Abstract: Pre-Oligocene Structure of Central Railroad Valley Area and Adjacent Pancake Range, Nevada Western Margin of the Paleogene Sheep Pass Basin

William J. Perry, Jr., James A. Peterson

Lacustrine rocks of the Paleocene to Eocene Sheep Pass Formation, penetrated in wells of the Eagle Springs oil field in eastern Railroad Valley, Nevada, rest unconformably on Pennsylvanian Fly Limestone and are buried beneath younger Tertiary volcanic rocks. Three miles to the west, the Meridian 32-29 Federal deep test penetrated a similar sequence above the Mississippian Chainman Shale. Farther west, in the Trap Springs Field, the Sheep Pass Formation is thin to absent; Devonian Guilmette Formation is buried beneath Tertiary volcanic rocks. Where present, the intervening Sheep Pass Formation appears to comprise basin-margin colluvial deposits, fault-separated from the Paleogene downthrown block to the east. Several hundred feet of Sheep Pass Formation (primarily redbeds and red-matri limestone conglomerates) are exposed and mapped along the western margin of Railroad Valley near 38° 45^prime N latitude. In the Pancake Range, four miles northwest of this exposure, a similar subvolcanic conglomeratic redbed rests on east-dipping Devonian Simonson Dolostone. On the next fault block to the west, pockets of similar subvolcanic Sheep Pass(?) rest on easterly dipping Lower Pennsylvanian Fly Limestone and underlying Mississippian Chainman Shale. Farther west in the Pancake Range, but still east of recognized contractional deformation, an east-dipping mass of Upper Mississippian, Pennsylvanian and basal Permian rocks rests in low-angle fault contact on the Chainman Shale and is interpreted as an extensional klippe. The northern part of this mass is also overlapped by la e Focene volcanics and pockets of subvolcanic Sheep Pass(?) conglomeratic redbeds. The geometry of the fault beneath the klippe is that of a nearly flat portion of a west-dipping listric normal fault. Observations indicate that eastward-dipping panels of Paleozoic rocks are bounded by west-dipping Paleogene or Late Mesozoic normal faults older than the late Eocene Stone Cabin volcanic rocks and older than the Sheep Pass of Railroad Valley and the adjacent Pancake Range to the west.

These extensional faults appear to have developed much later than the Early Cretaceous or older thrust faults and associated tight folds along the western margin of the Pancake Range. The contractional deformation (Eureka belt) may be as old as Sonoman and may represent the leading edge of the Sonoman fold-and-thrust belt in the western Pancake Range, in front of the Golconda allochthon.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994