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The isotopic composition of ore lead of the Creede mining district and vicinity, San Juan Mountains, Colorado: Text of a talk presented at the San Juan Mountains symposium to honor Thomas A. Steven; Rocky Mountain Section meeting of the Geological Society of America, May 2, 1987, Boulder, Colorado
Galenas from the major Creede veins and their northern extensions are remarkably homogeneous in Pb-isotopic composition and are too radiogenic to have been derived from any magma comparable in composition to the principal volcanic rocks. This pattern was identified by Doe et al. in 1979 who proposed that the lead was derived from the Precambrian basement. The homogeneity of the ore leads, however, requires a uniform reservoir; an unlikely prospect for lead from the Precambrian basement. We report on 16 new analyses of geographically and paragenetically dispersed galenas from the Creede district and other areas as far as 11 km to the north. The lead values range from 18.972 to 19.060 for 206 Pb/ 204 Pb, from 15.591 to 15.671 for 207 Pb/ 204 Pb, and from 37.781 to 37.921 for 208 Pb/ 204 Pb. These ranges overlap those previously reported for the main ore zone. Recent work allows us to extend the results of Doe et al. and to consider alternative processes to explain the widespread homogeneity and radiogenic nature of the ore lead: 1) David Matty (pers. commun., 1986) has shown that some minor volcanic units in the area have unusually radiogneic lead values; magmas comparable in composition to the units are a possible, though improbable, source of the ore lead. 2) The uniformity of the isotopic values of galenas may have resulted from homogenization during an extensive potassium-metasomatic event that predated the ores; this possibility is being tested in an on-going study of feldspars from metasomatized and unmetasomatized rocks. 3) Recent regional studies suggest the possibility of a prevolcanic, NNW-trending graben system filled by clastic sediments derived from the Precambrian basement, a process that would have an homogenizing effect on the lead isotopes. This interpretation implies importation, deep within the Creede hydrologic system, of fluids from remote sources. These alternatives show that the Pbisotope systematics may have a profound impact on the interpretation of the Creede hydrothermal system, and that further study is warranted.
Igneous rocks in the Fish Creek Mountains and environs, Battle Mountain area, north-central Nevada: A microcosm of Cenozoic igneous activity in the northern Great Basin, Basin and Range Province, USA
The Great Basin of the western United States, the northern component of the Basin and Range Province, is a region of Cenozoic lithospheric extension with multiple periods and types of igneous activity. The composition and volume of Cenozoic magmas reflect a complex interaction between mantle-derived magmas and highly diverse crust, where both mantle sources and magmatic processes were modulated by tectonic environment. The Fish Creek Mountains in north-central Nevada underwent multiple igneous events ranging from ca. 40 Ma to 1 Ma that span all of the complex Cenozoic tectono-magmatic episodes of the Great Basin. The Fish Creek Mountains, therefore, is an ideal location to evaluate the different sources and processes involved in magma generation. Many plutons were emplaced in the region between about 40 and 38 Ma, several of which host base and precious metal deposits. Between 36 and 33 Ma, lava fields and calderas of the 37–19 Ma Ignimbrite Flare-up were emplaced. Both these and the preceding plutons resulted from southwestward rollback of the Farallon plate beneath North America during by far the most voluminous phase of Cenozoic magmatism. The lavas range from rare basalt and basaltic andesite to andesite, dacite, and rhyolite, have continental arc-like incompatible element patterns, and high initial 87 Sr/ 86 Sr and low εNd that require a metasomatized lithospheric mantle source combined with minor crustal component. Ignimbrites of the 34.4 Ma Cove Mine (trachydacite to rhyolite) and 34.0 Ma Caetano calderas (rhyolite to high-silica rhyolite) are abundantly porphyritic, include hydrous phases, were largely derived from partial melts of crustal rocks, but likely include 20–30% of a mantle-derived component. Igneous activity ceased in the region as the rollback-arc migrated to the southwest, but at 24.9 Ma a new caldera formed in the southern Fish Creek Mountains that was filled by ignimbrites of the Fish Creek Mountains Tuff. Intracaldera rhyolite ignimbrites range from aphyric, pumice-rich deposits at the base to progressively more quartz-feldspar phyric ignimbrites at higher