Basaltic volcanism of the central and western Snake River Plain:... guide to fi eld relations between Twin Falls and Mountain...

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Geological Society of America
Field Guide 6
2005
Basaltic volcanism of the central and western Snake River Plain: A
guide to field relations between Twin Falls and Mountain Home, Idaho
John W. Shervais
Department of Geology, Utah 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, Louisiana 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, California 92182-1020, USA
ABSTRACT
Basaltic volcanism in the Snake River Plain of southern Idaho has long been associated with the concept of a mantle plume that was overridden by North America during
the Neogene and now resides beneath the Yellowstone plateau. This concept is consistent
with the time-transgressive nature of rhyolite volcanism in the plain, but the history of
basaltic volcanism is more complex. In the eastern Snake River Plain, basalts 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 flows that extend 50–60 km
from the central vent. The shields coalesce to form extensive plains of basalt 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 field relations between Twin Falls and Mountain Home, Idaho, in Pederson, J., and
Dehler, C.M., eds., Interior Western United States: Geological Society of America Field Guide 6, p. xxx–xxx, doi: 10.1130/2005.fld006(02). For permission to
copy, contact [email protected]. © 2005 Geological Society of America
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J.W. Shervais et al.
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high along the length of the plain. In contrast, basaltic volcanism in the western Snake
River Plain formed in two episodes: the first (ca. 7–9 Ma) immediately following the
eruption of rhyolites lavas now exposed along the margins of the plain, and the second
forming in the Pleistocene (≤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 sediments of Pliocene-Pleistocene Lake Idaho,
which filled the western Snake River Plain graben after the end of the first 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
flood.
INTRODUCTION
The Snake River Plain of southern Idaho is one of the most
distinctive physiographic features of North America (Fig. 1). The
topographic low that defines the plain cuts across the structural
grain of both the Idaho batholith and the Basin and Range province—even though formation of the plain coincided in time with
Basin and Range extension. Evolution of the eastern Snake River
Plain has been associated with the passage of North America over
a mantle hotspot, forming a time-transgressive volcanic province
that youngs from WSW to ENE (e.g., Morgan, 1972; Suppe et
al., 1975; Armstrong et al., 1975). The onset of hotspot-related
volcanism is marked by a series of overlapping caldera complexes, ignimbrites, and caldera-filling rhyolite lavas (Bonnichsen, 1982a, 1982b; Bonnichsen and Kauffman, 1987; Pierce and
Morgan, 1992; Christiansen et al., 2002). The early rhyolite complexes were followed by extensive eruptions of plains basalts,
which form a carapace on top of the earlier rhyolites (Malde and
Powers, 1962; Greeley, 1982; Leeman, 1982; Kuntz et al., 1982,
45º 30’
rn
P
2
SR
P
SR
MH
ste
n
3
Ea
W
es
te
r
1
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41º 59’
42º 00’
117º 02’
111º 05’
Figure 1. Physiographic map of the northwestern United States showing the Snake River Plain (SRP) and related features. Note the strong
topographic expression of the plume track and the absence of Basin
and Range extension across the axis of the plain. MH—Mountain
Home; TF—Twin Falls.
1992; Malde, 1991). The hotspot is currently located under the
Yellowstone Plateau, which also forms the locus of a gigantic
geoid anomaly that underlies much of western North America
(Smith and Braile, 1994; Pierce et al., 2002).
The hotspot or plume model for the Snake River Plain is
supported by studies of tectonic uplift and collapse along the
plume track (Pierce and Morgan, 1992; Anders and Sleep, 1992;
Smith and Braile, 1994; Rodgers et al., 2002), the 1000-km-wide
geoid anomaly centered under Yellowstone (Smith and Braile,
1994), seismic tomography of the underlying mantle (Saltzer
and Humphreys, 1997; Jordan et al., 2004), and helium isotopes
(Craig, 1997). Alternate models have been proposed, however,
such as localized asthenospheric upwelling associated with edge
effects of North American plate motion, and counter flow created
by the descending Farallon slab (Humphreys et al., 2000; Christiansen et al., 2002).
There are two aspects of Snake River Plain volcanism that
are not readily explained by the hotspot model. First, the eruption 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
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 Yellowstone hotspot, although models
that link volcanism in the western Snake River Plain indirectly
to the hotspot have been proposed (Geist and Richards, 1993;
Camp, 1995; Shervais et al., 2002; Camp and Ross, 2004).
Basalts of the Columbia Plateau, thought to represent the “head”
of the Yellowstone plume, also lie well north of the presumed
hotspot track. Several models have been proposed to explain
this anomaly, including deflection of the plume by the Farallon
plate (Geist and Richards, 1993), compression of the plume head
by North American lithospheric mantle (Camp, 1995; Camp
and Ross, 2004), or location of the plume head below northern
Nevada (Pierce and Morgan, 1992; Pierce et al., 2002).
The central Snake River Plain lies at the junction of the
physiographic western and eastern Snake River Plain and represents a critical link between the two volcanic provinces. The
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Basaltic volcanism of the central and western Snake River Plain
structural, geophysical, sedimentary, and volcanic features of
these provinces are distinct and require different origins. In this
guidebook, we present a brief synopsis of geologic relations
within and between the central and western Snake River Plain
and then examine the stratigraphy and volcanology of each province in the field guide section. A compendium of recent papers on
the Snake River Plain has recently been published by the Idaho
Geological Survey (Bonnichsen et al., 2002), and interested readers are referred to that volume for more detailed information.
THE SNAKE RIVER VOLCANIC PROVINCE:
GEOLOGIC SETTING
The central and western Snake River Plain comprise two distinct provinces with different crustal structure, stratigraphy, and
volcanic history. Both provinces are characterized by crust that
is thicker (40–45 km) than crust in the adjacent Basin and Range
province (≈33 km; Mabey, 1978, 1982; Iyer, 1984; Malde, 1991).
The western Snake River Plain is also characterized by a positive
gravity anomaly that trends parallel to the axis of the plain, and
magnetic anomalies that parallel its southern margin (Mabey,
1982). In contrast, gravity and magnetic anomalies in the eastern
Snake River Plain are subdued and define a NW-trending texture
that is normal to the trend of the eastern plain, but parallels structural trends in the adjacent Basin and Range province (Mabey,
1982; Malde, 1991). These contrasts reflect fundamental differences in the underlying structure and stratigraphy of the two terranes: the western Snake River Plain is a true graben, whereas the
eastern Snake River Plain is structurally downwarped with little
or no faulting along its margins (McQuarrie and Rodgers, 1998;
Wood and Clemens, 2002; Rodgers et al., 2002).
Central and Eastern Snake River Plain
The ENE-trending central and eastern Snake River Plain
begins as a gentle structural depression on the Owyhee Plateau
that deepens to the NE and merges with the physiographic Snake
River Plain near Twin Falls. This structural depression continues
to the NE until it merges with the Yellowstone Plateau (Rodgers
et al., 2002). The central Snake River Plain is defined loosely as
that part of the eastern Snake River Plain trend that lies between
the Owyhee Plateau and the Great Rift, or approximately between
116°W and 114°W along the axis of the plain.
The increase in elevation of the eastern Snake River Plain
from SW to NE is thought to result from thermal buoyancy in
the upper mantle under the hotspot (Dueker and Humphreys,
1990; Pierce and Morgan, 1992; Smith and Braile, 1994). The
progressive decay of this thermal anomaly with time has resulted
in tectonic collapse in the wake of the “deformation parabola”
that emanates from the hotspot (Anders and Sleep, 1992; Pierce
and Morgan, 1992; Smith and Braile, 1994). The elevation difference between the Owyhee Plateau and the eastern Snake River
Plain probably results from differences in the underlying crust:
the Owyhee Plateau is underlain by the southern extension of the
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Idaho batholith, whereas the eastern Snake River Plain transects
older crust of the Basin and Range province (Malde, 1991).
The eastern Snake River Plain is characterized by 1–2 km of
basalt that overlies rhyolite and welded tuff (e.g., Leeman, 1982;
Kuntz et al., 1988, 1992; Greeley, 1982). Scientific drill holes at
the Idaho National Laboratory (INL) site show that the basaltic
suite ranges from <100 m to over 1500 m thick, with rhyolite
basement extending to depths in excess of 3000 m (Malde, 1991;
Hughes et al., 1999, 2002). Sedimentary intercalations consisting of fluvial sands, lacustrine muds, windblown sand, and loess
range from 2 m to ≈25 m thick.
The rhyolite eruptive centers consist of overlapping caldera
complexes and ignimbrites that represent the initial volcanic
activity at any given location along the axis of the Snake River
Plain, and are thought to mark the arrival of the hotspot (e.g.,
Bonnichsen, 1982a; Christiansen, 1982; Draper, 1991; Morgan,
1992). Rhyolite eruptive centers become younger from SW to
NE: the Bruneau-Jarbidge eruptive center (12.5–11.3 Ma), the
Twin Falls eruptive center (10.0–8.6 Ma) the Picabo eruptive
center (10.2 Ma), the Heise eruptive center (6.7–4.3 Ma) and the
Island Park–Yellowstone eruptive center (1.8–0.6 Ma). The oldest
basalts in the central and eastern Snake River Plain are slightly
younger than the eruptive centers they mantle; the youngest
basalts erupted from a series of NW-trending volcanic rift zones
during the Holocene (e.g., the Shoshone lava flow, Craters of the
Moon, the Great Rift, Hells Half Acre, Wapi lava flow), with flows
as young as 2000 yr B.P. reported (Kuntz et al., 1986).
