ABSTRACT: Early Miocene Sedimentation and Tectonics in Western San Luis Obispo County, Central California

Marilyn E. Tennyson, M. A. Keller, M. Filewicz, M. L. Thornton, D. Vork

Lower Miocene rocks between Santa Maria and Ragged Point suggest that the Oceanic-West Huasna fault was active during early Miocene subsidence but fail to confirm or refute strike-slip offset on the Hosgri-San Simeon or Oceanic-West Huasna fault systems.

Nonmarine conglomerate and sandstone and overlying deep-water mudstones along the Oceanic-West Huasna fault may indicated that the fault was a locus of subsidence east of a Franciscan high. Abrupt subsidence is suggested by the presence along the fault of as much as a few hundred meters of lower Miocene mudstone directly overlying the Toro Formation, Cambria Felsite, nonmarine Lospe Formation, or shallow marine Vaqueros Formation. West of the fault, rocks of the Franciscan complex are overlain locally by Lospe and regionally by thin shelly sandstone and conglomerate of the Vaqueros beneath middle Miocene strata; thick lower Miocene bathyal mudstone is unknown. We have not identified any sections offset laterally across this fault system.

Nonmarine conglomerates mapped as Lospe differ across the San Simeon fault, as noted by earlier workers. West of the fault, the Lospe(?) consists mainly of ophiolite detritus, like the type Lospe near Point Sal. However, it is undated and does not contain volcanic rocks as the Lospe near Point Sal does, weakening the case for suggested offset of 80 km or more on the Hosgri fault. East of the San Simeon fault, the Lospe is composed of Franciscan or Great Valley detritus, and its age is bracketed between latest Oligocene (inferred for the Cambrian Felsite), and early Miocene (Vaqueros and Rincon formations).

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990