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Abstract

Over 165 species of megainvertebrates, mostly mollusks, from the lowest three (of approximately 17) emergent marine terraces along the southwestern coast of Santa Barbara County, California, provide the basis for both paleoecological and paleoclimatic interpretation. Most of the fossiliferous exposures on the two lowest terraces at Canada de Alegria, Gaviota, and Arroyo Hondo are situated at or very near to the original shoreline angle (i.e., base of the fossil sea cliff) of their respective terraces, physically constraining any interpretation of their depositional setting. The overall composition of their faunas, however, suggests sources from several near-shore marine habitats, all at intertidal to shallow inner sublittoral depths (0 to 18-27 m). Only the sandy bottom faunas from localities at Cojo Bay differ, representing a position about 700 to 750 m from shore and a water depth of about 15 m.

Composite faunas from localities on the first terrace at Cojo Bay, Gaviota, and Arroyo Hondo, and from the second terrace at Canada de Alegria contain from four to 12 extralimital northern species each, indicative of slightly cooler water paleoclimatic conditions, comparable to those occurring today in the vicinity of Monterey Bay, central California. The fauna from the third terrace at Arroyo Hondo contains four extralimital southern gastropods, whose zoogeographic ranges suggest a more southerly geographic equivalency and slightly warmer water conditions, comparable to those occurring today between San Diego, California, and Ensenada, Baja California.

On the basis of terrace mapping, uranium-series age estimates of bones, amino-acid racemization and epimerization analyses of bivalve mollusks, long-term uplift studies, and the zoogeographic signatures of the terrace faunas, the three lowest terraces are assigned ages of 85 to 80 ka (Cojo Bay, Gaviota, and lower Arroyo Hondo localities), 105 to 100 ka (Canada de Alegria second terrace locality), and 130 to 120 ka (Arroyo Hondo upper terrace localities), correlative with dated sea-level highstands recorded elsewhere as reef terraces, and in deep-sea sediments as marine oxygen isotope substages 5a, 5c, and 5e, respectively.

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