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| JOURNAL HOME | HELP | CONTACT PUBLISHER | SUBSCRIBE | ARCHIVE | SEARCH | TABLE OF CONTENTS |
1 U.S. Geological Survey, MS 176, Mackay School of Earth Sciences and Engineering, University of Nevada, Reno, Nevada 89557, USA
2 Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84112, USA
3 U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONREGIONAL GEOLOGIC SETTINGMIOCENE VOLCANISMGEOCHRONOLOGYSEDIMENTARY ROCKS IN THE...CHIMNEY BASINIVANHOE BASINCARLIN BASINELKO BASINPINE AND INDEPENDENCE VALLEYSREGIONAL PALEOGEOGRAPHIC...IMPLICATIONS FOR MINERAL...REFERENCES CITED |
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Relatively minor post-sedimentation extension produced early north-northwest–striking normal faults with variable amounts of offset, and later east-northeast–striking normal faults with up to several kilometers of vertical and left-lateral offset. The earlier faults are more pronounced east of the Tuscarora Mountains, possibly reflecting a hanging-wall influence related to uplift of the Ruby Mountains-East Humboldt core complex on the east side of the Elko basin. The later faults are concentrated along the north-northwest–trending northern Nevada rift west of the Tuscarora Mountains. The area west of the rift contains major tilted horsts and alluvium-filled grabens, and differential extension between this more highly extended region and the less extended area to the east produced the intervening east-northeast–striking faults.
The Humboldt River drainage system formed as the four basins became integrated after ca. 9.8 Ma. Flow was into northwestern Nevada, the site of active normal faulting and graben formation. This faulting lowered the base level of the river and induced substantial erosion in upstream regions. Erosion preferentially removed the poorly consolidated Miocene sediments, progressively reexposed the pre-middle Miocene highlands, and transported the sediments to downstream basins. Thus, some ranges in the upstream region are exhumed older highlands rather than newly formed horsts. In addition, the drainage system evolution indicates that northern Nevada has become progressively lower than central Nevada since the middle Miocene.
Mineral belts with large Eocene gold deposits are exposed in uplands and concealed beneath Neogene basin units in the study area. Also, numerous epithermal hot-spring deposits formed at and near the paleosurface in the Chimney, Ivanhoe, and Carlin basins as those basins were forming. The Neogene geologic and landscape evolution had variable effects on all of these deposits, including uplift, weathering, supergene enrichment, erosion, and burial, depending on the events at any particular deposit. As such, this study documents the importance of evaluating post-mineralization processes at both regional and local scales when exploring for or evaluating the diverse mineral deposits in this area and other parts of the Basin and Range region.
Keywords: sedimentary basins • tectonics • geomorphology • Nevada • Miocene • Pliocene • gold • Humboldt River
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONREGIONAL GEOLOGIC SETTINGMIOCENE VOLCANISMGEOCHRONOLOGYSEDIMENTARY ROCKS IN THE...CHIMNEY BASINIVANHOE BASINCARLIN BASINELKO BASINPINE AND INDEPENDENCE VALLEYSREGIONAL PALEOGEOGRAPHIC...IMPLICATIONS FOR MINERAL...REFERENCES CITED |
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Recent field and thermochronologic studies have demonstrated that the ages of basin-range pairs in this area are diverse. The Ruby Mountains (Fig. 1) experienced major uplift at ca. 15–14 Ma (Colgan and Metcalf, 2006), other ranges did not begin to form until ca. 10 Ma (Wallace, 1991, 2005; Colgan et al., 2004, 2006), and some ranges, such as the Adobe Range, may be relicts of early Tertiary uplands (Haynes, 2003; Wallace, 2005). In addition, some faulting events did not produce major uplift (Gordon and Heller, 1993; Wallace, 1993, 2005; Colgan et al., 2008).
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The study area includes world-class, Paleogene and older mineral deposits in the basement rocks and Miocene epithermal deposits in or adjacent to the Miocene basins (Fig. 1). Consequently, northern Nevada is the third-largest producer of gold in the world. An important goal of the study was to determine the effects of late Cenozoic landscape evolution on these mineral deposits, with implications for the formation and modification of the deposits and deposit- to regional-scale mineral exploration and assessment.
The study area includes four middle and late Miocene sedimentary basins. From west to east, they include what are referred to in this paper as the Chimney, Ivanhoe, Carlin, and Elko basins, which were actively receiving sediments between ca. 16.5 and 9.8 Ma (Figs. 1 and 2). All of the Miocene strata in each basin were examined on at least a reconnaissance basis, and appropriate samples of the sedimentary and coeval volcanic rocks were collected and dated to provide a time-stratigraphic framework. In addition, the Neogene sedimentary units in Pine and Independence Valleys (Fig. 1) were studied and dated on a reconnaissance basis.
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In all of the basins, the two principal components of the sediments are air-fall ash and pumice that were derived from distal to, less commonly, local eruptions, and materials that were eroded from pre-Miocene bedrock exposures in uplands that surrounded the basins. The bedrock-derived materials are generically referred to as epiclastic sediments to highlight their more local derivation in comparison to the air-fall materials. Air-fall deposits that landed on the uplands were redistributed during erosion and stream transport and were intermixed with epi-clastic materials at the depositional sites. Both sediment components were deposited in fluvial and lacustrine environments, and the distinctions between air-fall and bedrock-derived sediments in the strata are described in the text.
