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Although warmer water Cenozoic molluscan faunas of the Gulf of Alaska differ compositionally from those of the western United States, correlations are possible. Cooler water faunas are difficult to correlate because they contain many endemic and “pre-circumboreal”–circumboreal elements. The strong latitudinal temperature gradient of the Pliocene and Pleistocene prevents direct correlation with faunas of the conterminous United States, but some correlations with the Bering Sea transgressions are possible. Because of differing thermal histories and water depths, western Gulf faunas differ from northeastern Gulf faunas. Mollusk-bearing units in the western Gulf are the Stepovak (late Galvinian, Matlockian?), “Narrow Cape” of Sitkinak Island (Juanian), Narrow Cape Newportian), Unga Conglomerate (late Newportian? to early Wishkahan?), upper Bear Lake (Wishkahan), Tachilni (Graysian?), and Tugidak (Beringian-Anvilian). Mollusk-bearing units in the northeast Gulf are the Poul Creek (late Galvinian to Pillarian), mainland Yakataga (Newportian to Beringian-Anvilian), and Middleton Island Yakataga (Anvilian). Newportian and Wishkahan (Miocene) temperatures of the western Gulf indicate warm temperate water comparable to that of the western United States. Newportian and younger faunas of the northeast Gulf indicate anomalous cooler water, probably caused by local glaciation. Water temperatures colder than those of the present probably did not occur until the Beringian (late Pliocene–Pleistocene). “Pre-circumboreal” mollusks first appear in the Juanian (late Oligocene) of the Gulf, but the first opening of Bering Strait apparently occurred in Wishkahan (late Miocene) time; reopening in Beringian time caused significant changes in Gulf faunal composition.

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