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Distribution of Ammonoids in the Lower Triassic Union Wash Formation (Eastern California): Evidence for Paleoceanographic Conditions During Recovery from the End-Permian Mass Extinction

¹ University of Southern California, Department of Earth Sciences, Los Angeles, CA 90089-0740

The Lower Triassic (Smithian-Spathian) Union Wash Formation (east-central California), deposited along the western edge of Pangea, is generally barren of macrofossils with the exception of ammonoids. To better understand the paleoenvironmental conditions that existed in the region of Union Wash Formation deposition, data were gathered concerning the taphonomy, sedimentology, and distribution of these ammonoids. Results indicate Meekoceras bed deposition was due to a drop in background sedimentation rates, perhaps coupled with favorable environmental conditions. Disappearance of ammonoid shells in the stratigraphic interval above the Meekocerasbed appears to be the result of a shallow aragonite compensation depth (ACD); the return of ammonoids higher in the section is associated with the appearance of seafloor calcium carbonate cements that represent an increase in the degree of calcium carbonate supersaturation, related to the upwelling of anoxic, alkaline waters. The subsequent disappearance of seafloor cements suggests that seawater calcium carbonate chemistry returned to normal prior to deposition of the Parapopanocerasbed. The condensed nature of the Parapopanocerasbed, coupled with poor shell preservation, indicates deposition as a lag deposit. Bedding plane occurrences of ammonoids elsewhere in the sequence imply deposition as mass mortality events possibly related to anoxia (indicated by the depauperate and laminated nature of much of the unit) and/or hypercapnia (related to the growth of seafloor cements). The results of this study further attest to the unusual paleoceanographic conditions that were present off the western margin of Pangea during the Early Triassic recovery from the end-Permian mass extinction.

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