THE SHEEP PASS FORMATION, NEVADA: STRATIGRAPHIC EVIDENCE FOR A PALEOGENE TRANSITION FROM CONTRACTION TO EXTENSION?

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. Our new 1:12,000 scale mapping and stratigraphic measurements of the Egan Range type section indicate that the basal member of the Sheep Pass Formation (SPF) generally occupies a narrow trough that is bounded by a series of faults cutting the Paleozoic basement. The basal conglomeratic member of the SPF may be further divided into a lower, massive, poorly bedded boulder conglomerate containing abundant megabreccia blocks of upper Paleozoic limestone and sandstone, and an upper, well organized sequence of pebble to cobble conglomerate and siltstone. The lowermost beds of the basal SPF member appear to be confined to the fault-bounded trough, while the upper beds of the basal member overlap the faults but thin dramatically or pinch-out entirely outside of the trough. Upper members of the Sheep Pass Formation overlap these faults with only minor changes in thickness. Preliminary field observations suggest that the faults bounding this trough have an older over younger sense of offset. Additionally, we have identified a younger fault that juxtaposes the SPF against upper Paleozoic rocks, and mapping to date suggests a younger over older sense of offset. Indications are that this younger fault is overlapped by the Stinking Spring conglomerate and the volcanic sequence of the Garrett Ranch Group. If these observations are correct, this would bracket the younger fault between the Bridgerian (ca 52-46 Ma) fossil assemblage of the upper Sheep Pass Formation, and our 40Ar/39Ar sanidine age of 35.43 ± 0.11 Ma of the basal Garrett Ranch Group tuff in Sheep Pass Canyon, and would make the faults at the base of the SPF older than the Maastrichtian (ca. 70-65 Ma) fossil assemblage of the lower SPF. The implications are that the SPF may record a transition from contraction to extension within the Sevier hinterland during Paleogene time.