The Sheep Pass Formation, Nevada: Record of the transition from contraction to extension in the Sevier hinterland?
University of Nevada Las Vegas, Department of Geoscience
Las Vegas, Nevada
The Sheep Pass Formation of east-central Nevada is a >1 km thick sequence of latest Cretaceous to Eocene alluvial/fluvial and lacustrine strata deposited within the Sevier hinterland. Ongoing 1:12,000 scale mapping and stratigraphic measurements of the Egan Range type section indicate that the megabreccia-containing lowermost members of the Sheep Pass Formation (SPF) are confined to a trough bounded by faults involving the Mississippian Chainman Shale, and which display a reverse sense of offset. However, the upper members of the SPF overlap this trough and its bounding faults. Additionally, ongoing mapping has identified a younger fault that juxtaposes the SPF against upper Paleozoic rocks. Indications are that this younger fault displays a normal sense of offset, and is overlapped by the Stinking Spring conglomerate and the volcanic sequence of the Garrett Ranch Group. This would bracket movement on the younger fault between the Bridgerian (ca 52-46 Ma) fossil assemblage of the upper Sheep Pass Formation, and the newly obtained 40Ar/39Ar sanidine age of 35.43 ± 0.11 Ma of the basal Garrett Ranch Group tuff in Sheep Pass Canyon. The implications are that the SPF and the overlying Garrett Ranch Group may display a distinct two-part stratigraphy recording contraction within the Sevier hinterland during the latest Cretaceous to Eocene(?), and extension during the Middle to Late Eocene.
AAPG Search and Discovery Article #90060©2006 AAPG Foundation Grants-in-Aid