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1 U.S. Geological Survey, Menlo Park, California 94025, USA
2 Nevada Bureau of Mines and Geology, Reno, Nevada 89557, USA
3 U.S. Geological Survey, Menlo Park, California 94025, USA
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONGEOLOGIC SETTINGSTRUCTURAL RECONSTRUCTIONMIOCENE SEDIMENTARY BASINSDISCUSSION AND CONCLUSIONSREFERENCES CITED |
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42 km (east-west) width of the Caetano caldera has been extended 110%, resulting in 22 ± 3 km westward translation of the Fish Creek Mountains relative to the southern Cortez Range. Major normal faults mapped within the caldera continue south and north along strike into the surrounding Paleozoic basement rocks; therefore it is likely that parts of surrounding areas are also significantly extended. Miocene (16–12 Ma) sedimentary rocks in the hanging walls of major normal faults include both fluvial/lacustrine facies and coarser alluvial fan deposits. Where exposed, the bases of the Miocene sedimentary sections are in angular conformity with underlying 40°E tilted 34–25 Ma volcanic and sedimentary rocks. The distribution, composition, and geometry of these deposits are best explained by accumulation in a set of half-graben basins that formed in response to slip on basin-bounding faults. Extension thus appears to have taken place in the middle Miocene, beginning at or shortly after 16 Ma, and was mostly completed by 10–12 Ma. Fault blocks and basins formed during middle Miocene extension are cut by younger, more widely spaced, high-angle normal faults that began forming more recently than 10–12 Ma. These faults outline the modern basins and ranges in the study area and some have remained active into the Holocene.
Keywords: Basin and Range Province • Miocene • extensional tectonics • calderas
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONGEOLOGIC SETTINGSTRUCTURAL RECONSTRUCTIONMIOCENE SEDIMENTARY BASINSDISCUSSION AND CONCLUSIONSREFERENCES CITED |
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Basin and Range deformation in this region is difficult to quantify, because exposed bedrock consists primarily of Paleozoic siliceous "western facies" rocks of the Roberts Mountains allochthon. These highly deformed, thrust-imbricated slices of argillite, quartzite, chert, and greenstone are the product of a complex history of Late Paleozoic contractional deformation overprinted by Cenozoic extension (e.g., Roberts et al., 1958; Gilluly and Gates, 1965). These rocks tend to be poorly exposed and offer few reliable stratigraphic markers that can be used to restore slip along normal faults. To understand Cenozoic extension, it is thus desirable to find well-dated, regionally extensive, pre-extensional Cenozoic rocks that can be matched across normal faults and used to restore extension of the underlying Paleozoic units.
The most extensive Cenozoic unit in the study area is the late Eocene Caetano Tuff (Gilluly and Masursky, 1965), exposed in the northern Toiyabe and southern Shoshone Ranges (Fig. 1). It was originally interpreted as filling the eastern part of an east-west–trending, 90 x 20 km "volcano-tectonic trough" (Masursky, 1960; Burke and McKee, 1979). The western part of this "trough" has since been shown to consist of the structurally intact Fish Creek Mountains caldera (Fig. 1; McKee, 1970), dated to 24.72 ± 0.05 Ma (John et al., 2008). New geologic mapping, geochronology, and geochemical data indicate a similar caldera origin for the Caetano Tuff in the eastern part of the "trough," referred to hereafter as the Caetano caldera (this study; John et al., 2008). The present east-west elongation of the Caetano caldera (Figs. 1 and 2) is the result of east-west extension along north-striking normal faults, making the caldera an ideal strain marker for restoring later Cenozoic deformation.
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33.8 Ma age for the Caetano Tuff and demonstrate that it fills a structurally dismembered caldera (all 40Ar/39Ar dates in this paper are referenced to sanidine from the Fish Canyon Tuff with an assigned age of 28.02 Ma (Renne et al., 1998)). The purpose of this paper is to restore deformation of the caldera and thereby establish the timing and kinematics of Basin and Range extension in this region. We present a new geologic map and cross-sections (Plate 1) that document the magnitude and structural style of extension, together with new 40Ar/39Ar dates and tephra correlations from synextensional basins that establish the timing of faulting.
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110% (22 km) extension. Extension began ca. 16 Ma and took place in two main phases. We interpret 16–12 Ma sedimentary rocks to record large-magnitude middle Miocene extension and tilting of the Caetano caldera. Fault blocks and basins formed during middle Miocene extension were subsequently cut by a second generation of more widely spaced, high-angle normal faults that strike more northeast. The onset of younger faulting is not well established but is thought to be younger than ca. 10 Ma. |
GEOLOGIC SETTING |
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TOPABSTRACTINTRODUCTIONGEOLOGIC SETTINGSTRUCTURAL RECONSTRUCTIONMIOCENE SEDIMENTARY BASINSDISCUSSION AND CONCLUSIONSREFERENCES CITED |
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2 km thick in the middle Miocene, although an unknown amount may have been removed by erosion before that time.
Underlying lower plate rocks (Pzlc) of the lower Paleozoic (Cambrian–Devonian) continental shelf (predominantly carbonate) are locally exposed in structural windows beneath the siliceous upper plate (Plate 1). In the southern Cortez Range, this sequence is at least 1.8 km thick with Cambrian rocks exposed at the base (Gilluly and Masursky, 1965). Lower plate carbonate rocks are an important host rock for the major gold deposits in the area, including the giant Pipeline deposit (2005 reserves 11 Moz; past production 9 Moz; Muntean, 2006). In the southern Cortez Range, the lower plate was exposed by mid-Tertiary erosion prior to extensional faulting (Gilluly and Masursky, 1965; John et al., 2008), but we argue here that some lower-plate windows may be exhumed in the footwalls of Miocene normal faults; this interpretation is discussed in a later section.
Upper Paleozoic rocks deposited unconformably on the Roberts Mountains allochthon following its emplacement—the Antler overlap assemblage—have been described by Moore et al. (2000) and Racheboeuf et al. (2004) in the southwestern part of the study area (Pzo). Equivalent units are exposed in the south-central Cortez Range (Fig. 1) to the northeast of the study area (Muffler, 1964). In the southwestern Shoshone Range, these rocks were overthrust in the Late Permian–Early Triassic by deep-water siliceous rocks of the Golconda allochthon (Pzg; Moore et al., 2005).
