The Lower to Middle(?) Jurassic Dunlap Formation, west-central Nevada: Deposition in actively extending half-graben basins

Abstract

Autochthonous Lower Jurassic synorogenic strata (Dunlap and Water Canyon Formations) in the Pilot and Excelsior Mountains of west-central Nevada were deposited in actively extending half-graben basins. Northwest-trending syndepositional faults and rapid lateral variations in sedimentary facies from southwest to northeast indicate that the basins were northwesterly elongated. Mean northward paleotransport data indicates basins opened to the north. Sand-rich facies were funnelled northwestward down the axis of the basins by subaqueous tractive processes, whereas gravel-rich facies were shed northeastward by submarine fans which formed along the southwestern margins of the basins. Regional stratigraphic and petrographic evidence indicate that several basins existed, and that individual basins were isolated or poorly connected during deposition. Deposition of the Lower Jurassic Dunlap and Water Canyon Formations within half-grabens is in marked contrast to previous interpretations equating basin formation to mid-Mesozoic contraction. Dunlap sedimentation unequivocally predates the formation of Mesozoic folds and thrusts and cannot be used to date the onset of regional shortening. Synorogenic deposition of Lower Jurassic rocks in northwest-trending basins is consistent with widespread extensional systems in the western Great Basin, the Sierra Nevada and the Mojave.