--> ABSTRACT: Paleoecology and Biogeography of Late Cretaceous Molluskan Assemblages Near Loma Prieta, California, by Willam P. Elder; #91003 (1990).

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ABSTRACT: Paleoecology and Biogeography of Late Cretaceous Molluskan Assemblages Near Loma Prieta, California

Willam P. Elder

Deformed, fossiliferous turbidite deposits resembling the Great Valley sequence are present in a fault wedge just east of the San Andreas fault near Loma Prieta, Santa Cruz Mountains, California. Near the top of these deposits late Campanian to early Maastrichtian Baculites rex is found along with late Campanian B. inornatus and B. aff B. anceps. Turbidite deposits associated with a conglomerate near the base of the Upper Cretaceous strata contain abundant exogyrid oysters and many rare or uncommon taxa including Amphidonte parasitica, Lyriochlamys traskii, Spondvlus subnodosus, and Hipponix dichotomus, which also imply a late Campanian age.

Composition of the oyster-rich assemblage indicates a relatively high-energy, nearshore source area for those turbidite deposits. This source area consisted of mixed sand and rock or shell bed substrates. Fossils found in the overlying Upper Cretaceous turbidites are indicative of a more offshore shelf habitat, suggesting a seaward shift in sediment source area or transgression through time. Molluskan assemblages include North Pacific species having tectonically transported distributions ranging from Baja California to Vancouver Island. Some species in the oyster beds have been previously reported only to the north and some only to the south of Loma Prieta; thus, the unusual faunal composition is apparently due to the

preservation of a very nearshore assemblage, which is rare in strata of this age in California, rather than to biogeographic constraints. The apparent absence of rudists in this very nearshore assemblage, where they would be expected if present, suggests significant displacement of the Upper Cretaceous rudist-bearing rocks found to the west of the San Andreas fault relative to Loma Prieta.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990