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REGIONAL GEOLOGIC SETTING |
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TOPABSTRACTINTRODUCTIONREGIONAL GEOLOGIC SETTINGMIOCENE VOLCANISMGEOCHRONOLOGYSEDIMENTARY ROCKS IN THE...CHIMNEY BASINIVANHOE BASINCARLIN BASINELKO BASINPINE AND INDEPENDENCE VALLEYSREGIONAL PALEOGEOGRAPHIC...IMPLICATIONS FOR MINERAL...REFERENCES CITED |
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In the Eocene, moderate extension produced modest-relief uplands and lowlands. Broad, shallow lakes covered some lowlands, and basin-filling sediments flanked the uplands, including the southern Tuscarora Mountains, Adobe Range, East Humboldt Range, and Ruby Mountains (Haynes, 2003). Late Eocene flows and ash-flow tuffs were erupted onto and across this still-subdued landscape, in places filling east- and west-draining paleovalleys (Henry and Ressel, 2000; Henry, 2008). Virtually all of this igneous activity had ceased by ca. 38 Ma in northeastern Nevada (Haynes, 2003).
Major gold deposits, some of which formed near coeval late Eocene igneous centers, are hosted by Paleozoic rocks and comprise the northwest-trending Carlin and Battle Mountain-Eureka mineral-deposit trends, the north-trending Jerritt Canyon trend, and the north-northeast–trending Getchell trend (Fig. 1; Cline et al., 2005). The tops of the central Carlin trend deposits were 
500–1500 m below the late Eocene paleosurface (Haynes, 2003; Ressel and Henry, 2006), and the tops of the slightly younger porphyry-related systems near Battle Mountain were a few hundred meters below the paleosurface at that location (Fig. 1; Theodore and Blake, 1975).
Middle Tertiary extension affected much of northern Nevada, although the amounts varied from minimal (Colgan et al., 2008) to at least 50 percent (Muntean et al., 2001). In most places, the age of extension can only be constrained to between the late Eocene and middle Miocene (Smith and Ketner, 1976; Smith and Howard, 1977; Wallace, 1993, 2003c). At Mule Canyon (Fig. 1), tilting took place between ca. 34 and 16 Ma (John et al., 2003), and, at Marys Mountain near Carlin (Fig. 1), tilting occurred both before and after the emplacement of a 25 Ma welded tuff and before the deposition of 15.3 Ma rhyolite flows (Henry and Faulds, 1999). In the southern Tuscarora Mountains, more deeply formed plutonic rocks were exposed by 16.5 Ma and shed clasts into the western part of the Carlin basin. Supergene alunite dates from gold deposits along the Carlin and Getchell trends indicate that there was enough uplift and erosion to erode and weather the deposits between 30 and 18 Ma (Hofstra et al., 1999; Cline et al., 2005).
As extension relaxed the crust, over-thickened parts of the crust became more buoyant. This allowed deep-seated metamorphic rocks to ascend to shallower levels in the Ruby Mountains and East Humboldt Range and produced a major, west-northwest–dipping detachment fault along the west flanks of those ranges. Displacement along this fault may have been more than 50 km (Howard, 2003). The thermal, metamorphic, and igneous history of this complex dates back to before the Eocene, and major uplift related to high-angle faulting peaked between 14 and 15 Ma (Dokka et al., 1986; Snoke et al., 1997; Howard, 2003; Colgan and Metcalf, 2006). Uplift has continued into the Holocene (Wesnousky and Willoughby, 2003).
Much more subdued extension stretched the crust by 10% between the middle and late Miocene (Muntean et al., 2001), although some areas just to the south were extended more than 100% (Colgan et al., 2008). This extension generally was to the west, and pre-Miocene basement structures undoubtedly affected the apparent local extension direction. For instance, north-northwest–striking dikes along the northern Nevada rift (Fig. 1; see ensuing section) suggest a west-southwest extension direction (Zoback et al., 1994), whereas reconstructed offsets for a large number of Miocene fault blocks in northeastern Nevada indicate a generally northwest but locally highly variable extension direction (Muntean et al., 2001).
Extension and the ascent of the Yellowstone mantle plume into the crust along the Oregon-Nevada border induced widespread bimodal volcanism starting at ca. 16.5 Ma (Christiansen et al., 2002). In northern Nevada, some of the early mafic magmas ascended and erupted along generally north-northwest–striking crustal breaks, such as the northern Nevada rift and related mafic dike complexes (John et al., 2000; Pierce et al., 2002; Ponce and Glen, 2002). Partial melting of the crust, coupled with magma mixing, and fractional crystallization, created more widespread rhyolitic volcanism (Coats, 1987; John et al., 2000; Brueseke and Hart, 2007). The heat from this magmatism, coupled with conduits provided by the faulting and water from the wet climate and lakes, generated a number of small to very large, epithermal gold + silver ± mercury deposits. Most of these deposits formed near the interface between volcanic centers and lacustrine basins (Fig. 1; John, 2001; Wallace et al., 2004a, 2004b).