Cenozoic Rocks that Predate the Caetano Caldera
Few Cenozoic rocks in the study area are known to predate eruption of the Caetano Tuff and formation of the Caetano caldera. Sparse, undated andesitic lava flows are exposed on the caldera floor in the Toiyabe Range (Figs. 2 and 3; included in unit Tad; note that this unit also includes post-caldera lavas). Coarse clastic sediments containing abundant carbonate clasts derived from lower-plate rocks are exposed on the caldera floor near Wenban Spring (Fig. 2). These exposures may correlate with an extensive but poorly exposed >400-m-thick gravel deposit filling a paleovalley in the southern Cortez Range (Tog). To the north of our study area, local outcrops of outflow tuff previously mapped as Caetano Tuff (Stewart and McKee, 1977; Doebrich, 1995) are demonstrated by John et al. (2008) to be the tuff of Cove Mine, a similar but petrographically and geochemically distinct, slightly older (34.2 Ma) tuff that predates the Caetano caldera. Southwest of Wilson Pass in the Shoshone Range (Fig. 2), a small outcrop of the tuff of Cove Mine (Tcm) overlies a small outcrop of mafic lava (Tob). Both of these units are stratigraphically beneath the lower intracaldera cooling unit of the Caetano Tuff, and thus represent the floor of the caldera (discussed in detail by John et al., 2008).
Caetano Tuff and Related Units
Detailed petrographic data, geochemical analyses, and 40Ar/39Ar age data for the Caetano Tuff and related units are presented in a companion paper (John et al., 2008). Here we summarize aspects of these units relevant to our discussion of extensional faulting.
Intracaldera Tuff: Lower Unit
Most of the intracaldera Caetano Tuff is a single compound cooling unit as much as 3.4 km thick (Tcl), although the top and bottom of the caldera fill are never exposed in the same structural block. For the purpose of this study, we conservatively estimate that this unit is as thick as 3.6 km, although the maximum thickness may be slightly greater (John et al., 2008). The tuff in the eastern part of the caldera in the northern Toiyabe Range is relatively fresh, with vitro-phyre locally preserved at the base. In contrast, the intracaldera tuff west of Carico Lake Valley (Fig. 2) is strongly hydrothermally altered; in some cases, it is difficult to tell if the rock was formerly extrusive or a shallow intrusion.
Intracaldera Tuff: Upper Unit
A distinct cooling break is present in the upper part of the intracaldera Caetano Tuff, most prominently exposed between Rocky Pass and Red Mountain in a series of small fault blocks west of Tub Spring (Fig. 2). Above this cooling break, the Caetano Tuff consists of as much as 1000 m of generally poorly exposed, poorly welded ash-flow tuff, locally interbedded with finely laminated tuffaceous siltstone and sandstone. Thin (5–10 m) ledges of densely welded tuff within this unit are geochemically and petrographically similar to the main intracaldera Caetano Tuff, and yield statistically indistinguishable 40Ar/39Ar ages (John et al., 2008). Where exposed, this sequence is a useful marker for restoring slip on normal faults, and we designate all caldera fill above the top of the lower cooling unit (Tcl) as a separate map unit (Tcu). Both the upper and lower units are hydrothermally altered in the western part of the caldera, and, where we have not mapped them in detail, they are shown as intracaldera tuff, undivided (Tcc).
Breccia
Large (>10 m) breccia blocks and crudely bedded sheets composed of crushed and broken Paleozoic quartzite and chert (locally with a tuffaceous matrix) are exposed along the margins of the caldera, consistent with catastrophic collapse during eruption of the Caetano Tuff. Breccia deposits large enough to be shown in Plate 1 are designated "Tcb," but they are locally present elsewhere along the caldera margins.
Intrusive Rocks
The central part of the caldera in Carico Lake Valley (Fig. 2) is underlain by the Carico Lake pluton, a poorly exposed granite porphyry (Tcic) that intrudes and deforms the surrounding intra-caldera tuff. This granite is dated as 33.78 ± 0.05 Ma (40Ar/39Ar sanidine; John et al., 2008), and we interpret it to represent a resurgent intrusion derived from the same magma chamber that produced the Caetano Tuff. The Redrock Canyon pluton (Tcir) crops out sporadically across the western part of the caldera between Redrock Canyon and Carico Lake Valley (Fig. 2), where it intrudes the altered upper unit of the Caetano Tuff. This intrusion is strongly altered, and John et al. (2008) infer that it was the source of heat and/or hydrothermal fluids that altered the western part of the caldera.
Outflow Sheet
Outside the caldera, the Caetano Tuff is locally present within the study area as an outflow sheet generally deposited on pre-Cenozoic basement to the south and west of the caldera. The outflow sheet, shown as a separate unit (Tct) in Plate 1, is as much as 300 m thick where it fills paleo-topography in Golconda Canyon in the Tobin Range (Fig. 1; Gonsior and Dilles, 2008; John et al., 2008).
Oligocene Rocks Overlying the Caetano Tuff
Within the caldera, the Caetano Tuff is locally overlain by 500–1000 m of tuffaceous sedimentary rocks (Tcs). These rocks are present only within the mapped outline of the caldera and are thickest along its edges (primarily the northwestern margin). We interpret these rocks to have been deposited in the topographic depression created by caldera collapse, in lacustrine and fluvial environments (John et al., 2008). The sedimentary rocks are overlain by, and interbed-ded with, the 25–30 Ma Bates Mountain Tuff (Tbm, described below), and are thus early Oligocene in age. They are shown separately from Tbm in Plate 1 where they are extensive enough to be mapped as a distinct unit.