levels; all flow units lack hydrous phases. No contemporaneous mafic or intermediate igneous activity accompanied caldera formation, but initial 87 Sr/ 86 Sr values in the Fish Creek Mountains tuffs are lower than in the Caetano Tuff, suggesting a greater mantle contribution to the 24.9 Ma ignimbrites. After another hiatus in igneous activity, the region was intruded and overlain by basalt to rhyolite dykes and lavas of the northern Nevada rift between 16.8 and 15.1 Ma. The primarily tholeiitic igneous suite is of the same age, chemistry, and isotopic composition as the Grande Ronde Formation of the Columbia River flood basalts, and evolved members (trachydacite and rhyolite) are crustally contaminated. The youngest northern Nevada rift lava is an alkali olivine basalt with isotopic affinity to basalts of the eastern Snake River Plain. After 10 Ma of quiescence, the region was locally covered by mafic lava flows with high-alumina olivine tholeiite compositions, represented by the 5.4 Ma Pumpernickel Valley flows. Their mid-ocean ridge-like incompatible element compositions indicate a depleted mantle source for the lavas, but radiogenic isotopic compositions indicate that the lavas of this region include a significant contribution from a mafic to ultramafic, high- 87 Sr/ 86 Sr source. The final igneous event in the Fish Creek Mountains region, the 4.0 to 1.0 Ma Buffalo Valley volcanic field, includes flows and spatter cones of transitional to alkalic basalt that are divided into two geochemical groups with identical isotopic compositions. They represent variable, low percent partial melts of the asthenosphere at different depths, yielding different rare earth element characteristics. Similar to the Lunar Crater volcanic field, the Buffalo Valley rocks represent a rare case where the lithosphere in the central Great Basin is now thin enough to allow melting of the underlying asthenosphere. Cenozoic magmatism in the northern Great Basin exhibits several transitions in magma sources and tectonic setting with time. Magmatism began as pre-extension, subduction-related, primarily lithospherically-derived magmas emplaced on/in tectonically-thickened crust. The onset of extension was partially driven by impingement of the Yellowstone plume that resulted in emplacement of rift-related volcanic and intrusive rocks in the northern Nevada rift, followed by the eruption of extension-related HAOT lavas along the northwest margin of the Great Basin. Finally, lithospheric thinning allowed for partial melting of the asthenosphere and eruption of alkaline basaltic lavas.
Elk response to the La Mesa fire and current status in the Jemez Mountains
Faunal remains in local archeological sites and historic information suggest that elk populations in the Jemez Mountains were low from ca. 1200 A.D. through ca. 1900 A.D., when they were extirpated from this region. Elk were reintroduced to the Jemez country in 1948 and 1964- 1965, and their population apparently grew exponentially, reaching 1000 animals in the 1970's and about 7000 by 1991. Elk populations in Bandelier National Monument and adjoining areas increased rapidly after the 1977 La Mesa Fire. Winter use by elk in the La Mesa Fire area, centered on Bandelier, grew from about 100 animals in .1978 to around 1500 elk by 1992. The dramatic increase in the Bandelier elk herd (an annual growth rate of 21.3% and a 3.6 year population doubling time) was due in part to the creation of about 6000 hectares of grassy winter range in and around the park by the La Mesa Fire. Some of this local population increase reflects concentration of elk into this favorable wintering habitat from surrounding portions of the Jemez Mountains. Existing data are inadequate to determine whether elk populations are still growing rapidly in the Jemez Mountains. While annual aerial surveys since 1990 in Bandelier reveal no clear population trend, a variety of observations demonstrate increasing elk use of lower elevation areas. Negative resource impacts from today's high elk populations are beginning to be widely noted across the Jemez Mountains, especially in high-use portions of the Bandelier National Monument area. Affected resources range from plant communities to soils and even archeological sites. Given the large uncertainties associated with the current data on elk populations, care should be taken to avoid further population increases until the resource impacts of this new phenomenon (large numbers of elk) can be identified, desirable population levels identified (based to a significant degree upon ecological information and resource carrying capacities, as well as social considerations), and appropriate cooperative management strategies implemented.
