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 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 (airfalland/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, plagioclase-pyroxene-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 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 (0.5-5 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 (50-200 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 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 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 (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 40Ar-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 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 40Ar-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 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 (BBU-9-12) 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, densely-welded 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.