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The Roberts Mountain thrust is the oldest and probably the largest of many eastward-moving regional thrust plates that make up much of the Basin and Range province. The fault can be traced from the state of Washington through central Nevada into California. Movement commenced during Late Devonian-Early Mississippian time (Antler orogeny), probably as a result of plate collision and subduction farther west along the margin of the eastern Klamath geanticlinal arc.
Ordovician to Devonian age, eugeosynclinal, highly organic (2 to 5%), dark-colored shales, siltstones, cherts, and limestones--the "western siliceous facies"--perhaps originally totaling 20,000 ft (6,100 m) are thrust over "eastern-facies" carbonate and clastic shelf deposits. Movement along the thrust continues sporadically into middle Permian, by which time much of the eugeosynclinal rocks were eroded eastward as a flysch. These clastic deposits of Mississippian, Pennsylvanian, and Permian age are deposited along with shallow-marine units in local basins and sags.
Outcrops of thin-bedded, oil-bearing shales (25 gal/ton) and black cherts of the "western siliceous facies" Vinni Formation are present at Roberts Mountain. This unit is a potential source, seal, and possible reservoir rock, averaging several thousand feet in thickness, which "floors" many of the Miocene valley basins west of the leading edge of the thrust. Oil potential is considered to be good although fewer than 15 wells have been drilled in the area. Many had oil shows but none has tested the Vinni or the overlying Cretaceous-Eocene sedimentary units at depth. Free oil has flowed from perforations at approximately 7,200 ft (2,195 m) from possibly Mississippian or Devonian dolomites in the Amoco-Getty 3 Blackburn Unit, (Sec. 8, T27N, R52E, Pine Valley, Eureka County, Nevada). This s the first substantial oil recovery from the Paleozoic rocks in any valley basin other than Railroad Valley in the Nevada portion of the Basin and Range. Oil appears to be of Paleozoic source. In some valleys the overlying Cretaceous and Tertiary units may provide additional source, seal, and reservoir rocks.
East of the thrust trace, in several valley basins, Paleozoic source and reservoir rocks are present and intertongue with the flysch of the Roberts Mountain thrust. In some valleys, these units are also overlain by sedimentary Cretaceous and Tertiary rocks which may also be potential source, reservoir, and seal for any oil accumulations. Noncommercial Tertiary and Paleozoic free oil and gas have been recovered in several test wells drilled in T29N, R55E, Huntington Creek valley basin, Elko County, Nevada.
Local horst and graben structures, combined with Tertiary truncation similar to that found productive at Trap Spring and Eagle Springs fields, are present in valley basins both east and west of the thrust.
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