Abstract: Sedimentology of the Beechers Bay Formation, Santa Rosa Island, California

NUCCIO, ROBERT M., Pasadena, CA; andJOHN J. WOOLLEY, Venoco, Inc., Santa Barbara, CA

The Beechers Bay Formation of middle Miocene age consists of a sequence of generally coarse-grained volcaniclastic sediments which crop out on Santa Rosa Island, California. This unit was deposited on a deep-sea fan very near an active volcanic source.

The Beechers Bay Formation is the time-stratigraphic equivalent of the Monterey Formation. It is divided into 5 informal members, deposited as distinct lithofacies on the deep water fan.

Members A (oldest) and E (youngest) are composed of dacite-andesite conglomerate, basalt conglomerate, and interbedded pebbly sandstone, deposited on the inner (upper) fan. Members B and D were both deposited in the channeled portion of the upper mid-fan. Member B consists of feldspathic daciteandesite litharenite and interbedded siltstone. Member D is a true volcaniclastic sandstone that signifies an increase in volcanic activity at the nearby source. Member C is a rhythmically bedded sequence of feldspathic volcanic litharenite and siltstone, deposited on the distal mid-fan.

All members of the Beechers Bay Formation contain blue schist clasts thought to have been derived from the Catalina (Franciscan) schist. Paleoslope and paleocurrent indicators found in each member suggest the source area was located to the present-day east-northeast.

The source area was a volcanically active ridge, distinct from those which produced correlative volcanics on Santa Cruz Island and in the Santa Monica Mountains, located south of the present location of Santa Rosa Island. This area produced large quantities of dacitic and andesitic material which was deposited by sediment gravity flows on an adjacent submarine fan. The metamorphic material was shed from nearby Catalina Island.

AAPG Search and Discovery Article #90935©1998 AAPG Pacific Section Meeting, Ventura, California