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Lithiotis bioherms in the Pliensbachian (Lower Jurassic) of North America

University of British Columbia

In North America the aberrant pelecypod Lithiotis occurs in Upper Pliensbachian strata in Oregon, California and Nevada. The epifaunal, sessile bivalve formed massive, low diversity bioherms in shallow water marginal marine facies, which are most extensive in the Robertson Formation of east central Oregon. Lithiotis bioherms of the Robertson Formation comprise three biofacies; (1) a Reef Flank Assemblage; (2) a Death Assemblage and (3) a Life Assemblage. The Reef Flank Assemblage is characterized by Lithiotis shelly debris, the gastropod Nerinea and terebratulid brachiopods and marks the perimeter of the bioherms. The Death Assemblage is comprised of Lithiotis bivalves lying parallel to bedding and the Life Assemblage is made up of in situ vertical Lithiotis. Lithiotis is an elongate, stick-like bivalve that reaches over 30 cm in height and is made up of a thick, robust attached valve and a thin, fragile free valve. The free valve articulates with the attached valve by a ridge-and-groove structure on the cardinal face of the attached valve. Lithiotis is similar in morphology and occurs in a similar paleoenvironment as the Lower Jurassic bivalve, Cochlearites and the Upper Cretaceous oyster, Konbostrea. All three bivalves are found in low diversity bioherms that formed in a shallow water, marginal marine environment. The paleogeographic distribution of Lithiotis is widespread. Lithiotis occurs in the East Pacific, Circum-Mediterranean region and in the Eastern Tethys Ocean. An endemic center of the bivalves existed during the Pliensbachian in the East Pacific of North America and the Western Tethys Ocean, which indicates that Pliensbachian migration occurred through the Hispanic Corridor. During the Lower Toarcian Lithiotis migrated from the endemic center to the East Pacific of South America and to the Eastern Tethys Ocean. The absence of Lithiotis in the Canadian suspect terranes is likely due to collection failure or lack of appropriate shallow water marginal facies. Offshore exploration could well reveal Lithiotis in a shallow water marginal marine facies in Wrangellia in the Queen Charlotte Islands area.

Text

2010-06-27

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http://hdl.handle.net/2429/26012

Geological Sciences

University of British Columbia

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