Abstract: Tectonic Control on Deltaic Cyclicity in the Ridge Basin, California

Kenneth D. Ehman, Morgan D. Sullivan, Steve R. May

Upper Miocene sedimentary rocks of the Ridge basin provide an opportunity to evaluate the relationship between tectonics, subsidence history, and basin fill in an extensional strike-slip setting. Cyclicity of strata within the Ridge basin, which is expressed by the progradational members of the Ridge Route Formation and retrogradation within the Violin Breccia, records sedimentological response to basin margin faulting and variations in accommodation. This cyclicity is best developed within the members of the Peace Valley Formation where thick shaly lacustrine packages are punctuated by progradational sandy fluvial-deltaic units of the Ridge Route Formation. A geohistory curve constructed for the lower portion of the Ridge Basin Group based on magnetostratigraphic age control shows hi h rates of accommodation with dramatic changes in the interval subsidence rate. We interpret the variability of subsidence rates to reflect periods of movement along the San Gabriel fault. There is a significant correlation between increases in subsidence rate and the deposition of thick lacustrine shales in the basin axis and coeval progradation of the Violin Breccia along the western margin of the basin. The Violin Breccia strata were deposited contemporaneously with movement along the San Gabriel fault, and during periods of increased faulting, the Violin Breccia prograded into the basin in response to higher sedimentation rates associated with erosion of the uplifted footwall (high relief margin). Progradational intervals of the Violin Breccia, which correspond to the highest rates o subsidence within the basin, are out of phase with the progradation of the sand-prone members of the Ridge Route Formation being derived from the east (low relief margin) during periods of lower subsidence rate. As the Violin Breccia fan-delta complexes begin backstepping toward the western margin of the basin in response to waning tectonic subsidence and diminishing sediment supply from the uplifted footwall, the members of the Ridge Route Formation begin prograding into the basin from the northeastern margin due to decreasing accommodation. Decreases of subsidence rate generally correspond to periods of progradation of the sandstone members of the Ridge Route Formation and retrogradation of the Violin Breccia until the next period of tectonic subsidence which starts the cycle over aga n.

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