AN INTEGRATED PETROLEUM EVALUATION OF NORTHEASTERN NEVADA
Antler Orogeny and the Roberts Mountains Thrust
The Late Devonian and Early Mississippian Antler orogeny was the first major tectonic event to affect the passive margin that transected the evaluation area. It is traced regionally by the Roberts Mountains thrust and its accompanying allochthon. An oceanic tectonic assemblage of Ordovician through Devonian pelagic, turbiditic, and volcanic rocks, as well as small alpine type ultramafic fragments commonly less than 1 mile long, were transported and emplaced from 55-100 miles (90-160 km) over the Paleozoic continental shelf (Merriam and Anderson, 1942; Stewart and Poole, 1974; Evans and Theodore, 1978; Speed, 1983; Kleinhampl and Ziony, 1985).
The eastern or leading edge of the Roberts Mountains thrust can be traced with outcrops of the Ordovician Valmy and Vinini Formations and various Silurian-Devonian western assemblage units exposed from the southern Toiyabe and Toquima Ranges, north through the Fish Creek, Sulphur Spring, Pinon, and Adobe Ranges, and in a northeastward offset segment exposed within the Snake Mountains and HD Range. This offset segment in northern Elko County has drawn considerable attention and has been variously explained by an original irregularity in the Paleozoic margin, by right-lateral faulting, and by Mesozoic thrust faults reactivating segments of the Roberts Mountains thrust (Coats and Riva, 1983). Coats and Riva (1983) suggest that the northern continuation of the Roberts Mountains thrust actually trends north through the Independence Mountains and into the Mountain City Quadrangle where it is buried by Tertiary volcanics.
The Roberts Mountains thrust is actually a zone of thrusting, in part represented by imbricate thrusts within more competent parts of the allochthon. For example, at least 10 northwest-dipping imbricate thrust slices can be traced for several miles in the northern Toquima Range (McKee, 1976b). Movement along different portions of the thrust system was initiated during several pulses of movement.
Distinctive stratigraphic markers are not commonly present and thrust segments are offset by high-angle normal faults, making accurate tracing and reconstruction of fault segments difficult or impossible. Datable rocks overlap the thrusts in very few places making exact determination of the time of thrusting difficult. It seems well established from contact relationships along various portions of the thrust system that all of the tectonic units present in the Roberts Mountains allochthon were emplaced before the end of the Mississippian (Johnson and Pendergast, 1981; Speed and Sleep, 1982; Murphy and others, 1984).
The thrust surface itself is either a very sharp nearly knife-edge contact, or is marked by as much as several hundred feet of sheared, brecciated and silicified rocks. The basal surface commonly overlies the Devonian Devils Gate Formation or equivalent rocks. Deformation attenuates within 10's of feet beneath the thrust in the autochthon, suggesting that little of the allochthon was sheared off and "sandwiched" within the allochthonous rocks.
The entire Roberts Mountains allochthon is pervasively deformed by imbricate thrusts and dominantly concentric-style folds with steeply dipping or vertical axial planes which trend subparallel to the thrust (Evans and Theodore, 1978). The more incompetent portions of the allochthon containing thin-bedded cherts and limestones, show much more shearing, disharmonic folding, and chaotic faulting, but proportionally more of the strain was taken up in poorly exposed shales and argillites. Fabric analysis suggests that the apparent horizontal component of tectonic shortening within the allochthon was about North 70-75 West (Evans and Theodore, 1978).
Although internal deformation is commonly quite severe, individual thrust sheets are relatively coherent, leading some workers to suggest that the entire allochthon represents a pre-deformed accretionary sheet or wedge which was essentially obducted as an intact entity. Internal deformation of the Ordovician Vinini/Valmy sequence may be very similar to the siliceous Miocene Monterey Formation in coastal California where diagenetic heterogeneity controlled structural style.
Windows through the Roberts Mountains thrust include the Gold Acres and Goat windows in the northern Shoshone Range, the Petes Canyon, Ikes Canyon, and Callaghan windows in the northern Toquima Range, the Cortez window in the Cortez Mountains, the Tonkin, J-D and Windmill windows in the northern Simpson Park Range, the Lynn, Carlin, and Bootstrap windows in the Tuscarora Mountains (Evans and Mullens, 1976; Gilluly and Masursky, 1965; Gilluly and Gates, 1965; Roberts and others, 1967; McKee and Ross, 1969; Cress, 1972; Stewart and Palmer, 1967). Several unnamed windows through the thrust are present across the area including the northern Pinon Range, Lone Mountain, Devils Gate, Whistler Mountain, Roberts Mountains, southern Sheep Creek Range, and various portions of the Toiyabe and Toquima Ranges. These windows are discussed in detail within the regional structure section of this volume. The following paragraphs summarize what the thrust overlies within these windows.
