Authors: Michael W. Ressel and Seth Dee
Series: Open-File Report 2018-05
Format: plate: 37 x 30 inches, color, 2 cross sections; text: 20 pages, color
The Ravens Nest 7.5-minute quadrangle is located in the northern Piñon Range south of the town of Carlin in north-central Nevada. Mapping at Ravens Nest was undertaken to address several components of the complex geology of north-central Nevada, including the nature of sedimentation and deformation associated with the Early Mississippian Antler orogeny, the effects of post-Antler contractional deformation, development of the Eocene Elko Basin and slightly younger Robinson Mountain volcanic field, and deformation associated with Cenozoic extension in the hanging wall of the Ruby Mountains–East Humboldt Range metamorphic core complex. Importantly, the northern Piñon Range lies within the southern segment of the Carlin trend gold belt, which is one of the premier gold mining jurisdictions in the world. Gold produced from disseminated, sedimentary rock-hosted deposits, or Carlin-type deposits, in Nevada comprised about 90.5% of Nevada’s production and about 73.6% of U.S. production in 2016 (Muntean et al., 2017). The southern Carlin trend in the vicinity of the Ravens Nest quadrangle has seen several new Carlin-type gold discoveries since 2010 in non-traditional Paleozoic host strata. The Piñon Range and flanking Pine and Huntington valleys are also important areas for conventional and unconventional hydrocarbon resources. The Blackburn and Tomera Ranch oil fields in Pine Valley produce from Cenozoic and Paleozoic rocks widely distributed in the Ravens Nest quadrangle, and the Elko Basin has seen assessment of its oil-bearing shale by the U.S. Bureau of Mines in the 1970s and in 2012-15 for its hydrofracture potential by energy companies.
Mapping shows that three principal Paleozoic structural-stratigraphic domains exist at Ravens Nest. In particular, Late Devonian to Early Mississippian strata, which straddle the leading edge of the Roberts Mountains allochthon, are interpreted to comprise: 1) the deformed base of the allochthon, 2) the incipiently deformed foreland basin, and 3) the least-deformed passive margin. The allochthonous rocks are separated from rocks of the passive margin by a west-dipping thrust or thrusts. In addition, a fourth domain, the Antler overlap sequence, consists of relatively undeformed Late Mississippian through Permian strata widespread at Ravens Nest, which overlies all three other Paleozoic domains. The Early Mississippian Chainman Shale, previously mapped as a component of all three domains, is herein restricted to an allochthonous shale-dominant Lower Mississippian facies that occurs in the western half of the Ravens Nest quadrangle. The Chainman along with underlying deformed units of the Webb and Woodruff Formations were thrust upon a coarse chert- and quartzite-grain sandstone and fine conglomerate formerly assigned to the Chainman Shale by Smith and Ketner (1978) but herein attributed to the Melandco Sandstone. Two key Early Mississippian units, the Webb of allochthon derivation, and the Tripon Pass of passive margin derivation, straddle allochthon, foreland basin, and passive margin, and thus, provide important ties between Antler tectonic domains. Late south-vergent reverse faults cut some Lower Mississippian rocks, indicative of a change from Antler east vergence that may reflect either post-Early Mississippian (i.e., post-Antler) contraction, or local variation of Antler deformation through time and space.
Rocks of the Eocene Elko Basin unconformably overlie Paleozoic rocks at Ravens Nest. The basin developed as primarily lacustrine sedimentation from ~45 Ma to 38 Ma, and only a small part of the lacustrine facies is present in the far northeast corner of the Ravens Nest quadrangle. An Eocene basal fluvial-alluvial gravel is present in many places in and near Ravens Nest quadrangle, which in previous mapping was commonly assigned a Paleozoic age (Hollingsworth et al., 2017). By ~40 Ma at Ravens Nest, significant volcanic input is evident in strata of the Elko Basin, as distally derived small-volume ash-flow tuff and tephra. By ~38.5 Ma, significant local volcanic input began, and lacustrine deposition completely ceased by ~38 Ma, during major deposition of volcanic products of the nearby Robinson Mountain volcanic field (RMVF). Activity associated with the RMVF was short-lived, with abundant rhyolite and dacite lavas and domes with lesser tuff emplaced between ~38.5 and 37.5 Ma. Coeval with calc-alkaline volcanism was the emplacement of the Bullion granodiorite to high-silica rhyolite intrusion in the Railroad district as well as other smaller stocks and swarms of silicic to intermediate sills and dikes. The intrusions of the Railroad district are responsible for polymetallic skarn and carbonate-hosted replacement deposits. In addition, widespread marble and hornfels occurs over 40 km2 in the southern half of the quadrangle, indicative of a much larger underlying pluton. Carlin-type gold deposits are coeval with Eocene intrusions and occur peripherally to them and higher-temperature polymetallic deposits (Henry et al., 2015; Hollingsworth et al., 2017).
The mid-Miocene Humboldt Basin partly overlapped the Elko Basin in the vicinity of the Ravens Nest quadrangle. However, extensive areas of lacustrine shale and siltstone, tuff, and fluvial and alluvial gravels occur over a large area in the southwest part of the quadrangle where lacustrine deposition postdated emplacement of the Palisade Rhyolite (Wallace et al., 2008). Strata of the Humboldt Basin were faulted against Paleozoic bedrock along north striking normal faults on the eastern margin of Pine Valley. Pliocene and younger gravels and extensive tuffaceous lacustrine deposits of the Hay Ranch Formation were deposited synextensionally into a hydrologically closed Pine Valley. Quaternary normal faulting along the western margin of the Piñon Range has continued at least into the middle Pleistocene. The effects of the stream capture of Pine Creek and integration of Pine Valley into the Humboldt River watershed during the middle Pleistocene include deeply incised drainages, oversteepened hillslopes and extensive landslide deposits.
This geologic map was funded in part by the USGS National Cooperative Geologic Mapping Program under STATEMAP award number G17AC00212, 2018.
Read about the authors
Mike Ressel, author of two of these newly released maps, was featured in the Geological Society of Nevada “Faces of GSN” in November. You can read his story here:
NBMG staff pages
You can also read about the other geologic mappers and their work on the individual NBMG staff pages: