Geosphere; February 2008; v. 4; no. 1;
p. 1-35; DOI: 10.1130/GES00122.1
© 2008 Geological Society of America
Ash-flow tuffs and paleovalleys in northeastern Nevada: Implications for Eocene paleogeography and extension in the Sevier hinterland, northern Great Basin
Christopher D. Henry*,1
1 Nevada Bureau of Mines and Geology, University of Nevada, Reno, Nevada 89557, USA
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ABSTRACT
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Northeastern Nevada is generally interpreted as an area of large-magnitude Eocene extension possibly due to gravitational collapse of crust thickened during the Sevier orogeny. The extensional interpretation is based in part on the presence of widespread Eocene conglomerates and lacustrine basins, as well as on thermochronology-based evidence of major Eocene cooling and uplift of the Ruby Mountains–East Humboldt Range core complex.
The distribution of 45–40 Ma ash-flow tuffs and interbedded coarse conglomerates and lacustrine deposits, however, indicates they were predominantly deposited in a system of east-draining paleovalleys incised into a plateau or moderate-relief upland. A large, contiguous sedimentary basin probably was never present. Paleovalleys were as much as 10 km wide and 500 m to possibly as much as 1.6 km deep, based on the thickness of intra-valley deposits. Ash-flow tuffs are widely distributed near source calderas but are almost entirely confined to the paleovalleys as little as 20 km from their source. Basal, mostly pre-volcanic conglomerates contain clasts up to 6 m in diameter. The clasts are well rounded, indicating significant fluvial transport, not derivation from nearby fault scarps. Lacustrine deposits also are restricted to paleovalleys and accumulated during two periods that are interpreted to coincide with episodes of minor, northwest-directed extension, one before 41 Ma and possibly as old as 46 Ma, and another between 40 and 38 Ma. Extension formed small displacement, northeast-striking, mostly down-to-the-northwest faults that temporarily dammed the paleovalleys to form lakes. Lakes probably also formed where volcanic rocks or landslides dammed paleovalleys, a common process both in the Eocene and historically in the western United States. The absence of major Eocene extension suggests that gravitational collapse of overthickened crust, even assisted by thermal weakening of lithosphere by intense magmatism, was not sufficient to generate major extension.
Absolute elevation of the high plateau is uncertain, but it was high enough to have paleovalleys as much as 1.6 km deep. Based on published paleoflora data, interfluves could have been at elevations of 
4 km.
The Eocene paleovalleys in northeastern Nevada most likely drained eastward to remnants of the Uinta basin. An approximately north-south paleodivide through northeastern Nevada separated these east-draining paleovalleys from paleovalleys that drained westward to the Pacific Ocean.
Keywords: paleogeography • extension • Eocene • ash-flow tuff • Nevada
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INTRODUCTION
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The Eocene tectonic history and paleotopography of northeastern Nevada are uncertain and debated (Axelrod, 1966b; Christiansen and Yeats, 1992; McGrew and Snee, 1994; Snoke et al., 1997; Chase et al., 1998; Wolfe et al., 1998; McGrew et al., 2000; Rahl et al., 2002; Howard, 2003; Horton et al., 2004; Cline et al., 2005; Hickey et al., 2005). The region underwent multiple episodes of contraction in the Paleozoic and Mesozoic followed by extension in the mid- to late Cenozoic (Armstrong, 1968; Coney and Harms, 1984; Christiansen and Yeats, 1992; Wernicke, 1992; DeCelles, 2004; Dickinson, 2006). Eastern Nevada was in the hinterland of the Late Cretaceous Sevier orogenic belt (Armstrong, 1968; Miller and Gans, 1989; Camilleri et al., 1997; Vandervoort and Schmitt, 1990; Wright and Snoke, 1993; Howard, 2003; DeCelles, 2004). That the crust was 50–60 km thick in a belt through eastern Nevada and western Utah following contraction has been inferred from restoration of Tertiary extension (Coney and Harms, 1984), estimates of shortening in the overthrust belt (Thorman et al., 1991; Camilleri et al., 1997), and metamorphic mineral assemblages (Camilleri et al., 1997; McGrew et al., 2000; Lee et al., 2003).
The region was probably an eroding highland at the beginning of the Cenozoic (Armstrong, 1968; Coney and Harms, 1984; Christiansen and Yeats, 1992; Dilek and Moores, 1999; DeCelles, 2004). Sediments probably were carried eastward to the Uinta and Green River Basins (Baars et al., 1988; Hintze, 1993; Goldstrand, 1994), with little sedimentary record in Nevada until the Eocene (Fouch et al., 1979; Solomon et al., 1979; Vandervoort and Schmitt, 1990). However, interpretations of the absolute elevation, timing of uplift, and paleotopography vary widely. Coney and Harms (1984), Dilek and Moores (1999), and DeCelles (2004), drawing analogies to present-day Tibet and the Andean Plateau, inferred elevations of >3 km. They interpreted the high elevation to have resulted from thickening of crust, in large part during the Sevier orogeny. Absolute Eocene elevations inferred from fossil leaves from Copper Basin in northeastern Nevada, which has a present-day elevation of 2.2 km, range from 1.1 km (Axelrod, 1966b; Christiansen and Yeats, 1992), to 2.0 ± 0.2 km (Wolfe et al., 1998), to 1.6 ± 1.6 km or 2.8 ± 1.8 km (Chase et al., 1998). Although not providing absolute elevations, Horton et al. (2004) used stable isotope data to interpret that northeastern Nevada rose 2 km between the middle Eocene and early Oligocene, contemporaneous with the period of intense magmatism, then subsided 1–2 km since the middle Miocene. A rise of 2 km would seem either to require that northeastern Nevada was relatively low in the middle Eocene, contradicting the tie between Mesozoic crustal thickening and high elevation, or that it rose to elevations of as much as 5 km.