The Bates Mountain Tuff as originally defined by Stewart and McKee (1968) actually consists of several genetically unrelated tuffs. In the study area, these tuffs include Bates Mountain B (30.48 ± 0.06 Ma), equivalent to the tuff of Sutcliffe in western Nevada (Henry et al., 2004; Faulds et al., 2003), Bates Mountain C (28.64 ± 0.07 Ma), equivalent to the tuff of Campbell Creek in central and western Nevada (Henry et al., 2004; Faulds et al., 2003; McKee and Conrad, 1987), and Bates Mountain D (25.27 ± 0.07 Ma), equivalent to the widespread Nine Hill Tuff (Bingler, 1978; Deino, 1989). Not all units are present at all localities, and the individual ash-flow sheets are interbedded with sedimentary rocks (Tcs). No angular discordance is evident within this sequence of tuffs, or between them and the underlying sedimentary rocks and intracaldera Caetano Tuff. For the purpose of discussing subsequent deformation, it is convenient to refer to all rocks (including interbedded sedimentary rocks) between the stratigraphically lowest and highest Bates Mountain Tuff in a given section as a single unit, the 25–30 Ma Bates Mountain Tuff (Tbm). The composite sequence of Bates Mountain Tuff and interbed-ded sedimentary rocks may be as much as 300 m thick. Within the study area, the Bates Mountain Tuff is locally present outside the caldera, where it is deposited on outflow Caetano Tuff (Tct) or older lava flows (Tad).
In the western part of the study area, the Bates Mountain Tuff is interbedded with undated andestic and/or dacitic lava flows (Tad) mapped by Stewart and McKee (1977) and Moore et al. (2005). These flows are present both above and beneath the Bates Mountain Tuff, whereas similar lavas (the dacite of Wood Canyon) on the west side of the Cedars fault (Plate 1) are stratigraphically between the Bates Mountain Tuff and the Caetano Tuff (Moore et al., 2005).
Miocene Sedimentary Rocks
Miocene (16–12 Ma) sedimentary rocks locally overlie older volcanic rocks throughout the study area. These deposits (unit Ts, Plate 1) crop out between the larger fault blocks, and we interpret them to have been deposited in hanging-wall basins during fault slip. Because they bear directly on the timing of deformation, they are discussed in more detail in a later section.
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STRUCTURAL RECONSTRUCTION |
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TOPABSTRACTINTRODUCTIONGEOLOGIC SETTINGSTRUCTURAL RECONSTRUCTIONMIOCENE SEDIMENTARY BASINSDISCUSSION AND CONCLUSIONSREFERENCES CITED |
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Where faults are exposed or where they can be measured directly from map traces, they now dip moderately to gently west (25°–35°) and hanging-wall cutoffs suggest initial dips of 62°–72°. In other cases we estimate the minimum (present day) fault dip by projecting the fault plane above the footwall topography and using the dip of footwall rocks to infer initial dip, suggesting initial dips >55°–65°. Available constraints on the dip of major faults in the study area are listed in Table 1, and where necessary, we infer initial dip (usually 65°) using these known faults as a guide (details are given where relevant in the following discussion). The cross sections are restored as a series of rigid fault blocks, ignoring internal deformation (folding and/or bending) and possible slip on unidentified small faults. Locally, this leads to space problems where footwall tilt changes from block to block (Fig. 4), which we assume is accommodated by internal deformation of the fault blocks and/or the faults curving in the subsurface. In cross section, these uncertainties are assumed to be small relative to the uncertainty in fault dip.
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Extension of the Caetano Caldera (Sections A–A', B–B', C–C')
Restored Structure along Section A–A'
The western margin of the caldera is buried beneath Reese River Valley, and the westernmost units exposed along A–A' are post-caldera sedimentary rocks (Tcs) overlain by lava flows (Tad) and cut by many small-offset, west-dipping normal faults. These rocks are dropped down to the west along the west-dipping Moss Creek fault. To the east of the Moss Creek fault, a conformable, 40°–50°E dipping sequence of Caetano Tuff (Tcc) and post-caldera sedimentary (Tcs) and volcanic rocks (Tbm and Tad) is exposed on the west side of Redrock Canyon. The modern floor of Reese River Valley is 400 m lower than Redrock Canyon above Moss Creek, and Miocene sedimentary rocks (Ts) in Redrock Canyon are being eroded by modern streams that drain through Moss Creek canyon into Reese River Valley. We therefore infer that the Moss Creek fault is a late Miocene or younger fault with at least 400 m vertical offset along A–A' and show it dipping 60°W (based on exposed fault surfaces at the mouth of Moss Creek; Fig. 5A) and cutting an older, more gently dipping fault that accommodated tilting of the caldera sequence exposed at Moss Creek. This older fault may be a northern extension of the Cedars fault system exposed to the south, and we estimate it to dip 20°W, based on an assumed initial dip of 65° relative to a footwall tilt of 45°E.
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20° based on an assumed initial dip of 65° and the 45° tilt of its hanging wall and foot-wall blocks. After restoring 100 m of slip on the unnamed small fault, 6 km of slip on the Redrock Canyon fault is required to match the Bates Mountain Tuff on either side of Redrock Canyon.
Intracaldera tuff (Tcc and/or Tcu) and breccia (Tcb) are exposed east of Cooks Creek in the footwall of the Greystone fault, which is exposed 7 km north of A–A' in several roadcuts along the haul road to the Greystone Mine. There, it dips 35°W and places (Miocene?) alluvial fan deposits against brecciated Paleozoic chert and quartzite (Fig. 5B). Along Redrock Canyon, we assume a more gentle 20° dip for the Greystone fault based on 70°–40° dipping Caetano Tuff and breccia sheets exposed south of the line of section. The extensive breccia deposit along the caldera margin here appears to be interbedded with the upper cooling units of the Caetano Tuff (Tcu), so we restore this block to just below the inferred base of the post-caldera rocks (Tcs and Tbm) to the west, requiring 5 km of slip on the Greystone fault. The caldera margin is nearly parallel to line A–A' at this point and we do not attempt to accurately portray it on the cross section.
The footwall of the Greystone fault is covered by alluvium at the north end of Carico Lake Valley, and no known Cenozoic rocks are exposed farther east along A–A'. Undated, poorly exposed gravel deposits north of Rocky Pass may be Miocene sedimentary rocks (Ts?) in the hanging wall of the Rocky Pass fault system, although it is unclear where this fault (or faults) extends north into Paleozoic rocks. Approximately 3 km south of line A–A', 37°–43°E dipping Caetano Tuff (Tcl) crops out along the north wall of the caldera, suggesting that west-dipping faults continue at least several kilometers north in the Paleozoic rocks. Paleozoic rocks in this area are complexly faulted (Gilluly and Gates, 1965), and it is extremely difficult to tell whether faults have thrust or normal displacement, let alone their age. The eastern 19 km of line A–A' is buried beneath late Miocene(?) to Holocene basin fill in Crescent Valley. On line A–A', we show the approximate position of faults exposed along line B–B' 4–5 km to the south, as they would project if they continued north beneath Crescent Valley.
