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Ash-flow tuffs and paleovalleys in northeastern Nevada: Implications for Eocene paleogeography and extension in the Sevier hinterland, northern Great Basin

¹ Nevada Bureau of Mines and Geology, University of Nevada, Reno, Nevada 89557, USA

View larger version (132K): [in this window] [in a new window]   Figure 1. Digital elevation map of the Great Basin showing the distribution of known paleovalleys and a few segments (from Lindgren, 1911; Faulds et al., 2005; Garside et al., 2005; and this study), the Eocene paleodivide proposed from this study, and the paleodivide of Christiansen and Yeats (1992). The east-draining paleovalleys flowed to the Uinta basin; the west-draining paleovalleys flowed to the Pacific Ocean, which was in the Great Valley of California at the time. Arrows show flow direction where known or reasonably inferred.

View larger version (104K): [in this window] [in a new window]   Figure 2. Geologic map of northeastern Nevada (simplified from Coats, 1987), showing locations of more detailed maps.

View larger version (90K): [in this window] [in a new window]   Figure 3 (on this and following page). (A) Map of interpreted paleovalleys showing locations of simplified stratigraphic columns for paleovalley segments and correlations of tuffs along the paleovalleys (B).

View larger version (44K): [in this window] [in a new window]   Figure 3 (continued). (B) Dates in regular type are from this study; dates in italics are from Axelrod (1966a), Brooks et al. (1995a, 1995b), Henry and Faulds (1999), Mueller et al. (1999), Haynes (2003), and Wallace (2003a).

View larger version (23K): [in this window] [in a new window]   Figure 4. Probability plots of representative sanidine 40Ar/39Ar ages of ash-flow tuffs dated in this study showing different ages of the 45 Ma tuff, tuff of Nelson Creek (40.14 ± 0.06 Ma; n = 6), and tuff of Big Cottonwood Canyon (39.99 ± 0.08; n = 9). Unfilled data points were not used in age calculation (for example, analyses that are outside 2 uncertainty or are of plagioclase).

View larger version (53K): [in this window] [in a new window]   Figure 5. Compositional variation diagrams for whole-rock samples of Eocene ash-flow tuffs of northeastern Nevada. Data are from this study (Table 2) and Mueller (1992), Brooks et al. (1995a, 1995b), and Wallace (2003a). Major elements normalized to 100% volatile-free. (A) SiO2/Na2O+K2O. (B) SiO2/TiO2. (C) SiO2/P2O5. (D) Zr/Nb. (E) Zr/TiO2.

View larger version (13K): [in this window] [in a new window]   Figure 6. Ternary plot showing relative modal abundances of quartz, plagioclase, and sanidine phenocrysts in Eocene ash-flow tuffs of northeastern Nevada.

View larger version (52K): [in this window] [in a new window]   Figure 7. Simplified geologic map of the Tuscarora volcanic field with 40Ar/39Ar dates of ash-flow tuffs (Henry et al., 1999). Magmatism, including eruption of the tuff of Big Cottonwood Canyon at 40.0 Ma, occurred over a brief, intense period between 40.2 and 39.5 Ma. Any paleovalley is largely buried, but pre-volcanic conglomerate is exposed along the southern and southeastern flank of the volcanic field.

View larger version (79K): [in this window] [in a new window]   Figure 8. Simplified geologic map of the Eocene paleovalley near California Mountain, east side of the Independence Mountains (Muntean and Henry, 2006) with 40Ar/39Ar dates from this study (regular type) and Hofstra (1994; italics). The paleovalley, which was 5 km wide and at least 800 m deep, was filled by conglomerate containing rounded boulders to 1.5 m diameter, shale and limestone, the plagioclase-biotite tuff and tuff of Big Cottonwood Canyon, and a thick dacite sequence between the tuffs. The rocks were tilted gently southeastward after 40.0 Ma by several small-displacement, northeast-striking, mostly down-to-the-northwest normal faults. Dashed lines within unit Td are contacts between individual lavas and tuffs.

