and Clem- high in K2O and MgO (equivalent to the Boise River Group 2 of ens, 2002; Shervais et al., 2002). Overlying these basalts (and to Vetter and Shervais, 1992 and group M3 of White et al., 2002). some extent interbedded with them) are up to 2–3 km of lacus- This same sequence of basalts is observed in the Melba area, trine and fl uviatile sediments that form the “Idaho Group” of 100 km west of Mountain Home (White et al., 2002) and along Malde and Powers (1962): Chalk Hills Formation (oldest), Poi- the Boise River in the Smith area, 40 km north of Moun- son Creek Formation, Glenns Ferry Formation, and the Bruneau tain Home (Vetter and Shervais, 1992). Formation. Phreatomagmatic vents intercalated with sediments of the Idaho Group are common in the area west of Mountain HYDROGEOLOGY OF BASALTIC VOLCANISM IN Home, between Grand View and Marsing along the Snake River THE CENTRAL SNAKE RIVER PLAIN (Godchaux et al., 1992; Godchaux and Bonnichsen, 2002; Bon- nichsen and Godchaux, 2002). Detailed mapping in the central Snake River Plain has Mountain Home occupies an upland plateau above the revealed new details of groundwater recharge and fl ow, and Snake River fl oodplain, which is incised into fi ne-grained lacus- allows the formulation of new models for the formation of high trine sediments deposited by Lake Idaho during the late Mio- conductivity aquifers. cene–early Pliocene (Fig. 2). Late Pliocene to early Pleistocene Groundwater fl ow in the Snake River Plain is commonly basalts sit conformably on the lacustrine strata, or occur within thought to be controlled by relatively porous horizons between the uppermost few hundred feet. Flow of these basalts into the lava fl ows (Lindholm and Vaccaro, 1988; Welhan et al., 2002c). shallow margin of the lake resulted in deltas of hyaloclastite brec- These so-called interfl ow zones are characterized by complex cia and pillow lava, which pass upward into subaerial pahoehoe geometries refl ecting the fractal nature of the pahoehoe infl ation- fl ows. Farther inland, fl ows are separated by deposits of fl uvial ary process, the quasi-stratigraphic layering and interfi ngering of gravel and sand. These plateau-forming basalts are overlain by a fl ows within and between lava fi elds (shield volcanoes), and the series of younger lavas erupted from 13 central vents that cluster development of tension fi ssure networks along the margins of near the NE margin of the plateau. The shield volcanoes rise infl ated fl ow structures (Welhan et al., 2002b). These permeable 120–210 m above the surrounding plateau; several are capped zones can be mapped in the subsurface using core and geophysi- by central depressions that probably represent former lava lakes cal logs and their autocorrelation length scales used to construct (pit craters). Pleistocene cinder cones are the youngest volcanic stochastic models of subsurface pathways of preferential fl ow features, and may cap small shield volcanoes. (Welhan et al., 2002a). Direct evidence of the “pipeline” nature Recent tectonic activity is demonstrated by fault scarps with of this fl ow, as refl ected in extremely low dispersivities associated ~2–9 m of throw that crosscut all of the volcanic features. There with mass transport on scales of 25 km, has been documented by are two dominant fault sets: one trends N85W, the other trends Cecil et al. (2000) using chlorine-36. N60W (parallel to the range front faults) and appears to truncate The mapping of Cooke (1999) and Matthews (2000) shows against the N85W set. All of the faults are vertical or high angle that thick alluvial intervals are most common where adjacent normal faults that generally dip steeply to the south and are down- volcanic edifi ces abut one another and overlap. These overlap- thrown on the south or SW side (a few are downthrown to the N, ping fl ows create moats, which control the location of surface forming small grabens with adjacent faults). The young faulting stream drainages and fi ll with coarse . Younger lava may be related to Basin and Range extension that has refracted fl ows are channeled into these drainages, displacing the streams to more westerly orientation to exploit preexisting fault zones of and covering the alluvium with relatively thick, semi-permeable the western Snake River Plain. This relationship suggests that caps, forming elongate aquifers with extremely high conductivi- post–Lake Idaho volcanism in the western Snake River Plain ties that follow the preexisting paleo-drainage. Reconstruction of fl d006-02 page 8 of 26

8 J.W. Shervais et al. basaltic volcanism through time using detailed geologic maps the Hagerman Valley are shown on Figure 4; all Day 1 stops are allows us to predict the location of paleo-drainage and elongate shown on Figure 5. alluvial aquifers. This ability to predict aquifer location will On the way to Stop 1, we will drive by scour features and prove to be a valuable tool as increased demands are placed upon deposits created by the . The following paragraphs the groundwater supply by and population growth. briefl y describe some of the effects of the fl oodwaters (Fig. 4). In addition, our mapping reveals that young basalt vents seldom About fi fty years ago, geologists fi rst recognized that giant exhibit channelized drainage systems that connect with major gravel bars and stark erosional features common along the Snake through-going streams. Instead, the rugged volcanic topogra- River were caused by cataclysmic lowering of Pleistocene Lake phy of ridges, fl ow fronts, lava channels, and collapse pits trap Bonneville. The most complete descriptions and analyses of the precipitation, which must either evaporate or percolate into the Bonneville Flood were written by Malde (1968) and O’Connor fractured lavas to recharge the local groundwater. These young (1993). O’Connor (1993, p. 1) writes, “Approximately 14,500 volcanic features constitute “negative basins” of interior drain- years ago, Pleistocene discharged 4750 km3 of age, despite their topographic emergence. We suggest that these water over the divide between the closed Bonneville basin and young basalts represent recharge zones that can be easily mapped the watershed of the Snake River. The resulting fl ood, released and distinguished from older fl ows that display well-developed near Red Pass, Idaho, followed the present courses of external drainage (Cooke and Shervais, 1999). March Creek, the Portneuf River, and the Snake and Columbia On a more regional scale, the eastern and central Snake River before reaching the Pacifi c Ocean. For 1100 km between River Plain basalts and intercalated units form the Snake River and Lewiston, Idaho, the Bonneville Flood left a Plain aquifer, which along with the Snake River system sup- spectacular array of fl ood features.” O’Connor (1993) goes on to plies water to southern Idaho and are essential to the agricultural describe the hydrology, hydraulics, and of the development of the state. Water recharges from snow melt in the fl ood and provides a picture of the history and character of the north of the plain and in local basalt recharge areas. fl ood and its many and deposits. A major characteristic Groundwater fl ow is from the NE to SW and it discharges as of the fl ood is a stepwise drop in the water surface caused by the springs in the Thousand Springs and other reaches of the north diverse geologic and geomorphic environments along the fl ood and east wall of the Snake River between Twin Falls route. A repeated phenomenon is a narrow canyon with con- and King Hill. The spring water is used extensively by the aqua- stricted but high-discharge fl ow that opens up into a wider valley culture industry, but quantity and quality of the water has been with less restricted, lower-discharge fl ow. In many locations, a declining due to increased groundwater pumping upgradient, constriction -watered the fl ood causing inundation and over- drought, and changes in surface irrigation systems. Spring and land fl ow. East of Twin Falls fl ood water diverted north of the groundwater irrigation is supplemented by surface withdrawals Snake River canyon scoured a long stretch of basalt surfaces and from the Snake River, but acute water shortages are posing a rejoined fl ood waters in the canyon for ~17 km forming several political dilemma for Idaho politicians and water experts. Map- systems of cataracts and plunge pools (see Day 3, Stop 3.6). ping of springs in the northwall of the Snake River Canyon by The areas of Hagerman and Thousand Springs (Day 1) show Covington and Weaver (1989), the Idaho Geological Survey, and the stepwise nature of the fl ood’s energy and the resulting features. others suggests a geologic control. Covington et al. (1985) (and The Thousand Springs reach was a constriction that overfi lled and others before them) recognized that some springs discharge at the overland fl ow scoured and plucked basalt at the Sand Springs rubble zones at the base of where canyon-fi lling late Quaternary Nature Preserve, Box Canyon, and Blind Canyon. South of Thou- basalts entered a paleodrainage system. It also appears that the sand Springs a dry valley appears to have been the previous loca- topography developed on the older, slightly altered (Banbury) tion of the confl uence of and the Snake River. basalts may have infl uenced spring discharge, as many springs The fl ood may have scoured a new channel for the Snake River and seem to emerge through talus but just above the Quaternary- subsequently the river abandoned the previous valley. Just down- Tertiary unconformity. The older units may locally form a more stream of Thousand Springs, where the Hagerman valley opens up, impermeable horizon and basal aquitard. energy dropped and the fl oodwaters deposited a large expansion bar 5 km long with 4 m boulders at its upstream end. At maximum FIELD TRIP GUIDE discharge, the fl oodwaters would have been ~60 m deep at the location of the Hagerman Inn. In the bottom of the valley, thick Day 1 (Half Day) Hagerman Valley deposits of Yahoo clay that had buried basalt of Notch Butte were stripped away and the surface of the basalt was scoured. Directions to Stop 1.1 From Hagerman, drive south on Idaho Highway 30 ~7 mi Stop 1.1—Banbury Basalt and Bacon Butte Basalt: Basalt (11 km) across the bridge over the Snake River to ~1 mi south of Water Interaction [N 42°43.804′, W 114°51.079′] Thousand Springs Resort. Pull off to the right on a small dirt road across from the entrance gate to a big house on the river. Stop 1 Mapping and reexamination of the “older” volcanic units lies ~200 ft (60 m) uphill to the right. Bonneville fl ood effects in exposed south of Hagerman is one phase of the Idaho Geological fl d006-02 page 9 of 26

