URANIUM-SERIES AGES OF MARINE TERRACE CORALS FROM THE PACIFIC COAST OF NORTH-AMERICA AND IMPLICATIONS FOR LAST-INTERGLACIAL SEA-LEVEL HISTORY

Few of the marine terraces along the Pacific coast of North America ha ve been dated using uranium-series techniques. Ten terrace sequences f rom southern Oregon to southern Baja California Sur have yielded fossi l corals in quantities suitable for U-series dating by alpha spectrome try. U-series-dated terraces representing the approximately 80,000 yr sea-level high stand are identified in five areas (Bandon, Oregon; Poi nt Arena, San Nicolas Island, and Point Loma, California; and Punta Ba nda, Baja California); terraces representing the approximately 125,000 yr sea-level high stand are identified in eight areas (Cayucos, San L uis Obispo Bay, San Nicolas Island, San Clemente Island, and Point Lom a, California; Punta Banda and Isla Guadalupe, Baja California; and Ca bo Pulmo, Baja California Sur). On San Nicolas Island, Point Loma, and Punta Banda, both the approximately 80,000 and the approximately 125, 000 yr terraces are dated. Terraces that may represent the approximate ly 105,000 sea-level high stand are rarely preserved and none has yiel ded corals for U-series dating. Similarity of coral ages from midlatit ude, erosional marine terraces with coral ages from emergent, construc tional reefs on tropical coastlines suggests a common forcing mechanis m, namely glacioeustatically controlled fluctuations in sea level supe rimposed on steady tectonic uplift. The low marine terrace dated at ap proximately 125,000 yr on Isla Guadalupe, Baja California, presumed to be tectonically stable, supports evidence from other localities for a +6-m sea level at that time. Data from the Pacific Coast and a compil ation of data from other coasts indicate that sea levels at approximat ely 80,000 and approximately 105,000 yr may have been closer to presen t sea level (within a few meters) than previous studies have suggested . (C) 1994 University of Washington.