Contrasts in Early Miocene Subsidence History Across Oceanic-West Huasna Fault System, Northern Santa Maria Province, California

TENNYSON, M. E., U.S. Geological Survey, Denver, CO, M. A. KELLER, U.S. Geological Survey, Menlo Park, CA, and M. V. FILEWICZ and M. L. COTTON THORNTON, Unocal, Ventura, CA

Within the Oceanic-West Huasna fault system of western San Luis Obispo County, thick bathyal mudstone records abrupt early Miocene subsidence. Lower Miocene rocks west of the fault, in contrast, suggest a relative high.

Between fault splays, <= 1800 ft of mudstone (Rincon Formation) contains Saucesian lower bathyal foraminifera and CN2-CN1c (~17-21 Ma) nannofossils. The mudstone was deposited on a subsiding erosional surface cut on Great Valley and Franciscan basement overlain locally by late Oligocene rhyodacite (Cambria Felsite), nonmarine conglomerate (Lospe Formation), and nearshore sandstone (Vaqueros Formation). Basinal late early to middle Miocene Obispo and Monterey formations overlie the older mudstone.

West of the fault, major subsidence does not conclusively predate ~17 Ma submarine Obispo tuff. Northeast of Morro Bay, nearshore Vaqueros underlies Obispo or Monterey formations. Northeast of Santa Maria, Franciscan rocks are overlain by thin nonmarine conglomerate, sandstone with upper bathyal Saucesian foraminifera and upper CN2 nannofossils, and Obispo volcanics. Near Cambria, nonmarine conglomerate and Vaqueros sandstone were deposited on the Franciscan between ~27-27 Ma (Cambria Felsite) and 25 Ma (87Sr/86Sr on Vaqueros shells); only ~100 ft of basinal mudstone separates Vaqueros from Obispo tuff. In the Pismo syncline, the Franciscan was near sea level 25 Ma (87Sr/86Sr on shells); subsidence to bathyal depths cannot be demonstrated until just before Obispo eruption.

Paleobathymetric estimates suggest initiation of at least 6000 ft of subsidence along the fault system ~20-21 Ma. The block to the west, in contrast, either stayed high, or subsided but was uplifted and stripped, subsiding again as Obispo eruption began. We have not found evidence for major lateral offset on this fault.

AAPG Search and Discovery Article #91009©1991 AAPG-SEPM-SEG-SPWLA Pacific Section Annual Meeting, Bakersfield, California, March 6-8, 1991 (2009)