Late Quaternary Eustatic Sea-Level Changes Along TheMalibu Coast, Los Angeles County, California

Late Quaternary relative sea-level changes along the Malibu coast can be deciphered by study of geomorphic features and marine and associated stream deposits. Evidence for such changes includes (a) stream downcutting in large valleys during low stands of sea level, (b) stream aggradation in the same valleys during high stands of sea level, and (c) marine shoreline angles whose altitudes coincide with the tops of valley-fill stream deposits. This evidence, and open-system uranium-series dates for shells from the Corral and Dume terrace deposits, provide a time scale for certain sea-level changes. Evidence from other coasts of the United States suggests that sea level was close to its present position about 30,000 years ago. Such a post-Dume, preHolocene relatively high stand of sea level has not been recognized in marine deposits along the Malibu coast. There is evidence, however, for stream aggradation at possibly 30,000 years B.P. Sea-level position at that time cannot be determined, but considerations of tectonic rate of uplift suggest that it was no higher than about -30-foot altitude. Most of the relative sea-level changes identified here are probably due to eustatic variations in sea level. Tectonic movements complicate the interpretation of the stratigraphic and geomorphic record; however, because movements seem to have been consistently upward, relative rises of sea level are interpreted as eustatic and relative falls as partly eustatic. Eustatic origin is also suggested by the similarities in ages of sea-level fluctuations between the Malibu coast and the coasts of Alaska, Barbados, Mallorca, and New Guinea.