Post-glacial sediments of the Californian shelf from Cape San Martin to the US–Mexico border

Abstract

Previous workers of the Californian shelf from Cape San Martin to the US–Mexico border have assembled compilations of high-resolution acoustic seismic profiling data from a number of primary sources. To examine these data more effectively the region has been divided into four areas for convenience in mapping, and partly in response to natural geographic divisions. These data, plus additional detailed profiling off San Onofre south of Dana Point, have made possible calculations of the total volume of unconsolidated post-glacial sediments on this narrow, active transform margin shelf. The San Onofre survey shows that there are several well-defined buried terraces cut into the bedrock surface. These occur at about −20, −30 to −35, −40 to −50, and −60 to −80 m below present datum. Dating reported in the literature suggests that the bulk of the sediments are Holocene. As sea level shifts from glacial to interglacial, much of the sediment stored on the shelf at high stand is reworked to deeper water during the fall to the following low stand. Previous studies suggest that for the large deposits off the Santa Maria, Santa Ynez and Ventura–Santa Clara Rivers, some sediment may remain after a sea-level cycle. The estimated total Holocene (past 10,000 years) river discharge for the region, based on records of the past century, is about 130 km 3. Thus the total shelf deposit of ca. 60 km 3 represents about one-half of the Holocene siliciclastic contribution. However, the 60 km 3 includes basal sediments in the three large river lenses that are probably pre-Holocene. A more probable Holocene-contributed volume is about 30 km 3. Previous studies show that biogenic and fluvial contributions are generally <5% and 25–30% of the shelf sediments, so that the dominantly sandy shelf sediments of probable Holocene age approximate the coarse fraction of the river. The bulk of the fluvial contribution passes via lateral longshore transport into the adjacent basins via submarine canyons. Little sediment from the areas of relatively high input passes to the starved areas which remain as non-depositional, low-deposition rate, or erosional surfaces. The major portion of the shelf is sediment-starved since only a few small ephemeral streams enter most of the cliffed coastal areas, and rocky surfaces are exposed in the inner and outer shelf.