Cenozoic extension is generally interpreted to have begun in the Eocene, but the timing and amount of early extension are poorly constrained. Thermochronologic data have been interpreted to record initial exhumation of the Ruby Mountains metamorphic core complex between 63 and 49 Ma (Hodges et al., 1992; McGrew and Snee, 1994; McGrew et al., 2000). These data are difficult to interpret, however, and Howard (2003) concluded that the timing of early unroofing of the Ruby Mountains is uncertain. A gradient in K-Ar and 40Ar/39Ar biotite ages from 36 to 20 Ma west-northwest across the range is generally interpreted to reflect cooling during prolonged uplift (Kistler, 1981; Dokka et al., 1986; McGrew and Snee, 1994). Apatite fission-track and U-Th/He dating indicate the Ruby Mountains underwent rapid cooling and uplift in the middle Miocene (ca. 15 Ma; Colgan and Metcalf, 2006). Sedimentary basins in northeastern Nevada began filling as early as 46 Ma (Solomon et al., 1979; Haynes, 2003; Cline et al., 2005; Hickey et al., 2005). Most basins have been interpreted to be extensional (Axelrod, 1966b; Solomon et al., 1979; Clark et al., 1985; Vandervoort and Schmitt, 1990; Satarugsa and Johnson, 2000; Haynes, 2003; Cline et al., 2005; Hickey et al., 2005), while others have not—for example, the White Sage basin in westernmost Utah (Fig. 1; Potter et al., 1995; Dubiel et al., 1996).
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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.
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The goal of this study is to document the distribution of regional, Eocene ash-flow tuffs and character of the Eocene paleosurface in northeastern Nevada. The study grew out of an investigation of the magmatic-tectonic setting and origin of the Eocene Carlin-type gold deposits, whose relationship to magmatism or extension and the depth of formation are highly debated (Muntean et al., 2004; Cline et al., 2005; Hickey et al., 2005; Ressel and Henry, 2006). By analyzing the distribution of 45–40 Ma ash-flow tuffs and contemporaneous sedimentary deposits, I infer a system of paleovalleys that drained eastward to the remnants of the Uinta-Green River basins of Utah. Complementary paleoval-leys drained westward across the central Great Basin and Sierra Nevada to the Pacific Ocean. These data also indicate that Eocene extension consisted of possibly two episodes of relatively minor extension, one before ca. 41 Ma and another between 40 and 38 Ma. Most extension in the region probably occurred in the middle Miocene or later.
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GEOLOGIC SETTING
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Northeastern Nevada underwent multiple episodes of contraction from the Late Devonian-Early Mississippian Antler orogeny through the Late Cretaceous Sevier orogeny (Roberts et al., 1958; Armstrong, 1968; Miller et al., 1992; Poole et al., 1992; Camilleri et al., 1997; Taylor et al., 2000; DeCelles, 2004; Dickinson, 2006). During the Antler orogeny, deep-water siliciclastic rocks (western assemblage) were thrust eastward over coeval shelf- and slope-facies marine rocks (eastern assemblage) along the Roberts Mountains thrust (Fig. 1). A thick, clastic wedge was deposited in front of the thrust belt (Burchfiel et al., 1992). Subsequent thrust belts developed progressively eastward, and eastern Nevada was in the hinterland of the Sevier orogenic belt, the youngest contractional deformation in the region (Armstrong, 1968; Miller and Gans, 1989; Vandervoort and Schmitt, 1990; Wright and Snoke, 1993; Howard, 2003; DeCelles, 2004).
Jurassic and Cretaceous plutons are scattered through northeastern Nevada (Coats, 1987; Wright and Snoke, 1993; Barton, 1996; Mortensen et al., 2000). They are relatively localized near the surface but presumably are more abundant at depth, as indicated by the abundance of granitic rocks in the Ruby Mountains metamorphic core complex (Snoke et al., 1997; McGrew et al., 2000; Howard, 2003; Lee et al., 2003). Late Cretaceous peraluminous granites formed by crustal anatexis in the thick crust are common (Miller et al., 1990; Lee et al., 2003).
Cenozoic magmatism began ca. 45 Ma in northeastern Nevada and was part of a southward-migrating belt of magmatism that swept from Washington and Idaho, through northeastern Nevada and northwestern Utah, and into central Nevada in the Oligocene (Stewart, 1980; Christiansen and Yeats, 1992; Brooks et al., 1995a, 1995b; Humphreys, 1995; Henry and Ressel, 2000a; this study). Eocene magmatism in northeastern Nevada was dominated by andesitic to dacitic lavas and compositionally similar intrusions in numerous centers. Several rhyolitic ash-flow tuffs also erupted, notably from one or more calderas near Tuscarora (Figs. 1 and 2; Henry et al., 1999), but the total number of tuffs and, with few exceptions, the location of their source calderas are uncertain. The early phase of Cenozoic magmatism in northeastern Nevada ended by ca. 35 Ma, and, with the exception of 29 Ma sills in the Ruby Mountains core complex (Wright and Snoke, 1993; MacCready et al., 1997; Howard, 2000), magmatism did not resume until the middle Miocene.
Sedimentary basins began to form at about the same time as magmatism began and are mostly interpreted to have resulted from extension (Solomon et al., 1979; Clark et al., 1985; Solomon, 1992; Satarugsa and Johnson, 2000; Haynes et al., 2002; Haynes, 2003). Eocene sedimentary rocks crop out widely in the region (Fig. 2) and are variably interpreted as contiguous parts of a single, large basin, generally referred to as the Elko basin (Smith and Ketner, 1976; Solomon, 1992; Christiansen and Yeats, 1992), or several isolated basins (Solomon et al., 1979; Nutt and Good, 1998). Haynes (2003), Cline et al. (2005), and Hickey et al. (2005) show a large, composite Elko basin that encompasses most Eocene sedimentary deposits but is partly separated by local topographic highs. As discussed below, I interpret many of the Eocene sedimentary deposits to have accumulated in relatively small basins along paleovalleys.
