Abstract: Miocene Paleogeography of Southwestern California

FRITSCHE, A. EUGENE, California State University, Northridge, CA 91330-8266

At the end of the Oligocene epoch in southwestern California, deposition occurred in two, mostly nonmarine depositional basins that were aligned end-to-end in a northwest-southeast direction and were separated by a highland that existed in the San Gabriel-San Rafael (SG-SR) areas. The southern basin, extending from the SG-SR highland southeastward along the southern California coast to Mexico, was fed by a large river that entered the basin at its northeastern corner, south of the SG-SR highland. This basin, which contained a marine embayment at its southeastern end, is named the "AnaVent" basin because it is composed of rocks found today in the Santa Ana and Ventura structural basins. The northern basin, extending from the SG-SR highland northwestward along the California coast toward central California, was fed by a second large river that entered the basin at its southeastern corner, north of the SG-SR highland. This nonmarine basin is named the "DiSoCuMa" basin because it is composed of rocks found today in the Diligencia, Soledad, Cuyama, and Santa Maria structural basins. Subsidence occurred during the early Miocene and flooded the two basins with marine environments. As subsidence continued, the two basins were eventually joined by a strait that formed when a low spot in the SG-SR highland became flooded. During the medial Miocene, subsidence continued as the western side of the AnaVent basin (today's western Transverse Ranges) began to pull away from the southern California coast and rotate clockwise around its northern end. This transrotation process served to enlarge both of the basins by crustal extension, which in turn produced large volumes of submarine and in some places subaerial volcanics that became interbedded with the sedimentary rocks in the two basins. Throughout the late Miocene, transrotation and subsidence continued, the basins grew in size and depth to their present~ configuration, and internal isostatic adjustments produced several new marine depositional basins within the confines of the previous two-basin framework.

AAPG Search and Discovery Article #90935©1998 AAPG Pacific Section Meeting, Ventura, California