Restoring the Caetano caldera over the western part of section A–A', between the surface trace of the Moss Creek fault (a1) and the surface trace of Greystone fault (a2), yields 5.6 km of extension (10.5–4.9 km) or 115% strain. Given the lack of Cenozoic marker units northeast of the caldera and the extent of young cover in Crescent Valley, it is impossible to restore the eastern 30 km of the section.
Restored Structure along Section B–B'
West of the Moss Creek fault, line B–B' is covered by alluvium in Reese River Valley, although it is inferred at depth to consist of faulted post-caldera sedimentary rocks (Tcs) and lava flows (Tad) by analogy to line A–A'. The Moss Creek fault is assumed to dip 60°W and cut an older fault that dips 25°W (assumed initial dip of 65°), with 40°E dipping intracaldera Caetano Tuff, post-caldera sedimentary rocks (Tcs), and Bates Mountain Tuff (Tbm) in its footwall. This older fault may be the northern extension of the Cedars fault or fault system, and the footwall is locally complicated along B–B' by a minor down-to-the east fault that dies out to the north.
To the east of Redrock Canyon, line B–B' crosses the Redrock Canyon fault and two other west-dipping faults that drop 40°–45°E dipping sedimentary rocks (Tcs) and Bates Mountain Tuff down against highly altered intracaldera tuff (Tcc) and the Redrock Canyon pluton (Tcir). Redrock Canyon is bound on the east side by the Redrock Canyon fault, which places Miocene sedimentary rocks against the Redrock Canyon pluton. We assume a 35°W dip for the Redrock Canyon fault along B–B', somewhat less than along A–A', consistent with less steeply dipping Bates Mountain Tuff in its hanging wall. Approximately 2.5 km of slip on the Redrock Canyon fault, together with 2.5 km combined slip on the two gently west dipping faults to the east is required to restore the Bates Mountain Tuff (Tbm) on either side of Redrock Canyon. This system of west-dipping faults is cut at a high angle by two northeast-striking, down-to-the southeast faults that form prominent lineaments on aerial photographs but appear to have only a few hundred meters of offset at most.
Within Carico Lake Valley, line B–B' crosses alluvial cover and the Carico Lake pluton (Tci), interpreted as a resurgent intrusion within the Caetano caldera (John et al., 2008). In the low hills between lines B–B' and C–C', 35°E dipping Bates Mountain Tuff is exposed just south of the intrusion, requiring a west-dipping fault to bring it up relative to east-dipping exposures of Bates Mountain Tuff and sedimentary rocks (Tcs) to the west. We infer this fault to be a southern extension of the Greystone fault system extending south down Carico Lake Valley. Along B–B', this fault drops Caetano Tuff and overlying rocks down against the pluton, although it is unclear how far west the pluton extends in the subsurface. The east side of Carico Lake Valley is bound by the west-dipping Rocky Pass fault, which places 24°E dipping Miocene sedimentary rocks (Ts) against 40°–45°E dipping intra-caldera Caetano Tuff (Tcl and Tcu) exposed along the steep ridge south of Rocky Pass (Fig. 5C). The Rocky Pass fault is not exposed, but is inferred to dip 25°W, based on an assumed initial dip of 65° and a footwall tilt of 40° (Table 1). The exposed top of the upper Caetano Tuff (Tcu) in the footwall of the Rocky Pass fault is assumed to restore to a preextensional stratigraphic position just beneath the Bates Mountain Tuff. Approximately 8.5–9.0 km of total slip along the Rocky Pass fault and inferred Greystone fault is therefore required to restore the upper Caetano Tuff (Tcu) beneath the Bates Mountain Tuff (Tbm) in the southern Shoshone Range. To the east of Rocky Pass, the Caetano Tuff is assumed to be overlain by Miocene sedimentary rocks (Ts), which dip 10°–20°E where exposed along strike to the south.
The northern Toiyabe Range is bound to the west by the west-dipping Toiyabe Mine fault. The present dip of this fault is unknown, but we estimate 20°–25°, assuming an initial dip of 65°–70° and a footwall tilt of 45°E, based on measured dips in the upper Caetano Tuff (Tcu) and overlying sedimentary rocks (Tcs) exposed 6 km to the south near Caetano Ranch. The foot-wall of the Toiyabe Mine fault along line B–B' consists almost entirely of densely welded intra-caldera tuff (Tcl), which is cut by several small-offset, west-dipping faults assumed to have a geometry similar to that of the Toiyabe Mine fault. John et al. (2008) interpret a small exposure of conglomerate just east of the Toiyabe Mine fault to predate the Caetano Tuff, correlative with conglomerate (Tog) exposed on the floor of the caldera east of Caetano Ranch (2 km south of C–C'). The exposure of caldera floor portrayed in B–B' is based on this interpretation, assuming the conglomerate is no more than 400 m thick. Assuming that the lower Caetano Tuff is 3.6 km thick (consistent with its >3.4 km thickness in the footwall of the Caetano Ranch fault just to the east), 7–8 km of combined slip on the Toiyabe Mine fault and the next west-dipping fault to the east is required to exhume the caldera floor. We assume very little slip on the other two west-dipping faults, consistent with no additional exposures of the caldera floor to the east. Interpretation of the gravel deposit (Tog) as caldera floor affects the stratigraphic position of the restored footwall of the Toiyabe Mine fault (and thus the total slip on each fault), but has very little effect on the amount of extension.
