Abstract: Patterns of California Continental Borderland Sedimentation Derived from Late Pleistocene and Holocene Studies

D. S. Gorsline

Studies of cores from the basins of the California continental borderland show that the main sediment sources are the northern sectors of the adjacent mainland although that dominance has varied in its influence with changing climate and sea level. During interglacial, high-sea-level times the northern contribution is the largest. During glacial, low-sea-level times there is a marked increase in contribution from the entire adjacent mainland region, but the northern drainages remain the largest although their proportion of the total terrigenous contribution probably decreases.

Oceanographic factors position the area of highest planktonic productivity in the waters of the northern borderland just south of the northern Channel Island massif where bottom topography, wind drift, and surface-current interactions generate a large upwelling region. This region expands during the glacial times and contracts during the interglacial stages reflecting the shifts of climatic zones toward and away from the equatorial belt and a matching waxing and waning of thermal gradients in atmosphere and ocean.

Trends of thickness of the sedimentary fills in the borderland basins also reflect the dominance of the northern drainages. Inner basins have thick fills but central and outer basins show decreased fill south and west across the borderland. The pattern illustrated in the most recent cycles (ca. 30,000 years ago) probably has been characteristic of most of the borderland's history since its initiation in late Miocene time. It is probable that the greatest fluctuations in sediment-delivery rates and in basins' sedimentation rates have occurred during the periods of strongly developed glacial-interglacial variation during the past 1 to 2 m.y.

Major rivers that presently contribute are the Santa Ynez, Santa Clara, Ventura, Los Angeles, San Gabriel, and Santa Ana. Until the innermost basins filled (Ventura, Los Angeles, San Gabriel, San Fernando), the main locus of sedimentation probably was shifted landward toward these basins. When they were filled to overflow, the locus then probably shifted to essentially its present position. Upwelling always may have been centered in its modern position, for the older inner basins (now filled) would have been more isolated and perhaps essentially were landlocked, thus blocking circulation effects which were less affected by filling of the old basins.

AAPG Search and Discovery Article #90976©1976 AAPG-SEPM-SEG Pacific Section Meeting, San Francisco, California