Extension continued after 10 Ma and led to much of the fault-bounded horst-and-graben physiography of northeastern Nevada (Stewart, 1998). Uplift rates may have been greater in the late Neogene than in the Quaternary (Colgan et al., 2004; Personius and Mahan, 2005). Total extension across northern Nevada was not great, and the formation of the horst-graben pairs was more vertical than in parts of southern and western Nevada, where the horsts are highly tilted fault blocks that formed during significant extension (Anderson, 1971; Proffett, 1977).
Miocene Climate
As noted by Smith (1994), climate can have equal or greater influences on basin sedimentation than do other geologic processes and events. Precipitation causes erosion, creates streams that carry sediments, and fills confined basins with lakes. Modern annual precipitation in the region is less, often much less, than 25 cm (10 in), and a sage-grassland flora dominates the landscape. In contrast, studies of middle Miocene floras by Axelrod (1956) indicate that 50–64 cm (20–25 in) of annual precipitation in northeastern Nevada supported a mixed conifer and oak woodland flora, and pollen and other data indicate that pines were abundant and sage and grasslands were relatively minor (Davis and Moutoux, 1998; Retallack, 2001). As such, the middle Miocene basin sedimentation described in this paper was influenced by much greater amounts of rainfall and different vegetative cover, resulting in different runoff responses than the predominantly sheet-flood runoff found in the current semi-arid, sage-grassland setting.
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MIOCENE VOLCANISM |
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TOPABSTRACTINTRODUCTIONREGIONAL GEOLOGIC SETTINGMIOCENE VOLCANISMGEOCHRONOLOGYSEDIMENTARY ROCKS IN THE...CHIMNEY BASINIVANHOE BASINCARLIN BASINELKO BASINPINE AND INDEPENDENCE VALLEYSREGIONAL PALEOGEOGRAPHIC...IMPLICATIONS FOR MINERAL...REFERENCES CITED |
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In north-central Nevada, bimodal volcanism between ca. 16.8 and 14 Ma produced local to extensive volcanic fields composed of mafic flows to felsic domes, flows, and tuffs. The greatest amount of volcanic activity was focused along the north-northwest–trending northern Nevada rift and the related Santa Rosa-Calico volcanic field to the northwest (Fig. 1). Domes and flows of the Jarbidge rhyolite were erupted across the northeastern tier of the area as the mantle plume migrated to the east-northeast, in some areas producing extensive and thick masses of coarsely porphyritic rhyolite with a minor amount of underlying basalt. Volcanic activity occurred at the same time that the sedimentary basins described in this paper were forming and expanding, and, in many areas, volcanism and sedimentation overlapped in both time and space.
Northern Nevada Rift
The north-northwest–trending northern Nevada rift extends from east-central Nevada to near the Oregon border (Fig. 1), a distance of 500 km. The rift originally was identified by a pronounced positive aeromagnetic anomaly related to a narrow (7 km wide), deep-seated middle Miocene mafic dike complex that was intruded along a preexisting crustal structure (Zoback and Thompson, 1978; Hildenbrand et al., 2000). Intense bimodal volcanism occurred along the rift and adjacent areas from ca. 16.0 to 14.9 Ma (John et al., 2000; Leavitt et al., 2004). In general, thick mafic flow sequences formed early, followed in many, but not all, areas by the eruption of more viscous dacite to rhyolite flows and domes. The very similar and likely related Santa Rosa–Calico volcanic field is just west of the magnetic anomaly near the Oregon-Nevada border. This field was active from ca. 16.7 to 14 Ma (Brueseke and Hart, 2007), and volcanic units related to both systems overlap in the area of the Chimney basin. Volcanism along the northern Nevada rift was more extensive, especially to the east, than the aeromagnetic anomaly, indicating that the deeper crustal processes related to magma genesis were not confined to just the mafic dike complex.
Normal faulting along the rift occurred during the entire period of volcanism. Some of this faulting was related to the "rift," and some likely was related to regional extension. In the northern Shoshone Range and at the latitude of Midas, offset along north-northwest–striking faults along and near the locus of volcanism produced a central downdropped zone. This zone is 14–19 km wide in the northern Shoshone Range and 30 km wide near Midas. Displacement along the faults generally was about a kilometer, with little to no fault-related tilting of the Miocene volcanic units. Overall, the total amount of extension across the structural zone was no more than a few kilometers (John et al., 2000). Rift-related volcanism produced more than 1 km of volcanic rocks in 300,000 yr in the southwestern Sheep Creek Range (John et al., 2000) and at least that much over 500,000 yr in the central Snowstorm Mountains (Wallace, 1993; Leavitt et al., 2004; this study). With as much as 1 km of synvolcanic downdropping, the relief of the volcanic assemblage produced along the rift may have been slight. Specific details on the relation between volcanism, faulting, and sedimentation in the Snowstorm Mountains and the adjacent Chimney and Ivanhoe basins are presented in later sections of this paper.
Jarbidge Rhyolite
The Jarbidge rhyolite loosely includes coarsely porphyritic flows and domes, with some related tuffaceous units (Coats, 1987). The type area is in the Jarbidge area of northeastern Nevada (Fig. 1), and similar rhyolites are scattered throughout northeasternmost Nevada and as far east as the Utah border. Because of the highly viscous nature of the magmas and mode of emplacement, flow across the landscape probably was not great, and the distribution of the rhyolite likely reflects the distribution of the eruptive centers. Miocene basin sediments locally overlie, underlie, or encase rhyolite flows. North of the Independence Mountains, the rhyolite overlies 16.6 Ma basaltic andesite flows (Rahl et al., 2002), and some nearby rhyolite domes were erupted at ca. 15.8–16 Ma (Coats, 1987; C. Henry, 2004, written commun.). Jarbidge-like flows and dikes just southwest of Wells in the East Humboldt Range were emplaced between 14.8 and 13.4 Ma (Snoke et al., 1997), and some data suggest that rhyolitic volcanism at Jarbidge, the type area, occurred at ca. 14 Ma (Bernt, 1998).