Basaltic volcanism of the central and western Snake River Plain 9

Figure 4. Topographic map of Thousand Springs and the southern Hagerman Valley showing erosional and depositional features of the Bonnev- ille Flood. Large blue arrows show inferred fl oodwater fl ow directions. The arrow pointing northwest is placed to show the change from scoured rock to the bouldery expansion bar deposited in the broadening valley.

Survey’s STATEMAP project in the Twin Falls 30′ × 60′ quad- includes a fi eld of hydrovolcanoes, which appear to be localized rangle. The area includes Banbury Hot Springs, type locality for along a NW-trending structural zone that also infl uenced later the Tertiary Banbury Basalt, which Malde et al. (1963) mapped graben development and formation of Pliocene Lake Idaho, geo- over a wide area of . Our work and the excel- thermal activity, and canyon development. The inferred structure lent mapping of Malde and Powers (1972) show much hetero- is suspiciously close to a projection (S49E) of the northern mar- geneity within the “type section.” The lower Banbury Basalt gin of the western Snake River Plain. fl d006-02 page 10 of 26

10 J.W. Shervais et al.

Stop 1.1 42º43’ 48” N, 114º 51’ 05” W Stop 1.2 42º43’ 35” N, 114º 51’ 21” W

Figure 5. Topographic map of the Snake Stop 1.3 River canyon between Banbury Springs and Sand Springs showing locations 42º43’ 11” N, 114º 51’ 24” W for Stop 1 of the fi eld trip, and ero- sional and depositional features of the Bonneville Flood. Relict fl ood features include scoured and plucked basalt and abandoned valley with no established drainage.

Stop 1.4 42º42’ 24” N, 114º 50’ 23” W

Stratigraphically overlying Miocene rhyolite, the Miocene- lacustrine and fl uvial setting prior to Lake Idaho and probably Pliocene Banbury Basalt consists of a lower sequence of altered over a considerable time span (Gillerman, 2004). The heteroge- basalt fl ows and vent deposits, a middle layer of basaltic pyro- neity, polarity reversals, and spatial distribution of the fl ows also clastic deposits overlain by and/or interbedded with sediments, indicate a variety of sources for the Banbury units. As noted by which thicken to the south, and an upper sequence of basalt fl ows others, it is time to revise the nomenclature. The recent Idaho (Malde and Powers, 1972). The vent facies of the lower Banbury Geological Survey mapping, which will be displayed on the fi eld Basalt includes at least two exposed volcanic centers (Thousand trip, has renamed and more precisely subdivided many units Hill and Riverside Ferry vents), which are characterized by based on mapping and paleomagnetism, combined with chemis- tuff with laterally extensive and locally palagonitized try and a very few radiometric age dates. surge and deposits. At Stop 1.2, spectacular 10-m-high beds of tuff contain blocks larger than 1 m. At Stop 1.1, Stop 1.1A—Exposure of the Lower Banbury Distal Vent distal phreatomagmatic tuffs containing volcanic bombs, stream Facies Volcaniclastics pebbles, and siltstone xenoliths are overlain by a 1-m-thick bed of black spatter. The spatter is overlain by an oxidized tuff with The prominent outcrops to the right of the gully are massive small glass bombs, and above that are three upward-coarsening basaltic tuffs in 3–4 upward-coarsening cycles, 2–3 m thick, with cycles of air-fall tuff. Unconformably overlying the emergent laminated to cross-bedded tuff above and thin (6–8 cm), locally hydrovolcano is a baked siltstone and the upper Banbury Basalt, cross-bedded, fi ne tuff below (Fig. 6A). This sequence overlies which here consists of notably fresher, plagioclase-phyric, vesic- a juvenile conglomerate of basalt, hyaloclastites, and some ular basalt fl ows. Elsewhere in the map area, the upper Banbury rounded stream cobbles, that transitions up to a bleached/oxi- consists of altered or water-affected basalt (WAB of Godchaux dized (?) pumice bed (Fig. 6B). Below these beds are vent-like and Bonnichsen, 2002) with a different magnetic polarity than spatter and pillowy basalt with abundant fresh glass. Although the feldspar-rich fl ows. Pending geochronology and chemistry the base of the sequence here is unexposed, Malde and Powers may improve age constraints and correlations. Even at its type (1972) mapped the lower portion of the slope as lower Banbury locality, the Banbury Basalt is lithologically heterogeneous with sediments and tuff, most likely from better exposures ~1 mi to lateral facies changes indicating basaltic volcanism within a the north, near the Sligar’s Thousand Springs Resort. There, a fl d006-02 page 11 of 26

Basaltic volcanism of the central and western Snake River Plain 11

section of mixed tuffs, sandstones, and massive basaltic units A are exposed in a gully and in old road cuts. The massive and black mottled basalts are coarse grained, magnetic, and look almost gabbroic. An exposure of the mottled rock is just below the Stop 1.1B outcrop. Godchaux and Bonnichsen (2002) referred to these massive mottled rocks as “spotted” due to black augite crystals in a lighter brownish groundmass, which might include glass and fi brous and hydrous such as zeolites, chlorite, or . They interpret the spotted, massive lavas as water-affected basalt, which formed in deep water (over a few tens of feet). If so, then the volcaniclastic sediments should be indicative of such an environment. It may be that volcanic eruption of fl ows, tectonic activity, and climatic cycles created a rapidly changing and laterally heterogeneous distribution of B lakes and sedimentary facies during deposition of the “Banbury Basalt” and related units. At this stop, the spatter layers dip into the hill (strike N65E, dip 11°NW to EW and 14°N) and as seen from a distant photo (Fig. 6C) the entire lower Banbury Basalt section (renamed Tvd or vent deposits, distal), including the baked siltstone above, appears to be tilted 10–15°NNW – possibly faulted along a WNW normal fault. At this location, the upper Banbury lavas are clearly unconformable on top of the lower Banbury volcaniclas- tics. The upper Banbury lavas are fl at lying and mostly subaerial, but the lower part is glassy, popcorn-like, with quasi-pillows and glass. The fl ows here are distinguished by abundant feldspar phe- C nocrysts and normal magnetic polarity, and have been renamed the Basalt of Oster Lakes (Tob) for exposures farther east and several hundred feet topographically lower. Structural relation- ships, age, and exact correlations of these various exposures are problematic.

