Appendix 6. Stratigraphic framework of rhyolite units
Bruneau-Jarbidge region
The Bruneau-Jarbidge (BJ) region occupies the southwestern part of the central Snake
River Plain (CSRP) (text Fig. 1) and contains the most thoroughly documented
(Bonnichsen, 1982, Bonnichsen and Citron 1982, Bonnichsen and Godchaux, 2002,
Bonnichsen and Jenks, 1990, Jenks and Bonnichsen, 1990) of the CSRP silicic volcanic
units, in terms of geologic mapping, geochemical and petrographic studies, and radiometric
dating. The available geochronology indicates that silicic volcanism associated with this
region spanned at least 4.6 Ma, and occurred in two distinct phases. An early (12.7 to 10.5
Ma) and dominantly explosive phase produced voluminous high-temperature, typically
rheomorphic ignimbrites that constitute the Cougar Point Tuff (CPT). Correlative units are
exposed along the southern part of the region and extend many kilometers to the east and
west across southernmost Idaho as well as southward into Nevada. This recurrent activity
has been referred to as an ignimbrite flare-up (Perkins et al. 1995, 1998) and resulted in
collapse of the BJ center. The resulting topographic basin was partly filled by voluminous
rhyolitic lava flows and relatively minor pyroclastic deposits erupted locally during a
second phase of activity (10.5 to 8.1 Ma), although some rhyolite lavas erupted during
latest stages of the ignimbrite phase.
The Cougar Point Tuff
The CPT is named for its well-exposed occurrence in the canyon of the East Fork of the
Jarbidge River, near Cougar Point (Coats 1964). The best exposed and most complete
section occurs at Black Rock escarpment on the east side of Bruneau Canyon, just north of
the Idaho-Nevada border, where eight ignimbrite cooling units are exposed (Bonnichsen
and Citron 1982; Bernt, 1982). At this site (text Fig. 2) the CPT ranges up to 475 m thick,
whereas to the east in Jarbidge Canyon the exposed section is approximately 250 m thick.
In ascending order the CPT comprises nine ignimbrites (Appendices 2A, 4A) denoted as
units III (12.67±0.03 Ma), V (12.07±0.04 Ma), VII (11.81±0.03 Ma), IX (11.56±0.07 Ma),
X (~11.3 Ma), XI (11.22±0.07 Ma), XII (~11.1 Ma), XIII (10.79±0.04 Ma), and XV (~10.5
1
Ma). There are no units corresponding to Roman numerals I, II, IV, VI, VIII, or XVI and
above. Ages are known within 0.1 Ma or better and indicate that recurrence intervals
between the nine dominant explosive episodes ranged between ca. 200-300 ka. Two thin
welded tuff units, the Whiskey Draw and Rattlesnake Draw ignimbrites, are exposed
beneath the CPT section southwest of the BJ center (Bernt, 1982). Because they are
chemically similar to CPT unit III and appear to be part of the high-silica, Fe-Ti-poor,
initial phase of CSRP silicic volcanism, they have been assigned to the same 12.4-13.0 Ma
time interval as CPT III (text Table 3). Intercalated within the succession of CPT units is
the Black Rock Escarpment rhyolite lava flow (between CPT XII and XIII), and it is
probable that the nearby Marys Creek lava flow (Bonnichsen, 1982) located adjacent to the
Grasmere escarpment (west of the BJ region) also is approximately the same age as CPT
XII and the Black Rock Escarpment lava flow (text Table 3).
Except in distal localities or where thin, each CPT unit is a simple cooling unit formed
from one or more eruptive surges in a short period of time. Below each unit are bedded ash
layers (airfall and/or cross-bedded surge deposits) that are partially or completely fused to
dense vitrophyres. The overlying ignimbrite part of each unit typically grades from a dense
basal vitrophyre to a devitrified interior and a locally preserved upper vitrophyre zone. In
places the upper parts are devitrified and quite vesicular, with variable extents of vapor
phase crystallization. Interiors and tops of many CPT units are complexly folded –
presumably due to post-emplacement rheomorphic adjustment. Units VII, XI, and XIII
represent unusually large eruptions with estimated volumes exceeding 1000 km3.
Petrographic and other features of the CPT units (Bonnichsen and Citron 1982; Honjo et
al.1992; Cathey and Nash 2004) are summarized in Appendix 2A. All CPT ignimbrites
have similar mineralogy: primarily phenocrysts of quartz, sanidine, plagioclase, augite,
pigeonite, fayalite, magnetite and ilmenite with accessory zircon, monazite, and apatite
microphenocrysts. All CPT units lack hornblende and biotite phenocrysts, reflecting the
water-undersaturated character and high temperatures of the magmas. Notably, plagioclasepyroxene-Fe-Ti oxide glomerocrystic aggregates occur within most CPT units. In the more
felsic units, sanidine locally jacketed and partially replaced plagioclase or formed
micrographic intergrowths with quartz. Such intergrowths occur both as partial rims
2
attached to sanidine crystals and as broken fragments. Phenocryst content is typically low
<10%), but can reach 20% in the least felsic units.
Bruneau-Jarbidge rhyolite lava flows
Stratigraphic relationships between most BJ rhyolite lavas, as well as their internal
structures, are best exposed along the walls of Bruneau and Jarbidge Canyons in the
interior of the BJ center (Bonnichsen 1982; Bonnichsen and Kauffman 1987; Appendices
2A, 4A). The rhyolite lavas are characterized by vitrophyric basal and upper zones, much
thicker devitrified interiors, and little or no basal fallout ash. The flows typically are lobate,
thinning from the centers to the margins; thickness commonly exceeds 30 m, and in some
units exceeds 100 m. Most lavas are internally deformed and contain a myriad of flow and
fracture structures. Breccias commonly formed during eruption and subsequent flowage,
and tend to be most concentrated in the upper and marginal parts of the units. The lateral
extent of many individual flows exceeds 10 km, and one has been traced continuously for
40 km (Bonnichsen and Kauffman 1987; Bonnichsen and Jenks 1990; Jenks and
Bonnichsen 1990). Vents for individual flows are rarely exposed; they are presumably
dike-like with kilometer-scale lengths. Estimated volumes for most flows are in the range
of 10-20 km3, but some (e.g., Sheep Creek) may exceed 200 km3.