Regional metamorphism in the Condrey Mountain Quadrangle, north-central Klamath Mountains, California
A subcircular area of about 650 km 2 in northern California and southwestern Oregon is occupied by rocks of the greenschist metamorphic facies called the Condrey Mountain Schist. This greenschist terrane is bordered on the east and west by rocks belonging to the amphibolite metamorphic facies that structurally overlie and are thrust over the Condrey Mountain Schist. The amphibolite facies is succeeded upward by metavolcanic and metasedimentary rocks belonging to the greenschist metamorphic facies. The Condrey Mountain Schist is composed predominantly of quartz-muscovite schist and lesser amounts of actinolite-chlorite schist formed by the metamorphism of graywacke and spilitic volcanic rocks that may have belonged to the Galice Formation of Late Jurassic age. Potassium-argon age determinations of 141?4 m.y. and 155?5 m.y. obtained on these metamorphic rocks seem to be incompatible with the Late Jurassic age usually assigned the Galice. The rocks that border the amphibolite facies are part of an extensive terrane of metavolcanic and metasedimentary rocks belonging to the western Paleozoic and Triassic belt. The metavolcanic rocks include some unmetamorphosed spilite but are mostly of the greenschist metamorphic facies composed of oligoclase (An15-20) and actinolite with subordinate amounts of chlorite and clinozoisiteepidote. The interbedded sedimentary rocks are predominantly argillite and slaty argillite, less commonly siliceous argillite and chert, and a few lenticular beds of marble. On the south, high-angle faults and a tabular granitic pluton separate the greenschist metavolcanic terrane from the amphibolite facies rocks; on the east, nonfoliated amphibolite is succeeded upward, apparently conformably, by metasedimentary rocks belonging to the greenschist metavolcanic terrane. In the southern part of Condrey Mountain quadrangle, an outlier of a thrust plate composed of the Stuart Fork Formation overlies the metavolcanic and metasedimentary rocks. The Stuart Fork in this region is composed of siliceous phyllite and phyllitic quartzite and is believed to be the metamorphosed equivalent of rocks over which it is thrust. In the Yreka-Fort Jones area, potassium-argon determinations on mica from the blueschist facies in the Stuart Fork gave ages of approximately 220 m.y. (Late Triassic) for the age of metamorphism. Rocks of the amphibolite facies structurally overlie the Condrey Mountain Schist along a moderate to steeply dipping thrust fault. The amphibolite terrane is composed of amphibolite and metasedimentary rocks in approximately equal amounts accompanied by many bodies of serpentinite and a number of gabbro and dioritic plutons. Most of the amphibolite is foliated, but some is nonfoliated; the nonfoliated amphibolite has an amphibolite mineralogy and commonly a relict volcanic rock texture. The nonfoliated amphibolite occurs on the southern and eastern borders of the amphibolite terrane between the areas offoliated amphibolite and the overly ing metavolcanic and metasedimentary rocks. Hornblende and plagioclase (An30-35) are the characteristic minerals, indicating that the rocks are of the almandine-amphibolite metamorphic facies. The metasedimentary rocks interbedded with the amphibolites include siliceous schist and phyllite, minor quartzite, and subordinate amounts of marble. Potassium-argon age dates obtained on hornblende from foliated amphibolite yield ages of 146?4 and 148? 4 m.y., suggesting a Late Jurassic metamorphic episode. Mafic and ultramafic rocks are widespread in the amphibolite terrane but are almost entirely absent from the area of greenschist facies metavolcanic and metasedimentary rocks. The ultramafic rocks, predominantly serpentinite, occur as a few large bodies and many small tabular concordant bodies interleaved with the foliated rocks. The ultramafic rocks include harzburgite and d1lIlite and their serpentinized equivalents. In the Condrey Mountain quadrangle, probably more t