Rhyolite flows were erupted at ca. 15.3 Ma west and south of Carlin (Fig. 1) to form the Palisade Canyon rhyolite. These volcanic units have an uncertain relation to both the northern Nevada rift and Jarbidge systems, although their mineralogies and chemistries suggest an origin similar to rhyolites in those systems. This unit is described in more detail in the Carlin basin section.
Post-Basin Volcanic Units
Extensive sheets of Miocene rhyolite and basalt flows underlie much of the Owyhee Plateau in northernmost Nevada and are related to the Yellowstone mantle plume along the Snake River Plain to the north (Fig. 1; Wood and Clemens, 2002). These volcanic units were erupted between ca. 15.4 and 8 Ma, and they conceal virtually all underlying Miocene and older units, including the northern end of the northern Nevada rift and any basin sediments that may be present. These volcanic units are largely unfaulted and retain their original low to horizontal dips.
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GEOCHRONOLOGY |
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TOPABSTRACTINTRODUCTIONREGIONAL GEOLOGIC SETTINGMIOCENE VOLCANISMGEOCHRONOLOGYSEDIMENTARY ROCKS IN THE...CHIMNEY BASINIVANHOE BASINCARLIN BASINELKO BASINPINE AND INDEPENDENCE VALLEYSREGIONAL PALEOGEOGRAPHIC...IMPLICATIONS FOR MINERAL...REFERENCES CITED |
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provides all of the dates and correlation ages cited in this paper, and Appendix 1 gives brief descriptions of unpublished samples collected and dated for this study. Several figures in the text show the approximate locations where samples were collected, along with the date or date range. In addition, Table 2, introduced later in the paper, summarizes published and unpublished K-Ar and 40Ar/39Ar dates on supergene alunite from several gold deposits in the study area. All 40Ar/39Ar dates in the tables, including the dates used for the tephra correlations, were calculated or recalculated using 28.02 Ma for the Fish Canyon Tuff sanidine standard (Renne et al., 1998).
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The 40Ar/39Ar dates were obtained by laser-fusion analyses of multiple sanidine crystals from the same sample, and dates and uncertainties are the weighted means of multiple analyses (see Fleck et al. (1998) for details of the analytical procedures). Sample 049-21E from the Carlin basin (Table 1)
was dated using incremental-step heating on plagioclase. Tephra correlations were determined using the chemical and petrologic methods described in Perkins et al. (1998). Some of these samples produced more than one possible correlation age—some strong, some weak, and some very disparate. Because almost all tephra samples were collected either as pairs of tephra beds or as multiple samples from an unfaulted stratigraphic sequence, the correlation age that had the strongest chemical correlation and that was consistent with other dates (tephra or isotopic) in that pair or specific section was chosen. Some samples were dated using both isotopic and tephra correlation methods (cf. Wallace et al., 2007a), but most samples were dated using only one method.
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SEDIMENTARY ROCKS IN THE SNOWSTORM MOUNTAINS |
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TOPABSTRACTINTRODUCTIONREGIONAL GEOLOGIC SETTINGMIOCENE VOLCANISMGEOCHRONOLOGYSEDIMENTARY ROCKS IN THE...CHIMNEY BASINIVANHOE BASINCARLIN BASINELKO BASINPINE AND INDEPENDENCE VALLEYSREGIONAL PALEOGEOGRAPHIC...IMPLICATIONS FOR MINERAL...REFERENCES CITED |
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The older sedimentary units are most extensively exposed along Castle Ridge and at Snowstorm Mountain itself (Fig. 3; Wallace, 1993). The Castle Ridge units overlie and locally interfinger with 16.1 Ma basaltic andesite flow units and underlie 15.5–15.7 Ma rhyolite flow units (Table 1) . The units reach a thickness of 100 m and are continuous along strike for more than 15 km. They extend south into the Midas district, where they have been dated at 15.7 Ma (Table 1 ; Leavitt et al., 2004), and to the north to within 4 km of 16.1–15.5 Ma strata related to the Chimney basin.
At Snowstorm Mountain, the sedimentary units overlie and are interbedded with 16.1 Ma basaltic andesite flow units and underlie 15.5–15.7 Ma rhyolite flow units. The sedimentary units are as much as 100 m thick. Small areas of possibly related pyroclastic materials are inter-bedded with basaltic andesite flows southwest of Snowstorm Mountain. Sediments of this age are absent only in the west-southwestern part of the volcanic field, and the overall field relations suggest that the sedimentary rocks may be continuous beneath younger volcanic rocks throughout much of the field (Wallace, 1993).