Few post-CPT rhyolite lavas have been dated directly. New sanidine Ar-Ar dates (Table
1) for the Cedar Tree (10.16±0.09 Ma) and Bruneau Jasper (9.50±0.06 Ma) flows, and
whole-rock K-Ar dates for the Dorsey Creek flow (average 8.1 Ma; Hart and Aronson
1982) suggest that the main period of BJ rhyolite lava flow emplacement lasted about 2
million years; recurrence intervals between eruptions are poorly constrained at present. In
terms of apparent volume and age, rhyolite lava flows in the BJ region can be subdivided
into two general periods. An earlier group (10.2–9.5 Ma) includes the Cedar Tree, Triguero
Homestead, Indian Batt, Long Draw, and Bruneau Jasper flows, all of which are small (0.55 km3) to medium-sized (5-50 km3) in volume. The younger (ca. 9.5–8 Ma) Sheep Creek,
Poison Creek, Dorsey Creek and Juniper-Clover flows are relatively large in volume (50200 km3).
One of the youngest units in the BJ center, the Three Creek rhyolite
(Bonnichsen, 1982), has been reinterpreted as being an ignimbrite (cf. text Table 3); it is
3
very similar in character to the nearby Greys Landing ignimbrite (Andrews, 2007) in the
Rogerson graben.
Notably, several basalt flows are intercalated within the BJ rhyolites (Bonnichsen and
Godchaux 2002); these occur in the time interval between the youngest (Bruneau Jasper)
flow of the earlier group and the oldest (Sheep Creek) flow of the younger group
(Bonnichsen and Jenks 1990; Jenks and Bonnichsen 1990; Bonnichsen and Godchaux
2002). This ca. 9.5 Ma episode of basaltic volcanism may signify increased input of
mantle-derived magma, onset (or increased rate) of rifting in the CSRP, or both. We infer
that the earlier group of small to medium-sized rhyolite lava flows corresponds to a waning
stage of the main CPT magmatic system, and that production of the younger and larger
rhyolite lavas is in some way associated with southeastward migration of western SRP
rifting into the CSRP – perhaps signifying an increase in the local rate of extension and
basaltic intrusion.
All of the rhyolite lavas have similar mineralogy (Bonnichsen 1982; Honjo et al. 1992;
Hirt 2002). The dominant phenocrysts (plagioclase, augite, pigeonite, and magnetite) occur
in every flow and are accompanied by quartz in more than half and by sanidine in a few.
Ilmenite occurs in some units, and zircon, monazite, and apatite are common accessory
minerals. Phenocrysts of hornblende, biotite, hypersthene, and fayalite are absent in BJ
rhyolite lavas. Small glomerocrystic aggregates (plagioclase-pyroxene-opaque oxides)
occur in all flows; these commonly exhibit metamorphic or plutonic igneous textures, and
appear to be recrystallized xenolithic fragments of older underlying volcanic rocks.
Multiple texturally distinct variants of plagioclase phenocrysts commonly occur in
individual thin sections. Such features suggest complex petrogenetic histories for the
rhyolitic magmas.
Rogerson-Twin Falls region
The Rogerson-Twin Falls (RTF) region includes the area around the city of Twin Falls and
extends southward to the Rogerson graben and the Browns Bench (BB) escarpment that
forms its western margin (text Fig. 1). This region lies between the BJ center to the west,
the Cassia Mountains to the southeast, and the Mount Bennett Hills to the north. Some
4
CPT units extend eastward 50 km or more from the BJ center into the Rogerson region
(text Fig. 1). Although exact correlations remain to be established, new 40Ar-39Ar dates and
related investigations suggest equivalencies with major CPT units. Similarities in
phenocryst assemblages and chemical compositions between the CPT and RTF ignimbrites
indicate that most are related to the BJ magmatic system. As discussed by Andrews et al.
(2007) fault displacements along the BB escarpment expose ignimbrites as young as 10 Ma
(text Table 1), whereas the younger Greys Landing ignimbrite floors the graben and onlaps
older volcanic units. Clearly, graben development was partly concurrent with silicic
volcanism. Additional rhyolitic units are sporadically exposed beneath a cover of younger
basalt flows, in canyons of the Snake River and of Salmon Falls Creek (text Fig. 1). Silicic
volcanism in the RTF area extends the known duration of CSRP magmatism beyond that of
the BJ center proper and reflects expansion of activity to more easterly locations.
A nearly 400 m thick sequence of densely welded ignimbrites, airfall ash and
intercalated sediments is exposed for some 30 km along the BB escarpment (text Fig. 3) at
the western margin of the Rogerson graben, west of Salmon Falls Creek Reservoir. The
ignimbrites represent extracaldera or outflow facies deposits related to eruptions from the
BJ and RTF areas within the CSRP. For simplicity, the section is subdivided into 12
unnamed BB Units (BBU 1-12) in ascending order (Appendices 2B and 4B).