Monitoring mountains in a changing world: New horizons for the Global Network for Observations and Information on Mountain Environments (GEO-GNOME)
Mountains are globally distributed environments that provide significant societal benefits, a function that is increasingly compromised by climatic change, environmental stress, political and socioeconomic transformations, and unsustainable use of natural resources. Gaps in our understanding of these processes and their interactions limit our capacity to inform decisions, where both generalities of mountain regions (eg climate processes) and specificities (eg context-specific manifestations of climate risks) matter. The Global Network for Observations and Information on Mountain Environments (GEO-GNOME), a Group on Earth Observations initiative, aims to fill these gaps through accessible Earth Observation (EO) as well as in-situ data and information on global change drivers, conditions, and trends. A workshop convened by the Mountain Research Initiative (MRI) revised GEO-GNOME's work plan, galvanizing a network that promotes relevant monitoring of global change in mountains and is responsive to the integrated knowledge needs of policy, research, and management.
Bryozoa of the Murdock Mountain formation (Wordian, Permian), Leach Mountains, northeastern Nevada
A thin limestone tongue in the upper part of the Murdock Mountain Formation of northeastern Nevada contains abundant bryozoans of earliest Wordian age. This bryozoan fauna is close to the Kungurian-Kazanian boundary in Russia. These bryozoans are younger than those found in the Kaibab Formation of southern Nevada and slightly older than those in the Gerster Limestone of northern Nevada. This limestone tongue, herein referred to as the Stenodiscus beds, lies below the Thamnosia beds described by Wardlaw in the Murdock Mountain section of the Leach Mountains. This bryozoan-rich limestone tongue serves as a marker unit for mapping the Murdock Formation in the Leach Mountains. Nine new species of bryozoans occur in the Stenodiscus beds of the Murdock Mountain Formation: Hinganella felderi, Neoeridotrypella schilti, Stenopora parvaexozona, Stenodiscus murdockensis, Dyscritella acanthostylia, Pseudobatostomella irregularis, Streblotrypa ( Streblotrypa ) elongata, Morozoviella praecurriensis, and Thamniscus erraticus, Hinganella felderi, Neoeridotrypella schilti, Dyscritella acanthostylia, and Streblotrypa ( Streblotrypa ) elongata are very similar to species described from Russia.
Age of volcanism, intrusion, and mineralization in the Thomas Range, Keg Mountain, and Desert Mountain, western Utah
Twenty-six new age determinations by the fission-track method establish a chronology for volcanism, intrusion, and mineralization in the Thomas Range, Keg Mountain, and Desert Mountain, in western Utah. The fission-track ages confirm D. R. Shawe's three-fold classification of igneous rocks. The oldest group of rocks consists of flows, agglomerates, and some ash-flow tuffs that were deposited 38-39 m.y. ago. The middle group contains widespread rhyolitic ash-flow tuffs that originated from local volcanic centers 30-32 m.y. ago. A dike emplaced along the probable ring-fracture zone of the Keg caldera about 31 m.y. ago indicates that caldera collapse occurred soon after eruption of ash-flow tuffs. Rocks believed to belong-to the middle group also were intruded by the quartz monzonite of Desert Mountain 27-30 m.y. ago. Little or no igneous activity took place within the next 20 m.y., during which time the region was strongly broken by Basin-and-Range faulting. Rhyolites of the youngest group were extruded 8-10 m.y. ago in the Keg Mountain area and 6-7 m.y. ago in the Thomas Range. The beryllium-fluorite mineralization at Spor Mountain occurred after the rhyolitic volcanism in the Thomas Range.