The easternmost block in the northern Toiyabe Range is bound along B–B' by the southernmost trace of the Crescent fault, a young (late Miocene(?) to Holocene) high-angle fault that cuts the older, more gently dipping Caetano Ranch fault. We infer an initial dip of 65°W for the Caetano Ranch fault, which currently dips >15°–18° (Table 1). The amount of slip on the Crescent fault is not known, but is inferred to be small because it dies out 2–3 km to the south; we show 500 m of slip in the cross section. South of B–B', the Caetano Tuff is unbroken by major faults and dips 35°–45°E. This is the greatest exposed thickness of the lower Caetano Tuff (3.4 km), and we assume it to be close to the maximum thickness (inferred 3.6 km; John et al., 2008), although additional fill may be concealed upsection beneath northern Grass Valley. After restoring the Crescent fault, 2 km of slip on the Caetano Ranch fault is required to restore this thick section of intracaldera tuff with similar exposures to the west, although this is dependent upon the caldera floor being exposed in the footwall of the Toiyabe Mine fault. The eastern margin of the caldera is shown in Plate 1 as the subsurface extent of the Caetano Tuff defined by Barrick Gold Corporation from geophysical and drill-hole data (K. Hart, 2006, written commun.).
The breccia deposit (Tcb) along the caldera margin in B–B' was mapped by Gilluly and Masursky (1965) as in-place Paleozoic rocks, but John et al. (2008) interpret it as megabreccia blocks of Paleozoic rocks within the caldera, on the basis of field relationships and drilling data (R. Leonardson, 2007, oral commun.). Northern Grass Valley is bound to the east by the west-dipping Cortez fault, which drops Miocene conglomerate and sandstone down against lower-plate Paleozoic rocks intruded by the 158 Ma Mill Canyon Stock (Gilluly and Masursky, 1965). We estimate 4 km of slip on the Cortez fault. Both the Cortez fault and Miocene sedimentary rocks in its footwall are cut by the younger Crescent fault, which strikes nearly east-west and dips 60°N where it truncates the northern end of Grass Valley at the Cortez Mine (Fig. 6A). The Cortez Range footwall block is structurally intact and tilted gently east-southeast. Miocene (16.7–16.4 Ma; Table A1) basalt flows on the crest of the range dip <10°SE, and lower-plate carbonate rocks adjacent to the Cortez fault dip as much as 30°E (Gilluly and Masursky, 1965). We interpret the Cortez fault as the breakaway fault for the system of west-dipping faults that extended the Caetano caldera, with footwall tilt within the Cortez Range increasing toward the Cortez fault, possibly due to flexural uplift and bending of the Cortez Range footwall block during fault slip.
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112% strain.
Restored Structure along Section C–C'
The western end of line C–C' is covered by Reese River Valley and is close to the southern caldera margin, which parallels the section line. The west side of the Shoshone Range forms a steep front that rises 800 m above Reese River Valley, suggesting a somewhat greater amount of slip on the Moss Creek fault (we estimate 1 km) than to the north (see B–B'). East of the Moss Creek fault, the Caetano Tuff dips 30°–40°E and is cut by two prominent north-striking faults. The Cedars fault dips 35°W as it cuts down the range front north of the section line. Along C–C' it is entirely within altered intracaldera tuff (Tcc), although subcrop of what may be post-caldera sedimentary rocks (Tcs) in the hanging wall suggests that it drops down the upper part of the caldera fill. To the east, the west-dipping Elephant Head fault drops Bates Mountain Tuff and post-caldera sedimentary rocks (Tcs) down against altered intracaldera tuff. The dip of this fault is not known, but we assume it to be similar to the Cedars fault (35°W; Table 1).
The east side of Elephant Head is bound by the down-to-the east Stone Cabin fault, which forms the west side of Carico Lake Valley to the south. Slip on this fault is inferred to increase from north to south with increasing topographic relief; at the latitude of C–C' we assume 500 m of slip on this fault. To the east of the Stone Cabin fault, 30°–35°E dipping Bates Mountain Tuff is faulted against Caetano Tuff along a 26°W dipping fault exposed in a small drainage (Fig. 6B). This is one of the few outcrop exposures of the gently west dipping faults that dismember the Caetano caldera. Intracaldera tuff (Tcl and Tcu) and breccia in the footwall of this fault dip 50°E. The fault must be cut to the west by the Stone Cabin fault, but the upthrown trace (dashed on the cross section) has not been identified or is not currently exposed (and is not shown on the map). To the east, the upper Caetano Tuff is cut by a high-angle, down-to-the east fault that places Bates Mountain Tuff against Caetano Tuff 2 km north of line C–C'. The restored thickness of the upper Caetano Tuff between the Stone Cabin fault and Carico Lake Valley is >2 km (Fig. 4). Rather than being twice as thick here as its documented 500–1000 m thickness elsewhere, we think it more likely that the upper Caetano Tuff here is repeated by an unmapped down-to-the west fault. The west side of Carico Lake Valley is marked by young fault scarps that offset alluvial fans (Qaf) by several meters, and we show small offset of the fan deposits on the cross section.
Caetano Tuff and overlying rocks are exposed in a low hill just north of line C–C' in Carico Lake Valley. The Caetano Tuff here is strongly altered, brecciated, and displays an anomalous east-west strike and steep (70°–90°) dips, which we attribute to deformation during intrusion of the Carico Lake pluton (John et al., 2008). Deformed intracaldera tuff is overlain by Bates Mountain Tuff and post-caldera sedimentary rocks (Tcs) that dip 30°E, requiring that they be bound by a west-dipping fault to bring them up relative to exposures of the same units to the west, although the exact location and geometry of this fault are concealed by alluvium. We infer a southern extension of the Greystone fault system dipping 30°W. Approximately 5–6 km of total slip is required on the inferred Greystone fault and the exposed fault east of the Stone Cabin fault (shown in Fig. 6B) to restore the Bates Mountain Tuff in Carico Lake Valley with the outcrops just east of the Stone Cabin fault.
North of Red Mountain, the east side of Carico Lake Valley is bound by the Red Mountain fault, with a prominent scarp suggestive of recent (Holocene?) slip (Fig. 6C). The north-striking Red Mountain fault makes an 90° bend to strike east-west at the latitude of the caldera margin, and we infer that it reactivated an 2-km-long section of the original caldera-bounding fault. The Red Mountain fault cuts an older, 40°W dipping fault that is exposed for 3–4 km along the range front north of Red Mountain. This fault is entirely within altered intracaldera Caetano Tuff dipping 40°–45°E, so we infer the presence of an additional fault necessary to bring the Bates Mountain Tuff in Carico Lake Valley down relative to Red Mountain; this fault is shown on C–C' as a 25°W dipping extension of the Rocky Pass fault. Above the lower Caetano Tuff, the upper unit (Tcu) dips 40°–45°E, and, after restoring 200–300 m of slip on the Red Mountain fault, these exposures are restored by 2.6 km of slip along the Rocky Pass fault to a position just beneath the Bates Mountain Tuff to the west. Between Carico Lake Valley and Tub Spring, the upper unit (Tcu) is repeated by several small west-dipping faults; 1.5–2 km total slip is required on this set of faults to restore the upper cooling units with exposures to the west. To the east and southeast, these fault blocks are overlain by Miocene sedimentary rocks (Ts) that dip 10°–20°E.