Eocene sedimentary rocks in the Elko Hills and Piñon Range have been called the Elko Formation (Smith and Ketner, 1978; Solomon et al., 1979; Haynes, 2003) and are interpreted to have accumulated in the hanging wall of the Ruby Mountains detachment fault (Fig. 3
; Solomon et al., 1979; Satarugsa and Johnson, 2000; Haynes et al., 2002; Haynes, 2003). The Elko Formation in the Elko Hills consists of 200 m of basal conglomerate overlain by 600 m of lacustrine shale, oil shale, claystone, siltstone, and minor, water-laid tuff (Fig. 3; Solomon et al., 1979; Ketner and Alpha, 1992; Haynes, 2003). Zircon U-Pb ages on water-laid tuffs in the basal conglomerate and near the top of the lacustrine sequence are 46.1 ± 0.2 Ma and 38.9 ± 0.3 Ma, respectively (Haynes et al., 2002; Haynes, 2003). Outcrop, seismic-reflection, and borehole data indicate that Eocene sedimentary rocks considered equivalent to the Elko Formation are discontinuous in the subsurface along the Ruby Mountains front (Smith and Ketner, 1976; Satarugsa and Johnson, 2000; Haynes, 2003).
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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).
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The little studied Bull Run basin is the other large area of Eocene sedimentary rocks (Fig. 3)
. The Bull Run basin is interpreted to have formed through extension by ca. 44 Ma and existed until 36 Ma (Axelrod, 1966a, 1966b; Clark et al., 1985). Basin fill consisting of basal conglomerate overlain by lacustrine shale, oil shale, siltstone, and marl is more than 950 and possibly 1500 m thick (Decker, 1962).
Interpreted episodes of extension began with initial exhumation of the Ruby Mountains between the Late Cretaceous and Eocene (McGrew and Snee, 1994; Camilleri and Chamberlain, 1997; McGrew et al., 2000) and development of the Elko basin of the Elko Hills at ca. 46 Ma (Haynes et al., 2002; Cline et al., 2005; Hickey et al., 2005). An episode of extension that tilted the Elko Formation near Elko 10° to 15° to the southeast before ca. 38 Ma is well established (Brooks et al. 1995a; Henry and Faulds, 1999; Henry et al., 2001; Haynes, 2003; Hickey et al., 2005). Most cooling and presumed uplift from extension of the Ruby Mountains is interpreted to have occurred between 36 and 20 Ma based on K-Ar and 40Ar/39Ar ages (Kistler et al., 1981; Dokka et al., 1986; McGrew and Snee, 1994) and at ca. 15 Ma based on apatite fission-track and (U-Th)/He ages (Colgan and Metcalf, 2006). The apparent 36–20 Ma uplift generated no basins or sedimentary deposits, however, and any sediment was transported out of the region (Wallace et al., 2008). Major uplift and erosion of the Ruby Mountains in the middle Miocene (ca. 15 Ma) is also documented by the first appearance of clasts of Paleozoic metasedimentary rocks and garnet-muscovite–bearing granites in the Humboldt Formation west of the Ruby Mountains (Smith and Ketner, 1976). Extension of variable magnitude was widespread in northern Nevada at ca. 15 Ma and includes development of the northern Nevada rift beginning ca. 16 Ma (Zoback et al., 1994; John et al., 2000), extension of the nearby Shoshone and Toiyabe Ranges (Colgan et al., 2008), and major exhumation of the Snake Range metamorphic core complex (Miller et al., 1999).
Volcanism in northeastern Nevada renewed at 16.5 Ma with small eruptions of Steens-type basalt near Copper Basin (Coats, 1964; Rahl et al., 2002). The Jarbidge Rhyolite, a widespread and voluminous group of rhyolite lavas (Fig. 2), began to erupt by 16.2 Ma (my unpublished 40Ar/39Ar age in Bull Run basin).
The giant Carlin-type gold deposits formed contemporaneously with Eocene magmatism (for example, 40–37 Ma in the Carlin trend, Fig. 3; Ressel and Henry, 2006) and possibly are coeval with extension (Seedorff, 1991; Cline et al., 2005; Hickey et al., 2005). The origin of Carlin-type deposits and their relation to magmatism and extension are strongly debated (Muntean et al., 2004; Cline et al., 2005; Hickey et al., 2005; Ressel and Henry, 2006).
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ASH-FLOW TUFF STRATIGRAPHY
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Geochronologic, Geochemical, and Petrographic Data
I use the distribution and character of regional ash-flow tuffs and interbedded sedimentary rocks to determine the Eocene paleogeography and extensional history of the region. Ash-flow tuff correlation is based on my own and published geologic mapping, stratigraphy, petrography, geochemistry, and especially 40Ar/39Ar ages. This section focuses on 40Ar/39Ar ages, petrography, and geochemistry. Stratigraphy and other field relations are discussed in more detail in the descriptions of individual paleovalley segments.
Table 1 lists more than 30 new and published 40Ar/39Ar ages on ca. 40–45 Ma ash-flow tuffs in northeastern Nevada (analytical data are available in Table S11). All ages have been normalized to an age of 28.02 Ma for the common Fish Canyon Tuff sanidine monitor (Renne et al., 1998). Some published ages that were disturbed or could not be assigned unequivocally to a specific tuff because of location uncertainty are not listed. Ages obtained in this study are primarily on replicate single grains of sanidine, because they provide highly precise, reproducible ages that can distinguish volcanic events separated by as little as 100,000 yr at the ca. 40 Ma age of these rocks. The K/Ca ratio of sanidine also provides an additional correlation tool, and analyses of single grains permit the identification of xenocrysts (Fig. 4) (McIntosh et al., 1990; John et al., 2008). All published ages cited in Table 1 are by step-heating of bulk samples of sanidine or biotite (Hofstra, 1994; Brooks et al., 1995a, 1995b; Mueller et al., 1999). Most of these ages agree with the results obtained in this study, although bulk samples can contain xenocrysts. Fortunately, Mesozoic granitic rocks, the only significant potential source of biotite or feldspar xenocrysts other than other tuffs, are not major constituents of bedrock in the region. Biotite dates can be significantly disturbed by minor alteration, especially chloritization (DiVincenzo et al., 2003). Data presented in this paper illustrate that sanidine ages provide the most certain and straightforward correlations, whereas comparison between sanidine and biotite dates can be uncertain. Most published K-Ar ages are insufficiently precise to help with correlation and are not considered.