Stop 1.1B—Quaternary Basalt of Bacon Butte: Water Escape Features

The Basalt of Bacon Butte (formerly included in Sand Springs basalt by Malde and Powers, 1972) fi lls paleo-drainage here, with evidence for water interaction at base (pillows and hya- Figure 6. Banbury Basalt in Hagerman Valley. (A) Lower Banbury loclastites) and vertical dewatering conduits that penetrate entire Basalt, volcaniclastics and phreatomagmatic vent facies (Stop 1.1). fl ow (Fig. 7). The water-escape structures in this fl ow constitute a (B) Bleached pumice bed in lower Banbury Basalt. (C) Unconformity feature of volcanic rocks that have not been described previously. of upper Banbury Basalt (Basalt of Oster Lakes) over tilted section They consist of subvertical conduits ≈1 m in diameter (0.5–1.5 m of lower Banbury phreatomagmatic volcaniclastics with primary and probable structural dip (Stop 1.1). Fence posts for scale. range) and spaced 5–15 m apart, partly fi lled with spicaceous basaltic rubble. Columnar jointing in the adjacent basalt wraps from vertical along the base of the fl ow to horizontal adjacent to the conduits. Vesicles are more common in the wallrock of the conduits than in parts of the fl ow farther away, and the basalt is popcorn. This spicaceous rubble of “popcorn” basalt does not quenched to a glassy or aphanitic texture in the wallrock adjacent represent later fi ll or pieces broken from the wallrock, but rather to the conduit (Fig. 7A). In many of the conduits, basalt from the blobs of lava that were isolated within the conduit while steam adjacent wallrock forms a lattice or trellis arrangement within the was actively rising through the fl ow (Fig. 7C). conduit that is contiguous with basalt of the wallrock (Fig. 7B). The upper surface of the lava fl ow contains depressions as The basalt in this trelliswork has glassy margins and spikey much as 10 m across and 1.5 m deep centered above the water- projections along its surface. Spicaceous rubble is common in escape structures, which appear to represent areas where the all conduits, often forming discrete fragments that resemble surface lava collapsed into the conduit during water escape. Such fl d006-02 page 12 of 26

12 J.W. Shervais et al.

Figure 7. Water-escape structures in basalt of Bacon Butte, Stop 1.1B. (A) Panorama of outcrop showing water-escape conduits, rotated colum- nar jointing, and rubbly fi ll. (B) Close-up of water escape conduit showing trellis structure and spiny rubble with popcorn texture. (C) Popcorn texture basalt in water conduit. (D) Large (1–2 m) scale steam cavities at base of fl ow, just above section of megapillows.

a feature is well illustrated at another location by the occurrence (0.5 km) to farthest gully with trees and brush. Hike steeply of a ropey pahoehoe fl ow surface that is preserved deep inside a uphill 150′–200′ (45–60 m) to large, cliffy exposure of the mega- lava fl ow within a steam conduit. breccia at top of ridge on north side of gully. The subaerial lava fl ow with water escape structures is underlain by pillow lava that fi lls intervening depressions in the Stop 1.2—Banbury Basalt: Megabreccias [N 42°43.583′, paleosurface. Many of these are mega-pillows as much as 4 m W 114°51.35′] across, which may contain water-escape structures that continue up into the subaerial fl ow. In addition, the interface between the As you climb up to the ridgetop, there are poor exposures on pillow zone and the subaerial fl ow commonly contains small the slope of white breccias and palagonite tuffs, as well as lavas. caves and cavities that represent large steam bubbles (>1 m The outcrop on the ridge top is an ~10-m-thick “megabreccia” across) trapped below the lava fl ow (Fig. 7D). unit, which can be followed ~600′ to the west, where it termi- nates, perhaps on the crater rim, or is faulted. Directions to Stop 1.2 The megabreccia (Tvp) consists of unsorted tuff breccia Continue South 0.3 mi (0.5 km) on Hwy 30 to a prominent with large accidental blocks, many more than 1 m in length draw on the right with several dirt roads. Turn in to the right, go (Fig. 8A). Block compositions include several types of basalt through gate and close it, and park near end of drivable jeep trail (lower Banbury Basalt in appearance), tan fi ne-grained lake sedi- to right. Hike northward on cow trail along wire fence ~1/3 mi ment, sparse stream pebbles, and some juvenile spattery tephra. fl d006-02 page 13 of 26

Basaltic volcanism of the central and western Snake River Plain 13

Figure 8. Vent facies of the Banbury Basalt. (A) Megabreccias that probably represent a vent facies (Stop 1.3). (B) abRiverside Ferry vent (Stop 1.4).

Some clasts show evidence of hot reaction rims on spheroidally Directions to Stop 1.4 weathered basalt. The matrix is unstratifi ed with ~25% fi ne ash Drive south on Hwy 30, ~2 mi (3.2 km), and turn left at and many small (<1cm) clasts. The breccia unit appears to grade intersection with paved “River Road.” Follow River Road past upwards into tuff with fewer large clasts. The megabreccia and houses, ~1 mi (1.6 km) and turn left on paved road near power associated tuff are interpreted as explosive, subaerial crater-fi ll- line. Drive past sequence of orange-colored palagonitic tuffs on ing deposits of the Thousand Hill vent. right roadcut and continue to gravel road at top of hill. Turn right This small drainage basin (~1/2 square mile) is characterized on gravel road, go ~1/3 mi (0.5 km) and fork left. Drive carefully by outcrops of megabreccia, tuffs, red-orange sediment (which northward to edge of vent exposure. Some roads are very sandy always overlies the volcaniclastics), and lavas, and by faults. The and may need 4WD. There are at least two jeep trails that access area was mapped by Stearns et al. (1938) as “Fault complex of the vent area and overlook the Snake River. Hagerman lake beds and interstratifi ed tuff.” Malde and Powers (1972) mapped it as lower Banbury “vent deposits of basaltic Stop 1.4—Banbury Basalt: Riverside Vent (optional, if pyroclastic material,” without showing any internal detail. The time permits) [N 42°42.40′, W 114°50.383′] geology is very complex and diverse, and the volcanic and struc- tural relationships are obscured by a veneer of tan-colored Qua- This exposure in the north half of the section 29 “island” is a ternary Yahoo Clay (Bruneau sediments in earlier work), which hydrovolcano bisected by the Snake River and originally named unconformably overlies the Tertiary volcanics. the Riverside Ferry cone by Stearns et al. (1938). Bonnichsen and Godchaux (2002) referred to it as the Riverside Ferry cone but Directions to Stop 1.3 named the phreatomagmatic units “Tuff of Blue Heart Springs” Return toward vehicles and hike over to small hill nearby, after a spring located on the north side of the river adjacent to the labeled “Thousand” on topographic map. . They described the tuff as explosively erupted layers of cinder to spatter-rich deposits tilted near the vent constructs and Stop 1.3—Banbury Basalt: Tilted Hydromagmatic Vent transitioning to subhorizontal, fi ner-grained units more distally. and Vent Facies Breccias [N 42°43.183′, W 114°51.40′] Chemically they place the basaltic material in the general Snake River olivine tholeiite compositional range. They also place The outcrop consists of a 7-m-thick, unsorted and unstrati- some of the Banbury tuff units as intercalating with the Glenns fi ed, matrix-supported volcanic breccia with dull olive-green to Ferry Formation sediments, although the mapping of Malde and tan color containing accidental blocks of vesicular basalt (as large Powers (1972) would generally place the Banbury Basalt as as 0.5 m across) and numerous polished stream pebbles. Note the older than the thick pile of lake sediments. Certainly there are oblong blocks of undisturbed, fi ne-grained tan water-laid sedi- “Banbury-age” lake and fl uvial sediments interstratifi ed with ment, which must have been horizontal originally, suggesting that basaltic units (4–6 Ma) and there are basaltic tuffs and basalt dip is the result of structural tilting. This is a section of the vent (3.4–3.8 Ma) interbedded within the Glenns Ferry Formation at deposits, dipping 40 degrees north, probably due to faulting. the Hagerman National Monument (W.K. Hart and M.E. Hike around to top surface (now dipping) to look at strong Brueseke, 1999, personal commun.). Dating the Banbury units is linear features, which are interpreted as elutriation (gas escape) diffi cult due to the alteration, although a few recent Ar-Ar radio- features. metric dates are available, but not defi nitive. This outcrop may be the same unit as in Stop 1.2, but on the The core of the Riverside Ferry vent is well exposed by opposite side of vent. Note that the Thousand Hill vent and the (Fig. 8B). A massive tuff breccia, air-fall tuffs and brec- Riverside Ferry vent (Stop 1.4) are both along a strong north- cia, cindery beds, surge deposits, and palagonitized tuffs can be west-trending structural orientation. found within a short hike across the 1000 ft of exposed vent. fl d006-02 page 14 of 26