In the Toquima Range, windows through the Roberts Mountains thrust reveal the Ordovician Vinini Formation thrust over the Devonian-Silurian Roberts Mountains Formation or Masket Shale, the Devonian Tor and McMonnigal Limestones, the Devonian-Silurian Masket Shale (Roberts Mountains Formation), and locally the upper portion of the Ordovician Pogonip Group (Antelope Valley Limestone), or Middle and Upper Cambrian strata (Kay and Crawford, 1964; Stewart and Palmer, 1967; McKee and Ross, 1969).
In the northern Shoshone Range, Gilluly and Gates (1965) mapped several windows through the thrust. The Ordovician Valmy overlies undeformed unnamed Devonian Limestone and the Devonian-Mississippian Pilot Formation, as well as mildly metamorphosed and brecciated rocks which include the Cambrian Shwin Formation, Ordovician Hanson Creek Formation, and Devonian-Silurian Roberts Mountains Formation.
The northwesternmost exposure of the Roberts Mountains thrust is in Sec. 6, T. 45 N., R. 55 E., in the southeastern portion of the Mountain City Quadrangle. The thrust trends about North 40 West and places the Valmy Formation over tightly folded Ordovician Goodwin Limestone or Pogonip Group (Coats, 1985). In the nearby Mount Velma Quadrangle the Valmy is thrust on the Cambrian Prospect Mountain Quartzite. Locally the thrust may be modified either by Mesozoic thrusting or by low-angle normal faulting where it places the Valmy over the Pennsylvanian Strathearn Formation.
In the Snake Mountains the Roberts Mountains thrust places the Ordovician Valmy on the Ordovician Pogonip Group, Eureka Quartzite and Hanson Creek Formation, and the Devonian-Silurian Roberts Mountains Formation (Gardner and Peterson, 1968). In the nearby HD Range the Ordovician Valmy and Silurian Noh Formations appear to rest on the Mississippian Chainman Formation. Relationships mapped as thrusts in the Pennsylvanian and Permian units in the HD Range appear to be modified by Tertiary low-angle normal faulting.
Three windows in the Tuscarora Range expose contorted, sheared and brecciated Devonian Rabbit Hill Limestone and the Devonian-Silurian Roberts Mountains Formation beneath Ordovician and Silurian western assemblage rocks (Evans, 1974a, 1974b; Cress, 1972; Evans and Mullens, 1976). In the southern Cortez Mountains and northern Toiyabe Range the Cortez window reveals the Ordovician Vinini Formation thrust over the Devonian Wenban Limestone, the Devonian-Mississippian Pilot Formation, and the Ordovician Hanson Creek Formation (Gilluly and Masursky, 1965).
In the northern Simpson Park Mountains near McClusky Pass, several windows expose alternating slivers of the Ordovician Vinini and the Devonian Devils Gate and Nevada Group (Johnson, 1959; Roberts and others, 1967). In the Roberts Mountains and Sulphur Springs Ranges the Ordovician Vinini is thrust dominantly over the Devonian Devils Gate Limestone or Nevada Group, and the Devonian-Silurian Roberts Mountains Formation. At Lone Mountain and in the Adobe Range and Pinon Range the thrust locally overlies the Mississippian Chainman Formation, while at Devils Gate the Vinini is thrust on the Mississippian-Pennsylvanian Diamond Peak Formation.
In general, these windows define an overall geometry for the Roberts Mountains thrust surface and indicate that it slowly climbs section from west to east. In western exposures the thrust commonly lies on Cambrian and Ordovician autochthonous rocks. In eastern exposures it is dominantly emplaced over younger Devonian and locally Mississippian sediments. The windows show us that very little deformation is present below the thrust, although locally, the rocks have been sheared and brecciated and a few high-angle faults which may pre-date thrusting are present in the autochthon (Gilluly and Gates, 1965; Gilluly and Masursky, 1965).