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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).
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Thirty-five whole-rock, ash-flow tuff samples were analyzed for major and trace elements by XRF (X-ray fluorescence) at Washington State University (Table 2) and were combined with published analyses in Mueller (1992), Brooks et al. (1995a, 1995b), and Wallace (2003a). Comparison of the new data with published analyses of the same units and, in a few cases, the same outcrops, indicates some interlaboratory bias, which complicates correlation. Alteration, which has affected some samples, is another complication. For example, a plot of SiO2 versus Na2O + K2O, a standard classification scheme for volcanic rocks, shows some clustering but considerable overlap between different tuffs and that some samples of the tuff of Big Cottonwood Canyon are silicified (Fig. 5). Selected immobile elements from these analyses are still useful for correlation—for example, Zr versus TiO2. Obviously, analyses of altered samples should not be used for petrogenetic studies.
Regionally Widespread Ash-Flow Tuffs
The tuff of Big Cottonwood Canyon (Tbcc in all figures) is the youngest, most distinctive, and most widely distributed, and therefore, best marker of the major ash-flow tuffs of the region (Tables 1 and 2; Figs. 3, 5, and 6). The sparsely to moderately porphyritic tuff is high-silica rhyolite with distinctively low, whole-rock TiO2, P2O5, Ba, and Zr contents, although samples with more than 78% SiO2 are silicified (Fig. 5; Table 2). The compositionally most unusual sample is a pumice fragment from intracaldera tuff; the fragment has the lowest SiO2 content and is notably enriched in Ba and Zr. Nine sanidine 40Ar/39Ar ages of this study range from 39.92 ± 0.10 to 40.13 ± 0.10 Ma and give a mean and standard deviation of 39.99 ± 0.08 Ma (Table 1). Published biotite and sanidine step-heating ages are similar. Paleomagnetic data at Nanny Creek, Windermere Hills, and Oxley Peak support correlation (Palmer and MacDonald, 2002). The tuff erupted from a caldera in the northern part of the Tuscarora volcanic field, the only well-established caldera for the tuffs described in this paper (Figs. 2 and 7; Henry and Boden, 1998; Castor et al., 2003).
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Figure 6. Ternary plot showing relative modal abundances of quartz, plagioclase, and sanidine phenocrysts in Eocene ash-flow tuffs of northeastern Nevada.
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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.
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The tuff of Nelson Creek (Tnc) underlies the tuff of Big Cottonwood Canyon near the Tuscarora volcanic field (Figs. 3 and 7). It contains less sanidine and quartz and more biotite and hornblende than the tuff of Big Cottonwood Canyon (Fig. 6). It is a low-silica rhyolite that contains distinctly more TiO2, P2O5, Ba, Zr, and Nb than the tuff of Big Cottonwood Canyon (Fig. 5). Six sanidine 40Ar/39Ar ages range from 40.07 ± 0.08 to 40.23 ± 0.11 Ma and give a mean and standard deviation of 40.14 ± 0.06 Ma. The tuff of Nelson Creek has only been found within about a 50 km radius southeast and southwest of the Tuscarora volcanic field. A volcanic center at Lone Mountain could be the source (Henry and Ressel, 2000b). Henry et al. (1999) suggested that a large area of thick, undivided ash-flow tuff west of the Big Cottonwood Canyon caldera (Fig. 2) could be a caldera and source for the tuff of Nelson Creek. However, absence of the tuff in the central paleovalley (Fig. 3) argues against that area being the source.
The tuff of Coal Mine Canyon (Tcm) is a high-silica dacite characterized by abundant phenocrysts of plagioclase, biotite, and hornblende (Figs. 5 and 6; Table 2). Petrographically and compositionally similar tuff crops out in the Windermere Hills. Although compositionally variable, these tuffs form a group distinct from other tuffs, for example, in Zr and TiO2 abundances (Fig. 5). Reported ages from Coal Mine Canyon are 40.7 ± 0.4 Ma (hornblende, 40Ar/39Ar; Brooks et al., 1995a, 1995b) on densely welded tuff and 41.1 ± 0.2 (zircon, U-Pb; Haynes et al., 2002) on an immediately underlying poorly welded tuff that we interpret to be a basal part of the densely welded tuff. The probably correlative tuff in the Windermere Hills underlies the tuff of Big Cottonwood Canyon and overlies a tuff dated at 40.64 ± 0.20 Ma.
Ash-flow tuff dominated by plagioclase and biotite phenocrysts with lesser and variable amounts of hornblende, pyroxene, quartz, and sanidine is widespread through the central and eastern part of Figure 2. Despite minor phenocryst variations, these tuffs are compositionally similar and mostly occupy the same stratigraphic position (Table 2; Figs. 3, 5 and 6). Five 40Ar/39Ar ages fall into two groups (Table 1). Two ages, one on biotite and one on plagioclase, are ca. 40.7 Ma (40.64 ± 0.20, Windermere Hills; 40.69 ± 0.13 Ma, Nanny Creek), whereas three others, all on biotite, are 41.34 ± 0.22 (Nanny Creek), 41.9 ± 0.4 (California Mountain), and 41.6 ± 0.2 Ma (Copper Basin). Despite the two age groups, two separate plagioclase-biotite tuffs are not present at any location east of the Tuscarora volcanic field. Moreover, the 40.69 Ma plagioclase date and the 41.34 Ma biotite date are from the same tuff at Nanny Creek. The 40.64 Matuff crops out in the Windermere Hills, where it underlies the plagioclase-biotite-hornblende tuff that is probably the tuff of Coal Mine Canyon. Paleomagnetic data support correlation of the Nanny Creek and Windermere Hills plagioclase-biotite tuffs (Palmer and MacDonald, 2002). Haynes (2003) obtained a zircon U-Pb date of 40.8 ± 0.6 Ma on a thin, altered plagioclase-biotite tuff in Taylor Canyon. All these data support an age of ca. 40.7 Ma for the plagioclase-biotite tuff at Nanny Creek and in the Windermere Hills. Whether the two other tuffs with ca. 42 Ma ages are an older but petrographically and chemically similar tuff is uncertain. In the following discussions, all plagioclase-biotite tuffs are treated as one and informally termed the plagioclase-biotite tuff (Tpb). But the possibility that they are two separate tuffs must be kept in mind.