14 J.W. Shervais et al.

On the southeast side of the vent, a basaltic lava fl ow appears Directions to Stop 2.2 to overlie the vent deposits and may be related. Exposures on Return to I-84 and proceed west toward Boise; take the next the north side of the river contain abundant red cindery mate- available exit (exit number 90) then continue across and back on rial, but these deposits have not been studied in detail. A NW- to I-84 toward Boise (this will slow you down and position you trending fault may underlie the course of the Snake River, and for the next stop). Pull off to the right immediately after entering it is probable that there is a signifi cant northwest-trending fault the freeway at the fi rst roadcut encountered. under the “dry” river course south of the “island.” Evidence for this structure(s) includes small visible offsets of the older rhyo- Stop 2.2—Plagioclase Flotation Cumulates of Lockman lites and lower Banbury lavas, topographic features emphasized Butte, I-84 Roadcut [N 43°10.599′, W 115°45.601′] by the Bonneville Flood (Fig. 4), and alignment of geothermal wells and hot springs. This roadcut transects a major fl ow lobe from Lockman Return to Hagerman on Hwy 30. Butte that consists of multiple lava fl ows that are stacked one on top of (or beneath) another. Many of these fl ow units contain Day 2 (All Day) Mountain Home plagioclase fl otation cumulates that formed within covered lava channels and aphyric ferrobasalt residues that drained out the Directions to Stop 2.1 bottom of the cumulates (McGee and Shervais, 1997; Shervais Drive north on I-84 to Mountain Home; take Exit 95 on et al., 2002; Zarnetske and Shervais, 2004). Another interesting Idaho Hwy 20 north toward Camas Prairie and Sun Valley. Con- aspect of these fl ows are the gigantic vesicles found in some gas tinue N for ~7 mi (11 km) where the highway enters Rattlesnake accumulation horizons; these vesicles are up 2 m long and 0.6 m Creek canyon; park at the turnout on the right side of highway. high—big enough to lie down in! All Day 2 stops are shown on Figure 9A; detailed topography for Lava fl ows with fl otation cumulates comprise three zones: a stops 2.2 through 2.4 is shown in Figure 9B. central, plagioclase porphyry with intersertal to intergranular tex- tures in the groundmass, an upper diktytaxitic zone comprising pla- Stop 2.1—Danskin Mountains Rhyolite, Highway 68 North gioclase laths with large voids and minor intergranular mafi cs and of Mountain Home [N 43°12.052′, W 115°33.2′] glass, and a lower aphyric zone of ferrobasalt, with 16%–17% FeO* (Fig. 10A). Mass balance calculations show that the diktytaxic The oldest volcanic rocks exposed in the Mountain Home zone contains 30%–50% porosity, represented by the ferrobasalt area are rhyolite lava fl ows that form the Danskin Mountains base, if the central plagioclase porphyry is assumed to represent and the Mount Bennett Hills. Clemens and Wood (1993) the bulk composition. Detailed outcrop maps show that successive mapped the rhyolite here as the Danskin Mountains Rhyo- lava fl ows fl owed beneath previously emplaced fl ows, infl ating and lite, and mapped rhyolite which is exposed farther east as the plastically deforming the aphyric ferrobasalt zone in the overlying Mount Bennett Rhyolite. They determined a K-Ar age of 10.0 fl ow. Plagioclase fl otation cumulates beneath this ferrobasalt ceil- ± 0.3 Ma for sanidine in a vitrophyre from the summit area ing display a horizontal contact between the diktytaxitic and plagio- on Teapot Dome (Danskin Mountains Rhyolite), and a K-Ar clase porphyry zones; we interpret this horizon to represent the con- age of 11.0 ± 0.5 Ma for plagioclase in a rhyolite from near tact between interstitial melt (below) and volcanic gasses (above), Mount Bennett. Possible correlative units on the south side of and suggest that interstitial melt was displaced by the rising gasses the western Snake River graben include the Sheep Creek Rhyo- (McGee and Shervais, 1997; Zarnetske and Shervais, 2004). lite (9.88 ± 0.46 Ma; Hart and Aronson, 1983), the rhyolite of Plagioclase phenocrysts are An65 in both the porphyry Tigert Springs, the rhyolite of O X Prong, and the rhyolite of and diktytaxitic zones, suggesting formation of the plagioclase Rattlesnake Creek (Kauffman and Bonnichsen, 1990; Jenks et framework by simultaneous crystallization throughout the fl ow, al., 1993). Rhyolite in the eastern Mount Bennett Hills ranges followed by sinking of the dense, interstitial ferrobasaltic liquid in age from 9.2 ± 0.13 Ma to 10.1 ± 0.3 Ma (Armstrong et al., to the bottom of the fl ow. This interpretation is supported by

1980; Honjo et al., 1986, 1992). olivine phenocrysts (Fo68) trapped in the plagioclase framework,

The Danskin Mountains Rhyolite is typically a vitrophyre and by partially replaced intergrowths containing Fo80–89 olivine. with abundant phenocrysts of sanidine and quartz set in a red, The Fo-rich olivine relicts imply a parent magma that was signifi - gray-brown, or black volcanic glass. Flow banding appears as cantly more magnesian than observed. laminar variations in the color of the glass, or in its crystallinity. These fl otation cumulates provide an analogue for anor- The fl ow banding is commonly folded ptygmatically, indicat- thosite formation, both in the lunar magma ocean and in Earth’s ing rheomorphic mobilization of the rhyolite. Flow banding mid-crust. We suggest that planetary anorthosites represent pla- and axial foliation in the vitrophyre generally trend N50ºW to gioclase saturated melts that crystallize en mass to form buoyant N60ºW, and dip 15º–45°NE. There are no indications that these rafts of plagioclase plus trapped mafi cs; interstitial ferrobasalts rhyolites are rheomorphic ignimbrites; they appear to be rhyolite liquids are forced out by rising magmatic gasses, and sink lavas erupted from fi ssures that were subparallel to the current back into the underlying magma (McGee and Shervais, 1997; range-front faults (Bonnichsen, oral comm., 1996, 1997). Zarnetske and Shervais, 2004). fl d006-02 page 15 of 26

