Abstract: Miocene Paleogeography of Central, Western California: Offset Along the San Gregorio-Hosgri Fault Zone

Clarence A. Hall Jr., Raymond V. Ingersoll, Mark J. Sutherland

Three Miocene paleogeographic maps (Zemorrian-Saucesian, Relizian-Luisian, and Mohnian) largely of the region from near Santa Maria (Santa Barbara County) to Monterey (Monterey County), California, and west of the Rinconada-East Huasna-Hildreth/Cachuma fault, are based on a new compilation of onshore and offshore geology. An understanding of the San Gregorio-Hosgri fault (SG-H) fault is essential in making palinspastic maps of the region. New provenance data indicate that source areas for the upper Miocene Pismo Formation from the Point Sur area are now 90 to 160 km to the south relative to Point Sur on the east side of the SG-H fault. The presence of pebble and cobble clasts of felsitic and tuffaceous volcanic Oligocene Cambria Felsite and Morro Rock-Islay Hill Complex i the Pismo Formation of the Point Sur area further suggests that the Point Sur area has moved 100 to 160 km relatively north since late Miocene time.

Based on several lines of geologic and geophysical evidence from several stratigraphic horizons at four localities along the SG-H fault, and three Miocene paleogeographic maps, the slip along the SG-H fault averages 90 km between each matched stratigraphic package or unit, each set of shorelines, and offset fault trends.

During Zemorrian-Saucesian time, the Monterey Peninsula-Sierra de Salinas, Morro Bay, and Santa Maria areas of central California were topographic highs. During Relizian-Luisian time, the entire coastal area from Morro Bay to Carmel was a topographic high. During Mohnian time, the central coastal area from San Luis Obispo to Monterey was a topographic high, except for an embayment in the Point Sur/Cambria area. The pattern of sedimentation in the small Point Sur/Cambria embayment indicates a progradational deltaic depositional environment. Turbidites from the embayment represent sedimentation on the delta slope. The fine-grained rocks at the top of this upper Miocene section represent a subsequent retrogradation of the deltaic system.

AAPG Search and Discovery Article #90958©1995 AAPG Pacific Section Meeting, San Francisco, California