The two oldest units (BBU-1 and BBU-2), known only in the Corral Creek area, are thin
and show only limited lateral exposure, but undoubtedly extend for greater distances along
the escarpment. Their stratigraphic positions and compositions suggest they are correlative
with CPT units III and V, respectively. The lower of these units (BBU-1) is conspicuously
silicified and likely was emplaced in a wet environment, perhaps a shallow lake. Its base is
not exposed. The BBU-2 ignimbrite is about 20 m thick and shows complex rheomorphic
folding. The next higher unit (BBU-3) generally varies from 50 to 100 m thick and is a
compound cooling unit comprising 8 to 10 distinct layers. The stratigraphic position,
reverse paleomagnetic signature, and large volume of BBU-3 suggest that it is related to the
eruption(s) that formed CPT unit VII and related units farther west; however, on average,
its composition is slightly more mafic than CPT VII (e.g., 3.5 vs. 2.6% FeO*). It is overlain
by the thinner BBU-4 and BBU-5 units that we tentatively correlate with CPT IX and XI,
respectively, based on flux-gate paleomagnetic polarities for BBU-4 (normal) and BBU-5
5
(reverse). Above these is a 20-40 m slope interval consisting mainly of easily eroded ash
and perhaps reworked volcaniclastic sediments designated as unit BBU-6; this section
includes two or more thin (typically <10 m thick) resistant welded tuff layers that are partly
oxidized and silicified. BBU-6 represents a time interval of significant ash and sediment
accumulation, perhaps in a lacustrine environment.
Above this interval is a conspicuous unit (BBU-7; 2.5% FeO*) that ranges in thickness
from 50 to more than 100 m along the escarpment; it includes 5 to 7 distinct emplacement
layers, some of which show considerable internal rheomorphic deformation. BBU-7 is
equivalent to parts 1 to 6 of the Jackpot rhyolite member (exposed a few km to the SE;
Andrews et al. 2007). Our
40
Ar-39Ar date (10.91 Ma; text Table 1) suggests that BBU-7
may be equivalent to the tuff of Big Bluff, located farther east in the Cassia Mountains, and
to airfall tephra further east in the Trapper Creek area. Although chemically similar (~2.3%
FeO*), differences in age and physical characteristics suggest that BBU-7 is slightly older
than the 10.8 Ma CPT XIII unit to the west. Nevertheless, BBU-7 and CPT XIII may have
had their origins from the same magma source, and their collective volumes make it clear
that voluminous ignimbrites erupted at this time are widely distributed across southern
Idaho.
A thinner and more mafic unit (BBU-8; 3.5% FeO*) overlies BBU-7, forming a 20-40 m
high cliff. It is similar in most respects to both the uppermost (part 7) Jackpot rhyolite
member of Andrews et al. (2007) and the tuff of Steer Basin exposed farther east in the
Cassia Mountains. (Williams et al. 1990, 1991; Watkins et al. 1996; Watkins 1998); in
some respects BBU-8 also resembles the slightly less mafic (2.8% FeO*) CPT XV unit to
the west. Although not dated directly, available stratigraphic information and interpolation
of tephrochronology suggest an age of about 10.5 Ma for all of these units (Perkins et al.
1995, 1998; McCurry et al. 1996; Perkins and Nash 2002).
Unit BBU-9 occurs along the BB escarpment rim, but commonly has been eroded back
from it, forming a 15-30 m cliff. This unit has been dated at 10.22±0.09 Ma (text Table 1)
and has a relatively felsic composition (2.75% FeO*). It seemingly is equivalent to the
downfaulted Rabbit Springs ignimbrite member of the Jackpot formation that now floors
part of the Rogerson graben (Andrews et al. 2007); and although the latter unit has been
dated at 10.37±0.13 Ma (text Table 1), these ages overlap within error. Unit BBU-9 is
6
younger than any known CPT units, but is similar in age and composition to the Cedar Tree
rhyolite flow (10.16±0.09 Ma; Table 1) in the BJ region. Based on chemical and
stratigraphic similarities, BBU-9 also may be related to the House Creek ignimbrite
(Branney et al. 2004), exposed between the RTF and BJ areas. BBU-9 is overlain by three
thin, densely welded and glassy tuffs (designated as units BBU-10, -11, and -12) that
represent the last of the widespread ignimbrites deposited in this part of the southern CSRP.
Absence of sedimentary material between these units suggests they are relatively close in
age. They likely were deposited during the time interval when the Rogerson graben began
to develop. Where exposed at the northernmost part of the BB escarpment, they are
separated from underlying units by a down-to-north fault possibly related to CSRP regional
collapse. The uppermost of these units is probably equivalent to the Browns View
ignimbrite member of the Rogerson formation (Andrews et al. 2007) that floors the
Rogerson graben. Lower precision K-Ar dates (9.6 Ma, sample YU69-43L of Armstrong
et al. 1975; 9.85 Ma, sample SF-1 of Armstrong et al. 1980) for presumably equivalent
units from upper BB escarpment suggest that they erupted during late stages of the BJ
eruptive center activity. Because the lower and upper tuffs of Wooden Shoe Butte in the
Cassia Mountains (Williams et al. 1990, 1991; Parker 1996; Parker et al. 1996) also are
thought to be younger than 10.02 Ma (McCurry et al. 1997), and are similar in composition
and stratigraphic position, we tentatively propose that these units are roughly correlative.
West of Twin Falls, the rhyolites of Balanced Rock and Castleford Crossing are exposed
discontinuously along Salmon Falls Creek canyon for some 20 km southward from its
confluence with the Snake River (text Fig. 1). Although similar in composition, the
Balanced Rock rhyolite is a lava flow, whereas the overlying Castleford Crossing unit is a
very high-temperature, rheomorphic ignimbrite (Bonnichsen et al. 1988, 1989; McCurry et
al. 1997; Monani 1997). Compositions of these units are among the most mafic (4.1%
FeO*) of all CSRP rhyolites, and are distinguished by their hypersthene-bearing phenocryst
assemblage. The Castleford Crossing unit has a provisional
40
Ar-39Ar whole rock date of
8.13±0.29 Ma (John Kauffman, written communication 2005) and normal magnetic
polarity, whereas the somewhat older Balanced Rock unit has reverse polarization. We
suggest this latter unit is approximately 9 Ma old – in part because of its similarity to the
9.15 Ma City of Rocks rhyolite in the eastern Mount Bennett Hills (MBH). South and west
7
of the lower Salmon Falls Creek canyon are widely-distributed exposures of rhyolite (e.g.,
Horse Butte area; Bonnichsen 1982) with similar chemical and petrologic characteristics
(cf. sample I-693 in text Table 2).
