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ABSTRACT

The Beechers Bay Formation of medial 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 timestratigraphic equivalent of the Monterey Formation. It is divided into five informal members which were 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, all 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 dacite-andesite 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 which was deposited on the distal mid-fan.

All members of the Beechers Bay Formation contain blueschist 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 eastnortheast.

The source area was a volcanically active area distinct from that 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 a nearby exposure of Catalina Schist and related rocks.

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