Basaltic volcanism of the central and western Snake River Plain 15 A

Stop 2.3 Stop 2.2 Stop 2.1 Stop 2.4

Stop 2.5

B

Stop 2.3

Stop 2.2

Stop 2.4

Figure 9. (A) Topographic map of the western Snake River Plain around Mountain Home, Idaho, showing Day 2 stops. (B) Detail of Stops 2.2 through 2.4 NW of Mountain Home. fl d006-02 page 16 of 26

16 J.W. Shervais et al.

Directions to Stop 2.3 Stop 2.3—Crater Rings Pit Craters: Lava Lakes and Drive NW on I-84 for ≈14 mi (23 km) to the fi rst exit (Simco Spatter Ramparts [N 43°11.715′, W 115°50.866′] Road, Exit 74); cross the freeway and return to I-84 in the SE direction. Exit at the fi rst Mountain Home exit (exit number 90) The Crater Rings are a spectacular volcanic feature that con- and take the fi rst right turn encountered on Business I-84 (old sists of two large pit craters at the summit of a broad Hwy 30). Proceed along the frontage road parallel to the railroad (Fig. 10B). The western crater is ≈800 m across and 75 m deep; tracks for ≈6 mi (≈10 km), then turn left and cross the RR tracks the eastern crater is ≈900 m across and 105 m deep. The inner at a clearly marked crossing. Proceed ≈1.8 mi (2.9 km) along this walls of the pit craters consist of welded spatter, agglutinate, and wide gravel road to a dirt road turn-off on the left near the crest of minor intercalated lava fl ows; no fragmental horizons are exposed the grade. Turn left and follow this dirt road ≈1.3 mi (2.1 km) to in the crater walls, which argues against phreatomagmatic erup- the rim of Crater Rings. You will go past a stock pond (commonly tion. The welded spatter and agglutinate are easily identifi ed by dry) and through at least one wire fence; be sure to close all gates their characteristic textures, oxidized coloration, and hollow ring behind you. Note: Crater Rings is rattlesnake heaven, so watch when struck with a hammer. The eastern crater is surrounded on where you step and put your hands! three sides by spatter and agglutinate ramparts. The Crater Rings represent pit craters that were fi lled episodi- cally with lava lakes (Shervais et al., 2002). They are equivalent to similar features in Hawaii, such as Halemaumau pit crater on the summit of Kilauea volcano and the paired lava lakes of the 1972 Mauna Ulu eruption (Decker, 1987; Tilling et al., 1987). Fire A fountain eruptions in the lava lakes fed spatter to the rims, which were occasionally mantled by lava fl ows when the lava lakes over- fl owed their ramparts. The fi nal eruptive phase was confi ned to the eastern vent, where fi re fountains built ramparts on three sides of the vent that were not covered by subsequent lava fl ows, although lava from the eastern vent may have fl owed into the western vent during lava highstands at this time (Shervais et al., 2002).

Directions to Stop 2.4 Return to the frontage road and turn right; proceed SE along B the frontage road (toward Mountain Home) for ≈5 mi (8 km) and turn right across the railroad tracks (you will be almost due north of Union Butte cinder cone). Continue south ~1.1 mi (1.8 km) to the Union Butte cinder cone.

Stop 2.4—Union : Holocene Cinder Cones, Late Alkaline Basalts Equivalent to Boise River Group 2 of C Vetter and Shervais (1992) [N 43°9.565′, W 115°46.06′]

Union Buttes are the two most prominent volcanic vents west of Mountain Home. Their stratigraphic position (overlying all older vents) and relative preservation suggest an age of <500,000 yr. The basalt of Union Buttes erupted from these two vents but was confi ned between the large Rattlesnake Springs shield volcano to the south and the smaller Crater Rings and Lockman Buttes vents to the west and north. The western Union Butte is larger than the eastern vent, and fed a small fl ow that fl owed west toward Crater Figure 10. Volcanic features near Mountain Home. (A) Plagioclase Rings. Both vents consist of cinder cones built on top of small fl otation cumulates in covered lava channel, Lockman Butte—note shield volcanoes of similar composition basalt. diktytaxitic texture near top of cumulate layer where exsolved gas has The basalt of Union Buttes is distinct from almost all other forced residual ferrobasaltic liquid from the interstices. (B) The Crater basalts in the Mountain Home area because it contains large, Rings, two large pit craters that fed lava lake eruptions and Pelean- clear phenocrysts of olivine but lacks plagioclase phenocrysts. style fi re fountains. (C) Pillow and hyaloclastite delta in basalt of Little Joe Butte (Strike Dam Road), which sits on lacustrine sediments of The basalt of Union Buttes, and the similar basalt of Little Joe Glenns Ferry Formation (Lake Idaho); note water escape conduits, Butte (also known as the basalt of Strike Dam Road), are charac- steam cavities, and southeast dip of foreset beds terized by high K2O compared to other Mountain Home basalts, fl d006-02 page 17 of 26

Basaltic volcanism of the central and western Snake River Plain 17

along with lower TiO2 and high-fi eld strength trace element Return to Hagerman abundances. In terms of their chemical and age relations, they are Continue south on Strike Dam Road, across Snake River similar to the Boise River Group 2 basalts of Vetter and Shervais at Strike Dam Bridge, to Idaho Hwy 78. Drive E on Hwy 78 (1992) and to the M3 basalts of White et al. (2002). The transi- through Bruneau to Hammett. As time permits, we will make tion from high fi eld strength-rich, alkali-poor tholeiites to potas- stops along this route to view the stratigraphy of the former Lake sium-rich, high fi eld strength-poor transitional alkaline basalts Idaho and the effects of the Bonneville fl ood. At Hammett, rejoin around 600,000–700,000 yr ago is one of the most fundamental I-84 and return to motel. time-dependent transitions observed in the western Snake River Plain province, with the younger lavas characterized by OIB-like Day 3 (Partial Day) Twin Falls2 isotopic compositions. See Vetter and Shervais (1992) and White et al. (2002) for discussions. Directions to Stop 3.1 From Hagerman, drive north on Hwy 30 to Bliss, turn E Directions to Stop 2.5 (right) onto Hwy 26 to Shoshone. At main intersection in Sho- Continue south on gravel and paved roads ~2.6 mi (4.2 km) shone, turn N (left) on Hwy 75. Drive N ~2.3 mi (3.7 km) and to intersection with Idaho Hwy 67. Turn right onto Hwy 67 and park on right. Hike E into the Shoshone lava fl ow to the lava chan- proceed SW toward Mountain Home Air Force Base. Bear right nel. All of the stops for Day 3 are shown on Figure 11. Source at the turn-off into the air base (away from the base) and continue vents for the basalt fl ows are indicated on Figure 12, a hillshade SW on Hwy 67 for 6.5 mi (10.8 km) to a poorly marked intersec- topographic map of the Twin Falls 30′ × 60′ quadrangle. tion with Strike Dam Road. Turn left onto Strike Dam Road and proceed south for 6.75 mi (10.9 km) to the rim of the plateau; Stop 3.1—Lava Channel is Shoshone Lava Flow; a‘a park at the top and walk down hill to the next stop. Basalt Fills Channel in Pahoehoe Flow [N 42°58.936′, W 114°18.22′] Stop 2.5—Basalt of Little Joe Butte: Subaerial Basalt on a Pillow Lava + Hyaloclastite Delta on Lake Idaho The Shoshone basalt fl ow (basalt of Black Butte Crater Sediments [N 42°57.648′, W 115°58.526′] on the Idaho Geological Survey Twin Falls 30′ × 60′ geologic map) erupted from Black Butte Crater located in the Black The basalt of Little Joe Butte is olivine-phyric basalt that Butte Crater quadrangle. The basalt is black, fi ne grained, crops out along the western edge of the Mountain Home area vesicular to massive, and aphyric to olivine phyric. Olivine