The Greys Landing ignimbrite occurs in the northern part of the Rogerson graben; it
thickens northward toward a source buried in the CSRP. A whole-rock K-Ar age
(7.62±0.40 Ma; Hart and Aronson 1982) indicates that it is younger than the Balanced
Rock and Castleford Crossing units. It unconformably onlaps the tilted ignimbrites (BBU9-12) of BB escarpment and floors the northern part of the Rogerson graben, indicating it is
one of the youngest rhyolites in the RTF region. Its stratigraphic position, age, and
chemical composition are similar to those of the Dorsey Creek and Juniper-Clover area
rhyolite lavas of the BJ region as well as the Three Creek ignimbrite in the southeastern
part of the BJ region. Two thin ignimbrites (Coyote Creek and Sand Springs members;
Andrews et al. 2007) overlie the Greys Landing ignimbrite in the eastern part of Rogerson
Graben. The former is poorly sorted and unwelded, whereas the latter is a thin, denselywelded ignimbrite consisting principally of glassy and relatively mafic rhyolite - a
characteristic feature is an abundance of vitric clasts, some up to a few cm in diameter, that
are dispersed throughout the unit.
The Shoshone Falls rhyolite lava is exposed beneath younger basalt flows along a 12 km
stretch of Snake River canyon from Shoshone Falls to nearly 4 km downstream from the
Perrine Bridge (McCurry et al. 1997; Monani 1997). Although its base is hidden, drilling
near Shoshone Falls indicates that its thickness is nearly 200 m at that locality (Street and
DeTar 1987). K-Ar dates (averaging 6.4±0.3 Ma; Armstrong et al. 1975, 1980) indicate
that it is the youngest rhyolite in the area, excepting the Magic Reservoir system some 50
km to the north (Leeman 2004).
Additional rhyolite units between the BJ and RTF regions, exposed in the Crows Nest
area along the northeast margin of the BJ center (text Fig. 1) include a lower rhyolite unit,
probably an ignimbrite, and an upper unit thought to be a lava flow (Bonnichsen 1982).
The lower Crows Nest unit is similar in composition to BBU-9 and the Cedar Tree rhyolite
flow, whereas the upper Crows Nest unit compositionally resembles the Sheep Creek and
Poison Creek rhyolite flows.
8
Grasmere escarpment and Jacks Creek regions
To the west and northwest of the BJ center (text Fig. 1) are high-temperature rheomorphic
ignimbrites and rhyolite lava flows that are partly contemporaneous with the development
of the BJ volcanic center. These include rhyolites (probably lava-like ignimbrites) in the
Grasmere escarpment area and a series of rhyolite lava flows in the Jacks Creek area
(Bonnichsen and Kauffman, 1987; Kauffman and Bonnichsen, 1990).
The oldest two Grasmere escarpment units are the Buckhorn and Crab Creek ignimbrites
(Bonnichsen and Godchaux 2002). Their average compositions (Appendix 4D),
stratigraphic positions, and reverse magnetic polarities suggest they are related to, or even
extensions of, CPT VII. These units are overlain by the lava-like Grasmere escarpment
ignimbrite (Bonnichsen and Godchaux 2002), that extends about 30 km along the Grasmere
escarpment and continues northwestward into the Jacks Creek region, where it is
intercalated between two rhyolite lavas (Tigert Springs and O-X Prong flows; Kauffman
and Bonnichsen 1990). Its composition (Appendix 4D), stratigraphic position, and reverse
magnetic polarity suggest this unit is equivalent to CPT XI; a whole-rock K-Ar date for this
unit of 11.22±0.52 Ma (Hart and Aronson 1982) supports this interpretation. The Marys
Creek lava is exposed for about 25 km along the base of the Grasmere escarpment and in
one area flowed westward out of the BJ center through a small graben and over the
Grasmere Escarpment ignimbrite (Bonnichsen, 1982). Its age relative to other rhyolite
lavas in the BJ center is unknown but, based on its chemical similarity to the nearby Black
Rock Escarpment lava and CPT unit XII, we suggest that it is approximately the same age
as those units, and likely came from the same magma system.
Rhyolite lava flows erupted in the contiguous Jacks Creek area northwest of the BJ
center (text Fig. 1) include, in approximate stratigraphic order, the Rattlesnake Creek, O-X
Prong, Halfway Gulch, Tigert Springs, Perjue Canyon, and Horse Basin rhyolites
(Bonnichsen and Kauffman 1987; Kauffman and Bonnichsen 1990; Jenks et al. 1998).
Their compositions are given in Appendix 4D. The first three are similar in composition to
CPT VII, whereas the Tigert Springs and Perjue Canyon flows are similar in composition
and age (K-Ar ages of 9.4±2.0 and 9.7±1.5 Ma, respectively, Armstrong et al, 1980; Ekren
et al. 1981, 1982, 1984) to the Bruneau Jasper rhyolite flow (9.5 Ma; text Table 1) in the BJ
9
area. The youngest, the Horse Basin rhyolite, is compositionally similar to the Sheep Creek
rhyolite lava of the BJ region, but lies stratigraphically above it as exposed near Big Jacks
Creek Canyon.
At the northwestern side of the Jacks Creek area the Badlands rhyolite unit (Ekren et al
1981, 1982, 1984) is exposed near Poison Creek, some 30-35 km southwest of Grand
View. A sample of this unit yielded a K-Ar age of 12.0±0.2 Ma; this represents the lower
age limit for the succession of Jacks Creek area units noted in text Table 3, inasmuch as the
Badlands rhyolite of Poison Creek extends beneath the Jacks Creek units.