Geophysical studies of the eastern Snake River Plain
have documented differences in both the crustal structure and
lithosphere-asthenosphere boundary relative to both the adjacent
Basin and Range province and the Archean Wyoming craton
(Braile et al., 1982; Iyer, 1984; Dueker and Humphreys, 1990,
1993; Humphreys and Dueker, 1994; Peng and Humphreys,
1998; Saltzer and Humphreys, 1997; Humphreys et al., 2000).
The most significant crustal feature observed in seismic profiles
of the eastern Snake River Plain is a midcrustal “sill” ≈10 km
thick and 90 km wide that underlies the entire width of the eastern Snake River Plain, with seismic velocities (≈6.5 km/s) intermediate between the mafic lower crust and the more felsic intermediate crust (Braile et al., 1982; Peng and Humphreys, 1998).
This mafic “sill” is inferred to represent basaltic melts that were
intruded into the crust and either fractionated to form the high
temperature rhyolites of the eastern Snake River Plain (McCurry
and Hackett, 1999) or induced partial melting of the crust to form
these rhyolites (Leeman, 1982; Bonnichsen, 1982a, 1982b; Doe
et al., 1982). The crust is also underlain by a low velocity layer
that is thought to be partially molten (Peng and Humphreys,
1998). This low-velocity layer apparently thickens to the NE,
toward Yellowstone, where it dominates the lower crustal section
(Priestley and Orcutt, 1982).
Teleseismic tomography across the eastern Snake River
Plain and adjacent domains show that the subcontinental lithosphere directly beneath the plain has been eroded to form a deep
channel parallel to North American plate motion (Dueker and
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Humphreys, 1990; Humphreys and Dueker, 1994; Saltzer and
Humphreys, 1997; Humphreys et al., 2000). The channel consists of low velocity, partially molten asthenosphere buttressed by
levees of high-velocity, melt-depleted mantle. The depth of the
lithosphere-asthenosphere boundary beneath the plain is roughly
constant parallel to its axis (Saltzer and Humphreys, 1997). Since
the subcontinental mantle lithosphere is thicker toward the NE,
the sublithospheric channel must be more deeply eroded into the
lithosphere to the NE.
Tomographic images of the Yellowstone plume show that it
dips steeply to the north from Yellowstone and extends to a depth
of 400–600 km (Montelli et al., 2003; Jordan et al., 2004; Dueker
et al., 2004). This is consistent with recent models of deep-seated
mantle plumes, which show that they may tilt significantly from
vertical and follow complex flow lines within the mantle (e.g.,
King, 2004; Farnetani and Samuel, 2004).
Western Snake River Plain
In contrast to the eastern Snake River Plain, the western
Snake River Plain is a NW-trending structural graben, ≈70 km
wide and 300 km long, bounded by en echelon normal faults
and filled with up to 2–3 km of late Miocene through Pliocene
sediment (Wood and Clemens, 2002; Shervais et al., 2002).
Structural relief, based on deep drill holes that intercept basement, ranges from 2900 m near Mountain Home to over 3500 m
on the southwest side of the valley (Malde, 1991). Sedimentary
deposits in the western Snake River Plain range in age from
Miocene through Quaternary; these deposits are dominantly
lacustrine, with subordinate fluviatile and phreatomagmatic
deposits (Wood and Clemens, 2002; Godchaux and Bonnichsen,
2002; Shervais et al., 2002). Miocene sediments were deposited
in small, interconnected lakes, precursors to the large Pliocene
“Lake Idaho” (Kimmel, 1982; Smith et al., 1982; Jenks and Bonnichsen, 1989; Malde 1991; Godchaux et al., 1992; Jenks et al.,
1993; Wood and Clemens, 2002). The western Snake River Plain
is topographically lower than the eastern Snake River Plain, with
elevations ranging from 670 m to 1100 m (Malde, 1991), but
recent data suggest that an ancestral Snake River system flowed
southwards during the Miocene, implying higher elevations to
the NW (Smith and Stearley, 1999; Link and Fanning, 1999).
Paleontological evidence similarly suggests southward drainage
in the late Miocene (Repenning et al., 1995).
Volcanic activity in the western Snake River Plain began
with the eruption of the silicic volcanic rocks along the northern
and southern margins of the basin between ca. 11.8 and 9.2 Ma
(Clemens and Wood, 1993; Wood and Clemens, 2002). Major
basaltic activity in the western Snake River Plain occurred in two
time periods: 9–7 Ma, and <2.2 Ma (White et al., 2002; Bonnichsen and Godchaux, 2002). The earlier episode is represented
by basalt flows and phreatomagmatic vents intercalated with late
Miocene sediments and by the acoustic basement that underlies
much of the western graben (Malde and Powers, 1962, 1972;
Amini et al., 1984; Malde, 1991; Wood and Clemens, 2002;
White et al., 2002; Shervais et al., 2002). The older (7–9 Ma)
lavas and late Miocene to Pliocene sediments comprise the Idaho
Group of Malde and Powers (1962). Young volcanic activity
(<2.2 Ma) in the western Snake River Plain consists of: (1) plateau forming eruptions of tholeiitic basalt that form the volcanic
uplands north and south of the Snake River, (2) tholeiitic shield
volcanoes that sit on top of these uplands, and (3) young shield
and cinder cone vents of alkaline to transitional alkaline basalt
(Shervais et al., 2002; White et al., 2002). The younger lavas
comprise the Snake River Group of Malde and Powers (1962)
and correlate with the more abundant young volcanic rocks of
the eastern Snake River Plain (Leeman, 1982). Volcanic rocks
of similar age and character are also found in the Boise River
drainage 40 km north of Mountain Home (Howard and Shervais,
1973; Howard et al., 1982; Vetter and Shervais, 1992).
The physiographic junction between the western and eastern
portions of the plain is located west of Twin Falls, Idaho (Fig. 1).
Structurally, boundary faults of the western Snake River Plain
graben extend SE into the eastern Snake River Plain plume track
as far as Hagerman, and linears extend as far as Buhl, near Twin
Falls. The axial Bouguer gravity high of the western Snake River
Plain also extends into the plume track, and is deflected eastward
south of the Mount Bennett Hills (Mabey, 1976, 1982). The
positive magnetic anomaly along the SW margin of the western
Snake River Plain curves to the NE where it intersects the plume
track (Mabey, 1982), possibly outlining the southern margin of
the Bruneau-Jarbidge eruptive center (Bonnichsen, 1982a). The
NW-trending gravity and magnetic anomalies, which characterize the eastern Snake River Plain, are absent in this area.
BASALTIC VOLCANISM: FIELD RELATIONS AND
GEOCHEMISTRY
Central Snake River Plain
Late Neogene basalts of the central Snake River Plain north
of Twin Falls form large shield volcanoes (Fig. 2) clustered along
the axis of the plain, which overlie rhyolite of the Twin Falls
eruptive center (Bonnichsen and Godchaux, 2002). This area
has been mapped in detail during the past decade by the Idaho
Geological Survey, by graduate students from the University of
South Carolina and Utah State University, and by undergraduates from Centenary College. The results of this work have been
compiled by the Idaho Geological Survey and are being prepared
for publication as the Twin Falls 30′ × 60′ quadrangle geological
map. Readers are referred to the preliminary copy of that map for
the names, and extent of volcanic units in the central Snake River
Plain and their relation to sedimentary units.
All of the hills seen on this part of the plain are constructive
volcanic vents. The shield volcanoes of the oldest vents have subdued topography and radial drainage and a thick cover of loess
with well-developed soils, and they are typically farmed almost
all of the way to the vent summits (e.g., Flat Top Butte, Johnson
Butte). Many of the older vents appear smaller than the largest
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Basaltic volcanism of the central and western Snake River Plain
young vents because their flanks have been partially buried by
younger lava flows (e.g., Skeleton Butte, Bacon Butte, Lincoln
Butte). The surfaces of flows from younger vents are characterized by extremely rugged, chaotic topography, with inflated flow
fronts, collapsed flow interiors, ridges, and collapse pits (e.g.,
Owinza Butte, Rocky Butte, Notch Butte, Wilson Butte). These
surfaces lack established drainages but thin loess is present in
local depressions. The surfaces of flows from vents of intermediate age have nearly continuous loess mantles of variable thickness
and well-developed soils, but lack well defined surface drainage
and are rarely farmed (e.g., Crater Butte, Dietrich Butte). Where
the Bonneville Flood flowed overland, loess and soil were commonly stripped from basalt surfaces yielding surface morphology
that appears younger then it should, which complicates identification of basalt units based on surface characteristics.
Shield sizes vary and are difficult to constrain with confidence because the onlapping of younger lava flows often
conceals the true size of the older shields. Even so, the largest
are commonly 15–20 km across the main shield edifice—not
counting elongate flows that may extend 50–60 km from the
vent. Perhaps the most extensive shield in the region is Black
Ridge Crater, located at the eastern edge of the central plain
(113.96°W), which is ≈32 km across and covers ≈680 km2—all
or parts of eight 7.5′ quadrangles.
Shield volcanoes with extremely long flows include Black
Ridge Crater, Notch Butte, Wilson Butte, and Rocky Butte. The
Black Ridge Crater vent has a major sinuous lobe that flowed
over 40 km from the vent, terminating just south of Notch Butte.
Rocky Butte and Wilson Butte both fed extensive flow fields.
Flows from Notch Butte extend west as far as Hagerman, a distance of ~42 km.
