Sedimentation in the north San Clemente basin, California continental borderland

Three broad sedimentary environments within the north San Clemente Basin have been defined. The south slope of San Clemente Island and the northernmost part of the San Clemente Basin receive predominantly siliciclastic sediments shed from San Clemente Island. Clastic sediments deposited in San Clemente Canyon are sorted and transported by the action of tidally generated currents. Currents measured in the canyon show upcanyon and downcanyon reversals of flow nearly every 12 hours, and currents reached velocities of 18 cm/second during periods of peak flow. The currents have winnowed silt and clay from sediments at the head of the canyon, leaving a sandy lag deposit. Pine-grained sediment is transported into the basin and slowly deppsited as the currents lose velocity. Three piston cores from the northern part of the basin penetrated the Holocene-Plelstocene boundary. Slightly more than a meter of Holocene sediment was measured in each core, as determined by mlcropaleontologic methods. The central part of the north San Clemente Basin is dominated by pelagic sedimentation. Basin physiography effectively blocks transport of clastic sediments from San Clemente Island and mainland California to the area. Hemipelagic sediments and distal turbidites are transported through the Navy Fan of the south San Clemente Basin and into the southern part of the study area. The observed distal turbidites may be a result of Early Holocene sea-level regressions. Sediments within the north San Clemente Basin are in a reducing environment, as indicated by their color and the presence of authigenic sulfides. Micropaleontologic data from this study indicate that kummerform Globlgerina pachyderma tests are a result of displacement into unfavorable environments