Cassia Mountains
In the Cassia Mountains., which bound the southeastern part of the CSRP (text Fig. 1), is a
succession of late Miocene, densely welded, rhyolite ignimbrites with petrologic and
physical characteristics very similar to those of the BJ, BB, and Rogerson graben areas.
These include, from older to younger, the tuffs of Magpie Basin, Big Bluff, Steer Basin,
Wooden Shoe Butte, and McMullan Creek (Mytton et al. 1990; Williams et al. 1990, 1991;
Perkins et al. 1995; Parker et al. 1996; McCurry et al. 1996, 1997; Watkins 1998; Wright et
al. 2002). Earlier we noted the possible regional correlations for the Big Bluff, Steer Basin,
and Wooden Shoe Basin units. The overlying McMullan Creek tuff comprises at least five
separate cooling units (Williams et al. 1990, 1991; Wright et al. 2002). Ages for these
ignimbrites (8.9-8.4 Ma) are based on K-Ar dates (Armstrong et al. 1980) and
tephrochronology studies (Perkins et al. 1995, 1998). The oldest McMullan Creek unit,
exposed in the Dry Gulch quarry and in Rock Creek Canyon near the northern margin of
the Cassia Mountains, is relatively felsic, bears sanidine and hypersthene, and is chemically
distinct from the overlying McMullan Creek units, which are relatively mafic (Appendix
4B) and sanidine free (Wright et al. 2002). The oldest McMullan Creek unit, noted as the
Dry Gulch ignimbrite in text Table 3, was referred to as McMullan Creek unit 1 by Wright
et al. (2002), but was not recognized by Williams et al. (1990, 1991). Stratigraphically
higher McMullan Creek units (1-4 of Williams et al. 1990, 1991) are noted herein (text
Table 3) as McMullan Creek units 2-5 (cf. Wright et al. 2002). Units similar to the
10
McMullan Creek tuff extend to the northeast side of the Rogerson graben and are exposed
along the northern SRP margin between the eastern MBH and Lake Hills.
A thick succession of silicic airfall ash in the Trapper Creek area, on the southeastern
flank of the Cassia Mountains, evidently accumulated in a Miocene lacustrine setting. This
sequence also includes several primary surge and ignimbrite deposits. Compositions and
ages of the ash layers indicate that many represent distal deposits related to silicic eruptions
associated with development of the CSRP (Perkins et al. 1995, 1998; Perkins and Nash
2002). Moreover, Perkins et al. (1998) show that some of the SRP pyroclastic eruptions
distributed similar fine-grained airfall ash many thousands of kilometers from Idaho.
Northern margin of Snake River Plain
Many rhyolitic units along the northern margin of the SRP are best exposed in the MBH
(text Fig. 1). Because of lateral changes in the stratigraphy, they are discussed in terms of
geographic area. Below, we describe the principal units and, based on similarities in
composition, age, stratigraphic position, and paleomagnetic polarity (Appendices 2C and
4C), suggest tentative correlations across the CSRP.
The thickest (>700 m), most prominent sequence of rhyolitic units occurs in the western
MBH, in the general vicinity of West Bennett Mountain (text Fig. 1). Two sequences of
silicic rocks have been defined: an older and topographically higher Bennett Mountain
group, and a younger, onlapping Danskin Mountain group (Wood and Gardner 1984;
Wood 1989; Honjo 1990; Clemens and Wood 1993). Drilling in the Mountain Home area
clearly establishes the existence of significant down-to-south normal faulting in the
subsurface (cf. Wood and Clemens 2002, references therein), corresponding to km-scale
subsidence of the interior of the adjacent SRP. Many rhyolite units in both groups likely
were emplaced as voluminous ignimbrites as first noted by Wood and Gardner (1984). For
example, some are underlain by poorly welded airfall tephra and surge deposits. However,
many are rheomorphic and have lithologic characteristics similar to high-temperature
rhyolite lava flows (cf. Bonnichsen and Kauffman 1987) which, combined with limited
lateral exposure in many cases, complicates interpretation of depositional processes and
11
sources. It is probable that extensive post-emplacement flowage occurred due to eruption of
many West Bennett Mountain rhyolite tuffs onto steep or uneven topography.
Wood and Gardner (1984) identified six units in the Bennett Mountain group. From
oldest to youngest these were designated as follows: (1) rhyolite of Willow Creek for
which both reversed (Wood and Gardner 1984) and normal (Honjo 1990) magnetic
polarities have been reported; Clemens and Wood (1993) report a plagioclase K-Ar age of
11.0±0.5 Ma; (2) lower rhyolite of normal polarity; (3) reversed polarity rhyolite; (4)
rhyolite of Windy Gap with reverse polarity and a K-Ar age of 11.0±0.6 Ma (Armstrong et
al. 1980); (5) tuff of Bennett Mountain with reverse polarity, and (6) rhyolite of Henley
with normal polarity. The Danskin Mountain group includes, from oldest to youngest: (1)
rhyolite of Frenchman Springs; (2) tuff of Rattlesnake Springs; (3) tuff of Dive Creek; and
(4) rhyolite of High Spring with a sanidine K-Ar date of 10.0±0.3 Ma (Clemens and Wood
1993). Recent investigations (Will Starkel, 2006 personal communication) suggest that the
Danskin group section has been repeated by normal faulting and actually comprises only
two units: the rhyolites of Frenchman Springs and Dive Creek being the same older unit
and the rhyolites of Rattlesnake Springs and High Spring being the same younger unit.
This interpretation is supported by nearly identical sample compositions in each pair of
units (Appendix 4C). All Danskin Mountain units have normal polarity. As noted in the
main text, magnetic polarity data indicate that the K-Ar ages reported for these rocks
(Appendix 2C) appear to be slightly low.
The thick Windy Gap unit has reverse magnetic polarity, chemical composition, and
radiometric age that suggest it is the approximate time equivalent to the widespread and
voluminous CPT XI and Grasmere Escarpment ignimbrite units in the southwestern CSRP.