A few radiometric dates are available for basalts in the central 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 flows, and typically have subdued morphology
and radial drainage patterns. Remanent magnetic polarity of most
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 characteristics 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%), TiO2 (1.6%–4.3%), P2O5 (0.4%–1.2%), and Al2O3
(14%–17%). They are also higher in FeO* than similar basalts of
the eastern Snake River Plain (e.g., Idaho National Engineering
Laboratory or INEL site) but less Fe-rich than typical ferrobasalts
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of the western Snake River Plain near Mountain Home (Fig. 3).
The wide range in K, P, and K/P ratios at constant MgO implies a
range of parent magmas derived from a similar source by variable
degrees of partial melting. Fe8 values (≈13) imply deep melting
or a source higher in FeO than MORB asthenosphere, while Na8
values (2.4–3.2) imply moderate but variable degrees of partial
melting. Partial melting models based on 18 incompatible trace
elements indicate 5%–10% melting of a spinel lherzolite source
similar in composition to the E-MORB source. Garnet-bearing
sources are ruled out by the slope of the rare earth element patterns, implying pressures less than 20–25 Kb, i.e., within the
sublithospheric channel that has been imaged seismically.
Most of the chemical variation within flows from single
vents can be explained by low-pressure fractionation of the
observed phenocrysts (olivine + plagioclase). Line scans of
olivine phenocrysts show no reversals in composition or other
evidence of magma mixing. Line scans of plagioclase phenocrysts show minor reversals that indicate fluctuations in magma
chamber vapor pressures (Cooke, 1999; Matthews, 2000). The
occurrence of cumulate gabbro xenoliths (ol+cpx+plg+oxide) in
the Sid Butte vent is consistent with high-pressure fractionation
at midcrustal levels, within the “basaltic sill” imaged seismically
beneath the eastern plain (Matthews, 2000). The incompatible element ratios K/P and K/Ti decrease with mg# ( = molar
100*Mg/[Mg+Fe]), ruling out extensive assimilation of older,
A
B
Figure 2. Field photographs contrasting the stratigraphy and topography of the central and western Snake River Plain. (A) Shield volcanoes
of the central Snake River Plain north of Twin Falls. (B) Lacustrine
sediments of the Glenns Ferry Formation near Mountain Home, overlain by plateau-forming Pleistocene basalts.
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Shoshone
Mtn Home Tholeiites
Mtn Home Alkali
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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.
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Basaltic volcanism of the central and western Snake River Plain
felsic crust; these trends may be due to assimilation of previously
injected gabbroic dikes at midcrustal depths. We infer that these
basalts represent a mixed asthenospheric-lithospheric source that
formed in response to the Yellowstone melting anomaly; these
melts evolved by a combination of high-pressure and low-pressure crystal fractionation, with possible assimilation of previously intruded midcrustal ferrogabbros (Shervais et al., 2004).
Western Snake River Plain
The western Snake River Plain graben formed over a short
time period ca. 10–12 Ma, coincident with the eruption of the
extensive rhyolites that now form the flanks of this structure
(Wood and Clemens, 2002). Late Miocene basalts (9–7 Ma)
underlie the central part of the graben, as shown by geophysical
surveys and deep drilling results (Wood, 1994; Wood and Clemens, 2002; Shervais et al., 2002). Overlying these basalts (and to
some extent interbedded with them) are up to 2–3 km of lacustrine and fluviatile sediments that form the “Idaho Group” of
Malde and Powers (1962): Chalk Hills Formation (oldest), Poison Creek Formation, Glenns Ferry Formation, and the Bruneau
Formation. Phreatomagmatic vents intercalated with sediments
of the Idaho Group are common in the area west of Mountain
Home, between Grand View and Marsing along the Snake River
(Godchaux et al., 1992; Godchaux and Bonnichsen, 2002; Bonnichsen and Godchaux, 2002).
Mountain Home occupies an upland plateau above the
Snake River floodplain, which is incised into fine-grained lacustrine sediments deposited by Lake Idaho during the late Miocene–early Pliocene (Fig. 2). Late Pliocene to early Pleistocene
basalts sit conformably on the lacustrine strata, or occur within
the uppermost few hundred feet. Flow of these basalts into the
shallow margin of the lake resulted in deltas of hyaloclastite breccia and pillow lava, which pass upward into subaerial pahoehoe
flows. Farther inland, flows are separated by deposits of fluvial
gravel and sand. These plateau-forming basalts are overlain by a
series of younger lavas erupted from 13 central vents that cluster
near the NE margin of the plateau. The shield volcanoes rise
120–210 m above the surrounding plateau; several are capped
by central depressions that probably represent former lava lakes
(pit craters). Pleistocene cinder cones are the youngest volcanic
features, and may cap small shield volcanoes.
Recent tectonic activity is demonstrated by fault scarps with
~2–9 m of throw that crosscut all of the volcanic features. There
are two dominant fault sets: one trends N85W, the other trends
N60W (parallel to the range front faults) and appears to truncate
against the N85W set. All of the faults are vertical or high angle
normal faults that generally dip steeply to the south and are downthrown on the south or SW side (a few are downthrown to the N,
forming small grabens with adjacent faults). The young faulting
may be related to Basin and Range extension that has refracted
to more westerly orientation to exploit preexisting fault zones of
the western Snake River Plain. This relationship suggests that
post–Lake Idaho volcanism in the western Snake River Plain
7
may be related to reactivated partial melting of plume-modified
lithosphere, in response to the onset of Basin and Range extension (Wood and Clemens, 2002; Shervais et al., 2002).
Basaltic lavas of the western Snake River Plain are generally
distinct from lavas of the eastern Snake River Plain trend, and are
commonly ferrobasalts with up to 17% FeO* (Fig. 3; Shervais et
al., 2002; White et al., 2002). This is clearly seen in the BruneauJarbidge eruptive center, where basalts of the eastern Snake River
Plain trend are overlain by younger ferrobasalts of Salmon Falls
Butte, which represents the SE extent of western Snake River
Plain magmatism (Vetter and Shervais, 1997). In general, the
oldest (pre–Lake Idaho) basalts are the most Fe-rich; younger
basalts include ca. 2.0–0.7 Ma Fe-Ti basalts (equivalent to the
Boise River Group 1 of Vetter and Shervais, 1992 and group M2
of White et al., 2002) and the <0.7 Ma alkali basalts which are
high in K2O and MgO (equivalent to the Boise River Group 2 of
Vetter and Shervais, 1992 and group M3 of White et al., 2002).
This same sequence of basalts is observed in the Melba area,
100 km west of Mountain Home (White et al., 2002) and along
the Boise River in the Smith Prairie area, 40 km north of Mountain Home (Vetter and Shervais, 1992).
HYDROGEOLOGY OF BASALTIC VOLCANISM IN
THE CENTRAL SNAKE RIVER PLAIN
Detailed mapping in the central Snake River Plain has
revealed new details of groundwater recharge and flow, and
allows the formulation of new models for the formation of high
conductivity aquifers.
Groundwater flow in the Snake River Plain is commonly
thought to be controlled by relatively porous horizons between
lava flows (Lindholm and Vaccaro, 1988; Welhan et al., 2002c).
These so-called interflow zones are characterized by complex
geometries reflecting the fractal nature of the pahoehoe inflationary process, the quasi-stratigraphic layering and interfingering of
flows within and between lava fields (shield volcanoes), and the
development of tension fissure networks along the margins of
inflated flow structures (Welhan et al., 2002b). These permeable
zones can be mapped in the subsurface using core and geophysical logs and their autocorrelation length scales used to construct
stochastic models of subsurface pathways of preferential flow
(Welhan et al., 2002a). Direct evidence of the “pipeline” nature
of this flow, as reflected in extremely low dispersivities associated
with mass transport on scales of 25 km, has been documented by
Cecil et al. (2000) using chlorine-36.
The mapping of Cooke (1999) and Matthews (2000) shows
that thick alluvial intervals are most common where adjacent
volcanic edifices abut one another and overlap. These overlapping flows create moats, which control the location of surface
stream drainages and fill with coarse alluvium. Younger lava
flows are channeled into these drainages, displacing the streams
and covering the alluvium with relatively thick, semi-permeable
caps, forming elongate aquifers with extremely high conductivities that follow the preexisting paleo-drainage. Reconstruction of
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J.W. Shervais et al.
basaltic volcanism through time using detailed geologic maps
allows us to predict the location of paleo-drainage and elongate
alluvial aquifers. This ability to predict aquifer location will
prove to be a valuable tool as increased demands are placed upon
the groundwater supply by agriculture and population growth.
In addition, our mapping reveals that young basalt vents seldom
exhibit channelized drainage systems that connect with major
through-going streams. Instead, the rugged volcanic topography of ridges, flow fronts, lava channels, and collapse pits trap
precipitation, which must either evaporate or percolate into the
fractured lavas to recharge the local groundwater. These young
volcanic features constitute “negative basins” of interior drainage, despite their topographic emergence. We suggest that these
young basalts represent recharge zones that can be easily mapped
and distinguished from older flows that display well-developed
external drainage (Cooke and Shervais, 1999).
On a more regional scale, the eastern and central Snake
River Plain basalts and intercalated units form the Snake River
Plain aquifer, which along with the Snake River system supplies water to southern Idaho and are essential to the agricultural
development of the state. Water recharges from snow melt in the
mountains north of the plain and in local basalt recharge areas.
