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Depositional Setting of the Fossil Hill Member, an Organic-Rich Unit of Middle Triassic Age, in the Western U.S.

T. T. Huynh
University of Southern California, Department of Earth Sciences, Los Angeles, CA

The Fossil Hill Member is a laterally uniform unit composed of interstratified calcareous mudstone and black limestone that contains a distinctive and exceptionally well-preserved Anisian cephalopod and bivalve fauna. Dense accumulations of halobiid bivalves within this member are a characteristic flat clam assemblage typical of Mesozoic strata. Other flat clam facies, such as “Lucina“ in the Jurassic Kimmeridge Clay (England), Halobia in the Triassic Shublik Formation (Alaska), and Bositra in the Jurassic Posidonschiefer (Germany), are similarly preserved in organic-rich units interpreted to represent deposition in dysoxic or anoxic environments. The aim of this research is to determine the conditions under which the Fossil Hill Member was deposited, with an emphasis on the contribution the organisms make to its sedimentary environment.

Paleoenvironmental analysis will rely on detailed field observations of physical and biogenic structures and thin section petrographic and stable isotopic analyses of bulk samples. Paleocommunity analysis of the bulk samples will form the basis for comparison with other wellstudied organic-rich units of similar character from Mesozoic strata. Attempts will be made to find a suitable modern analog of the fauna to better assess the interplay between the organisms and their sedimentary environment. The results from this research will better define the conditions under which high-density flat clam assemblages are accumulated in Mesozoic organic-rich strata, as well as add to our limited understanding of Triassic fauna from the western U.S.

AAPG Search and Discovery Article #90902©2001 AAPG Foundation Grants-in-Aid