The tuff of Fir Grove, exposed in the eastern MBH, has similar properties, has reverse
magnetic polarity and has a precise
39
Ar/40Ar date of 11.17±0.08 Ma; it overlies the
rhyolite of Deer Springs which has been dated similarly at 11.21±0.07 Ma (Oakley and
Link 2006). These units are essentially coeval with CPT XI and XII (as well as the
Grasmere Escarpment ignimbrite) and may be equivalent to the tuffs of Bennett Mountain
and Windy Gap, respectively. Intervening younger sediments and basalts preclude direct
tracing of these MBH units across the CSRP, but the aforementioned features and relative
thicknesses strongly suggest that all resulted from a related sequence of eruptions that
12
accompanied the emptying of a large magma chamber and formation of a now-buried
caldera system. Assuming these inferred stratigraphic equivalences to be correct, we use
available data to infer provisional correlations between other West Bennett Mountain and
BJ units. Because available paleomagnetic data are based on limited field flux-gate
magnetometer studies (albeit using multiple samples from multiple sites), greater emphasis
is placed on other criteria. The rhyolite of Willow Creek is likely related to CPT VII –
particularly if its reverse paleomagnetic polarity is correct. The unnamed normal-polarity
lower rhyolite unit may be equivalent to CPT IX, whereas the overlying unnamed reversepolarity unit may be equivalent to CPT X. The tuff of Bennett Mountain is compositionally
similar to CPT XII; both are relatively mafic (~3.15% FeO*) and the only significant
compositional difference is lower Ba in CPT XII. The fact that CPT XII has normal
polarity whereas the tuff of Bennett Mountain is reversed could signify that the related
eruptions straddled a reversal in the Earth’s magnetic field or that the polarities are
unreliable.
An important observation is that the uppermost (Henley) rhyolite of the Bennett
Mountain group and the earliest rhyolite (Frenchman Springs/Dive Creek) of the onlapping
Danskin Mountain group have similar characteristics. Both may represent a sequence of
eruptions from a large magma chamber whose partial evacuation led to widespread
topographic collapse. Onlap of Frenchman Springs/Dive Creek on Henley units implies
that major subsidence was associated with the eruptive event(s) that produced them. Both
are plausibly related to eruption of compositionally and magnetically similar units to the
south – CPT XIII, BBU-7 (Jackpot rhyolite parts 1-6), and the tuff of Big Bluff. Small
differences in 40Ar/39Ar ages for the latter units, if real, could signify piecemeal collapse of
their eruptive center over a significant but unresolved time interval.
The overlying rhyolites of Rattlesnake Spring/High Spring are approximately the same
age as CPT XV in the BJ region and the BBU-8 and Steer Basin units in the RTF region
(text Table 3). These units are relatively mafic (3.3 and 2.7% FeO*) and smaller in volume
compared to their immediate precursors. The upper (Rattlesnake Spring/High Spring)
rhyolite of the Danskin Mountain group chemically, mineralogically and paleomagnetically
most closely resembles the Cedar Tree rhyolite in the BJ region and the BBU-9 and Rabbit
13
Springs units in the RTF region, all of which are similar in age (~10 Ma). If correct, these
correlations imply that the Danskin Mountain group emplacement took about 500 ka.
In the central MBH, are exposed several informally named rhyolites (or tuffs) of Cold
Spring Creek, and King Hill Creek, that onlap some of the western MBH units (e.g., tuff of
Bennett Mountain; Honjo 1990); the first two units comprise several cooling units that
likely correspond to different eruptions. Several map units (upper Cold Springs units,
middle King Hill, Knob, and Gwin Springs tuffs) have comparable iron-rich compositions
(~4% FeO*) and occupy similar stratigraphic positions (i.e., overlying the 11.17 Ma tuff of
Fir Grove). Only the Knob tuff has been dated (10.63±0.23 Ma; Oakley and Link 2006);
this unit is approximately coeval with CPT XV. Although undated, ages of the other units
are probably around 10 Ma based on ages (Armstrong et al. 1980; Honjo et al. 1986; Honjo
1990) of bracketing units including: rhyolites of Thorn Creek (10.1 Ma) and City of Rocks
(9.15 Ma); magnetic polarities are inconsistent within most of these units, precluding their
use in refining the age estimate. The City of Rocks rhyolite is the youngest known in the
eastern MBH. It is unique in containing orthopyroxene and having the lowest known SiO2
content (~70%); it is also mineralogically and compositionally similar to the slightly
younger Balanced Rock and Castleford Crossing units to the south. Generally, eastern
MBH units have normal magnetic polarities, ages (~10 to 9 Ma) and/or chemical
compositions that suggest they are equivalent to uppermost parts of the BJ section and
younger units in the RTF area and Cassia Mountains.
Finally, in the Picabo and Lake Hills (text Fig. 1) is exposed a package of at least four
ignimbrite cooling units that have physical, chemical, and petrographic similarities to the
Tuff of McMullen Ck. (Moye et al. 1988; Honjo 1990; Wright et al. 2002). Plagioclase KAr dates for two samples of the uppermost unit average 9.0 Ma (Honjo et al. 1986; Honjo
1990). New
40
Ar/39Ar dates for the Lake Hills section range between 9.2-8.6 Ma (D.
Rodgers, 2006 personal communication). These units onlap an eroded basement of Eocene
Challis volcanics and locally rest unconformably on a poorly preserved, nearly aphyric
rhyolite cooling unit that fills paleotopography; based on preliminary data the latter unit
(11.19±0.05 Ma; Honjo, 1990) resembles CPT XI, and could represent distal remnants of
this massive eruptive event. In general, we have not attempted serious correlations across
the CSRP for units younger than 10 Ma; most have the same (normal) paleomagnetic
14
polarity and similar compositions, making them hard to distinguish. However, ages and
field relations strongly suggest that they are in part correlative with units of the tuff of
McMullen Creek.