Groundwater flow is from the NE to SW and it discharges as
springs in the Thousand Springs and other reaches of the north
and east wall of the Snake River Canyon between Twin Falls
and King Hill. The spring water is used extensively by the aquaculture industry, but quantity and quality of the water has been
declining due to increased groundwater pumping upgradient,
drought, and changes in surface irrigation systems. Spring and
groundwater irrigation is supplemented by surface withdrawals
from the Snake River, but acute water shortages are posing a
political dilemma for Idaho politicians and water experts. Mapping of springs in the northwall of the Snake River Canyon by
Covington and Weaver (1989), the Idaho Geological Survey, and
others suggests a geologic control. Covington et al. (1985) (and
others before them) recognized that some springs discharge at
rubble zones at the base of where canyon-filling late Quaternary
basalts entered a paleodrainage system. It also appears that the
topography developed on the older, slightly altered (Banbury)
basalts may have influenced spring discharge, as many springs
seem to emerge through talus but just above the QuaternaryTertiary unconformity. The older units may locally form a more
impermeable horizon and basal aquitard.
FIELD TRIP GUIDE
Day 1 (Half Day) Hagerman Valley
Directions to Stop 1.1
From Hagerman, drive south on Idaho Highway 30 ~7 mi
(11 km) across the bridge over the Snake River to ~1 mi south of
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
lies ~200 ft (60 m) uphill to the right. Bonneville flood effects in
the Hagerman Valley are shown on Figure 4; all Day 1 stops are
shown on Figure 5.
On the way to Stop 1, we will drive by scour features and
deposits created by the Bonneville Flood. The following paragraphs
briefly describe some of the effects of the floodwaters (Fig. 4).
About fifty years ago, geologists first recognized that giant
gravel bars and stark erosional features common along the Snake
River were caused by cataclysmic lowering of Pleistocene Lake
Bonneville. The most complete descriptions and analyses of the
Bonneville Flood were written by Malde (1968) and O’Connor
(1993). O’Connor (1993, p. 1) writes, “Approximately 14,500
years ago, Pleistocene Lake Bonneville discharged 4750 km3 of
water over the divide between the closed Bonneville basin and
the watershed of the Snake River. The resulting flood, released
near Red Rock Pass, Idaho, followed the present courses of
March Creek, the Portneuf River, and the Snake and Columbia
River before reaching the Pacific Ocean. For 1100 km between
Red Rock Pass and Lewiston, Idaho, the Bonneville Flood left a
spectacular array of flood features.” O’Connor (1993) goes on to
describe the hydrology, hydraulics, and geomorphology of the
flood and provides a picture of the history and character of the
flood and its many landforms and deposits. A major characteristic
of the flood is a stepwise drop in the water surface caused by the
diverse geologic and geomorphic environments along the flood
route. A repeated phenomenon is a narrow canyon with constricted but high-discharge flow that opens up into a wider valley
with less restricted, lower-discharge flow. In many locations, a
constriction back-watered the flood causing inundation and overland flow. East of Twin Falls flood water diverted north of the
Snake River canyon scoured a long stretch of basalt surfaces and
rejoined flood waters in the canyon for ~17 km forming several
systems of cataracts and plunge pools (see Day 3, Stop 3.6).
The areas of Hagerman and Thousand Springs (Day 1) show
the stepwise nature of the flood’s energy and the resulting features.
The Thousand Springs reach was a constriction that overfilled and
the overland flow scoured and plucked basalt at the Sand Springs
Nature Preserve, Box Canyon, and Blind Canyon. South of Thousand Springs a dry valley appears to have been the previous location of the confluence of Salmon Falls Creek and the Snake River.
The flood may have scoured a new channel for the Snake River and
subsequently the river abandoned the previous valley. Just downstream of Thousand Springs, where the Hagerman valley opens up,
energy dropped and the floodwaters deposited a large expansion
bar 5 km long with 4 m boulders at its upstream end. At maximum
discharge, the floodwaters would have been ~60 m deep at the
location of the Hagerman Inn. In the bottom of the valley, thick
deposits of Yahoo clay that had buried basalt of Notch Butte were
stripped away and the surface of the basalt was scoured.
Stop 1.1—Banbury Basalt and Bacon Butte Basalt: Basalt
Water Interaction [N 42°43.804′, W 114°51.079′]
Mapping and reexamination of the “older” volcanic units
exposed south of Hagerman is one phase of the Idaho Geological
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Basaltic volcanism of the central and western Snake River Plain
page 9 of 26
9
Figure 4. Topographic map of Thousand Springs and the southern Hagerman Valley showing erosional and depositional features of the Bonneville Flood. Large blue arrows show inferred floodwater flow 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′ quadrangle. The area includes Banbury Hot Springs, type locality for
the Tertiary Banbury Basalt, which Malde et al. (1963) mapped
over a wide area of southwestern Idaho. Our work and the excellent mapping of Malde and Powers (1972) show much heterogeneity within the “type section.” The lower Banbury Basalt
includes a field of hydrovolcanoes, which appear to be localized
along a NW-trending structural zone that also influenced later
graben development and formation of Pliocene Lake Idaho, geothermal activity, and canyon development. The inferred structure
is suspiciously close to a projection (S49E) of the northern margin of the western Snake River Plain.
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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
River canyon between Banbury Springs
and Sand Springs showing locations
for Stop 1 of the field trip, and erosional and depositional features of the
Bonneville Flood. Relict flood features
include scoured and plucked basalt and
abandoned valley with no established
drainage.
Stop 1.3
42º43’ 11” N, 114º 51’ 24” W
Stop 1.4
42º42’ 24” N, 114º 50’ 23” W
Stratigraphically overlying Miocene rhyolite, the MiocenePliocene Banbury Basalt consists of a lower sequence of altered
basalt flows and vent deposits, a middle layer of basaltic pyroclastic deposits overlain by and/or interbedded with sediments,
which thicken to the south, and an upper sequence of basalt flows
(Malde and Powers, 1972). The vent facies of the lower Banbury
Basalt includes at least two exposed volcanic centers (Thousand
Hill and Riverside Ferry vents), which are characterized by
tuff breccias with laterally extensive and locally palagonitized
surge and tephra deposits. At Stop 1.2, spectacular 10-m-high
beds of tuff breccia contain blocks larger than 1 m. At Stop 1.1,
distal phreatomagmatic tuffs containing volcanic bombs, stream
pebbles, and siltstone xenoliths are overlain by a 1-m-thick bed
of black spatter. The spatter is overlain by an oxidized tuff with
small glass bombs, and above that are three upward-coarsening
cycles of air-fall tuff. Unconformably overlying the emergent
hydrovolcano is a baked siltstone and the upper Banbury Basalt,
which here consists of notably fresher, plagioclase-phyric, vesicular basalt flows. Elsewhere in the map area, the upper Banbury
consists of altered or water-affected basalt (WAB of Godchaux
and Bonnichsen, 2002) with a different magnetic polarity than
the feldspar-rich flows. Pending geochronology and chemistry
may improve age constraints and correlations. Even at its type
locality, the Banbury Basalt is lithologically heterogeneous with
lateral facies changes indicating basaltic volcanism within a
lacustrine and fluvial setting prior to Lake Idaho and probably
over a considerable time span (Gillerman, 2004). The heterogeneity, polarity reversals, and spatial distribution of the flows also
indicate a variety of sources for the Banbury units. As noted by
others, it is time to revise the nomenclature. The recent Idaho
Geological Survey mapping, which will be displayed on the field
trip, has renamed and more precisely subdivided many units
based on mapping and paleomagnetism, combined with chemistry and a very few radiometric age dates.
Stop 1.1A—Exposure of the Lower Banbury Distal Vent
Facies Volcaniclastics
The prominent outcrops to the right of the gully are massive
basaltic tuffs in 3–4 upward-coarsening cycles, 2–3 m thick, with
laminated to cross-bedded tuff above and thin (6–8 cm), locally
cross-bedded, fine tuff below (Fig. 6A). This sequence overlies
a juvenile conglomerate of basalt, hyaloclastites, and some
rounded stream cobbles, that transitions up to a bleached/oxidized (?) pumice bed (Fig. 6B). Below these beds are vent-like
spatter and pillowy basalt with abundant fresh glass. Although
the base of the sequence here is unexposed, Malde and Powers
(1972) mapped the lower portion of the slope as lower Banbury
sediments and tuff, most likely from better exposures ~1 mi to
the north, near the Sligar’s Thousand Springs Resort. There, a
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Basaltic volcanism of the central and western Snake River Plain
section of mixed tuffs, sandstones, and massive basaltic units
are exposed in a gully and in old road cuts. The massive brown
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 fibrous and hydrous minerals such as zeolites,
chlorite, or amphiboles. 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 flows, tectonic activity, and climatic cycles created
a rapidly changing and laterally heterogeneous distribution of
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 volcaniclastics. The upper Banbury lavas are flat lying and mostly subaerial,
but the lower part is glassy, popcorn-like, with quasi-pillows and
glass. The flows here are distinguished by abundant feldspar phenocrysts 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 relationships, age, and exact correlations of these various exposures are
problematic.
page 11 of 26
11
A
B
C
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) fills paleo-drainage
here, with evidence for water interaction at base (pillows and hyaloclastites) and vertical dewatering conduits that penetrate entire
flow (Fig. 7). The water-escape structures in this flow constitute a
feature of volcanic rocks that have not been described previously.
They consist of subvertical conduits ≈1 m in diameter (0.5–1.5 m
range) and spaced 5–15 m apart, partly filled with spicaceous
basaltic rubble. Columnar jointing in the adjacent basalt wraps
from vertical along the base of the flow to horizontal adjacent to
the conduits. Vesicles are more common in the wallrock of the
conduits than in parts of the flow farther away, and the basalt is
quenched to a glassy or aphanitic texture in the wallrock adjacent
to the conduit (Fig. 7A). In many of the conduits, basalt from the
adjacent wallrock forms a lattice or trellis arrangement within the
conduit that is contiguous with basalt of the wallrock (Fig. 7B).
