Sediments and Growth Pattern of Navy Deep-Sea Fan, San Clemente Basin, California Borderland

Abstract

Navy Fan is a deep-sea fan forming as a result of the overflow of sediment from San Diego Trough into San Clemente Basin. It appears to have preserved its growth pattern unmodified by the effects of the Holocene transgression or of tectonic activity. Since the beginning of the last glaciation about 56 of sediment has been deposited on the fan. The fan is fed by Navy Channel, a deep gorge which cuts through the basement volcanic rock of a threshold 1,500 m deep which separates San Clemente Basin from San Diego Trough. Although it is situated 50 km offshore, Navy Channel contains gravels. The upper fan is crossed by a leveed depositional fan-valley which opens out onto the suprafan: an area of rapid sedimentation characterized by many shallow, shifting channels. Seaward of the suprafan, the lower fan passes into two small ponded basins within San Clemente Basin. The sediments of the fan comprise turbidite sands and muds and hemipelagic muds. Near-surface distribution of the sediments, determined from more than 100 cores, shows that the thickness, abundance, and grain size of turbidite sands decrease distally on the fan and that fine-grained sediments become proportionally more important. The depth of acoustic penetration with high-frequency (3.5 kHz) reflection profiling is low over the fan-valley and suprafan and increases distally. These and other spatial variations generally support concepts of proximality developed for ancient turbidites. The hemipelagic muds have a sand fraction of radiolaria and foraminifera; the sand fraction of the turbidite muds is mainly mica. The turbidite muds are siltier than the hemipelagic muds and are found in graded beds up to 30-cm thick.