The oldest recognized, Cenozoic ash-flow tuff in Nevada, informally termed the 45 Ma tuff (T45), crops out in a belt from Copper Basin westward past Cornwall Mountain to a possible caldera source north of Bull Run basin (Fig. 3). It overlaps in SiO2 content with several other tuffs but has lower TiO2 and P2O5 contents (Table 2; Figs. 5 and 6). Moreover, two sanidine 40Ar/39Ar ages are 44.90 ± 0.07 and 45.10 ± 0.09 Ma (Table 1). Although the two ages do not overlap at 2 sigma, their similarity to each other, their difference from all other tuff ages, and the stratigraphic, petrographic, and compositional evidence for correlation seem incontrovertible. The tuff may have erupted from a caldera at the north end of Bull Run basin, where petrographically and compositionally similar tuff is at least 1000 m thick and contains probable andesite megabreccia. However, descriptions by Decker (1962) suggest the tuff north of Bull Run basin may occupy a northeast-trending paleovalley (discussed in the next section). Axelrod (1966a) reported a biotite K-Ar age of 44.2 ± 2.1 Ma from this tuff in the northern Bull Run basin. Although not diagnostic, this K-Ar age is one of the oldest in the region and supports correlation.
Two minor, plagioclase-biotite tuffs (Tt) in the western part of the Tuscarora volcanic field do not correlate with any of the more extensive tuffs to the east. Both of these tuffs overlie the tuff of Nelson Creek; therefore, they are younger than the plagioclase-biotite tuff(s), and one has a plagioclase 40Ar/39Ar age of 40.01 ± 0.49 Ma (Table 1). They are compositionally similar to the plagioclase-biotite tuff but are somewhat distinguished by high Nb concentrations (Table 2; Fig. 5).
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PALEOVALLEY GEOLOGY
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Ash-flow tuffs are widely distributed and thickest around Tuscarora, Taylor Canyon, and north of Bull Run basin, areas of known or suspected calderas, and generally thin away from the source areas (Figs. 2 and 3). At distances of only 20 km from these areas, the tuffs and sedimentary rocks are confined to narrow channels cut into Paleozoic bedrock. This section describes three paleovalley systems, starting with the best understood, central paleovalley. All paleovalleys are discontinuously exposed across a series of ranges that formed during middle Miocene or later Basin and Range extension. Figure 3 shows the generalized stratigraphy and 40Ar/39Ar and U-Pb ages of tuffs and lavas in the paleovalleys, and Table 3 summarizes the characteristics of paleovalleys, conglomerates, and lacustrine deposits at each location. The thickness of tuffs, lavas, and sedimentary rocks in the paleovalleys is used as the best approximation of the depth of the paleovalley. Obviously, paleovalleys could be either underfilled or overfilled, although overfill other than by intermediate lavas is probably minor.
These descriptions are based on my detailed mapping of the Tuscarora volcanic field, California Mountain, and Nanny Creek, and on detailed published maps in Willow Creek, Windermere Hills, Oxley Peak, and Coal Mine Canyon, and on more generalized or regional maps north of Bull Run basin, at Cornwall Mountain, Copper Basin, Double Mountain, and Taylor Canyon. In all areas, I examined stratigraphy and rock types and, where appropriate, collected samples for chemical analysis or 40Ar/39Ar dating. In some cases noted below, my interpretations are significantly different than those of the original workers.
Central Paleovalley
The central paleovalley can be traced 150 km from near the Tuscarora volcanic field eastward over the Independence Mountains at California Mountain as far as Nanny Creek in the Pequop Mountains (Fig. 3). Although gaps exist across several valleys, continuity is based on proximity and trend of individual segments and similar stratigraphy, especially the presence of the tuff of Big Cottonwood Canyon.
Tuscarora Volcanic Field
The Tuscarora volcanic field, the largest Eocene volcanic center in the region, erupted rhyolitic to dacitic tuffs and andesitic to dacitic lavas during several episodes between ca. 40.2 and 39.5 Ma (Fig. 7; Henry et al., 1999; Castor et al., 2003). Basal, pre-volcanic conglomerate crops out in two locations near the south edge of the field and may be covered elsewhere. However, the presence of a buried paleovalley in this area is speculative. Otherwise, the oldest volcanic rocks, the tuff of Nelson Creek and andesite lavas, rest directly on a low-relief surface developed on Paleozoic rocks west of the field. An extensive dacite-andesite complex, a group of dacite lava domes and small-volume ash-flow tuffs, and the tuff of Big Cottonwood Canyon erupted sequentially but all indistinguishably at 40.0 Ma. Numerous small to moderate-sized intrusions, including a northeast-striking dike swarm, were emplaced between ca. 39.8 and 39.6 Ma, and the final igneous activity consisted of more dacite lavas and domes erupted at 39.5 Ma.