(Cinder Cone Butte, Crater Rings SW quadrangles) and con- phenocrysts (Fo58–73) are visible in hand sample and are up to tinues into adjacent quadrangles to the west (Little Joe Butte, 2.0 mm in size. Most plagioclase (An31–72) displays a normal Dorsey Butte) and south (Grand View, C.J. Strike Dam; Jenks et chemical zoning. al., 1993). The source of this fl ow is Little Joe Butte, but it has Black Butte Crater is a broad shield volcano that rises also been mapped as the basalt of Strike Dam Road (Jenks et al., 135 m above the surrounding topography. The volcano contains 1993). It consists of at least two major fl ow units, with a mappable two large craters that are ~300 m across and 100 m in depth. fl ow front preserved in the upper unit, and appears to fl ow south This young lava fi eld extends south to the Dietrich Butte quad- along a former channel of the Canyon Creek drainage. Collapsed rangle and then west into the Shoshone, Tunupa, and Gooding areas of this fl ow are commonly overlain surfi cially by “intermit- quadrangles, and terminates just west of the town of Gooding. tent lake deposits,” i.e., dry lake beds (Shervais et al., 2002). Eruption of lava disrupted the confl uence of the Big and Little This unit is largely subaerial, but its base is commonly pil- Wood , which was probably near Shoshone, and separated lowed along its southern margin, demonstrating the fl ow of sub- the two rivers to their present confl uence at Gooding. Radio- aerial lava into standing water of Lake Idaho. The subaqueous carbon dating of underlying sediment baked by the fl ow (Kuntz portion of the fl ow consists of pillow lava and hyaloclastite breccia et al., 1986) has yielded a date of 10,130 ± 350 yr B.P. for this forming foreset delta sequences with individual “rolled” pillows basalt. Aside from a few areas covered by thin loess deposits or interspersed (Fig. 10C). The delta foresets dip to the SE, indicating alluvium, most of the basalt is completely exposed, and almost the direction the lava fl owed when it entered the lake. The subaque- all of the original fl ow morphology can be seen. The fl ow is ous lava delta is ≈10 m thick in the bluffs overlooking C.J. Strike dominated by a series of large lava tube and lava channel sys- reservoir, where it fl owed over fl at-lying lake deposits—indicating tems that can be seen on aerial photographs and topographic the exact water depth in the lake at the time of eruption. maps. Most of the Shoshone fl ow consists of smooth pavement Like the Bacon Butte Basalt at Stop 1.1B, the subaerial lava outcrops of pahoehoe, overlain by sporadic a‘a fl ows; a‘a fl ows that overlies the pillow delta contains water escape conduits also fi ll lava channels and collapsed lava tubes. and giant vesicles (steam bubbles). The conduits and vesicles We plan to visit a large lava channel in the pahoehoe fl ow can be seen from the road but a close-up examination requires that is now fi lled with a‘a lava. This lava channel has levees that climbing up the pillow delta, which is inherently unstable—be rise 60–70 ft above the surrounding lava, and the channel is as careful if you go! much as 70 ft deep (Fig. 13A). fl d006-02 page 18 of 26

18 J.W. Shervais et al.

Stop 3.1 Stop 3.2 Stop 3.3

Day One Stop 3.4 Stops Stop 3.5 Stop 3.6

Figure 11. Topographic map of central Snake River Plain showing Day 3 stops.

Figure 12. Hillshade of the Twin Falls area showing the names and locations of volcanic source buttes and the topographic expression of different basalt fl ows revealed through vertical exaggera- tion. The solid lines are main highways. The dashed line is the perimeter of the Twin Falls 30′ × 60′ quadrangle. fl d006-02 page 19 of 26

Basaltic volcanism of the central and western Snake River Plain 19 fl d006-02 page 20 of 26

20 J.W. Shervais et al.

layered spatter and massive fl ows. Most outcrops along the fl anks of the volcano can be found along terraced ridges concentric to the main crater. These terraces show the same interlayering of spatter and massive basalt, indicating that they may mark the position of older crater rims. The basalt of Crater Butte is dark gray, vesicular to massive, and fi ne grained, containing phenocrysts of plagioclase and oliv- ine. Some of the samples taken from the massive layers display a slightly diktytaxitic texture but most are intergranular in texture. Olivine phenocrysts (Fo ) are small (<0.6 mm) and rounded. A 45–74 Plagioclase phenocrysts (An55–71) are generally 2.5 mm in size. Several large plagioclase phenocrysts display an oscillatory zon- ing, which may be due to pressure fl uctuations during eruption or to magma mixing. Electron microprobe line scans across plagio- clase phenocrysts reveal a strong normal chemical zonation.

Directions to Stop 3.3 Return to paved road, turn right (S) and continue 3.3 mi (5.3 km) to Hwy 24. Turn right (W) onto Hwy 24 and continue to intersection at south end of Shoshone (≈8.3 mi/13.4 km). Turn left (S) onto Hwy 93 and drive 2.85 mi (4.6 km) to a gravel road on the left (east side) of road that leads to Notch Butte. B Stop 3.3—Notch Butte [N 42°53.036′, W 114°24.98′] Figure 13. Volcanic features of the central Snake River Plain north of Twin Falls, Idaho. (A) Lava channel in the Sho- Notch Butte is located in the southeastern corner of the shone basalt. (B) Pit crater in Crater Butte. Shoshone quadrangle and rises 110 m above the surrounding topography. The lava fl ows from this broad shield volcano cover 150 km2, including the southern half of the Shoshone quadrangle, west into the Tunupa and Gooding SE quadrangles, south into the Shoshone SW and SE quadrangles, and east into the Dietrich Directions to Stop 3.2 quadrangle. Several lobes fl owed west to Gooding, continued Return to Shoshone and proceed NE on Hwy 93 toward south and west around Gooding Butte, and fl owed into the ances- Richfi eld. Turn right (S) ≈8.8 mi (14.2 km) from intersection in tral Snake River canyon near Hagerman. The unit is equivalent Shoshone onto a paved road. Drive south 2.5 mi (4.0 km) to the to the Wendell Grade basalt of Malde et al. (1963), except for second dirt road on the right side of road. Turn right onto this dirt the canyon fi lling part at Hagerman, which they mapped as Sand road and drive to the top of Crater Butte (~0.9 mi/1.5 km). Note: Springs basalt. The unit has been renamed during recent mapping if we cannot get the vans up Crater Butte, we will drive to the top to follow the convention of naming fl ows after their source vent. of nearby Dietrich Butte instead. The fl ows display a relatively young volcanic morphology with many fl ow features such as pressure ridges, collapsed lava Stop 3.2—Crater Butte [N 42°57.582′, W 114°16.724′] tubes, and fl ow fronts still visible beneath a thin soil and loess layer. The loess mantle is mostly confi ned to depressions in the Crater Butte, located in the northeastern portion of the fl ow surfaces. Note the contrast between fl ows from this vent and Dietrich quadrangle, is a steeply sloping shield volcano that rises those from the older Crater Butte vent and the younger Shoshone ~140 m above the surrounding topography. The most striking fl ow lavas. feature of this volcano is the large 80 m deep crater in its center. Mapping revealed three to four lobes of varying mineralogy This bowling pin-shaped crater is ~1300 m × 1000 m in size within the fl ow fi eld. In general, higher lobes closer to the vent and trends in a NW–SE direction. The inner walls of the crater were found to be rich in plagioclase, while those at lower eleva- show thin layers of spatter interbedded with thicker beds of more tions farther away from the vent contain less plagioclase and are massive basalt (Fig. 13B). The tops of the more massive beds rich in olivine. The higher lobes are interpreted to be younger, are highly vesicular which indicates gaseous escape from lava more fractionated, and therefore more viscous basalt (due to the channels beneath the cooler, more viscous upper crust. When the plagioclase content), while those found at lower elevations are volcano was active, the crater probably contained a lava lake that older, less fractionated, and less viscous. This difference in vis- would periodically spill over the spatter rim to create the inter- cosity between the older and younger lava accounts for the fact fl d006-02 page 21 of 26