Chemistry of unsaturated zone gases sampled in open boreholes at the crest of Yucca Mountain, Nevada: Data and basic concepts of chemical and physical processes in the mountain
Boreholes open to the unsaturated zone at the crest of Yucca Mountain, Nevada, were variously sampled for CO 2 (including 13 C and 14 C), CH 4 , N 2 , O 2 , Ar, CFC-11, CFC-12, and CFC-113 from 1986 to 1993. Air enters the mountain in outcrops, principally on the eastern slope, is enriched in CO 2 by mixing with soil gas, and is advected to the mountain crest, where it returns to the atmosphere. The CFC data indicate that travel times of the advecting gas in the shallow Tiva Canyon hydrogeologic unit are ≤5 years. The 14 C activities are postbomb to depths of 100 m, indicating little retardation of 14 CO 2 in the shallow flow systems. The 14 C activities from 168 to 404 m in the Topopah Spring hydrogeologic unit are 85–90 pMC at borehole USW-UZ6. The CFC data show that the drilling of USW-UZ6 in 1984 has altered the natural system by providing a conduit through the Paintbrush Nonwelded unit, allowing flow from Topopah Spring outcrops in Solitario Canyon on the west to USW-UZ6, upward in the borehole through the Paintbrush, to the shallow Tiva Canyon flow systems, and out of the mountain.
Nonmarine facies in the Late Triassic(?) to Early Jurassic Horn Mountain Tuff member of the Talkeetna Formation, Horn Mountain, lower Cook Inlet basin, Alaska
The Talkeetna Formation is a prominent lithostratigraphic unit in south-central Alaska. In the Iniskin–Tuxedni area, Detterman and Hartsock (1966) divided the formation into three mappable units including, from oldest to youngest, the Marsh Creek Breccia, the Portage Creek Agglomerate, and the Horn Mountain Tuff Members. The Horn Mountain Tuff Member was thought to include rocks deposited in a nonmarine setting based on the presence of “tree stumps in an upright position” (Detterman and Hartsock, 1966, p. 19) near the top of the type section at Horn Mountain. Bull (2015) recognized possible nonmarine volcaniclastic rocks in the member during the 2014 field season in a saddle on the north side of Horn Mountain (figs. 2-1 and 2-2). The authors visited this location in 2015 and measured a short stratigraphic section to document facies, interpret depositional setting, and constrain age. This report summarizes our field observations and presents preliminary interpretations.
Erosion of an ancient mountain range, the Great Smoky Mountains, North Carolina and Tennessee
Analysis of 10 Be and 26 Al in bedrock (n=10), colluvium (n=5 including grain size splits), and alluvial sediments (n=59 including grain size splits), coupled with field observations and GIS analysis, suggest that erosion rates in the Great Smoky Mountains are controlled by subsurface bedrock erosion and diffusive slope processes. The results indicate rapid alluvial transport, minimal alluvial storage, and suggest that most of the cosmogenic nuclide inventory in sediments is accumulated while they are eroding from bedrock and traveling down hill slopes. Spatially homogeneous erosion rates of 25 - 30 mm Ky −1 are calculated throughout the Great Smoky Mountains using measured concentrations of cosmogenic 10 Be and 26 Al in quartz separated from alluvial sediment. 10 Be and 26 Al concentrations in sediments collected from headwater tributaries that have no upstream samples (n=18) are consistent with an average erosion rate of 28 ± 8 mm Ky −1 , similar to that of the outlet rivers (n=16, 24 ± 6 mm Ky −1 ), which carry most of the sediment out of the mountain range. Grain-size-specific analysis of 6 alluvial sediment samples shows higher nuclide concentrations in smaller grain sizes than in larger ones. The difference in concentrations arises from the large elevation distribution of the source of the smaller grains compared with the narrow and relatively low source elevation of the large grains. Large sandstone clasts disaggregate into sand-size grains rapidly during weathering and downslope transport; thus, only clasts from the lower parts of slopes reach the streams. 26 Al/ 10 Be ratios do not suggest significant burial periods for our samples. However, alluvial samples have lower 26 Al/ 10 Be ratios than bedrock and colluvial samples, a trend consistent with a longer integrated cosmic ray exposure history that includes periods of burial during down-slope transport. The results confirm some of the basic ideas embedded in Davis’ geographic cycle model, such as the reduction of relief through slope processes, and of Hack’s dynamic equilibrium model such as the similarity of erosion rates across different lithologies. Comparing cosmogenic nuclide data with other measured and calculated erosion rates for the Appalachians, we conclude that rates of erosion, integrated over varying time periods from decades to a hundred million years are similar, the result of equilibrium between erosion and isostatic uplift in the southern Appalachian Mountains.