Offset along predominantly north-northwest–striking normal faults occurred during volcanism in the Snowstorm Mountains. As exposed in both the Snowstorm Mountain and Castle Ridge areas, faulting offset and tilted the 15.7 Ma and older volcanic and sedimentary units prior to the eruption of 15.5 Ma and younger rhyolite flows, some of which filled a broad, fault-controlled basin between Castle Ridge and Snowstorm Mountain. The younger units are relatively unfaulted and have very modest dips, and a few ca. 15.1 Ma flows near Snowstorm Mountain contain directional flow-related folds that indicate flow westward away from the central part of the volcanic center (Wallace, 1993).
The younger sedimentary rocks on the northeastern side of the Snowstorm Mountains overlie the 15.5 Ma Little Humboldt rhyolite, and they underlie and interfinger with 9.8 Ma basalt flows of the Big Island Formation (Wallace, 1993). A broader study of the mixed sedimentary and basaltic Big Island Formation in northernmost Nevada showed that sedimentation closely preceded and coincided with the eruption of the basalt flows (Coats, 1985). On the northeastern side of the Snowstorm Mountains, the fine- to medium-grained sedimentary units are weakly to moderately consolidated. Most of the materials are air-fall tuff and reworked tuffaceous material, although some pebble-rich layers contain rhyolitic clasts. Clast imbrications and compositions, as well as cross-bedding directions, indicate that these clasts were derived from the Snowstorm Mountains area to the south. Additional post-Little Humboldt rhyolite sedimentary units are widespread throughout the Snowstorm Mountains area, but their correlation to those in the Big Island Formation can only be inferred from their general stratigraphic position.
These younger sedimentary and basaltic units in the Snowstorm Mountains area are not faulted, and they overlie faulted and tilted volcanic and sedimentary units related to the older assemblage. Although these Big Island-related units are somewhat younger than those in other basins described in this paper, they do indicate that 10 Ma streams drained northeastward from a modest highland that had been faulted by that time.
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CHIMNEY BASIN |
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TOPABSTRACTINTRODUCTIONREGIONAL GEOLOGIC SETTINGMIOCENE VOLCANISMGEOCHRONOLOGYSEDIMENTARY ROCKS IN THE...CHIMNEY BASINIVANHOE BASINCARLIN BASINELKO BASINPINE AND INDEPENDENCE VALLEYSREGIONAL PALEOGEOGRAPHIC...IMPLICATIONS FOR MINERAL...REFERENCES CITED |
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. In general, the Chimney basin was active from ca. 16.3 Ma until 14.2 Ma (Fig. 2). The basin was a broad lowland, and coeval volcanic activity blocked westward streamflow to form a shallow, ephemeral lake. The basin in its early stages extended to the southeast across the northern Nevada rift and connected with the Ivanhoe basin near Midas. Continued volcanism along the fringes of the basin limited expansion of basin sedimentation, and late-stage uplift in the Snowstorm Mountains produced fluvial sedimentation in the eastern part of the basin. Most fault activity and uplift, however, took place after sedimentation, and uplift of two major horsts just to the south did not affect the basin. Despite a temperate, moist climate, no sediments were deposited after 14.2 Ma, and the basin presumably began to drain externally at that time.
Stratigraphy
The majority of the exposed sedimentary rocks in the Chimney basin are composed of fine-grained, evenly bedded, tuffaceous sediments and somewhat coarser unwelded, pumiceous, air-fall deposits. The finer grained sedimentary layers generally are thinly bedded, whereas the coarser, more pumiceous beds are thicker and structureless (Fig. 4A); bedding is usually planar. The sediments are almost entirely air fall in origin; epiclastic materials are rare, except adjacent to the Snowstorm Mountains to the east, where clasts of Snowstorm-derived volcanic rocks are scattered through the otherwise ash-rich sediments. Evidence of fluvial reworking, such as graded bedding, cross bedding, and channel features, is also uncommon, except in the area between the Snowstorm Mountains and Chimney Reservoir. Mud cracks and siliceous sinter deposits with reed and leaf fragments are common in sedimentary rocks in the northern and western parts of the basin and indicate periodic subaerial exposure. Diagenetic zeolites and Magadi-type chert replaced many of the fine-grained, ash-rich sediments (Sheppard and Gude, 1983).
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In the eastern and northeastern parts of the basin, the ash-rich sedimentary rocks both underlie and interfinger with flow units of the Little Humboldt rhyolite sequence. Farther east, toward the Little Humboldt ranch and at Rodear Flat (Fig. 3), ash-rich strata overlie basaltic andesite flow units that, in the Midas area to the southeast, were dated at 16.1 Ma (Leavitt et al., 2004), and the strata interfinger with and underlie the 15.5 Ma Little Humboldt rhyolite. Near the Little Humboldt ranch, the earliest rhyolite flow contains a thick phreatic breccia at its basal contact with the underlying strata, and several tens of meters of additional sedimentary units overlie the brecciated flow unit; these sediments, in turn, underlie a second, unbrecciated rhyolite eruptive unit (Fig. 4B). The sedimentary units may extend for an unknown distance to the northeast beneath the upper eruptive unit, and the Rodear Flat strata likely correlate with the fine-grained sediments exposed just to the south at the north end of Castle Ridge (see the previous section on the Snowstorm Mountains). In all of these northeastern exposures, the sediments are ash rich, fine grained, and altered to zeolites and chert, similar to those in the main part of the basin.
