Tectonic Control on Stratal Architecture and Cyclicity in the Ridge Basin, California

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

The dramatic cyclicity and thickness of coarse clastics within the late Miocene-aged Ridge Basin was driven by movement along the San Gabriel Fault. Through the evaluation of the subsidence history of the basin, a direct relationship between the fault history and the cyclicity in the Ridge Basin can be demonstrated. The cyclicity is developed between the shaly lacustrine members of the Peace Valley Formation and the sandy fluvial-deltaic units of the Ridge Route Formation. A geohistory curve constructed for the lower portion of the Ridge Basin Group shows high rates of accommodation with dramatic changes in the subsidence rate. There is a significant correlation between increases in subsidence rate, the deposition of thick lacustrine shales in the basin axis, and coeval p ogradation 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. During periods of increased fault movement, the Violin Breccia prograded into the basin in response to higher sedimentation rates associated with the erosion of the uplifted footwall (high relief margin). Progradational intervals of the Violin Breccia, which correspond to the highest rates of subsidence within the basin, are out of phase with the progradation of the sand-prone members of the Ridge Route. As the Violin Breccia fan-delta complexes backstepped toward the western margin of the basin, the members of the Ridge Route Formation prograded into the basin from the northeastern margin due to decreasing accommoda ion.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995