Geochemical map of the Pond Mountain and Pond Mountain Addition Roadless Areas, Carter County, Tennessee
In the spring of 1980, K. A. Duttweiler, J. W. Whitlow, and W. R. Griffitts of the U.S. Geological Survey conducted a reconnaissance geochemical survey of the Pond Mountain and Pond Mountain Addition Roadless Areas. The purpose of the study was to determine the geochemical nature of the study area as part of the mineral resource assessment. This report presents a brief summary of the geochemistry of the study area. The evaluation of the mineral resource potential is covered in a separate report (Griffitts, Bitar, and Chatman, in press). The Pond Mountain and Pond Mountain Addition Road less Areas encompass 6,668 acres in the Cherokee National Forest in northeastern Tennessee. They are approximately 7 mi southeast of Elizabethton, and 1 mi east of Hampton, Tenn. (fig. 1). The study area is fairly accessible by U.S Highway 321 north of the area and Forest Service Route 50 along the southern boundary. Relief is moderate, but the vegetation is very thick in most places.
Surficial geologic map of parts of the Misheguk Mountain and Baird Mountains quadrangles, Noatak National Preserve, Alaska
The map area, which comprises part of the Noatak National Preserve, includes approximately the southern two-thirds of the Misheguk Mountain quadrangle and the northern one-third of the Baird Mountains quadrangle. It is centered on a belt of west-trending lowlands along the Noatak River which separates the De Long Mountains to the north from the Baird Mountains to the south (Burch, 1990, p. 196-201). The map area extends between the drainage divides which bound the Noatak drainage system to the north and south, separating that network from streams that flow north into the Arctic Ocean and south into the Kobuk River. An additional small segment in the southwest corner of the map area covers the upper drainage basin of Eli River, which flows west and then south to intersect the Noatak River about 50 km upvalley from Kotzebue Sound.
Morphotectonic architecture of the Transantarctic Mountains rift flank between the Royal Society Range and the Churchill Mountains based on geomorphic analysis
Extensional forces within the Antarctic Plate have produced the Transantarctic Mountains rift-flank uplift along the West Antarctic rift margin. Large-scale linear morphologic features within the mountains are controlled by bedrock structure and can be recognized and mapped from satellite imagery and digital elevation models (DEMs). This study employed the Antarctic Digital Database DEM to obtain slope steepness and aspect maps of the Transantarctic Mountains (TAM) between the Royal Society Range and the Churchill Mountains, allowing definition of the position and orientation of the morphological axis of the rift-flank. The TAM axis, interpreted as a fault-controlled escarpment formed by coast-parallel retreat, provides a marker for the orientation of the faulted boundary between the TAM and the rift system. Changes in position and orientation of the TAM axis suggests the rift flank is segmented into tectonic blocks bounded by relay ramps and transverse accommodation zones. The transverse boundaries coincide with major outlet glaciers, supporting interpretation of rift structures between them. The pronounced morphological change across Byrd Glacier points to control by structures inherited from the Ross orogen.