In the east-central part of the basin, just west of the Snowstorm Mountains and south of the Little Humboldt River, a thick sequence of epiclastic sediments conformably overlies the ash-rich lacustrine sediments (Figs. 4C and 4D). The contact between the two facies is sharp, and the base of the sequence above fine-grained ash beds contains abundant volcanic cobbles. A tephra layer, 2 m beneath the contact, produced a tephra correlation age of 14.7 Ma (Table 1) . Near the Snowstorm Mountains, the epiclastic strata include alternating cobble-rich and sand-pebble beds. The coarser beds contain cross bedding, graded bedding, and channel cut-and-fill structures. Clasts in all parts of the fluvial section are composed of volcanic lithologies exposed in the Snowstorm Mountains directly to the east.
To the west, this sequence becomes finer grained and is composed of planar-bedded, structureless sandstones and thin, pebble-rich beds with weak cross bedding and graded bedding. This finer grained sequence projects west across the incised Little Humboldt River valley to the top of the sedimentary section exposed on the north shore of Chimney Reservoir. That section grades upward from thinly bedded, ash-rich bedded sediments into the epiclastic strata derived from the Snowstorm Mountains. This section, albeit finer grained and more gradational, mimics the transition from ash-rich to coarser epiclastic facies near the Snowstorm Mountains.
At the spillway for Chimney Reservoir (Fig. 3), sand- and pebble-rich clastic units overlie the 16.3 Ma rhyolite flow unit, and angular clasts derived from the rhyolite are abundant at the basal contact. Additional fluvial sediments are exposed in low hills along the Little Humboldt River downstream from the reservoir. All of these deposits are isolated from other fluvial deposits; they could be related to the Miocene epiclastic units described above or are much younger deposits related to the late Miocene to Pliocene development of the Little Humboldt River.
Miocene units are poorly to not exposed south of the Little Humboldt River. This area includes the Hot Springs Range, the Osgood Mountains, and the intervening Eden and Kelly Creek Valleys. Strata exposed between the Dry Hills (Fig. 3) and the Little Humboldt River are composed of fine-grained, poorly exposed sediments. Both epiclastic and ash-rich sedimentary units are exposed near the Snowstorm Mountains in the southeastern part of the basin, but the section thins and laps onto Paleozoic basement to the south. At the Twin Creeks mine (Fig. 3), Miocene units above the Paleozoic basement include basal colluvial and regolith deposits and overlying epiclastic sand and gravel deposits derived from the nearby Dry Hills; a 14.2 Ma, air-fall ash is interbedded with these units (Breit et al., 2005). Ash-rich basin strata extend a few kilometers south into Eden Valley between the Osgood Mountains and Hot Springs Range, but most of that area has an extensive cover of Pliocene and younger alluvial sediments that masks the southern extent of the Miocene units.
Faulting
Numerous small, normal faults offset the Miocene strata throughout the northern half of the Chimney basin. Alluvial cover and very poor exposures conceal any possible faults in the southern part of the basin. Dips on all Miocene sedimentary units generally are less than 15°, and many units are nearly horizontal. The basal sediments dip as gently as strata higher in the section, indicating little or no synsedimentary, fault-related tilting.
Faults strike predominantly to the north to north-northeast, although faults of all orientations are present in the basin. Offsets range from a few tens of meters to 100 m. Several down-to-the-east normal faults strike northerly through the middle of the basin, including the Chimney Reservoir area. The most obvious of these faults forms the east side of the Dry Hills (Fig. 3), extends north to near the dam at Chimney Reservoir, and continues to the north. Juxtaposed pre-sedimentation volcanic units and the basal parts of the Miocene section indicate offset of only 100 m. Along the southern projection of this fault at the Twin Creeks mine, a scarp along the fault appears to have controlled the deposition of Pliocene alluvial deposits (Breit et al., 2005). A series of both down-to-the-east and down-to-the west normal faults parallel the west side of the Snowstorm Mountains, but displacement along each fault was less than 100 m.
Along the west side of the basin, east of Paradise Valley (Fig. 3), down-to-the-west normal faults with individual offsets of less than 100 m offset the basin strata and underlying Miocene volcanic rocks and tilted them modestly to the east. At Martin Creek, a series of west-dipping normal faults tilted 16.1 Ma volcanic units 20°, but produced only minor offset of overlying, nearly horizontal 5 Ma basalt flows. To the south, these faults tilted the Hot Springs Range to the east. To the west, the bulk of the uplift of the Santa Rosa Range and formation of the east-dipping Paradise Valley half graben took place between ca. 10 Ma and 5 Ma (Colgan et al., 2004). The Martin Creek faults continue northwest and bisect the Santa Rosa Range, and the southern part of the range is now structurally lower than the northern half.
The Osgood Mountains are composed of two, west-dipping blocks. The largest is the main north-northeast–trending, west-tilted range, with a major range-front fault along its east side (Fig. 3). The north-striking Getchell normal fault truncates the northeast end of this block (Hotz and Willden, 1964). This fault is not evident in the main part of the Chimney basin to the north. Early Miocene (ca. 22 Ma) andesite flow units on the west side of the range likely were deposited at a very low angle, now dip 20° to the west, and project east to just above the crest of the range (Hotz and Willden, 1964). As such, the range crest was just below the early Miocene paleosurface, and the Osgood Mountains were not a highland at 22 Ma. The second block is the Dry Hills, which is bounded by the Getchell fault to the west and a relatively minor, north-striking fault to the east. This fault, as noted above, does cut the Miocene strata to the north. West-dipping, 22 Ma andesite flows (Wallace and McKee, 1994), identical to those along the west side of the Osgood Mountains, are present in the Dry Hills and may be the downfaulted remnants of a broader volcanic field.
