Abstract

Within coastal outcrop sections west of Santa Barbara, California, the upper part of the Middle Miocene is condensed into a narrow stratigraphic interval that is characterized by phosphatic hardgrounds. Foraminiferal biostratigraphy shows that the base of the condensed interval is diachronous, varying from a position within the Luisian Stage at Gaviota Beach, to a position just above the Luisian-Mohnian boundary at Naples Beach and probably at El Capitan Beach. Termination of the main episode of hardground deposition occurred within the Early Mohnian, and is synchronous within the resolution of the biostratigraphic correlations. Phosphatic hardground sections at Gaviota Beach, El Capitan Beach, and Naples Beach correlate to a biogenic silica-rich interval characterized by chert, porcelanite, and organic phosphatic shale at Arroyo Burro Beach. The Upper Luisian to Lower Mohnian siliceous sediments at Arroyo Burro Beach correlate in part or in whole to the massive chert member of the Monterey Formation, the main fractured chert reservoir interval of Monterey oil fields in Santa Barbara Channel.