Basaltic volcanism of the central and western Snake River Plain 21

that the older fl ows spread out over a large area away from the northeast toward Owinza Butte and Black Ridge Crater. Older vent while the younger fl ows did not. buttes to the southeast include Hansen Butte, Skeleton Butte, The basalt of Notch Butte is a black, vesicular to massive, Hazelton Butte, Milner Butte, and Burley Butte. medium- to fi ne-grained, plagioclase- ± olivine-phyric basalt. Looking south toward the Snake River canyon and Twin Samples taken from some fl ow lobes contain large glomero- Falls City, the track of Bonneville Flood overland fl ow was east crysts of plagioclase and olivine, which are visible in hand to west obliquely across I-84. Along the canyon wall south of sample. The basalt is intergranular to intersertal in texture and I-84 the overland fl oodwaters rejoined the component follow-

contains olivine phenocrysts (Fo35–73); large glomerocrysts of ing the course of the river, forming cataracts in several places.

plagioclase (An59–71) and olivine are visible in thin section and Although the basalt of Rocky Butte typically has loess fi lling are often up to 5 mm in size. The larger plagioclase phenocrysts surface depressions, in the track of the fl oodwaters virtually all are typically normally zoned. the loess is stripped away and patchy gravel deposits record local The view from the top of Notch Butte includes all of the deposition during the fl ood. major volcanoes in this part of the central Snake River plain: Crater Butte, Dietrich Butte, Owinza Butte, and Black Ridge Directions to Stop 3.5 Crater to the east; Lincoln Butte to the west; Bacon Butte, and Return to Hwy 25; turn left (E) onto Hwy 25 and proceed Flat Top Butte to the south, and Wilson Butte and Rocky Butte to 4.6 mi (7.4 km) to intersection with paved road. Turn S (right) the southeast. From here, it is easy to contrast the relative ages of and drive 5.4 mi (8.7 km) south—road goes from paved to gravel the fl ows, which can be estimated by the amount of cover indi- after ~2 mi (3.2 km). Continue on dirt road to SE around end of cated by farming. We can also see the western fl ow from Notch Wilson Butte fl ow lobe, ≈1 mi (1.6 km). Park anywhere. Butte, which extends some 40 km to the west where it fl owed into the ancestral valley of the Snake River. At the time of Notch Stop 3.5—Wilson Butte Flow Lobe [N 42°37.82′, W 114°21.76′] Butte eruptions, the ancestral Snake River was already at about its present position and elevation. Wilson Butte is a large shield volcano (≈10 km across). Its vent, composed of three small peaks, is rather unimpressive when Directions to Stop 3.4 compared to the estimated 250 km2 of lava that fl owed down the Return to Hwy 93. Turn left (S) onto Hwy 93 and drive south slopes of the shield. The fl ows cover the majority of the Shoshone 11.8 mi (19 km) to Hwy 25; turn left (E) onto Hwy 25 and after SE and half of the Star Lake quadrangles. The fl ow continues into ≈0.6 mi (1 km) turn left (N) onto paved road; follow for 0.3 mi the Shoshone SW and Hunt quadrangles, and it crosses Hwy 25 (0.5 km), then turn left (N) onto paved/gravel road that goes to in the Twin Falls NE quadrangle and continues into the Kimberly top of Flat Top Butte (~1 mi/1.6 km from highway). quadrangle, possibly emptying into the Snake River around the Twin Falls rapids or Devil’s Corral area. Stop 3.4—Flat Top Butte [N 42°43.722′, W 114°24.75′] The mode of the basalt is 35%–40% plagioclase, 10%–15% olivine, 10%–20% , 5%–8% oxides, and 15%–20% Flat Top Butte is an extremely large (≈450 km2) shield vol- glass. Plagioclase shows typical euhedral lath-shaped crystals

cano located ~20 km north of Twin Falls in the Falls City 7.5′ with An content ranging from An66 to An36. Olivine ranges from topographic map quadrangle. The vent has a large (200 m), shal- Fo71 to Fo33. low depression at its summit that represents the former summit At this stop we will view a major fl ow lobe from Wilson crater; the rim of this crater is now home to a forest of microwave Butte that stands 10–20 m above the surrounding lava fi elds and transceivers. The fl anks of the volcano are covered with a thick contains a giant complex of lava tubes that fed secondary fl ow mantle of loess and soil that obscures the underlying basalt. This lobes that fl owed primarily west and south. The fl ow lobe is char- butte is farmed almost to its summit, showing that it is one of the acterized by steep sides and a nearly fl at upper surface that can older vents in this part of the plain. Radiometric ages for Flat Top be traced for tens of kilometers. This lava tube–channel system Butte are in the 0.330–0.395 Ma range (Tauxe et al., 2004; Idaho fed fl ows adjacent to the Snake River that have been mapped as Geological Survey, 2005, unpublished data). Sand Springs basalt—in addition to fl ows from Rocky Butte (Hill The basalt of Flat Top Butte fl owed south and west fi ll- 4526), which may dominate the Sand Springs unit. Wilson Butte ing the course of the ancestral Snake River as far as Thousand and Rocky Butte have lavas that are nearly identical chemically, Springs, where Malde and Powers (1972) mapped the fl ows as and both appear to have erupted at essentially the same time, the Thousand Springs Basalt. Flows from Flat Top Butte defi ned although the Wilson Butte fl ow lobe appears to be younger than the present position of the Snake River by forcing the drainage the adjacent fl ows from Rocky Butte. Tauxe et al. (2004) report southward against the regional north slope of the older shield vol- radiometric age for “Sand Springs Basalt,” probably from Rocky canoes. Subsequently, the Snake River canyon was cut. Butte, as 0.095 Ma. The lava tube system is exposed in a series of From the summit area of Flat Top Butte we can look east windows that are open mainly on the eastern margin of the fl ow. toward the younger vents Wilson Butte, Rocky Butte, and Sitting on the Wilson Butte fl ow lobe here are coarse sedi- Kimama Butte, north toward Bacon Butte and Notch Butte, and ments deposited by the Bonneville fl ood. These sediments seem fl d006-02 page 22 of 26