Surficial geologic map of the Spirit Mountain SE and part of the Spirit Mountain NE 7.5' quadrangles, Nevada and Arizona
This geologic map includes a trove of stratigraphic and geomorphic information that chronicles the inception and evolution of the lower Colorado River. The map area is located near the south end of the Lake Mead National Recreation Area about 80 km (50 mi) downstream from Hoover Dam. It spans parts of northwestern Arizona and southern Nevada near the south end of Cottonwood Valley. The map includes the Spirit Mountain SE 7.5' quadrangle and the southern part of the Spirit Mountain NE 7.5' quadrangle. The map area contains well-exposed Neogene and Quaternary strata and associated geomorphic features that record and are critical in dating the arrival of the Colorado River in the early Pliocene and the subsequent history of the river and its landscape through the Holocene. The valley is bounded on the west by the Newberry Mountains (Nevada) and on the east by the Black Mountains (Arizona) and includes part of Lake Mohave, a reservoir created by the completion of Davis Dam in 1951. This map does not include the geology of the reservoir floor and focuses only on surficial deposits.
Cold tolerance of mountain stoneflies (Plecoptera: Nemouridae) from the high Rocky Mountains
How aquatic insects cope with cold temperatures is poorly understood. This is particularly true for high-elevation species, which often experience a seasonal risk of freezing. In the Rocky Mountains, nemourid stoneflies (Plecoptera: Nemouridae) are a major component of mountain stream biodiversity and are typically found in streams fed by glaciers and snowfields, which are rapidly receding due to climate change. Predicting the effects of climate change on mountain stoneflies is difficult because their thermal physiology is largely unknown. We investigated cold tolerance of several alpine stoneflies ( Lednia tumana , Lednia tetonica , and Zapada spp.) from the Rocky Mountains, USA. We measured the supercooling point (SCP) and tolerance to ice enclosure of late-instar nymphs collected from a range of thermal regimes. SCPs varied among species and populations, with the lowest SCP measured for nymphs from an alpine pond, which was much more likely to freeze solid in winter than flowing streams. We also show that L. tumana cannot survive being enclosed in ice, even for short periods of time (<3 h) at relatively mild temperatures (–0.5 °C). Our results indicate that high-elevation stoneflies at greater risk of freezing may have correspondingly lower SCPs, and despite their common association with glacial meltwater, these stoneflies appear to be living near their lower thermal limits.
Paleozoic stratigraphy and kinematics of the Roberts Mountains allochthon in the Independence Mountains, northern Nevada
New biostratigraphic, stratigraphic, and structural data from deformed lower Paleozoic sedimentary units of the Roberts Mountains allochthon (RMA), Independence Mountains, Nevada, have enabled the identification of stratigraphic units within the allochthon that range in age from Late Cambrian through Late Devonian. The McAfee thrust fault emplaced a relatively thick (>200 m) sheet of Ordovician Valmy quartzite (locally termed the McAfee Quartzite) on complexly deformed units of the RMA (locally termed the Snow Canyon Formation) comprised of the Vinini Formation, Elder Sandstone, and Slaven Chert. The rocks were affected by multiple phases of deformation during and after the Devonian-Mississippian Antler orogeny, including: emplacement of the Roberts Mountains allochthon with south-vergent folding and faulting of the upper and lower plate rocks to the Roberts Mountains thrust; emplacement of Upper Mississippian (324 Ma) basalt dikes that cross-cut the upper (allochthonous) and lower (para-autochthonous) plate rocks and south-vergent folds; emplacement of the McAfee Quartzite; and upright north-trending folding after the deposition of Antler overlap rocks. Recognition of this history is required to determine the internal structure and thickness of the allochthon and location of high-angle structures within it that, respectively, may conceal and localize Carlin-type gold deposits in underlying carbonate rocks.