Abstract: Middle Cenozoic Paleogeography of the Western Transverse Ranges, California

Jeffrey L. Howard, Wallace D. Lowry

The provenance and depositional history of the nonmarine upper middle Eocene to lower Miocene Sespe Formation were studied to determine its paleogeographic significance. It is concluded that tectonism, erosion of local basement sources, and sediment recycling played minor roles during sedimentation, thus conglomerates in the Oligocene part of the Sespe Formation were not deposited as alluvial fans in response to syndepositional rifting as previously thought. The detritus is mainly first cycle and derived from sources in the Mojave Desert, including distant areas in eastern California and Arizona. Conglomerates in both the Eocene and Oligocene parts of the formation were deposited in an alluvial coastal plain-fore-arc basin setting by one or more areally extensive braided river systems Eustasy was probably responsible for unconformities, changes in provenance, and depositional cycles within the Sespe Formation.

These results, and palinspastic reconstructions, suggest that coastal California was an alluvial braid plain bounded on the east by the Mojave-Sonora Desert region and Mogollon highlands during the Eocene and Oligocene. An ancestral Salt-Gila River probably drained into the western Transverse Ranges area from southwestern Arizona and deposited the Sespe Formation. During the early Miocene, rifting and detachment faulting created isolated nonmarine basins in the Mojave-Sonora Desert region. This tectonism also resulted in the first appearance of locally derived detritus in the uppermost part of the Sespe Formation, and the ultimate disruption of the postulated Sespe-Gila river system.

AAPG Search and Discovery Article #90981©1994 AAPG Pacific Section Meeting, Ventura, California, April 27-29, 1994