The Sheep Pass Formation, Nevada: A late Cretaceous to Eocene basin within the Sevier hinterland

Peter Druschke, University of Nevada Las Vegas, Department of Geoscience, Las Vegas, Nevada, [email protected]

The Sheep Pass Formation of east-central Nevada is a late Cretaceous to Eocene fluvio-lacustrine sequence that serves as a petroleum reservoir rock and potential source rock in Railroad Valley, Nevada. The age and distribution of the Sheep Pass Formation within the Sevier hinterland provides a critical record which spans Sevier contraction to Basin and Range extension. Current paleogeographic models for the region envision a low-relief plateau following the late Cretaceous cessation of contraction within the central Nevada fold and thrust belt, and prior to the Oligocene onset of Basin and Range extension. New 1:12,000 mapping and stratigraphic measurement of the Sheep Pass Formation in the Egan Range type section indicate the presence of megabreccia within the basal member (latest Cretaceous) of the Sheep Pass Formation. Also, high angle faults within the Paleozoic basement appear synchronous with the deposition of the basal Sheep Pass Formation, but are overlapped by the upper members.  Elsewhere within the Egan and Schell Creek Ranges, tuffaceous fluvio-lacustrine deposits have been previously correlated with the Sheep Pass Formation despite the lack of tuff within the type section.  New ⁴⁰Ar/³⁹Ar ages of 36.38 ± 0.11 Ma from a tuff within a Sheep Pass-correlative section, and 35.97 ± 0.10 Ma for a capping ignimbrite bracket an angular unconformity with ~15^o of dip discordance. These new data indicate that at least two phases of tectonism influenced the deposition of the Sheep Pass Formation.  Ongoing work will examine the tectonic setting under which the Sheep Pass Formation was deposited.