California Mountain
Eocene rocks occupy an 5-km–wide, 
800-m–deep, southeast- and east-tilted paleovalley at California Mountain in the Independence Mountains (Fig. 8; Muntean and Henry, 2006). A basal gravel is marked by a lag of well-rounded to subangular boulders of quartzite, chert, and chertquartzite-pebble conglomerate up to 70 cm in diameter. Thin (
5 m-thick) lenses of finely laminated shale (Fig. 9A) and micrite are interbedded with the gravel, and a thicker (20 m) section of shale overlies gravel. The whole sequence is 40 m thick. The plagioclase-biotite tuff dated at 41.9 ± 0.2 Ma (Hofstra, 1994) is the oldest volcanic rock. More gravel containing rounded boulders of quartzite and conglomerate to 1.5 m overlies this tuff in the center of the paleovalley. A thick sequence of locally derived, dacitic lavas and tuffs is dated at 40.4 ± 0.2 Ma (Hofstra, 1994). The tuff of Big Cottonwood Canyon is the youngest Eocene rock. Maximum thickness of the Eocene rocks is 800 m, and units thin and wedge out against paleovalley walls. A 6-km–long, 500-m–deep swale in the crest of the Independence Mountains is just west of the east-tilted paleovalley and may indicate its westward continuation (Fig. 9B).
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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.
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The Eocene rocks at California Mountain have undergone two episodes of faulting and tilting. All units with the possible exception of the tuff of Big Cottonwood Canyon were tilted 20° to the southeast along a series of east-northeast–striking, mostly northwest-dipping faults (Fig. 8). The faults are spaced every 1–2 km and have displacements of 300–500 m. Faulting is younger than 40.4–40.0 Ma. Later (probably post-middle Miocene), the rocks were tilted eastward 20° during formation of the present Independence Mountains. Scarps in Quaternary alluvial fans demonstrate that a similar style of faulting has continued to the present day (Muntean and Henry, 2006).
Double Mountain
The central paleovalley continues eastward to an unnamed, west-tilted range at Double Mountain, 20 km east of California Mountain (Fig. 3). The Eocene section here is identical to that at California Mountain, with a basal sedimentary sequence overlain by the plagioclase-biotite tuff, andesite lava, and the tuff of Big Cottonwood Canyon. However, the basal sedimentary section is considerably thicker. Unpublished details of mapping by Coats (1987; Nevada Bureau of Mines and Geology information office files) show a basal conglomerate overlain by calcareous to silicified shale, oil shale, sandstone, limestone, and minor pyroclastic-fall tuffs. Finely laminated deposits showing soft-sediment deformation are common. The sedimentary rocks are so poorly exposed that even measuring an attitude was not possible, but based on an estimated dip of 25° and outcrop width, the section is 300 m thick. The ash-flow tuffs are overlain unconformably by nearly flat-lying Miocene Jarbidge Rhyolite.
Oxley Peak
Two parts of what is probably a single paleovalley are separated by 4 km near Oxley Peak in the southern Snake Mountains. The northern channel is 1.7 km wide and has a 1-m–thick, oolitic limestone at the base. This is overlain by the plagioclase-biotite tuff and the tuff of Big Cottonwood Canyon, conglomerate or breccia containing clasts of the tuffs, and dacite lavas dated at 40.0 ± 0.2 Ma (Brooks et al., 1995a, 1995b; Thorman et al., 2003). Eocene(?) tuffaceous siltstone and shale overlie the dacite, apparently conformably but on an eroded surface; thickness varies greatly from 1 to 100 m (Thorman et al., 2003). Jarbidge Rhyolite overlies the sedimentary rocks without angular discordance (Thorman et al., 2003).
The southern channel is only 1 km wide. It has a basal pebble conglomerate containing well-rounded Paleozoic and volcanic clasts, including the plagioclase-biotite tuff, but the tuff is not exposed in the area examined by me. The overlying tuff of Big Cottonwood Canyon is discontinuously exposed and locally has been eroded and reworked into coarse, fluvial conglomerate containing angular blocks of the tuff up to 1 m in diameter. Some blocks are only partly broken away from outcrop, and none have undergone significant transport. Field relations between the tuff of Big Cottonwood Canyon and overlying conglomerate are well preserved and exposed because both were silicified.
Although Oxley Peak is 70 km east of Double Mountain, the correlation of the tuff of Big Cottonwood Canyon is certain based on composition and age (Tables 1 and 2), and the sequence of the plagioclase-biotite tuff overlain by tuff of Big Cottonwood Canyon is identical to that at Double Mountain and California Mountain.
Nanny Creek
Nanny Creek is a key locality in understanding paleovalleys and their significance in northeastern Nevada because it has the most distal known outcrops of the tuff of Big Cottonwood Canyon and it best illustrates megabreccia consisting of reworked ash-flow tuff. Brooks et al. (1995a, 1995b) produced a sketch geologic map of the Nanny Creek paleovalley and recognized several of the key relationships. They identified a coarse, basal conglomerate containing some clasts that could not be locally derived, overlain by two ash-flow tuffs and a sequence of andesite flows and flow breccias, all these overlain by a heterogeneous unit containing blocks of ash-flow tuff. C.H. Thorman (2003, personal commun.) recognized that the rocks occupied a paleovalley that could be as much as 1.6 km deep based on the dip of the volcanic rocks and the distribution of surrounding Paleozoic rocks. This study (Fig. 10) expands upon their interpretations.
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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).
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The exposed width of the Nanny Creek paleovalley is 6 km, but it is faulted against Paleozoic rocks on the south side, and the original width is thus unknown. The 20- to 30-m–thick basal conglomerate consists of a lag of rounded boulders commonly up to 1.5 m in diameter with one 6 m across (Figs. 9C and 9D). Most clasts, including the largest ones, are chert ± quartzite-pebble conglomerate similar to adjacent Paleozoic bedrock. However, numerous clasts are present of rocks that do not crop out locally, including coarse granite up to 1.7 m in diameter, andesite, and silicified, finely laminated, lake-bed sediments (Brooks et al., 1995a).