Eden Valley separates the northeast-trending Osgood Mountains and the north-trending Hot Springs Range, which converge to the south. East-dipping, early Miocene andesite flows on the east side of the Hot Springs Range mirror those along the west side of the Osgood Mountains (Jones, 1997). Gravity data in Eden Valley indicate that the thickness of Pliocene and Miocene units above Paleozoic bedrock is generally less than 1 km, consistent with the valley being a shallow synform between the two opposite-tilted ranges. This synform is narrow at the south end of the ranges, and it broadens to the north as the ranges diverge. North of the Little Humboldt River (north of the limits of the ranges), all Miocene strata dip very gently or are flat lying, and abundant small normal faults with widely varying strikes obscure the synform, if present. Farther north, west of Whiskey Springs (Fig. 3), any semblance of a synform in 10 Ma and older volcanic units is absent. As with the Osgood Mountains, the Eden Valley synform appears to die out into and is not a noticeable structural element in the Chimney basin.
Paleogeography and Sedimentation
Deposition of ash-rich, waterlain sediments in the Chimney basin began sometime after 16.3 Ma and was actively taking place from 16.1 Ma to ca. 14.7 Ma. Deposition of sands and coarser materials in the southeastern part of the basin occurred from 14.7 to 14.2 Ma. At limits of the exposed basin sediments, the sedimentary units interfinger with and pinch out against coeval volcanic flow units along the western, northwestern, and northern margins of the basin, and the volcanic activity likely limited the extent of the basin in those areas. The early sedimentary environment may have extended to the northeast, but eruption of the 15.5 Ma Little Humboldt rhyolite created a new, more proximal basin margin in that direction. In the eastern part of the basin and in all Snowstorm Mountain exposures, sediments were deposited on 16.1 Ma basaltic andesite flow units until ca. 15.7 Ma.
With the exception of the late epiclastic sediments in the southeastern part of the basin, the generally planar, laterally continuous bedding in the ash-rich units indicates deposition in a low-energy, lacustrine environment, and mud cracks and sinter deposits point to episodic subaerial exposure. The general absence of epiclastic material or evidence of fluvial reworking until late in the basin history suggests that the highlands that constrained the margins of this lacustrine environment had low relief. The shallow lacustrine connection of the Chimney and Ivanhoe basins across the northern Nevada rift in the Snowstorm Mountains demonstrates that rift-related volcanic and fault activity did not create any substantial positive or negative relief.
The lacustrine environment was shallow, became ephemeral, and eventually transitioned into a fluvial environment. The sinter deposits and mud cracks indicate periodic subaerial exposure during the lacustrine stage, and early, but not later, flows of the Little Humboldt rhyolite were erupted onto wet or submerged sediments. The upsection increase in diagenetic minerals indicative of more saline and alkaline conditions may reflect a progressively more evaporative lacustrine environment (Sheppard and Gude, 1983; R.A. Sheppard, 1992, personal commun.), and the transition from lacustrine to fluvial environments at and east of Chimney Reservoir point to possible external drainage starting at ca. 14.7 Ma.
The evolution of the western part of the Snowstorm Mountains volcanic field significantly affected the eastern margin of the basin. At Snowstorm Mountain (Fig. 3), the eruption of 15.7 Ma rhyolite flows ended sedimentation in that area, and post–15.5 Ma, pre–15.1 Ma faulting tilted the older units to the west (Wallace, 1993). However, this faulting did not generate epiclastic sediments, which did not appear in the basin until ca. 14.7 Ma; the sudden influx of coarse epiclastic sediments at that time indicates some uplift in the Snowstorm Mountain area, although ca. 15.1 Ma flow units at Snowstorm Mountain dip only 5° to the west. Faults along the west side of the range may have been active at this time, although there is no direct evidence of such activity. Farther south, at the Twin Creeks mine, sediments were shed from the Dry Hills at ca. 14.2 Ma (Breit et al., 2005), but the relation of fluvial sedimentation there and that west of Snowstorm Mountain is unknown due to the lack of intervening exposures.
On the basis of the southward thinning of the basin strata onto Paleozoic rocks, the southern margin of the basin at the time of sedimentation was most likely a low Paleozoic-cored upland or bedrock sill. Overall, the available stratigraphic and structural data indicate that the Osgood Mountains and Hot Springs Range formed after Miocene sedimentation but did not significantly affect the Chimney basin, despite its proximity and the projection of major structures toward the basin. Uplift possibly started near the end of sedimentation, producing 14.4 Ma supergene alunite at the Twin Creeks mine (Arehart and O'Neil, 1993) and the nearby 14.2 Ma clastic sediments. Alternatively, the major uplift took place between ca. 10 and 5 Ma, based upon evidence of that uplift age in the Santa Rosa Range and the tilting constraints near Martin Creek. Sedimentary and fault-scarp data (Breit et al., 2005; Wesnousky et al., 2005) indicate that uplift continued into the Pliocene and Quaternary.