22 J.W. Shervais et al. to represent a lag deposit from water fl owing over or around the Looking north across the river we can see over 4 m.y. of end of the lobe. At this elevation (~1173 m) the Bonneville Flood volcanic history (Fig. 14). At the base of the section is the Twin waters were relatively shallow. The relief of this fl ow lobe was Falls rhyolite (ca. 6.25 Ma; Armstrong et al., 1975) that forms the just suffi cient to divert most of the overland fl oodwater around basement here, and which underlies east of Twin the feature, but a few low spots allowed water to spill through and Falls. Sitting on the rhyolite (and on some patchy exposures of create small plunge pools and gravel deposits. The fl ood waters rhyolite breccia) are sediments that may correlate with the Plio- lost energy on the lee side of the fl ow lobe and deposited fi ner cene Glenns Ferry Formation (Covington et al., 1985; Covington sediment that forms fl at surfaces in broad depressions. and Weaver, 1989). If this correlation holds, these sediments would represent lake and fl ood-plain sediments that were depos- Directions to Stop 3.6 ited in an extension of Lake Idaho. Basalts intercalated with these Return to Hwy 25, turn W (left) and return to Hwy 93. Turn sediments just upstream from here have been dated at ca. 4 Ma left onto Hwy 93 and drive south 8.6 mi (13.8 km) to Perrine by Armstrong et al. (1975). Bridge in Twin Falls (you will cross I-84). Cross the bridge and Sitting on the sediment and rhyolite is a thick section of enter turnout into viewpoint-tourist information area immedi- basalt erupted from large vents to the south (Hub Butte and ately on south side of bridge. Park in visitor center parking lot other southern buttes). These fl ows are overlain by fl ows from next to bridge. Hansen Butte and other volcanoes to the southeast, which pushed the river farther north and built the north slope of a large shield Stop 3.6—Perrine Bridge Overlook [N 42°35.8966′, complex. Evidence downstream shows subsequent fl ows from W 114°27.266′] Flat Top Butte burying the preexisting northward slope, presum- ably fi lling any previous east-west drainage, and reestablishing The Perrine Bridge viewpoint is one of the classic views the course of the ancestral Snake River at the south edge of the of volcanic stratigraphy in the central Snake River Plain. The Flat Top shield. By the time of eruptions from Wilson Butte and Snake River gorge is over 120 m deep here and contains two golf Rocky Butte, the Snake River had incised the older basalts along courses within the gorge just downstream from the bridge. Evil the margin of the Flat Top Butte shield. Evidence in the canyon Kneivel’s famous attempt to jump the gorge on a rocket-powered wall shows local fi lling of the canyon by the younger fl ows. motorcycle took place just upstream from here. Subsequent fl ows from Flat Top Butte, Wilson Butte, and Rocky Butte followed the northern margin of these fl ows. The course of the Snake River then became established along the contact of the younger and older fl ows. The basalt of Flat Top Butte was formerly mapped as Thousand Springs Basalt by Malde and Sand Springs Basalt (Rocky or Wilson Buttes) Powers (1972), while the overlying Sand Springs Basalt (Malde and Powers, 1972) is now known to represent fl ows from at least Basalt of Flat Top Butte two vents in this area, Wilson Butte and Rocky Butte. However, valley-fi lling Sand Springs Basalt mapped along Cedar Draw Hub Butte Basalt (?) south of the Snake River (Malde and Powers, 1972) is probably from Flat Top Butte. At this vantage point near Perrine Bridge, the Bonneville Flood at maximum discharge would have been mostly confi ned to the canyon except that overland fl oodwaters from the north- Glenns Ferry Fm. east were rejoining the canyon along here and the water depth exceeded the top of the canyon by ~15 m. Areas of cataracts and plunge pools on the north side of the canyon, such as the Blue Lake alcove, show the erosive power of the overland fl ow (Fig. 15). In the canyon bottom, the golf courses have been built Twin Falls Rhyolite on fl ood-stripped and molded surfaces of older basalt and rhyo- lite that have thin deposits of fl ood gravel and sand. The erosional and depositional features of the Bonneville Flood are time transgressive, albeit a short time period of only Figure 14. Volcanic stratigraphy at the Perrine Bridge, Twin Falls, Ida- a few months. Unlike Glacial that is thought to ho. Dark rock at bottom of canyon is the 6.25 Ma Twin Falls rhyolites. have emptied and produced a catastrophic fl ood duration of only The thickest section of basalt erupted from vents south of the river several days, the catastrophic fl oodwater from Pleistocene Lake (Hub Butte, Sonnickson Butte), and is overlain by a thinner section of basalt from north of the current river (mostly Flat Top Butte with basalt Bonneville was more complex (O’Connor, 1993). The early of Rocky Butte and/or Wilson Butte) on top. Basalt and rhyolite are phases were in low discharge as the lake gently overtopped the separated by thin wedges of clastic sediment in middle of section. divide at Red Rock Pass and it began to erode. As erosion accel- fl d006-02 page 23 of 26

Basaltic volcanism of the central and western Snake River Plain 23

Figure 15. Topographic map of the area near Perrine Bridge showing features formed by the Bonneville Flood, includ- ing cataracts, plunge pools, scoured canyon walls, and giant gravel bars. Large arrows show inferred fl oodwater fl ow directions.

Armstrong, R.L., Leeman, W.P., and Malde, H.E., 1975, K-Ar dating, Quater- erated, the discharge grew to catastrophic proportions and was nary and Neogene volcanic rocks of the Snake River Plain, Idaho: Ameri- sustained for many weeks. However, as the level of Lake Bonn- can Journal of Science, v. 275, p. 225–251. eville dropped, gradually the discharge lowered. Ultimately, an Armstrong, R.L., Harakal, J.E., and Neill, W.M., 1980, K-Ar dating of Snake River plain (Idaho) volcanic rocks; new results: Isochron/West, v. 27, equilibrium was achieved between the lake and the outlet, and the p. 5–10. lake continued to drain into the Snake River drainage for at least Bonnichsen, B., 1982a, The Bruneau-Jarbidge eruptive center, southwestern hundreds of years. Given this backdrop, the overland fl oodwaters Idaho, in Bonnichsen, B., and Breckenridge, R.M., eds., Cenozoic Geology of Idaho: Idaho Bureau of Mines and Geology Bulletin 26, entering the canyon from the northeast represent fl ow during the p. 237–254. greatest discharges, and would have ceased when the canyon Bonnichsen, B., 1982b, Rhyolite fl ows in the Bruneau-Jarbidge eruptive center, could accommodate the fl ood. Features in the canyon, therefore, in Bonnichsen, B., and Breckenridge, R.M., eds., Cenozoic Geology of Idaho: Idaho Bureau of Mines and Geology Bulletin 26, p. 283–320. range from high-discharge rock scouring and deposition of giant, Bonnichsen, B., and Godchaux, M.M., 2002, Late Miocene, Pliocene, and bouldery gravel bars, to lower-discharge thin sand and gravel Pleistocene geology of southwestern Idaho with emphasis on basalts in deposits in the bottom of the canyon. the Bruneau-Jarbidge, Twin Falls, and western Snake River Plain , in Bonnichsen, B., White, C.M., and McCurry, M., eds., Tectonic and Return to I-84. Enter freeway in east-bound direction Magmatic Evolution of the Snake River Plain Volcanic Province: Idaho (right turn). Return to Salt Lake City (approximate driving Geological Survey Bulletin 30, p. 233–312. time: 4 hours). Bonnichsen, B., and Kauffman, D.F., 1987, Physical features of rhyolite lava fl ows in the Snake River plain volcanic province, southwestern Idaho, in Fink, J.H., ed., The Emplacement of Silicic Domes and Lava Flows: REFERENCES CITED Geological Society of America Special Paper 212, p. 119–145. Bonnichsen, B., White, C.M., and McCurry, M., eds., 2002, Tectonic and Magmatic Evolution of the Snake River Plain Volcanic Province: Idaho Amini, M.H., Mehnert, H.H., and Obradovich, J.D., 1984, K-Ar ages of late Geological Survey Bulletin 30, 482 p. Cenozoic basalts from the western Snake River Plain, Idaho: Isochron/ Braile, L.W., Smith, R.B., Ansorge, J., Baker, M.R., Sparlin, M.A., Prodehl, C., West, v. 41, p. 7–11. Schilly, M.M., Healy, J.H., Meuller, S., and Olsen, K.H., 1982, The Yel- Anders, M.H., and Sleep, N.H., 1992, Magmatism and extension: the thermal lowstone–Snake River Plain seismic profi ling experiment: Crustal struc- and mechanical effects of the Yellowstone plume: Journal of Geophysical ture of the eastern Snake River Plain: Journal of Geophysical Research, Research, v. 97, p. 15,379–15,393. v. 87, p. 2597–2609. fl d006-02 page 24 of 26

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