Structure and stratigraphy of the Potosi Mountain area, Southern Spring Mountains, Nevada
Cameron, Christopher Scott
Burchfiel, B. C.
Master of Arts
The Potosi Mountain area is located 25 miles southwest of Las Vegas, Nevada, along the eastern margin of the Cordilleran foreland thrust belt. Three structural blocks are exposed, including, from lowest to highest, the autochthon, the Contact thrust plate and the Keystone thrust plate. Both thrust plates contain a similar sequence of Middle Cambrian through Permian shelf carbonates. The presence of the Upper Ordovician (?) Mountain Springs Formation in both indicates a similar marginal miogeoclinal paleogeographic affinity. Triassic to Latest Jurassic (?) strata of cratonic facies are exposed in the autochthonous block. The Contact thrust, exposed east of Potosi Mountain, carries Paleozoic carbonate rocks east-northeastward over autochthonous Jurassic Aztec Sandstone, or locally, Latest Jurassic (?) synorogenic conglomerates. The Contact thrust plate is truncated north of Potosi Mountain by the northwest trending Cottonwood fault. The Keystone thrust plate overrides the Contact thrust plate south of this fault, and the autochthon to the north. The Cottonwood fault downdrops the Keystone thrust only 2 feet. The following sequence of structural events was deduced: 1) inferred high angle faulting of the autochthon; 2) east vergent folding within the future Contact thrust plate; 3) emplacement of the Contact and intraplate Potosi thrusts; 4) high angle faulting of the autochthon and Contact thrust plates (development of Cottonwood fault and Ninetynine fault zone); 5) emplacement of the Keystone thrust; 6) minor high angle faulting; and 7) minor gravity sliding. Event 1 probably predates Latest Jurassic (?) synorogenic conglomerates. Event 3 postdates these deposits and probably correlates with a folding and thrusting event dated at 135±5 M.Y.B.P. in the Clark Mountains (Burchfiel and Davis, 1971). Event 5 predates a 95±5 M.Y.B.P. post-tectonic pluton in the Clark Mountains (Burchfiel and Davis, 1971). The Keystone thrust is localized near the base of the Banded Mountain Member of the Middle Cambrian Bonanza King Formation, and similar stratigraphic control is inferred for the Contact thrust in subsurface. Cross sections indicate thrust faulting has produced a minimum 1 miles of shortening of miogeoclinal rocks in the Potosi Mountain area. The occurrence of synorogenic chert and quartzite pebble conglomerate restricted to a zone below the Contact thrust and unconformably overlying Aztec Sandstone is interpreted as evidence that the Contact thrust plate moved over an erosional surface.