Abstract

Basement terranes of Catalina Schist, which includes blueschist-facies and glaucophanic greenschist rocks, first began to shed detritus into adjoining basins during provincial middle Miocene time (upper Saucesian through Luisian Stages). The San Onotre Breccia and stratigraphically related units contain Catalina Schist debris. Some exposures on the Channel Islands and along the mainland coast contain fairly conclusive evidence for sediment-transport directions, depositional environments, and paleontologic age.

Along the mainland coast from the Santa Monica Mountains to the Tijuana area, abundant sedimentologic criteria suggest Catalina Schist sources to the west and south, and fossils indicate deposition of these materials in Relizian time. Except for the northern Channel Islands area, little is known about the age and conditions of deposition in the earliest schist-bearing sedimentary units. On Santa Cruz and Santa Rosa Islands, blueschist grains are present in strata that possibly are as old as provincial early Miocene but are most abundant in units of middle Miocene age. These strata indicate derivation from the north and east relative to their present outcrop area. On Anacapa Island, blueschist debris interbedded with middle Miocene volcanic rocks show west-northwest-directed sediment transport features. Elsewhere on the borderland and southern islands, few criteria are available to judge paleoenvironments.

Samples from the east edge of the Patton Ridge indicate local derivation from ultramafic basement rocks, which are unlike the Catalina Schist, and zeolite-cemented lithic sandstone somewhat similar to rocks of the Coast Ranges Franciscan Complex.

From the distributional pattern and age assignments of these deposits containing Catalina Schist, it is concluded that basins on the borderland and along the adjacent mainland coast were receiving sediments derived in large part from local, structurally controlled areas of high relief elevated above surf base during intervals of middle Miocene time.

In southern California, movement on the San Andreas fault may have begun about 12 m.y. ago, and the Pacific and North American plates may have become juxtaposed along a right-shear boundary as early as 29 m.y. ago. Eastward shifting of the plate boundary in middle Miocene time from a location near the top part of the old subduction zone, probably at the base of the Patton Escarpment, across the borderland region may have created the basin and ridge topography reflected by the distribution of debris from the Catalina Schist.