Why and when sedimentation ended in the Chimney basin are unclear. The climate was moist and temperate, the depositional environment was becoming increasingly subaerial and fluvial in the eastern part of the basin, and major rhyolite eruptions were significantly disrupting the northern and eastern parts of the basin. The absence of younger sediments argues against inflowing streams, which would have carried sediments, simply evaporating when they reached the basin. This absence of younger sediments in this topographically low area, despite the presence of uplands to the east, strongly suggests that the confining volcanic or topographic dams along the southwestern part of the basin were breached and the basin began to drain externally. Unfortunately, uplift of the Osgood Mountains and Hot Springs Range and Pliocene and younger alluvial sedimentation in Eden and Kelly Creek Valleys have largely obliterated the middle Miocene record in those areas.
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TOPABSTRACTINTRODUCTIONREGIONAL GEOLOGIC SETTINGMIOCENE VOLCANISMGEOCHRONOLOGYSEDIMENTARY ROCKS IN THE...CHIMNEY BASINIVANHOE BASINCARLIN BASINELKO BASINPINE AND INDEPENDENCE VALLEYSREGIONAL PALEOGEOGRAPHIC...IMPLICATIONS FOR MINERAL...REFERENCES CITED |
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Stratigraphy
The oldest sediments in the basin were deposited shortly before 16.1 Ma in the Willow Creek Reservoir area (Fig. 5; Table 1; Perkins et al., 1998; Wallace, 2003c). Sedimentation in that area continued until after 15.4 Ma, depositing more than 200 m of sediments above late Eocene and Paleozoic units. The basal Miocene sediments are composed of relatively minor epiclastic sand and silt and moderate amounts of reworked ash. The majority of the overlying units are composed of thinly planar-bedded, air-fall ash and pumice with local to pronounced soft-sediment deformation textures. One of these ash beds was dated at 16.1 Ma (Table 1; Wallace, 2003a). Thin conglomerate beds with clasts derived from the Tuscarora Mountains to the northeast locally are interbedded with the ash-rich units (Henry and Boden, 1999; Wallace, 2003a), and some beds are a mixture of reworked small pumice and epiclastic sand, indicating periodic influxes of non-ash material. The ash-rich section grades up into thin-bedded mudstones, thick-bedded sandstone, and a sub-aerial, 15.4 Ma vitric tuff that is a widespread marker unit throughout the northern part of the basin. Very thin-bedded, air-fall ash beds with locally pronounced, soft-sediment deformation, as well as some fine-grained epiclastic sediments, were deposited above the vitric tuff. Faulting truncated the top of the section at Willow Creek Reservoir, but a poorly exposed, thick section of fine-grained, ash-rich sediments overlies the vitric tuff southeast of the reservoir (Wallace, 2003a).
The Willow Creek Reservoir section extends discontinuously 15 km north and northeast, where it overlies both Eocene volcanic rocks and Paleozoic rocks. Closer to the Tuscarora Mountains (Fig. 5), the lower part of the section thins to 20 m above Eocene volcanic rocks and underlies the 15.4 Ma vitric tuff, which is the youngest preserved unit in that area (Henry and Boden, 1999).
Coeval volcanic activity produced rhyolite flow units and domes west of Willow Creek Reservoir. These include the Rock Creek rhyolite in the central part of the Ivanhoe basin and the June Bell rhyolite in the Midas area (Fig. 5). Neither rhyolite has been dated, but surface exposures and drilling data indicate that they are inter-bedded with or underlie the oldest sedimentary units in both areas and thus were erupted just before or during early sedimentation (Wallace, 1993; Goldstrand and Schmidt, 2000; Wallace, 2003c). The Rock Creek rhyolite is exposed over a large area and, based on exposed thicknesses, likely had relief on the order of several hundred meters, greater than the thickness of the sediments to the east.
The lower part of the Willow Creek Reservoir section thins to the south into the Ivanhoe mining district (Figs. 5, 6A, and 6B), where only 50 m of thin-bedded, ash-rich strata lie between the widespread 15.4 Ma vitric tuff and the Paleozoic basement rocks. Units above the vitric tuff in this area include thin- to thick-bedded, air-fall ash and epiclastic sand beds; locally abundant pebble conglomerate and debris-flow units contain clasts derived from Paleozoic rocks that cropped out a few kilometers to the east (Wallace, 2003b).
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The volcanic flow units in the district were emplaced subaerially, although the distal ends of a few andesite flow units have hyaloclastic breccias. Sinter deposits are common throughout most of the pre– and post–15.4 Ma sedimentary units (Fig. 6C; Wallace, 2003c), and they also indicate periodic, subaerial exposure during sedimentation. The locus of sinter activity shifted to the east with time, reflecting changes in the ground-water table and paleotopography produced by progressive volcanic activity, sedimentation, and minor late synsedimentary faulting (Wallace, 2003c).
In the Santa Renia Fields area east and southeast of Ivanhoe (Fig. 5), the oldest sediments were deposited shortly before 15.4 Ma, and sedimentation continued to after 14.7 Ma (Fleck et al., 1998). The sedimentary units are composed of a mixture of thin-bedded, air-fall ash and pumice deposits and fine-grained, epiclastic sediments derived from the Tuscarora Mountains to the east, including materials eroded from exposed gold deposits along the Carlin trend (Theodore et al., 1998, 2006). Air-fall materials are common in the lower half of the section,