The basalt in this trelliswork has glassy margins and spikey
projections along its surface. Spicaceous rubble is common in
all conduits, often forming discrete fragments that resemble
Figure 6. Banbury Basalt in Hagerman Valley. (A) Lower Banbury
Basalt, volcaniclastics and phreatomagmatic vent facies (Stop 1.1).
(B) Bleached pumice bed in lower Banbury Basalt. (C) Unconformity
of upper Banbury Basalt (Basalt of Oster Lakes) over tilted section
of lower Banbury phreatomagmatic volcaniclastics with primary and
probable structural dip (Stop 1.1). Fence posts for scale.
popcorn. This spicaceous rubble of “popcorn” basalt does not
represent later fill or pieces broken from the wallrock, but rather
blobs of lava that were isolated within the conduit while steam
was actively rising through the flow (Fig. 7C).
The upper surface of the lava flow contains depressions as
much as 10 m across and 1.5 m deep centered above the waterescape structures, which appear to represent areas where the
surface lava collapsed into the conduit during water escape. Such
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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 columnar jointing, and rubbly fill. (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 flow, just above section of megapillows.
a feature is well illustrated at another location by the occurrence
of a ropey pahoehoe flow surface that is preserved deep inside a
lava flow within a steam conduit.
The subaerial lava flow with water escape structures is
underlain by pillow lava that fills intervening depressions in the
paleosurface. Many of these are mega-pillows as much as 4 m
across, which may contain water-escape structures that continue
up into the subaerial flow. In addition, the interface between the
pillow zone and the subaerial flow commonly contains small
caves and cavities that represent large steam bubbles (>1 m
across) trapped below the lava flow (Fig. 7D).
Directions to Stop 1.2
Continue South 0.3 mi (0.5 km) on Hwy 30 to a prominent
draw on the right with several dirt roads. Turn in to the right, go
through gate and close it, and park near end of drivable jeep trail
to right. Hike northward on cow trail along wire fence ~1/3 mi
(0.5 km) to farthest gully with trees and brush. Hike steeply
uphill 150′–200′ (45–60 m) to large, cliffy exposure of the megabreccia at top of ridge on north side of gully.
Stop 1.2—Banbury Basalt: Megabreccias [N 42°43.583′,
W 114°51.35′]
As you climb up to the ridgetop, there are poor exposures on
the slope of white breccias and palagonite tuffs, as well as lavas.
The outcrop on the ridge top is an ~10-m-thick “megabreccia”
unit, which can be followed ~600′ to the west, where it terminates, perhaps on the crater rim, or is faulted.
The megabreccia (Tvp) consists of unsorted tuff breccia
with large accidental blocks, many more than 1 m in length
(Fig. 8A). Block compositions include several types of basalt
(lower Banbury Basalt in appearance), tan fine-grained lake sediment, sparse stream pebbles, and some juvenile spattery tephra.
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Basaltic volcanism of the central and western Snake River Plain
a
b
Some clasts show evidence of hot reaction rims on spheroidally
weathered basalt. The matrix is unstratified with ~25% fine ash
and many small (<1cm) clasts. The breccia unit appears to grade
upwards into tuff with fewer large clasts. The megabreccia and
associated tuff are interpreted as explosive, subaerial crater-filling deposits of the Thousand Hill vent.
This small drainage basin (~1/2 square mile) is characterized
by outcrops of megabreccia, tuffs, red-orange sediment (which
always overlies the volcaniclastics), and lavas, and by faults. The
area was mapped by Stearns et al. (1938) as “Fault complex of
Hagerman lake beds and interstratified tuff.” Malde and Powers
(1972) mapped it as lower Banbury “vent deposits of basaltic
pyroclastic material,” without showing any internal detail. The
geology is very complex and diverse, and the volcanic and structural relationships are obscured by a veneer of tan-colored Quaternary Yahoo Clay (Bruneau sediments in earlier work), which
unconformably overlies the Tertiary volcanics.
Directions to Stop 1.3
Return toward vehicles and hike over to small hill nearby,
labeled “Thousand” on topographic map.
Stop 1.3—Banbury Basalt: Tilted Hydromagmatic Vent
and Vent Facies Breccias [N 42°43.183′, W 114°51.40′]
The outcrop consists of a 7-m-thick, unsorted and unstratified, matrix-supported volcanic breccia with dull olive-green to
tan color containing accidental blocks of vesicular basalt (as large
as 0.5 m across) and numerous polished stream pebbles. Note the
oblong blocks of undisturbed, fine-grained tan water-laid sediment, which must have been horizontal originally, suggesting that
dip is the result of structural tilting. This is a section of the vent
deposits, dipping 40 degrees north, probably due to faulting.
Hike around to top surface (now dipping) to look at strong
linear features, which are interpreted as elutriation (gas escape)
features.
This outcrop may be the same unit as in Stop 1.2, but on the
opposite side of vent. Note that the Thousand Hill vent and the
Riverside Ferry vent (Stop 1.4) are both along a strong northwest-trending structural orientation.
13
Figure 8. Vent facies of the Banbury
Basalt. (A) Megabreccias that probably
represent a vent facies (Stop 1.3). (B)
Riverside Ferry vent (Stop 1.4).
Directions to Stop 1.4
Drive south on Hwy 30, ~2 mi (3.2 km), and turn left at
intersection with paved “River Road.” Follow River Road past
houses, ~1 mi (1.6 km) and turn left on paved road near power
line. Drive past sequence of orange-colored palagonitic tuffs on
right roadcut and continue to gravel road at top of hill. Turn right
on gravel road, go ~1/3 mi (0.5 km) and fork left. Drive carefully
northward to edge of vent exposure. Some roads are very sandy
and may need 4WD. There are at least two jeep trails that access
the vent area and overlook the Snake River.
Stop 1.4—Banbury Basalt: Riverside Vent (optional, if
time permits) [N 42°42.40′, W 114°50.383′]
This exposure in the north half of the section 29 “island” is a
hydrovolcano bisected by the Snake River and originally named
the Riverside Ferry cone by Stearns et al. (1938). Bonnichsen and
Godchaux (2002) referred to it as the Riverside Ferry cone but
named the phreatomagmatic units “Tuff of Blue Heart Springs”
after a spring located on the north side of the river adjacent to the
volcano. They described the tuff as explosively erupted layers of
cinder to spatter-rich deposits tilted near the vent constructs and
transitioning to subhorizontal, finer-grained units more distally.
Chemically they place the basaltic material in the general Snake
River olivine tholeiite compositional range. They also place
some of the Banbury tuff units as intercalating with the Glenns
Ferry Formation sediments, although the mapping of Malde and
Powers (1972) would generally place the Banbury Basalt as
older than the thick pile of lake sediments. Certainly there are
“Banbury-age” lake and fluvial sediments interstratified with
basaltic units (4–6 Ma) and there are basaltic tuffs and basalt
(3.4–3.8 Ma) interbedded within the Glenns Ferry Formation at
the Hagerman Fossils National Monument (W.K. Hart and M.E.
Brueseke, 1999, personal commun.). Dating the Banbury units is
difficult due to the alteration, although a few recent Ar-Ar radiometric dates are available, but not definitive.
The core of the Riverside Ferry vent is well exposed by
erosion (Fig. 8B). A massive tuff breccia, air-fall tuffs and breccia, cindery beds, surge deposits, and palagonitized tuffs can be
found within a short hike across the 1000 ft of exposed vent.
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J.W. Shervais et al.
On the southeast side of the vent, a basaltic lava flow appears
to overlie the vent deposits and may be related. Exposures on
the north side of the river contain abundant red cindery material, but these deposits have not been studied in detail. A NWtrending fault may underlie the course of the Snake River, and
it is probable that there is a significant northwest-trending fault
under the “dry” river course south of the “island.” Evidence for
this structure(s) includes small visible offsets of the older rhyolites and lower Banbury lavas, topographic features emphasized
by the Bonneville Flood (Fig. 4), and alignment of geothermal
wells and hot springs.
Return to Hagerman on Hwy 30.
Day 2 (All Day) Mountain Home
Directions to Stop 2.1
Drive north on I-84 to Mountain Home; take Exit 95 on
Idaho Hwy 20 north toward Camas Prairie and Sun Valley. Continue N for ~7 mi (11 km) where the highway enters Rattlesnake
Creek canyon; park at the turnout on the right side of highway.
All Day 2 stops are shown on Figure 9A; detailed topography for
stops 2.2 through 2.4 is shown in Figure 9B.
Stop 2.1—Danskin Mountains Rhyolite, Highway 68 North
of Mountain Home [N 43°12.052′, W 115°33.2′]
The oldest volcanic rocks exposed in the Mountain Home
area are rhyolite lava flows that form the Danskin Mountains
and the Mount Bennett Hills. Clemens and Wood (1993)
mapped the rhyolite here as the Danskin Mountains Rhyolite, and mapped rhyolite which is exposed farther east as the
Mount Bennett Rhyolite. They determined a K-Ar age of 10.0
± 0.3 Ma for sanidine in a vitrophyre from the summit area
on Teapot Dome (Danskin Mountains Rhyolite), and a K-Ar
age of 11.0 ± 0.5 Ma for plagioclase in a rhyolite from near
Mount Bennett. Possible correlative units on the south side of
the western Snake River graben include the Sheep Creek Rhyolite (9.88 ± 0.46 Ma; Hart and Aronson, 1983), the rhyolite of
Tigert Springs, the rhyolite of O X Prong, and the rhyolite of
Rattlesnake Creek (Kauffman and Bonnichsen, 1990; Jenks et
al., 1993). Rhyolite in the eastern Mount Bennett Hills ranges
in age from 9.2 ± 0.13 Ma to 10.1 ± 0.3 Ma (Armstrong et al.,
1980; Honjo et al., 1986, 1992).
