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Description
A J.5KHz High Resolution Continuous Subbottom Seismic Profiling survey of the San Diego County mainland shelf has delineated an approximate 125 kilometer long by 2 to 7 kilometer wide mainland shelf accumulation of primarily Holocene unconsolidated sediments paralleling the north-northwest trending coastline and shelf break. Generally, the sediment accumulation exhibits a wedge-shaped cross section between the water depths of 15 to 90 meters, and a mean maximum thickness of 10 meters between the water depths of 21 and 90 meters. Pleistocene wave-cut terraces are discontinuous over the shelf at six depth ranges below sea level (22 to 25, 32 to 39, 43 to 49, 52 to 59, 62 to 69, and 74 to 80 meters). Their sediment filled topographies locally thicken the wedge by up to 4.5 meters. Sediment accumulations up to 7 meters thick cover and are influenced by paleochannel remnant topographies offshore of San Onofre Bluff, Cardiff, Mount Soledad and Silver Strand. Waves are diverged over La Jolla submarine canyon topographies creating littoral sediment transport in rip currents offshore. Deposition occurs at 15 to 21 meter thick sediment accumulations adjacent to the Torrey Pines Cliffs sediment source and on the intercanyon shelf. Mid-shelf and deeper, shore paralleling bedrock highs of probable fault origin dam up to 5 meters of sediment shoreward. They are adjacent to San Onofre Bluff, Del Mar, Torrey Pines Cliffs and Point Loma. A structural graben, with 7 to 8 meter east-west striking scarps, traps up to 9 meters of sediment offshore of Mission Beach. The Null Line Theory of wave and sediment interaction (transportation and sorting of unconsolidated materials shoreward) explains the wedge build-up of sediment in 21 to 42 meters of water. Increasing wave action in less than 21 meters of water transports and sorts coarser materials landward at a faster rate, thinning the sediment wedge to bedrock in about 15 meters of water. Coarsest and best sorted materials are transported over exposed bedrock into the littoral zone supplying the beaches with sediment.
