Sedimentological studies of the Miocene San Onofre Breccia, which contains blueschist clasts of Catalina Schist facies in abundance, suggest that Catalina-type basement rocks may underlie part of the Santa Barbara Basin north of the Channel Islands. The San Onofre Breccia exposed on Santa Cruz Island was derived from the northeast; this conclusion is based on paleocurrent indicators and a north-eastward increase in blueschist clast size. On Santa Rosa Island, San Onofre Breccia is restricted to the northern tip and the sea floor north of the island; rocks of the same age to the south are of a finer grained, deeper water facies. The size and angularity of San Onofre clasts suggest that the southern margin of the Catalina basement block was near the Santa Cruz Island fault.

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