The Toiyabe Range along section C–C' is bound by the Toiyabe Mine fault, which (as along B–B') is inferred to dip 20°W beneath Miocene sedimentary rocks, based on 40°–60° dips in Caetano Tuff and in post-caldera sedimentary rocks in its footwall and an assumed 65° initial dip. The footwall of the Toiyabe Mine fault exposes 1.4 km of intracaldera Caetano Tuff overlain by 100–200 m of post-caldera sedimentary rocks (Tcs); the sedimentary rocks may continue upsection beneath the large area of alluvium north of the caldera margin. Approximately 5–6 km of slip is required on the Toiyabe Mine fault to restore these exposures adjacent to the upper cooling unit west of Tub Spring, although the displacement could be partitioned onto additional faults buried beneath Miocene sedimentary rocks (or that cut the sediments but are not exposed).
The northern Toiyabe Range is split by the north-striking Caetano Ranch fault, which is cut along line D–D' by a northwest-striking, down-to-the southwest fault with <500 m of offset. The Caetano Ranch fault drops upper Caetano Tuff down against Paleozoic rocks overlain by 200 m of Tertiary andesite lava flows. This is the only significant exposure of the caldera floor in the study area, and is discussed in more detail by John et al. (2008). To the east of the Caetano Ranch fault, another west-dipping fault offsets the caldera floor 400–500 m; the subsurface geometry of this fault is projected onto section C–C' from 2 km to the south. The footwall of this fault exposes 2 km of intracaldera tuff, although a greater thickness is likely present beneath Grass Valley (as shown in the cross section). The location of the eastern caldera margin beneath Grass Valley is shown as determined by Barrick Gold Corporation from potential field and drill-hole data (K. Hart, 2006, written commun.). A minimum of 3.5 km of slip is required on the Caetano Ranch fault, but this would make the main intracaldera cooling unit only 2.4 km thick. If instead the main cooling unit is assumed to be 3.6 km thick, comparable to the incomplete 3.4 km section exposed along strike to the north, then 5 km of slip is required.
The footwall of the Cortez fault along our section C–C' was originally mapped by Gilluly and Masursky (1965) as an upright fold in the Roberts Mountains thrust, with upper-plate units exposed along the range front in the western limb. We reinterpret these exposures of upper-plate rocks as being displaced down by 2 km of slip on a west-dipping normal fault that intersects line D–D' obliquely and displays an apparent shallow dip in cross section. We estimate a combined 3.5–4 km of slip on this fault and the Cortez fault.
Restoration of section C–C', between the surface trace of the Moss Creek fault (c1) and the surface trace of the Cortez fault (c2), yields 21.7 km of extension (40.9–19.2), or 113% strain.
Structural Reconstruction: Summary
The primary source of uncertainty in drawing and restoring the cross sections arises from the need to make assumptions about the dip of many of the major normal faults. To translate this uncertainty in fault geometry into uncertainty in the magnitude of extension, we consider that extension of the caldera occurred along a set of parallel, rotating, domino-style normal faults, which is a good approximation of the mapped geology. The magnitude of strain then depends on the present tilt of the fault blocks (
) and the initial fault dip (
) by the following relationship (modified from Thompson, 1960):
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Figure 7 is a plot of all compaction foliation and bedding attitudes from pre–25 Ma Cenozoic rocks within the Caetano caldera measured during this study; they strike roughly north (N11°E) and dip 30°–50°E. These measurements are not evenly distributed across the study area and thus may be a better indication of the range of possible dips rather than the average, but for the sake of this exercise we estimate 40° of block rotation across the caldera (the average of all measurements). Figure 8 then shows the effect of assumed initial fault dip on the restored width of the (now) 42-km-wide caldera. Based on the data in Table 1, we assume that most major faults in the study area had initial dips of 60°–70°, yielding an original caldera width of 17–23 km, corresponding to 90%–150% strain (technically, equation 1 yields an original width of 19.6 +2.7/–3.0 km). Available constraints on the dip of major normal faults (Table 1) appear to rule out initial fault dips <60°, but dips >70° are permitted for some of the unknown faults, which would lead to somewhat less total strain.
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110% strain for assumed initial fault dips of 65°; Fig. 4), and the above analysis (±3 km extension for initial fault dips of 60°–70°), we conclude that the present 42 km east-west dimension of the Caetano caldera has undergone roughly 110% strain, representing 22 ± 3 km westward translation of the Fish Creek Mountains relative to the Cortez Range (Fig. 1). The preextensional Caetano caldera was thus roughly equant, 10–17 km north-south by 20 km east-west, with the east side (10 km north-south) apparently narrower than the west (17 km north-south). Extension was taken up on a set of initially high angle, closely spaced (1–3 km), west-dipping normal faults that underwent 35°–50° of block rotation. The consistent north-strike (N11°E) of tilted units in the footwall blocks of these faults (Fig. 7) is consistent with major extension oriented approximately east-west or slightly northwest. These earlier faults are cut, commonly at oblique angles, by a second set of more widely spaced, high-angle faults that locally have significant vertical offset but do not appear to have accommodated much horizontal extension. In restoring the cross sections, we assume negligable tilting associated with slip on this younger set of faults. The largest of them, the Crescent fault (Plate 1), has 6° of footwall tilt despite 2–3 km of vertical offset (Muffler, 1964; Gilluly and Masursky, 1965). Therefore, we believe this is a reasonable assumption for the much smaller high-angle faults on our cross sections. The major faults appear to cut the caldera margin and continue north and south into the surrounding Paleozoic rocks, and we discuss the probability of extension outside the caldera in a later section. |
MIOCENE SEDIMENTARY BASINS |
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TOPABSTRACTINTRODUCTIONGEOLOGIC SETTINGSTRUCTURAL RECONSTRUCTIONMIOCENE SEDIMENTARY BASINSDISCUSSION AND CONCLUSIONSREFERENCES CITED |
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10–20 cm) lenticular beds of conglomerate, with 1–5 cm clasts of (variably) quartzite, chert, and volcanic rocks (Fig. 10B). Less commonly, the section contains thinly laminated (<1 cm) tuffaceous shales or siltstones. The most conspicuous layers in the Miocene basins consist of 2–5 m thick layers of silver-gray and white tephra (Fig. 10C) in 5-cm-thick to 1-m-thick beds that often consist of nearly pure volcanic glass shards. Tephra deposits are locally cross-bedded at the scale of 10–50 cm and mixed with coarser crystal and lithic fragments, which we interpret as evidence of local reworking. Tephra and sandstone layers are locally bioturbated and sometimes display conspicuous folds interpreted as soft-sediment deformation (Fig. 10C). We interpret the tuffaceous sandstones and shales to have been deposited in fluvial, lacustrine, and medial to distal alluvial-fan settings with frequent pyroclastic input, both from direct ash fall and material washing in from the surrounding highlands. Based on tephra correlations (this study; Table 2), the pyroclastic material was derived from eruptions in northern Nevada and southwestern Idaho (Perkins and Nash, 2002; Perkins et al., 1998) and does not represent local volcanic activity during deposition of the Miocene sedimentary rocks.
