THE SHEEP PASS FORMATION, NEVADA: A LATE CRETACEOUS TO EOCENE BASIN DEVELOPED ON THE SEVIER PLATEAU

The Sheep Pass Formation of east-central Nevada is a ~1 km thick fluvio-lacustrine sedimentary sequence previously interpreted as late Cretaceous to middle Eocene based on pollen and non-marine invertebrates. The depositional age and setting of the Sheep Pass Formation upon the Sevier Plateau provides a record of the transition of Sevier contraction to Basin and Range extension, and serves as an example of an ancient basin developed atop an orogenic plateau. However, the tectonic and depositional history of the Sheep Pass Formation is still poorly understood. Many paleogeographic models for the Great Basin 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 early Oligocene onset of Basin and Range extension. New 1:12,000 scale mapping and stratigraphic measurement of the Sheep Pass Formation in the Egan Range type section indicate the presence of megabreccia blocks of the Pennsylvanian Ely Limestone and Mississippian Scotty Wash Sandstone within the basal member of the Sheep Pass Formation. Also, paleochannels backfilled with coarse breccia of the basal Sheep Pass Formation are cut into the underlying Mississippian Chainman Shale, and rapid lateral thinning of the lower members of the Sheep Pass Formation indicate high relief and a strongly asymmetrical basin. Shale and limestone deposited within a lacustrine setting during Maastrictian to Eocene times suggests lower depositional energy following basin initiation. High angle faults within the Paleozoic basement appear to be synchronous with the deposition of the basal Sheep Pass Formation, but are overlapped by the upper members, indicating tectonic quiescence during later stages of deposition. Within its type section, the Sheep Pass Formation is unconformably overlain by the informally named Stinking Spring conglomerate and the volcaniclastic Garrett Ranch Group. A new 40Ar/39Ar age of 35.43 ± 0.11 Ma (late Eocene) was obtained from a tuff overlying the Stinking Spring conglomerate. Megabreccia blocks of the Ely Limestone within the Stinking Spring conglomerate indicate the rejuvenation of high relief possibly related to extension during the late Eocene. Ongoing work will examine the tectonic setting under which the Sheep Pass Formation was deposited.