Conglomerate is overlain by the plagioclase-biotite tuff and tuff of Big Cottonwood Canyon (Tables 1 and 2; Fig. 10; Brooks et al., 1995a, 1995b; Palmer and MacDonald, 2002). The thickness of the tuffs is difficult to estimate. Foliation in the tuffs dips 30°–60° eastward, but dips are probably partly primary, resulting from compaction of the tuffs against paleovalley walls (Henry et al., 2004). Using the 30°–60° range, the plagioclase-biotite tuff and tuff of Big Cottonwood Canyon could be 60–140 and 90–190 m thick, respectively. Both values are significantly greater than the thicknesses of 25 and 55 m, respectively, reported by Brooks et al. (1995a). The tuff of Big Cottonwood Canyon here is 150 km from its source in the Tuscarora volcanic field, yet it is at least 55 m thick and restricted to a narrow paleovalley within which it wedges out against the sides (north of the area of Fig. 10; Brooks et al., 1995a). Andesite and andesite flow breccia dated at 39.5 ± 1.0 Ma (Brooks et al., 1995a, 1995b) overlie the tuff of Big Cottonwood Canyon.
The uppermost unit in Nanny Creek (Fig. 10) is a breccia composed of blocks, mostly of silicified tuff of Big Cottonwood Canyon with lesser plagioclase-biotite tuff and andesite (Fig. 9E). Paleozoic clasts are rare. Clasts of tuff of Big Cottonwood Canyon are up to 12 m across, plagioclase-biotite tuff is up to 8 m long, and andesite is as much as 2 m across. Blocks range from angular to rounded, and many are internally broken but not disaggregated. Rarely exposed matrix consists of coarse sand and variably rounded pebbles and cobbles. Brooks et al. (1995a) recognized the hummocky character of this unit and the possibility that the hummocks might be landslide blocks. Chemical analysis and dating of one clast confirm that it is tuff of Big Cottonwood Canyon (Sample H05-38, Tables 1 and 2; Figs. 4 and 10). Furthermore, highly discordant compaction foliations and scattered magnetization directions from several breccia clasts (M.R. Hudson, in Brooks et al., 1995a; Palmer and MacDonald, 2002, their sites W02, W15, and W18) confirm that these blocks are not in place.
Windermere Hills
The Windermere Hills are also a key area to illustrate paleovalley relationships. Mueller (1992, 1993) mapped the area, and Mueller et al. (1999) used the mapping and 40Ar/39Ar geochronology to conclude that a thick sequence of sedimentary rocks accumulated in a half graben resulting from major extension between ca. 39 and 35 Ma. These sediments overlie a pre-extensional volcanic sequence that includes the regional ash-flow tuffs of this study. I reinterpret the area to be in an 10-km–wide part of the central paleovalley (Figs. 3 and 11) that was dammed by minor extension (see Discussion).
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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.
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The volcanic and volcaniclastic strata are similar across the width of the paleovalley but, as noted by Mueller (1992), show abrupt changes in rock type and thickness both along and across strike. A basal conglomerate and andesite are exposed only in the south and are overlain by the plagioclase-biotite tuff and tuff of Big Cottonwood Canyon. According to Mueller et al. (1999), the oldest synextensional deposits are pebble-cobble conglomerate and sandstone that overlie the tuffs (Tdc in Fig. 11). Clasts in the conglomerate are well rounded and mostly consist of the underlying volcanic rocks with some Paleozoic quartzite. Mueller (1992) found algal limestone and laminated siltstone in the conglomerate unit and imbricated clasts that indicate deposition in an east-flowing stream. A thick sequence of tuffaceous sandstone, siltstone, and shale overlies the conglomerate. Mueller et al. (1999) obtained dates of 35.01 ± 0.36 and 39.20 ± 0.50 Ma on sanidine and biotite, respectively, from a pyroclastic-fall tuff in the upper part of this sequence and favored the less disturbed sanidine age. They interpreted the sedimentary rocks to have been deposited in lacustrine fan-delta and lacustrine environments in a half graben.
Middle Miocene, tuffaceous sedimentary rocks overlie the Eocene rocks in the paleovalley but rest directly on Paleozoic rocks just to the south in the northwestern Pequop Mountains (Mueller, 1993). Both Eocene and Miocene rocks are similarly tilted 30° to 40° west (Fig. 11; Mueller, 1993; Mueller et al., 1999).
Mueller et al. (1999) interpreted exposures in the western part of Figure 11 as intact ashflow tuffs (plagioclase-biotite tuff and tuff of Big Cottonwood Canyon of this study) that were repeated by several closely spaced, east-dipping normal faults. However, where examined by the author, both in the area of Figure 11 and farther north in the map area of Mueller (1993), these outcrops are actually lenses of megabreccia (unit Tx) consisting of variably disaggregated blocks of the tuffs. The blocks form hummocks in a matrix of variably coarse clastic deposits, similar to those in the upper unit at Nanny Creek. As at Nanny Creek, the blocks have highly variable and discordant compaction foliations and scattered magnetization directions (Palmer and MacDonald, 2002, their sites W06 and W07) that confirm that they are not in place. Conglomerates composed of well-rounded pebbles of quartzite and chert are interbedded with the megabreccias, but megabreccias contain no clasts of Paleozoic rocks. The age and stratigraphic position of these breccias and the amount of repetition by faulting are therefore uncertain, but no fault repetition is required. The angular conformity of all Eocene rocks and at least the lower part of the middle Miocene rocks indicates negligible tilting during their deposition.
The Windermere Hills lie north of, and parallel to, the paleovalley at Nanny Creek (Fig. 3). It seems unlikely that both locations are part of a single, east-trending paleovalley. They could, however, be tributaries of the same paleovalley that joined downstream, which I conclude was to the east (see Discussion).
North Paleovalley
The north paleovalley has been investigated only in reconnaissance, and published maps commonly combined Tertiary volcanic rocks into one or two units interpreted to be Miocene (Decker, 1962; Coash, 1967). Therefore, interpretations about the geology are more tentative than for the central paleovalley. Nevertheless, the presence of a distinctive ash-flow tuff and sedimentary sequence filling narrow valleys cut into Paleozoic rocks over a 60 km length from Bull Run to the Copper Basin support the paleovalley interpretation. Miocene or younger rocks cover older rocks west of the Bull Run area.