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Miocene Basin at Redrock Canyon
In Redrock Canyon, Miocene sedimentary rocks overlie units C and D of the Bates Mountain Tuff with no apparent angular discordance. At the upper (east) end of Moss Creek Canyon, the base of the section consists of coarse sandstone and conglomerate dipping 42°E and overlying unit C (28.8 Ma) of the Bates Mountain Tuff. The basal conglomerate contains clasts of Paleozoic quartzite and Tertiary volcanic rocks (to 50 cm). The coarse deposits are only a few meters thick, and are overlain by as much as 200 m of poorly exposed, thinly bedded, white tuffaceous shale. Overlying the shale, an 20-m-thick sequence of ash-rich beds crops out for 500 m along strike in a low ridge. Tephra beds are locally cross-bedded and chaotically folded by what we infer to be soft-sediment deformation (Fig. 10C). Sanidine from a layer of relatively pure, silver-gray volcanic glass in this sequence (sample CT107) yielded multi-grain laser fusion ages ranging from 26.30 to 15.73 Ma, indicating contamination by older feldspar grains. A weighted mean of the four youngest analyses gives an age of 15.84 ± 0.15 Ma (Table 3), but this should be considered a maximum age for the sample. This tephra was independently correlated with 16.0–15.3 Ma Buffalo Canyon–type tephra (Table 2; Perkins and Nash, 2002). Sanidine from a second tephra sample in this sequence (sample CT120) yielded an age of 15.68 ± 0.14 Ma based on six reproducible multigrain laser fusion analyses (Table 3). From these dates, we conclude that deposition of the middle Miocene sedimentary rocks at Redrock Canyon began at or shortly before 15.8–15.6 Ma. Although we interpret the conglomerate and shale between the dated tephra horizon and the Bates Mountain Tuff to be middle Miocene (close to 16 Ma), it is important to note that geologic relationships permit it to be anywhere between 29 and 16 Ma.
To the south of Moss Creek Canyon, the base of the Miocene section overlies unit D (25.3 Ma) of the Bates Mountain Tuff (unit C is also present). Approximately 2 km southeast of Moss Creek, an isolated outcrop of relatively fresh white tephra (sample CT106; Fig. 9) was correlated with the 12.07 Ma Cougar Point V tuff (Table 2; Perkins and Nash, 2002). This exposure would be 600–700 m upsection from the Bates Mountain Tuff, although most of the intervening section is covered and we cannot rule out repetition by unexposed faults. Although the rest of the section to the east (upsection) is also covered by younger alluvium, we estimate at least 200–300 m of additional section above the ca. 12 Ma tephra. From this date, we conclude that deposition in the Redrock Canyon basin continued up to and after 12 Ma. The low hills on the east (fault bound) side of Redrock Canyon consist of soft, sandy and silty soil weathering out small (<10 cm) angular chips of tuff and Paleozoic quartzite. We interpret these deposits as poorly consolidated conglomerate within the Miocene sedimentary section, but we have not observed any actual outcrops. These deposits are concealed and truncated by a pediment surface covered with large (up to several meters) boulders of Caetano Tuff presumably derived from the adjacent highlands.
Miocene Basin at Wilson Canyon
At the mouth of Wilson Canyon, 40°E dipping tuffaceous sandstones with interbedded tephra layers conformably overlie unit D (25.3 Ma) of the Bates Mountain Tuff. Sanidine from a prominent, white, thinly bedded (2 cm), 1-m-thick layer of tephra 10 m above the Bates Mountain Tuff (sample CT116; Fig. 9) yielded multigrain laser-fusion 40Ar/39Ar ages ranging from 15.09 ± 0.05 to 20.0 ± 4.6 Ma (Table 3), indicating contamination by older feldspar grains. Excluding the 20 Ma date and calculating a weighted mean of the remaining five multigrain analyses (15.67–15.09 Ma; Table 3) yields an age of 15.38 ± 0.30 Ma, although the eruptive age may be as young as 15.09 Ma. This sample was independently correlated with any of several Yellowstone hotspot–type tephras ranging in age from 13 to 15 Ma (Table 2; Perkins and Nash, 2002). Upsection from this tephra bed, the section grades to tan, medium-grained sandstone in beds 20–150 cm thick, with a silty matrix and local small lenticular conglomerate beds (Fig. 10B). Sanidine from a tephra layer interbedded with these sandstones (sample CT121) yielded an 40Ar/39Ar age of 15.03 ± 0.10 Ma (Table 3) from the weighted mean of six reproducible multigrain laser-fusion analyses. From these dates, we conclude that deposition of the middle Miocene sedimentary rocks at Wilson Canyon began ca. 15.4–15.0 Ma, after a 10 m.y. hiatus following deposition of the Bates Mountain Tuff.
