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Range zones of pectinid bivalves provide a principal means of age determination and correlation of shallow-water, inshore facies of middle and late Tertiary marine basins of Oregon and Washington. During the Paleogene, pectinids were represented by a few species of Chlamys, Vertipecten, and some thin-shelled deep-water mud pectens of the Eburneopecten and Amusium groups. Neogene pectinids are much more numerous than Paleogene species in this region, although in each case, taxonomic diversity is far lower than in the warmer-water Neogene faunas of coastal California to the south. Giant pectinids, species that regularly attain at least 90 mm in diameter, appear near the Oligocene-Miocene boundary. Range zones of giant pectinids are useful in provincial age determination and correlation of marine Neogene strata in Oregon, Washington, and coastal British Columbia: Vertipecten fucanus (Dall)—lower lower Miocene (Pillarian Stage); Patinopecten propatulus (Conrad)—upper lower Miocene to middle Miocene (Newportian Stage); Patinopecten oregonensis (Howe)—lower upper Miocene (Wishkahan Stage); Yabepecten condoni (Hertlein)—upper upper Miocene (Graysian Stage); and Lituyapecten dilleri (Dall)—Pliocene (Moclipsian Stage). Certain widespread species facilitate interprovincial correlation between the California Neogene molluscan province, to the south, and the Gulf of Alaska Neogene molluscan province, to the north. Vertipecten fucanus, an early Miocene species, ranged southward to southern California and northward to the head of the Gulf of Alaska. Leptopecten andersoni (Arnold), a late early Miocene to middle Miocene species, ranged southward to Baja California Sur. Lituyapecten dilleri (Dall) ranged northward to near the head of the Gulf of Alaska and southward to Cedros Island, off Baja California, Mexico, during the Pliocene.

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