North Bull Run Area
An area of 30 km2 at the northeast edge of the Bull Run basin is underlain by ash-flow tuff (Decker, 1962). A section across the northern part of this area appears to be a single ash-flow tuff that is petrographically similar to the 45 Ma tuff at Cornwall Mountain and Copper Basin (Fig. 6). The tuff dips uniformly 20°–30° westward and is repeated by one, north-striking, east-dipping fault. Based on the dip and outcrop width, tuff in each fault block is 600–800 m thick, and the base is not exposed. Total displacement on the fault is uncertain but does not appear to be more than 200 m. Therefore, the tuff may total more than 1 km thick. The easternmost part of the tuff contains several lenses of andesite boulders. Although Decker (1962) shows this tuff in depositional contact with Paleozoic rocks to the west, this contact must be a fault because the tuff dips uniformly into the contact.
In describing the overall area, Decker (1962) stated that tuff fills a 4 x 8 km, northeast-trending "ancient topographic sag" and also noted steep irregular attitudes in the tuff, which he thought could be primary. Decker also noted that an exploration well 3 km west of a down-to-the-west fault bounding the southwestern edge of ash-flow tuff encountered only Eocene sedimentary rocks of the Bull Run basin above Paleozoic rocks (Fig. 3A).
I interpret that most of this tuff is the 45 Ma ash-flow tuff based on its similar phenocryst assemblage and composition (Fig. 5), as well as its location only 30 km west of dated 45 Ma tuff at Cornwall Mountain (see discussion of Cornwall Mountain and Copper Basin). Also, Axelrod (1966a) reported a biotite K-Ar age of 44.2 ± 2.1 Ma from tuff near the southwestern edge of outcrops. Although he did not describe the tuff and the age is not diagnostic, it is one of the oldest K-Ar ages in the region.
The tuff fills either a paleovalley or possibly a caldera. The paleovalley interpretation is consistent with the northeast-elongate "topographic sag" and with possible primary dips. The caldera interpretation is consistent with the apparent great thickness, which seems too large for a paleovalley, and with the andesite breccia lenses, which could be mesobreccia. Abrupt termination of the tuff to the southwest could be consistent with either interpretation.
Cornwall Mountain
The Tertiary section at Cornwall Mountain occupies a paleovalley that has been tilted moderately westward and folded into a west-plunging syncline (Fig. 12; Coash, 1967). Based on an average 40°–50° west dip and width of the outcrop belt, the overall section is 1.5 km thick. However, the section is repeated by at least one major, northwest-striking fault.
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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.
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A discontinuous basal conglomerate contains well-rounded boulders up to 4 x 6 m, mostly of quartzite, chert-pebble conglomerate, limestone, and granite. Overlying the conglomerate is a thick, ash-flow tuff that is zoned from an 30-m–thick lower part with relatively abundant sanidine and quartz phenocrysts to an upper part 500 m thick with relatively more abundant plagioclase and mafic phenocrysts (Figs. 5 and 6). A sample from the lower part has a sanidine 40Ar/39Ar age of 45.10 ± 0.09 Ma (Table 1).
The ash-flow tuff is overlain by a thick sequence of sedimentary rocks consisting of two distinct rock types (Fig. 12). Fine-grained, commonly tuffaceous siltstone, shale, sandstone, and minor pebble conglomerate make up most of the section. Interbedded with these fine deposits are numerous breccia lenses composed of ash-flow tuff indistinguishable from the upper part of the underlying tuff. Clasts are up to 4 m in diameter and are subrounded to subangular. Paleozoic clasts are absent; therefore, the foot-wall of a fault that exposed basement rock was not the source.
Copper Basin
Copper Basin is an 12 km2 area of Eocene ash-flow tuff and tuffaceous sedimentary rocks in the hanging wall of the Copper Basin fault, a major, east-dipping normal fault (Coats, 1964; Axelrod, 1966a, 1966b; Rahl et al., 2002). Eocene strata are divided into the Dead Horse Formation and the overlying Meadow Fork Formation (Coats, 1964; Rahl et al., 2002). The Dead Horse Formation is as much as 1600 m thick and consists of ash-flow tuff overlain by tuffaceous, lacustrine sedimentary rocks. The Meadow Fork Formation is as much as 400 m thick and consists of coarse conglomerate containing clasts of quartzite, marble, phyllite, and granitic rocks up to 1 m in diameter. The entire section is conformable but overlain with angular unconformity by Steens-type, coarsely porphyritic basalt, dated at 16.6 ± 0.4 Ma, and Jarbidge Rhyolite (Coats, 1964; Rahl et al., 2002). Shale in the Dead Horse Formation not far above the ash-flow tuffs contains the fossil plant assemblage that is variably interpreted to have lived at 1.1–2.8 km (Axelrod, 1966b; Chase et al., 1998; Wolfe et al., 1998).
Three ash-flow tuffs are present in the lower part of the Dead Horse Formation. They consist of the 45 Ma tuff, a plagioclase-biotite-hornblende tuff, and the tuff of Big Cottonwood Canyon (Table 1). Rahl et al. (2002) obtained a biotite 40Ar/39Ar age of 41.6 ± 0.2 Ma on what may be the middle, plagioclase-biotite tuff, which I tentatively correlate with the regional plagioclase-biotite tuff. A channel cut into the tuff of Big Cottonwood Canyon contains pebbles up to 4 cm of Paleozoic quartzite and chert and Tertiary volcanic rocks, mostly the tuff of Big Cottonwood Canyon. Paleozoic clasts are mostly well rounded, whereas the Tertiary clasts are subrounded to subangular. A pyroclastic-fall tuff from the uppermost part of the Dead Horse Formation gave an age of 37.6 ± 0.4 Ma (Rahl et al., 2002). The base of the Tertiary section is not exposed.
Coats (1964) interpreted the Meadow Fork Formation to have accumulated
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ABSTRACT
INTRODUCTION