Much of the remaining section to the east is covered by younger alluvium at the northwest end of Carico Lake Valley, but a small sandstone outcrop 1.5 km to the southeast (an unknown distance upsection) dips 18°E. On the east side of this basin, along the haul road to the Greystone Mine, sandstone and conglomerate are exposed in roadcuts in the hanging wall of the Greystone fault, locally in direct contact with the fault (Fig. 5B). Although much of the section is covered by younger alluvium, we estimate it to be >1 km thick, assuming no repetition by unmapped faults.
Miocene Basin at Wood Spring Canyon
The most extensive and best-exposed (relative to the others) Miocene basin occupies the hanging wall of the Toiyabe Mine fault, between the Rocky Pass–Red Mountain block and the northern Toiyabe Range (Fig. 9). In the west-central part of the basin, east of Tub Spring, tuffaceous sandstones and shales (Ts) are mostly covered by a thin layer of younger alluvium (shown as Qaf in Plate 1). These units dip 20°–5°E where they are exposed in modern drainages (Fig. 10A). Two samples from this section (CT101 and CT102; Fig. 9) were correlated with the Cougar Point III (12.69 Ma) and Cougar Point V (12.07 Ma) tephras, respectively (Table 2; Perkins and Nash, 2002). The section is cut by at least one small fault (Plate 1) that appears to offset the ground surface as much as 100 m and is therefore assumed to be young (Pleistocene?), with essentially no displacement at the scale of cross section C–C' (Fig. 4). Assuming the section is not repeated by additional unmapped faults, 300–400 m of section probably overlies the 12 Ma tephra layer. Much of it is covered by younger alluvium, but on the east side of the basin, close to the Toiyabe Mine fault, fine-grained tuffaceous sandstones and shales are overlain by medium- to coarse-grained, silty sandstones with uncommon matrix-supported clasts of Paleozoic quartzite. From the above dates, we conclude that deposition of the middle Miocene sedimentary rocks at Wood Spring Canyon continued up to and after 12 Ma.
The southwestern part of the basin, in the hanging wall of the Dry Canyon fault, consists primarily of gently east dipping (12°) tuffaceous sandstone and shale, with the base of the section largely obscured by younger alluvium. Just west of the Dry Canyon fault, the basin contains coarse sandstone and conglomerate with subangular clasts of greenish Paleozoic chert (>80% of clasts), quartzite, and (presumably Cenozoic) volcanic rocks (to 50 cm). The chert clasts are probably derived from similar-looking Paleozoic chert in the footwall of the Dry Canyon fault exposed <100 m to the east. These deposits are locally calcite cemented and crop out in highly resistant ledges, which we interpret as fossil spring deposits.
The southeast part of the basin, adjacent to the Toiyabe Mine fault, appears to consist primarily of medium-grained sandstone with a silty matrix, with varying amounts of matrix-supported angular Paleozoic chert and (predominantly) quartzite clasts (Fig. 10D). Outcrops of this deposit are very rare, but where exposed they exhibit poorly developed 50–150 cm bedding that dips gently (<10°) northeast. These deposits form a range of deeply dissected hills that rise to 7250 ft (2210 m) west of the Toiyabe Mine, and we estimate the section to be at least 1 km thick, with an unknown, but probably significant, thickness removed by erosion. The Toiyabe Mine fault occupies a deep canyon (200–300 m) between Paleozoic footwall rocks and hanging-wall basin sediments (Plate 1). The base of this section appears to (along D–D') overlie Bates Mountain Tuff (units C and D), but it is unclear whether the bulk of the overlying deposits is older or younger than the 12 Ma tuffs in the central part of the basin 5 km to the north (along C–C'). Although the deposits west of Toiyabe Mine are now topographically more prominent than the northern part of the basin, they dip gently northeast and may project under the 12 Ma tuffs to the north.
Other Miocene Basin Exposures
Several other exposures of Miocene sedimentary rocks in the study area yielded useful age constraints, although they are not from sections as complete or extensive as those described above. Along the east side of Carico Lake Valley, 2 km southwest of Rocky Pass, fine-grained tuffaceous sandstones dip 24°E in the hanging wall of the Rocky Pass fault. Sanidine from a tephra interbedded with these sandstones yielded an 40Ar/39Ar age of 15.21 ± 0.07 Ma, from a weighted mean of six reproducible multigrain laser-fusion analyses (sample H05–62; Fig. 9). This tephra was independently correlated with Twin Falls–type Yellowstone hotspot tephra, with a possible age ranging from 13 to 15.3 Ma (Table 2; Perkins and Nash, 2002).
Barrick Gold Corporation provided a tephra sample from a drill hole in the hanging wall of the Cortez fault (sample DC99–15; Fig. 9). Sanidine from this tephra yielded multigrain laser-fusion ages ranging from 32.1 to 15.2 Ma, indicating significant contamination by older feldspar grains. The two youngest analyses give a weighted mean age of 15.24 ± 0.17 Ma, and we consider this the best estimate for the age of the sample, although it may be slightly younger. We infer that the conglomerates and sandstones exposed in the footwall of the Crescent fault 4 km to the north (Fig. 6A) are correlative with the sedimentary rocks in this drill hole.
An additional sample was collected from exposures in the pit wall at the Pipeline Mine, where Miocene sedimentary rocks are deposited on lower-plate Paleozoic basement. Sanidine from a tephra within these deposits (sample CJV2; Fig. 9) yielded an 40Ar/39Ar age of 15.88 ± 0.10 Ma (John et al., 2008), and the same sample was independently correlated with 16.0–15.3 Ma Buffalo Canyon–type tephra (Table 2; Perkins and Nash, 2002).
Miocene Basins: Interpretation
From the geologic relationships and age data discussed above, we infer that Miocene sedimentation across the study area began at or shortly before 16–15 Ma, at which time the underlying 25–35 Ma volcanic and sedimentary rocks were essentially flat lying. With the exception of the paleovalley in the southern Cortez Range (John et al., 2008), pre–middle Miocene Cenozoic sedimentary rocks in the study area are older than 25 Ma and are confined to the topographic depression left by the Caetano caldera (Fig. 9). The absence of 25–16 Ma rocks in the study area suggests a period of tectonic quiescence, consistent with the presence of a low-relief land surface incised by paleovalleys, possibly with external drainage to the west (e.g., Henry, 2008;
