Multibeam bathymetry and Deep Tow sidescan sonar surveys SW of San Clemente Island, CA

Abstract

In March 1999, about 170 square nm of bathymetry were mapped from the 40- to 700-m contour lines using a 50-kHz Elac BottomChart multibeam echo-sounder temporarily installed aboard Scripps’ R/V NEW HORIZON. This survey provided the base map for a near-bottom 110-kHz sidescan sonar and 4-kHz subbottom profiler survey with the Marine Physical Lab. Deep Tow package. Notable seafloor geological features include a broad terrace, left over from the last glacial low sea level stand and sloping gently from the island’s shore to the 120-m depth contour, canyons and sediment aprons below, steep rocky extensions of China Point (island’s southern-most point), and the sheer wall of the San Clemente Escarpment to the east. The sidescan sonar map has poor contrast over the sandy bottom along the island’s western shore, whereas the rock spurs off China Point and a number of erosional channels carving the slopes between 200- and 800-m depth correspond to areas of high acoustic backscatter. Deep Tow subbottom profiles along the southern slope provide evidence of erosional channels cutting through sediment bedforms that are presumably compressional side effects of the strike-slip faulting along the San Clemente Escarpment.