The Danskin Mountains Rhyolite is typically a vitrophyre
with abundant phenocrysts of sanidine and quartz set in a red,
gray-brown, or black volcanic glass. Flow banding appears as
laminar variations in the color of the glass, or in its crystallinity.
The flow banding is commonly folded ptygmatically, indicating rheomorphic mobilization of the rhyolite. Flow banding
and axial foliation in the vitrophyre generally trend N50ºW to
N60ºW, and dip 15º–45°NE. There are no indications that these
rhyolites are rheomorphic ignimbrites; they appear to be rhyolite
lavas erupted from fissures that were subparallel to the current
range-front faults (Bonnichsen, oral comm., 1996, 1997).
Directions to Stop 2.2
Return to I-84 and proceed west toward Boise; take the next
available exit (exit number 90) then continue across and back on
to I-84 toward Boise (this will slow you down and position you
for the next stop). Pull off to the right immediately after entering
the freeway at the first roadcut encountered.
Stop 2.2—Plagioclase Flotation Cumulates of Lockman
Butte, I-84 Roadcut [N 43°10.599′, W 115°45.601′]
This roadcut transects a major flow lobe from Lockman
Butte that consists of multiple lava flows that are stacked one
on top of (or beneath) another. Many of these flow units contain
plagioclase flotation cumulates that formed within covered lava
channels and aphyric ferrobasalt residues that drained out the
bottom of the cumulates (McGee and Shervais, 1997; Shervais
et al., 2002; Zarnetske and Shervais, 2004). Another interesting
aspect of these flows are the gigantic vesicles found in some gas
accumulation horizons; these vesicles are up 2 m long and 0.6 m
high—big enough to lie down in!
Lava flows with flotation cumulates comprise three zones: a
central, plagioclase porphyry with intersertal to intergranular textures in the groundmass, an upper diktytaxitic zone comprising plagioclase laths with large voids and minor intergranular mafics and
glass, and a lower aphyric zone of ferrobasalt, with 16%–17% FeO*
(Fig. 10A). Mass balance calculations show that the diktytaxic
zone contains 30%–50% porosity, represented by the ferrobasalt
base, if the central plagioclase porphyry is assumed to represent
the bulk composition. Detailed outcrop maps show that successive
lava flows flowed beneath previously emplaced flows, inflating and
plastically deforming the aphyric ferrobasalt zone in the overlying
flow. Plagioclase flotation cumulates beneath this ferrobasalt ceiling display a horizontal contact between the diktytaxitic and plagioclase porphyry zones; we interpret this horizon to represent the contact between interstitial melt (below) and volcanic gasses (above),
and suggest that interstitial melt was displaced by the rising gasses
(McGee and Shervais, 1997; Zarnetske and Shervais, 2004).
Plagioclase phenocrysts are An65 in both the porphyry
and diktytaxitic zones, suggesting formation of the plagioclase
framework by simultaneous crystallization throughout the flow,
followed by sinking of the dense, interstitial ferrobasaltic liquid
to the bottom of the flow. This interpretation is supported by
olivine phenocrysts (Fo68) trapped in the plagioclase framework,
and by partially replaced intergrowths containing Fo80–89 olivine.
The Fo-rich olivine relicts imply a parent magma that was significantly more magnesian than observed.
These flotation cumulates provide an analogue for anorthosite formation, both in the lunar magma ocean and in Earth’s
mid-crust. We suggest that planetary anorthosites represent plagioclase saturated melts that crystallize en mass to form buoyant
rafts of plagioclase plus trapped mafics; interstitial ferrobasalts
liquids are forced out by rising magmatic gasses, and sink
back into the underlying magma (McGee and Shervais, 1997;
Zarnetske and Shervais, 2004).
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Basaltic volcanism of the central and western Snake River Plain
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.
15
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J.W. Shervais et al.
16
Directions to Stop 2.3
Drive NW on I-84 for ≈14 mi (23 km) to the first exit (Simco
Road, Exit 74); cross the freeway and return to I-84 in the SE
direction. Exit at the first Mountain Home exit (exit number 90)
and take the first right turn encountered on Business I-84 (old
Hwy 30). Proceed along the frontage road parallel to the railroad
tracks for ≈6 mi (≈10 km), then turn left and cross the RR tracks
at a clearly marked crossing. Proceed ≈1.8 mi (2.9 km) along this
wide gravel road to a dirt road turn-off on the left near the crest of
the grade. Turn left and follow this dirt road ≈1.3 mi (2.1 km) to
the rim of Crater Rings. You will go past a stock pond (commonly
dry) and through at least one wire fence; be sure to close all gates
behind you. Note: Crater Rings is rattlesnake heaven, so watch
where you step and put your hands!
A
Stop 2.3—Crater Rings Pit Craters: Lava Lakes and
Spatter Ramparts [N 43°11.715′, W 115°50.866′]
The Crater Rings are a spectacular volcanic feature that consists of two large pit craters at the summit of a broad shield volcano
(Fig. 10B). The western crater is ≈800 m across and 75 m deep;
the eastern crater is ≈900 m across and 105 m deep. The inner
walls of the pit craters consist of welded spatter, agglutinate, and
minor intercalated lava flows; no fragmental horizons are exposed
in the crater walls, which argues against phreatomagmatic eruption. The welded spatter and agglutinate are easily identified by
their characteristic textures, oxidized coloration, and hollow ring
when struck with a hammer. The eastern crater is surrounded on
three sides by spatter and agglutinate ramparts.
The Crater Rings represent pit craters that were filled episodically 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
fountain eruptions in the lava lakes fed spatter to the rims, which
were occasionally mantled by lava flows when the lava lakes overflowed their ramparts. The final eruptive phase was confined to the
eastern vent, where fire fountains built ramparts on three sides of
the vent that were not covered by subsequent lava flows, although
lava from the eastern vent may have flowed 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
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.
B
C
Figure 10. Volcanic features near Mountain Home. (A) Plagioclase
flotation cumulates in covered lava channel, Lockman Butte—note
diktytaxitic texture near top of cumulate layer where exsolved gas has
forced residual ferrobasaltic liquid from the interstices. (B) The Crater
Rings, two large pit craters that fed lava lake eruptions and Peleanstyle fire fountains. (C) Pillow and hyaloclastite delta in basalt of Little
Joe Butte (Strike Dam Road), which sits on lacustrine sediments of
Glenns Ferry Formation (Lake Idaho); note water escape conduits,
steam cavities, and southeast dip of foreset beds
Stop 2.4—Union Buttes: Holocene Cinder Cones, Late
Alkaline Basalts Equivalent to Boise River Group 2 of
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
confined 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 flow that flowed west toward Crater
Rings. Both vents consist of cinder cones built on top of small
shield volcanoes of similar composition basalt.
The basalt of Union Buttes is distinct from almost all other
basalts in the Mountain Home area because it contains large,
clear phenocrysts of olivine but lacks plagioclase phenocrysts.
The basalt of Union Buttes, and the similar basalt of Little Joe
Butte (also known as the basalt of Strike Dam Road), are characterized by high K2O compared to other Mountain Home basalts,
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Basaltic volcanism of the central and western Snake River Plain
along with lower TiO2 and high-field strength trace element
abundances. In terms of their chemical and age relations, they are
similar to the Boise River Group 2 basalts of Vetter and Shervais
(1992) and to the M3 basalts of White et al. (2002). The transition from high field strength-rich, alkali-poor tholeiites to potassium-rich, high field strength-poor transitional alkaline basalts
around 600,000–700,000 yr ago is one of the most fundamental
time-dependent transitions observed in the western Snake River
Plain province, with the younger lavas characterized by OIB-like
isotopic compositions. See Vetter and Shervais (1992) and White
et al. (2002) for discussions.
Directions to Stop 2.5
Continue south on gravel and paved roads ~2.6 mi (4.2 km)
to intersection with Idaho Hwy 67. Turn right onto Hwy 67 and
proceed SW toward Mountain Home Air Force Base. Bear right
at the turn-off into the air base (away from the base) and continue
SW on Hwy 67 for 6.5 mi (10.8 km) to a poorly marked intersection 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;
park at the top and walk down hill to the next stop.
Stop 2.5—Basalt of Little Joe Butte: Subaerial Basalt
on a Pillow Lava + Hyaloclastite Delta on Lake Idaho
Sediments [N 42°57.648′, W 115°58.526′]
The basalt of Little Joe Butte is olivine-phyric basalt that
crops out along the western edge of the Mountain Home area
(Cinder Cone Butte, Crater Rings SW quadrangles) and continues into adjacent quadrangles to the west (Little Joe Butte,
Dorsey Butte) and south (Grand View, C.J. Strike Dam; Jenks et
al., 1993). The source of this flow is Little Joe Butte, but it has
also been mapped as the basalt of Strike Dam Road (Jenks et al.,
1993). It consists of at least two major flow units, with a mappable
flow front preserved in the upper unit, and appears to flow south
along a former channel of the Canyon Creek drainage. Collapsed
areas of this flow are commonly overlain surficially by “intermittent lake deposits,” i.e., dry lake beds (Shervais et al., 2002).