Concluding remarks
Data and observations from the aforementioned sections have been used to establish a
regional stratigraphic model that is portrayed in text Table 3. This model subsequently is
used to evaluate eruptive volumes and variations in magmatic composition over the
duration of CSRP Miocene magmatism (see main text).
References to this Appendix
Andrews GDM, Branney MJ, Bonnichsen B, McCurry M (2007) Rhyolitic ignimbrites in
the Rogerson Graben, southern Snake River Plain volcanic province: volcanic
stratigraphy, eruption history and basin evolution. Bull Volcanol (this issue)
Armstrong RL, Leeman WP, Malde HE (1975) K-Ar dating of Quaternary and Neogene
volcanic rocks of the Snake River Plain, Idaho. Am J Sci 275:225-251
Armstrong RL, Harakal, JE, Neill WM (1980) K-Ar dating of Snake River Plain (Idaho)
volcanic rocks--new results. Isochron West 27:5-10
Bernt JD (1982) Geology of the southern J-P Desert, Owyhee County, Idaho. MS thesis,
University of Idaho, Moscow
Bonnichsen B (1982) Rhyolite lava flows in the Bruneau-Jarbidge eruptive center,
southwestern Idaho. In: Bonnichsen B, Breckenridge, RM (eds) Cenozoic geology of
Idaho. Idaho Bur Mines and Geol Bull 26:283-320
Bonnichsen B, Citron GP (1982) The Cougar Point Tuff, southwestern Idaho and vicinity.
In: Bonnichsen B, Breckenridge, RM (eds) Cenozoic geology of Idaho. Idaho Bur Mines
and Geol Bull 26:255-281
Bonnichsen B, Kauffman DF (1987) Physical features of rhyolite lava flows in the Snake
River Plain volcanic province, southwestern Idaho. In: Fink JH (ed), The Emplacement
of silicic domes and lava flows. Geol Soc Am Spec Pap 212:119-145
15
Bonnichsen B, Jenks MD (1990) Geologic map of the Jarbidge River wilderness study
area, Owyhee County, Idaho. US Geol Surv Misc Field Studies Map MF-2127
Bonnichsen B, Godchaux MM (2002) Late Miocene, Pliocene, and Pleistocene geology of
southwestern Idaho with emphasis on basalts in the Bruneau-Jarbidge, Twin Falls, and
western Snake River Plain regions. In: Bonnichsen B, McCurry M, White CM (eds)
Tectonic and magmatic evolution of the Snake River Plain volcanic province. Idaho
Geol Surv Bull 30:233-312
Bonnichsen B, Leeman WP, Jenks MD, Honjo N (1988) Geologic field trip guide to the
central and western Snake River Plain, Idaho, emphasizing the silicic volcanic rocks. In:
Link PK, Hackett WR (eds) Guidebook to the geology of central and southern Idaho.
Idaho Geol Surv Bull 27:247-281
Bonnichsen B, Christiansen RL, Morgan LA, Moye FJ, Hackett WR, Leeman WP, Honjo
N, Jenks MD, Godchaux MM (1989) Excursion 4A: Silicic volcanic rocks in the Snake
River Plain-Yellowstone Plateau province. New Mexico Bur Mines and Min Res Mem
47:135-182
Branney MJ, Barry TL, Godchaux M (2004) Sheathfolds in rheomorphic ignimbrites. Bull
Volcanol 66:485-491
Cathey HE and Nash BP (2004) The Cougar Point Tuff: Implications for thermochemical
zonation and longevity of high-temperature, large-volume silicic magmas of the
Miocene Yellowstone hotspot. J Petrol 45:27-58
Clemens DM, Wood SH (1993) Late Cenozoic volcanic stratigraphy and geochronology of
the Mount Bennett Hills, central Snake River Plain, Idaho. Isochron West 60:3-14
Coats RR (1964) Geology of the Jarbidge quadrangle, Nevada-Idaho. US Geol Surv Bull
1141-M:24 p
Ekren EB, McIntyre DH, Bennett EH, Malde HE (1981) Geologic map of Owyhee County,
Idaho, west of longitude 116 degrees W. US Geol Surv Misc Invest Ser Map I-1256
Ekren EB, McIntyre DH, Bennett EH, Marvin RF (1982) Cenozoic stratigraphy of western
Owyhee County, Idaho. In: Bonnichsen B, Breckenridge, RM (eds) Cenozoic geology of
Idaho. Idaho Bur of Mines and Geol Bull 26:215-235
16
Ekren EB, McIntyre DH, Bennett EH (1984) High-temperature, large-volume, lava-like ashflow tuffs without calderas in southwestern Idaho. US Geol Surv Prof Pap 1272:76 p
Hart WK, Aronson JJ (1982) K-Ar ages of rhyolites from the western Snake River Plain
area, Oregon, Idaho, and Nevada. Isochron West 36:17-19
Hirt WH (2002) Transition from ash-flow to voluminous lava-flow activity, BruneauJarbidge eruptive center, southwestern Idaho. In: Bonnichsen B, McCurry M, White CM
(eds) Tectonic and magmatic evolution of the Snake River Plain volcanic province.
