KELLER, MARGARET A., and JOHN A. BARRON, U.S. Geological Survey, Menlo Park, CA

ABSTRACT: Re-Evaluation of the Miguelito Member of the Pismo Formation of Montaua de Oro State Park, California, Including New Diatom Age Data

The Miguelito Member (basal member) of the Pismo Formation is a marine sequence of predominantly hemipelagic biosiliceous rocks. Within Montaua de Oro State Park along the central California coast north of Point Buchon, the stratigraphic sequence consists mainly of thin- and well-bedded porcelanite, chert, siliceous shale, and mudstone; it also includes clay shale, stratified and nodular dolostone, sandstone, and rare strata with phosphate nodules. In overall aspect and lithology, the Miguelito Member at Montaua de Oro is similar to the upper part of the Monterey Formation of nearby basins.

Originally included within the Monterey Formation, the rocks comprising the Montaua de Oro coastal sequence were later reassigned to the Miguelito Member of the Pismo Formation. In

this study of the Miguelito on the coast, diatom and silicoflagellate assemblages extracted from nodular dolostones indicate a late Miocene (10.4-6 Ma) age. Although the uppermost part is eroded and may be slightly younger, youngest exposed strata are 6.7-6. Ma in age; and strata near the base are 10.4-9 Ma in age, constrained by diatoms in conformably underlying Monterey strata. Thus, on the basis of diatom and silicoflagellate assemblages, the coastal Miguelito section is thought to be equivalent in age to the upper part of the Monterey of the nearby Santa Maria and Santa Barbara-Ventura basins and not to the Sisquoc Formation, as previously thought. Biostratigraphy further suggests that the Messinian sea level fall at 6.3 Ma corresponds to the upper part of the Miguelito Member rat er than to its base as previously interpreted.

AAPG Search and Discovery Article #90992©1993 AAPG Pacific Section Meeting, Long Beach, California, May 5-7, 1993.