Abstract

Petrographic comparison of volcanic clasts in the Blanca Formation on Santa Cruz and Santa Rosa Islands with the Santa Cruz Island Volcanics suggests that the Blanca Formation was not derived by erosion of the Santa Cruz Island Volcanics. More probably, the Blanca Formation, which includes primary flow and pyroclastic units, was deposited during an episode of active andesitic and dacitic volcanism. The observed textural and mineralogical differences between clasts in the Blanca Formation and the Santa Cruz Island Volcanics suggest possible strike-slip movement along the Santa Cruz Island fault, although the direction and magnitude of movement remain in doubt.

Dacite from an intrusive dike complex on the north part of Santa Cruz Island and a volcanic plug on San Miguel Island are similar in texture and mineralogy to some dacite clasts in the Blanca Formation. Dacite of probable Miocene age on Santa Catalina and San Clemente Islands bears little petrographic resemblance to dacite clasts or intrusive dacite on Santa Cruz Island.