Idaho Geol Surv Bull 30:195-204
Honjo N (1990) Geology and stratigraphy of the Mount Bennett Hills, and the origin of
west-central Snake River Plain rhyolites. PhD thesis, Rice University, Houston
Honjo N, McElwee KR, Duncan RA, Leeman WP (1986) K-Ar ages of volcanic rocks
from the Magic Reservoir eruptive center, Snake River Plain, Idaho. Isochron West
46:9-14
Honjo N, Bonnichsen B, Leeman WP, Stormer JC Jr (1992) High-temperature rhyolites
from the central and western Snake River Plain. Bull Volcanol 54:220-237
Jenks MD, Bonnichsen B (1990) Geologic map of the Bruneau River wilderness study
area, Owyhee County, Idaho. US Geol Surv Misc Field Studies Map MF-2128
Jenks MD, Bonnichsen B, Godchaux MM (1998) Geologic map of the Grand ViewBruneau area, Owyhee County, Idaho. Idaho Geol Surv Tech Rep T-98-1
Kauffman DK, Bonnichsen B (1990) Geologic map of the Little Jacks Creek, Big Jacks
Creek, and Duncan Creek wilderness study areas, Owyhee County, Idaho. US Geol Surv
Misc Field Studies Map MF-2142
Leeman WP (2004) Evolution of Snake River Plain (SRP) silicic magmas – the Magic
Reservoir eruptive center. Geol Soc Am Abst with Prog 36(5):24
McCurry M, Watkins AM, Parker JL, Wright K, Hughes SS (1996) Preliminary
volcanological constraints for sources of high-grade, rheomorphic ignimbrites of the
Cassia Mountains, Idaho: Implications for the evolution of the Twin Falls volcanic
17
center. In: Hughes SS, Thomas RC (eds) Geology of the crook in the Snake River Plain,
Twin Falls and vicinity, Idaho. Northwest Geology 26:81-91
McCurry M, Bonnichsen B, White C, Godchaux MM, Hughes SS (1997) Bimodal basaltrhyolite magmatism in the central and western Snake River Plain, Idaho and Oregon.
Brigham Young Univ Geol Studies 42:381-422
Monani S (1997) Rhyolites of the Twin Falls eruptive center, the Snake River Plain
volcanic province, southern Idaho: a petrographic and geochemical study. BA thesis,
Mount Holyoke College, South Hadley
Moye FJ, Leeman WP, Hackett WR, Bonnichsen B, Honjo N, Clarke C (1988) Cenozoic
volcanic stratigraphy of the Lake Hills, Blaine Co., Idaho, and bearing on the
development of the Snake River Plain Province. Geol Soc Am Abst With Prog
20(6):434
Mytton JW, Williams PL, Morgan WA (1990) Geologic map of the Stricker 4 quadrangle,
Cassia County, Idaho. U.S. Geol Surv Misc Invest Series Map I-2052
Oakley WL, Link PK (2006) Geologic map of the Davis Mountain Quadrangle, Gooding
and Camas Counties, Idaho. M.S. Thesis, Idaho State University, Pocatello
Parker JL (1996) Physical volcanology and geochemistry of the tuff of Wooden Shoe
Butte, Cassia Mountains, Idaho. MS thesis, Idaho State University, Pocatello
Parker JL, Hughes SS, McCurry M (1996) Physical and chemical constraints on the
emplacement of the tuff of Wooden Shoe Butte, Cassia Mountains, Idaho. In: Hughes
SS, Thomas RC (eds) Geology of the crook in the Snake River Plain, Twin Falls and
vicinity, Idaho. Northwest Geology 26:92-106
Perkins ME, Nash WP (2002) Explosive silicic volcanism of the Yellowstone hotspot: The
ash fall tuff record. Geol Soc Am Bull 114:367-381
Perkins ME, Nash WP, Brown FH, Fleck RJ (1995) Fallout tuffs of Trapper Creek, Idaho-A record of Miocene explosive volcanism in the Snake River Plain volcanic province.
Geol Soc Am Bull 107:1484-1506
18
Perkins ME, Brown FH, Nash WP, McIntosh W, Williams SK (1998) Sequence, age, and
source of silicic fallout tuffs in middle to late Miocene basins of the northern Basin and
Range province. Geol Soc Am Bull 110:344-360
Street LV, DeTar RE (1987) Geothermal resource analysis in Twin Falls County, Idaho. In:
Geothermal investigations in Idaho. Idaho Dept Water Res Info Bull 30 Part 15:1-95
Watkins AM (1998) Geochemistry, petrography, and stratigraphy of the tuff of Steer Basin,
Twin Falls and Cassia Counties, Idaho. MS thesis, Idaho State University, Pocatello
Watkins AM, McCurry M, Hughes SS (1996) Preliminary report on the stratigraphy and
geochemistry of the tuff of Steer Basin, Cassia Mountains, Idaho. Northwest Geol
26:107-120
Williams PL, Mytton JW, Covington HR (1990) Geologic map of the Stricker 1
quadrangle, Cassia, Twin Falls, and Jerome Counties, Idaho. US Geol Surv Misc Invest
Ser Map I-2078
Williams PL, Covington HR, Mytton JW (1991) Geologic map of the Stricker 2
quadrangle, Twin Falls and Cassia Counties, Idaho. US Geol Surv Misc Invert Ser Map
I-2146
Wood SH (1989) Silicic volcanic rocks and structure of the western Mount Bennett Hills
and adjacent Snake River Plain, Idaho. In: KL Ruebelmann (ed) Snake River PlainYellowstone volcanic province. 28th International Geological Congress Field Trip
Guidebook T305, Am Geophys Union:69-77
Wood SH, Gardner JN (1984) Silicic volcanic rocks of the Miocene Idavada Group,
Bennett Mountain, southwestern Idaho. Final contract report to the Los Alamos National
Laboratory from Boise State University
Wood SH, Clemens DM (2002) Geologic and tectonic history of the western Snake River
Plain, Idaho and Oregon. In: Bonnichsen B, McCurry M, White CM (eds) Tectonic and
magmatic evolution of the Snake River Plain volcanic province. Idaho Geol Surv Bull
30:69-103
Wright KE, Michael McCurry M, Hughes SS (2002) Petrology and geochemistry of the
Miocene tuff of McMullen Creek, central Snake River Plain. In: Bonnichsen B,
19
McCurry M, White CM (eds) Tectonic and magmatic evolution of the Snake River Plain
volcanic province. Idaho Geol Surv Bull 30:177-194
20