This unit is largely subaerial, but its base is commonly pillowed along its southern margin, demonstrating the flow of subaerial lava into standing water of Lake Idaho. The subaqueous
portion of the flow consists of pillow lava and hyaloclastite breccia
forming foreset delta sequences with individual “rolled” pillows
interspersed (Fig. 10C). The delta foresets dip to the SE, indicating
the direction the lava flowed when it entered the lake. The subaqueous lava delta is ≈10 m thick in the bluffs overlooking C.J. Strike
reservoir, where it flowed over flat-lying lake deposits—indicating
the exact water depth in the lake at the time of eruption.
Like the Bacon Butte Basalt at Stop 1.1B, the subaerial lava
that overlies the pillow delta contains water escape conduits
and giant vesicles (steam bubbles). The conduits and vesicles
can be seen from the road but a close-up examination requires
climbing up the pillow delta, which is inherently unstable—be
careful if you go!
17
Return to Hagerman
Continue south on Strike Dam Road, across Snake River
at Strike Dam Bridge, to Idaho Hwy 78. Drive E on Hwy 78
through Bruneau to Hammett. As time permits, we will make
stops along this route to view the stratigraphy of the former Lake
Idaho and the effects of the Bonneville flood. At Hammett, rejoin
I-84 and return to motel.
Day 3 (Partial Day) Twin Falls2
Directions to Stop 3.1
From Hagerman, drive north on Hwy 30 to Bliss, turn E
(right) onto Hwy 26 to Shoshone. At main intersection in Shoshone, turn N (left) on Hwy 75. Drive N ~2.3 mi (3.7 km) and
park on right. Hike E into the Shoshone lava flow to the lava channel. All of the stops for Day 3 are shown on Figure 11. Source
vents for the basalt flows are indicated on Figure 12, a hillshade
topographic map of the Twin Falls 30′ × 60′ quadrangle.
Stop 3.1—Lava Channel is Shoshone Lava Flow; a‘a
Basalt Fills Channel in Pahoehoe Flow [N 42°58.936′,
W 114°18.22′]
The Shoshone basalt flow (basalt of Black Butte Crater
on the Idaho Geological Survey Twin Falls 30′ × 60′ geologic
map) erupted from Black Butte Crater located in the Black
Butte Crater quadrangle. The basalt is black, fine grained,
vesicular to massive, and aphyric to olivine phyric. Olivine
phenocrysts (Fo58–73) are visible in hand sample and are up to
2.0 mm in size. Most plagioclase (An31–72) displays a normal
chemical zoning.
Black Butte Crater is a broad shield volcano that rises
135 m above the surrounding topography. The volcano contains
two large craters that are ~300 m across and 100 m in depth.
This young lava field extends south to the Dietrich Butte quadrangle and then west into the Shoshone, Tunupa, and Gooding
quadrangles, and terminates just west of the town of Gooding.
Eruption of lava disrupted the confluence of the Big and Little
Wood rivers, which was probably near Shoshone, and separated
the two rivers to their present confluence at Gooding. Radiocarbon dating of underlying sediment baked by the flow (Kuntz
et al., 1986) has yielded a date of 10,130 ± 350 yr B.P. for this
basalt. Aside from a few areas covered by thin loess deposits or
alluvium, most of the basalt is completely exposed, and almost
all of the original flow morphology can be seen. The flow is
dominated by a series of large lava tube and lava channel systems that can be seen on aerial photographs and topographic
maps. Most of the Shoshone flow consists of smooth pavement
outcrops of pahoehoe, overlain by sporadic a‘a flows; a‘a flows
also fill lava channels and collapsed lava tubes.
We plan to visit a large lava channel in the pahoehoe flow
that is now filled with a‘a lava. This lava channel has levees that
rise 60–70 ft above the surrounding lava, and the channel is as
much as 70 ft deep (Fig. 13A).
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J.W. Shervais et al.
18
Stop 3.1
Stop 3.2
Stop 3.3
Day One
Stops
Stop 3.4
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 flows revealed through vertical exaggeration. The solid lines are main highways. The dashed line is the perimeter of the Twin Falls 30′ ×
60′ quadrangle.
page 18 of 26
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Basaltic volcanism of the central and western Snake River Plain
page 19 of 26
19
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J.W. Shervais et al.
20
A
layered spatter and massive flows. Most outcrops along the flanks
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 fine grained, containing phenocrysts of plagioclase and olivine. Some of the samples taken from the massive layers display a
slightly diktytaxitic texture but most are intergranular in texture.
Olivine phenocrysts (Fo45–74) are small (<0.6 mm) and rounded.
Plagioclase phenocrysts (An55–71) are generally 2.5 mm in size.
Several large plagioclase phenocrysts display an oscillatory zoning, which may be due to pressure fluctuations during eruption or
to magma mixing. Electron microprobe line scans across plagioclase 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
Figure 13. Volcanic features of the central Snake River Plain
north of Twin Falls, Idaho. (A) Lava channel in the Shoshone basalt. (B) Pit crater in Crater Butte.
Directions to Stop 3.2
Return to Shoshone and proceed NE on Hwy 93 toward
Richfield. Turn right (S) ≈8.8 mi (14.2 km) from intersection in
Shoshone onto a paved road. Drive south 2.5 mi (4.0 km) to the
second dirt road on the right side of road. Turn right onto this dirt
road and drive to the top of Crater Butte (~0.9 mi/1.5 km). Note:
if we cannot get the vans up Crater Butte, we will drive to the top
of nearby Dietrich Butte instead.
Stop 3.2—Crater Butte [N 42°57.582′, W 114°16.724′]
Crater Butte, located in the northeastern portion of the
Dietrich quadrangle, is a steeply sloping shield volcano that rises
~140 m above the surrounding topography. The most striking
feature of this volcano is the large 80 m deep crater in its center.
This bowling pin-shaped crater is ~1300 m × 1000 m in size
and trends in a NW–SE direction. The inner walls of the crater
show thin layers of spatter interbedded with thicker beds of more
massive basalt (Fig. 13B). The tops of the more massive beds
are highly vesicular which indicates gaseous escape from lava
channels beneath the cooler, more viscous upper crust. When the
volcano was active, the crater probably contained a lava lake that
would periodically spill over the spatter rim to create the inter-
Stop 3.3—Notch Butte [N 42°53.036′, W 114°24.98′]
Notch Butte is located in the southeastern corner of the
Shoshone quadrangle and rises 110 m above the surrounding
topography. The lava flows 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
quadrangle. Several lobes flowed west to Gooding, continued
south and west around Gooding Butte, and flowed into the ancestral Snake River canyon near Hagerman. The unit is equivalent
to the Wendell Grade basalt of Malde et al. (1963), except for
the canyon filling part at Hagerman, which they mapped as Sand
Springs basalt. The unit has been renamed during recent mapping
to follow the convention of naming flows after their source vent.
The flows display a relatively young volcanic morphology
with many flow features such as pressure ridges, collapsed lava
tubes, and flow fronts still visible beneath a thin soil and loess
layer. The loess mantle is mostly confined to depressions in the
flow surfaces. Note the contrast between flows from this vent and
those from the older Crater Butte vent and the younger Shoshone
flow lavas.
Mapping revealed three to four lobes of varying mineralogy
within the flow field. In general, higher lobes closer to the vent
were found to be rich in plagioclase, while those at lower elevations farther away from the vent contain less plagioclase and are
rich in olivine. The higher lobes are interpreted to be younger,
more fractionated, and therefore more viscous basalt (due to the
plagioclase content), while those found at lower elevations are
older, less fractionated, and less viscous. This difference in viscosity between the older and younger lava accounts for the fact
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Basaltic volcanism of the central and western Snake River Plain
that the older flows spread out over a large area away from the
vent while the younger flows did not.
The basalt of Notch Butte is a black, vesicular to massive,
medium- to fine-grained, plagioclase- ± olivine-phyric basalt.
Samples taken from some flow lobes contain large glomerocrysts of plagioclase and olivine, which are visible in hand
sample. The basalt is intergranular to intersertal in texture and
contains olivine phenocrysts (Fo35–73); large glomerocrysts of
plagioclase (An59–71) and olivine are visible in thin section and
are often up to 5 mm in size. The larger plagioclase phenocrysts
are typically normally zoned.
The view from the top of Notch Butte includes all of the
major volcanoes in this part of the central Snake River plain:
Crater Butte, Dietrich Butte, Owinza Butte, and Black Ridge
Crater to the east; Lincoln Butte to the west; Bacon Butte, and
Flat Top Butte to the south, and Wilson Butte and Rocky Butte to
the southeast. From here, it is easy to contrast the relative ages of
the flows, which can be estimated by the amount of cover indicated by farming. We can also see the western flow from Notch
Butte, which extends some 40 km to the west where it flowed
into the ancestral valley of the Snake River. At the time of Notch
Butte eruptions, the ancestral Snake River was already at about
its present position and elevation.
Directions to Stop 3.4
Return to Hwy 93. Turn left (S) onto Hwy 93 and drive south
11.8 mi (19 km) to Hwy 25; turn left (E) onto Hwy 25 and after
≈0.6 mi (1 km) turn left (N) onto paved road; follow for 0.3 mi
(0.5 km), then turn left (N) onto paved/gravel road that goes to
top of Flat Top Butte (~1 mi/1.6 km from highway).