View larger version (169K): [in this window] [in a new window]   Figure 9. (A) Finely laminated, "paper" shale in the California Mountain paleovalley (Fig. 8). (B) Photo looking west at an 6-km–wide, 500-m–deep swale in the Independence Mountains west of the exposed paleovalley sequence near California Mountain (Fig. 8). The swale is probably the western continuation of the paleovalley. (C) Rounded, 6-m–wide boulder of chert-pebble conglomerate in basal Tertiary conglomerate of the Nanny Creek paleovalley (Fig. 10). (D) Well-rounded, 1-m–wide boulder of quartzite in the Nanny Creek paleovalley (Fig. 10). (E) Isolated block of tuff of Big Cottonwood Canyon in megabreccia in Nanny Creek paleovalley (Fig. 10). (F) Scattered blocks of tuff of Big Cottonwood Canyon in megabrecccia in Taylor Canyon paleovalley (Fig. 3). Compaction foliation is oriented differently in each block, showing that each is a separate block.

View larger version (61K): [in this window] [in a new window]   Figure 10. Geologic map of the paleovalley at Nanny Creek, east side of the Pequop Mountains (Brooks et al., 1995b; this study) with 40Ar/39Ar dates from this study (regular type) and Brooks et al. (1995a, 1995b; italics). The paleovalley, which was at least 6 km wide and possibly 1.6 km deep, has basal conglomerate containing rounded boulders up to 6 m in diameter, overlain by the plagioclase-biotite tuff and tuff of Big Cottonwood Canyon, andesite lavas, and a thick megabreccia consisting of angular blocks mostly of the tuff of Big Cottonwood Canyon. That megabreccia consists of reworked blocks is confirmed by petrographic, chemical, 40Ar/39Ar, and paleomagnetic data (this study; M.R. Hudson, in Brooks et al., 1995a; Palmer and MacDonald, 2002).

View larger version (58K): [in this window] [in a new window]   Figure 11. Geologic map of part of the Eocene paleovalley in the Windermere Hills (modified from Mueller, 1993, with 40Ar/39Ar dates from Mueller et al., 1999). Imbricated clasts in cobble conglomerate (Tdc) indicate eastward flow (Mueller, 1992). Mueller (1993) and Mueller et al. (1999) interpreted major normal faults repeating the ash-flow tuffs along the west side of this map. I reinterpret these as megabreccia (Tx) composed of blocks of the plagioclase-biotite tuff and tuff of Big Cottonwood Canyon. That they are blocks is confirmed by petrography, chemical analysis, discordant compaction foliations, and by scattered magnetization directions (Palmer and MacDonald, 2002). Therefore, their stratigraphic position and the amount of repetition by faulting are uncertain.

View larger version (80K): [in this window] [in a new window]   Figure 12. Simplified geologic map of the Eocene paleovalley near Cornwall Mountain (modified from Coash, 1967). Basal conglomerate with rounded clasts up to 4 x 6 m is overlain by the zoned, 45 Ma tuff (40Ar/39Ar date from this study). An upper, probably lacustrine sequence of tuffaceous siltstone and shale has numerous interbedded breccia lenses containing clasts up to 4 m diameter of the upper, less silicic part of the 45 Ma tuff.

View larger version (91K): [in this window] [in a new window]   Figure 13. Geologic map of the paleovalley at Coal Mine Canyon, southeast flank of the Adobe Range (modified from Moore et al., 1982). The sedimentary sequence consists of fluvial, boulder-to-cobble conglomerate overlain by lacustrine shale, claystone, and limestone. The tuff of Coal Mine Canyon is dated at 40.7 ± 0.4 Ma (40Ar/39Ar, hornblende; Brooks et al., 1995a, 1995b) and 41.1 ± 0.2 Ma (U-Pb, zircon; Haynes, 2003).