Stop 3.4—Flat Top Butte [N 42°43.722′, W 114°24.75′]
Flat Top Butte is an extremely large (≈450 km2) shield volcano located ~20 km north of Twin Falls in the Falls City 7.5′
topographic map quadrangle. The vent has a large (200 m), shallow depression at its summit that represents the former summit
crater; the rim of this crater is now home to a forest of microwave
transceivers. The flanks of the volcano are covered with a thick
mantle of loess and soil that obscures the underlying basalt. This
butte is farmed almost to its summit, showing that it is one of the
older vents in this part of the plain. Radiometric ages for Flat Top
Butte are in the 0.330–0.395 Ma range (Tauxe et al., 2004; Idaho
Geological Survey, 2005, unpublished data).
The basalt of Flat Top Butte flowed south and west filling the course of the ancestral Snake River as far as Thousand
Springs, where Malde and Powers (1972) mapped the flows as
the Thousand Springs Basalt. Flows from Flat Top Butte defined
the present position of the Snake River by forcing the drainage
southward against the regional north slope of the older shield volcanoes. Subsequently, the Snake River canyon was cut.
From the summit area of Flat Top Butte we can look east
toward the younger vents Wilson Butte, Rocky Butte, and
Kimama Butte, north toward Bacon Butte and Notch Butte, and
page 21 of 26
21
northeast toward Owinza Butte and Black Ridge Crater. Older
buttes to the southeast include Hansen Butte, Skeleton Butte,
Hazelton Butte, Milner Butte, and Burley Butte.
Looking south toward the Snake River canyon and Twin
Falls City, the track of Bonneville Flood overland flow was east
to west obliquely across I-84. Along the canyon wall south of
I-84 the overland floodwaters rejoined the component following the course of the river, forming cataracts in several places.
Although the basalt of Rocky Butte typically has loess filling
surface depressions, in the track of the floodwaters virtually all
the loess is stripped away and patchy gravel deposits record local
deposition during the flood.
Directions to Stop 3.5
Return to Hwy 25; turn left (E) onto Hwy 25 and proceed
4.6 mi (7.4 km) to intersection with paved road. Turn S (right)
and drive 5.4 mi (8.7 km) south—road goes from paved to gravel
after ~2 mi (3.2 km). Continue on dirt road to SE around end of
Wilson Butte flow lobe, ≈1 mi (1.6 km). Park anywhere.
Stop 3.5—Wilson Butte Flow Lobe [N 42°37.82′, W 114°21.76′]
Wilson Butte is a large shield volcano (≈10 km across). Its
vent, composed of three small peaks, is rather unimpressive when
compared to the estimated 250 km2 of lava that flowed down the
slopes of the shield. The flows cover the majority of the Shoshone
SE and half of the Star Lake quadrangles. The flow continues into
the Shoshone SW and Hunt quadrangles, and it crosses Hwy 25
in the Twin Falls NE quadrangle and continues into the Kimberly
quadrangle, possibly emptying into the Snake River around the
Twin Falls rapids or Devil’s Corral area.
The mode of the basalt is 35%–40% plagioclase, 10%–15%
olivine, 10%–20% pyroxene, 5%–8% oxides, and 15%–20%
glass. Plagioclase shows typical euhedral lath-shaped crystals
with An content ranging from An66 to An36. Olivine ranges from
Fo71 to Fo33.
At this stop we will view a major flow lobe from Wilson
Butte that stands 10–20 m above the surrounding lava fields and
contains a giant complex of lava tubes that fed secondary flow
lobes that flowed primarily west and south. The flow lobe is characterized by steep sides and a nearly flat upper surface that can
be traced for tens of kilometers. This lava tube–channel system
fed flows adjacent to the Snake River that have been mapped as
Sand Springs basalt—in addition to flows from Rocky Butte (Hill
4526), which may dominate the Sand Springs unit. Wilson Butte
and Rocky Butte have lavas that are nearly identical chemically,
and both appear to have erupted at essentially the same time,
although the Wilson Butte flow lobe appears to be younger than
the adjacent flows from Rocky Butte. Tauxe et al. (2004) report
radiometric age for “Sand Springs Basalt,” probably from Rocky
Butte, as 0.095 Ma. The lava tube system is exposed in a series of
windows that are open mainly on the eastern margin of the flow.
Sitting on the Wilson Butte flow lobe here are coarse sediments deposited by the Bonneville flood. These sediments seem
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J.W. Shervais et al.
22
to represent a lag deposit from water flowing over or around the
end of the lobe. At this elevation (~1173 m) the Bonneville Flood
waters were relatively shallow. The relief of this flow lobe was
just sufficient to divert most of the overland floodwater around
the feature, but a few low spots allowed water to spill through and
create small plunge pools and gravel deposits. The flood waters
lost energy on the lee side of the flow lobe and deposited finer
sediment that forms flat surfaces in broad depressions.
Directions to Stop 3.6
Return to Hwy 25, turn W (left) and return to Hwy 93. Turn
left onto Hwy 93 and drive south 8.6 mi (13.8 km) to Perrine
Bridge in Twin Falls (you will cross I-84). Cross the bridge and
enter turnout into viewpoint-tourist information area immediately on south side of bridge. Park in visitor center parking lot
next to bridge.
Stop 3.6—Perrine Bridge Overlook [N 42°35.8966′,
W 114°27.266′]
The Perrine Bridge viewpoint is one of the classic views
of volcanic stratigraphy in the central Snake River Plain. The
Snake River gorge is over 120 m deep here and contains two golf
courses within the gorge just downstream from the bridge. Evil
Kneivel’s famous attempt to jump the gorge on a rocket-powered
motorcycle took place just upstream from here.
Sand Springs Basalt (Rocky or Wilson Buttes)
Basalt of Flat Top Butte
Hub Butte Basalt (?)
Glenns Ferry Fm.
Twin Falls Rhyolite
Figure 14. Volcanic stratigraphy at the Perrine Bridge, Twin Falls, Idaho. Dark rock at bottom of canyon is the 6.25 Ma Twin Falls rhyolites.
The thickest section of basalt erupted from vents south of the river
(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
of Rocky Butte and/or Wilson Butte) on top. Basalt and rhyolite are
separated by thin wedges of clastic sediment in middle of section.
Looking north across the river we can see over 4 m.y. of
volcanic history (Fig. 14). At the base of the section is the Twin
Falls rhyolite (ca. 6.25 Ma; Armstrong et al., 1975) that forms the
basement here, and which underlies Shoshone Falls east of Twin
Falls. Sitting on the rhyolite (and on some patchy exposures of
rhyolite breccia) are sediments that may correlate with the Pliocene Glenns Ferry Formation (Covington et al., 1985; Covington
and Weaver, 1989). If this correlation holds, these sediments
would represent lake and flood-plain sediments that were deposited in an extension of Lake Idaho. Basalts intercalated with these
sediments just upstream from here have been dated at ca. 4 Ma
by Armstrong et al. (1975).
Sitting on the sediment and rhyolite is a thick section of
basalt erupted from large vents to the south (Hub Butte and
other southern buttes). These flows are overlain by flows from
Hansen Butte and other volcanoes to the southeast, which pushed
the river farther north and built the north slope of a large shield
complex. Evidence downstream shows subsequent flows from
Flat Top Butte burying the preexisting northward slope, presumably filling any previous east-west drainage, and reestablishing
the course of the ancestral Snake River at the south edge of the
Flat Top shield. By the time of eruptions from Wilson Butte and
Rocky Butte, the Snake River had incised the older basalts along
the margin of the Flat Top Butte shield. Evidence in the canyon
wall shows local filling of the canyon by the younger flows.
Subsequent flows from Flat Top Butte, Wilson Butte, and Rocky
Butte followed the northern margin of these flows. The course
of the Snake River then became established along the contact of
the younger and older flows. The basalt of Flat Top Butte was
formerly mapped as Thousand Springs Basalt by Malde and
Powers (1972), while the overlying Sand Springs Basalt (Malde
and Powers, 1972) is now known to represent flows from at least
two vents in this area, Wilson Butte and Rocky Butte. However,
valley-filling Sand Springs Basalt mapped along Cedar Draw
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 confined
to the canyon except that overland floodwaters from the northeast 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 flow
(Fig. 15). In the canyon bottom, the golf courses have been built
on flood-stripped and molded surfaces of older basalt and rhyolite that have thin deposits of flood gravel and sand.
The erosional and depositional features of the Bonneville
Flood are time transgressive, albeit a short time period of only
a few months. Unlike Glacial Lake Missoula that is thought to
have emptied and produced a catastrophic flood duration of only
several days, the catastrophic floodwater from Pleistocene Lake
Bonneville was more complex (O’Connor, 1993). The early
phases were in low discharge as the lake gently overtopped the
divide at Red Rock Pass and it began to erode. As erosion accel-
fld006-02
Basaltic volcanism of the central and western Snake River Plain
page 23 of 26
23
Figure 15. Topographic map of the area
near Perrine Bridge showing features
formed by the Bonneville Flood, including cataracts, plunge pools, scoured
canyon walls, and giant gravel bars.
Large arrows show inferred floodwater
flow directions.
erated, the discharge grew to catastrophic proportions and was
sustained for many weeks. However, as the level of Lake Bonneville dropped, gradually the discharge lowered. Ultimately, an
equilibrium was achieved between the lake and the outlet, and the
lake continued to drain into the Snake River drainage for at least
hundreds of years. Given this backdrop, the overland floodwaters
entering the canyon from the northeast represent flow during the
greatest discharges, and would have ceased when the canyon
could accommodate the flood. Features in the canyon, therefore,
range from high-discharge rock scouring and deposition of giant,
bouldery gravel bars, to lower-discharge thin sand and gravel
deposits in the bottom of the canyon.
Return to I-84. Enter freeway in east-bound direction
(right turn). Return to Salt Lake City (approximate driving
time: 4 hours).
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