Late Triassic invertebrate trace fossils from lacustrine shoreline deposits, Smith Clark quarry, Milford, New Jersey

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A moderately diverse invertebrate ichnofauna has been recovered from lacustrine shoreline deposits of the Late Triassic Perkasie Member (Passaic Formation), Smith Clark quarry, Milford, New Jersey. Dark-gray to black sandstones, siltstones, and mudstones comprising the lower interval of the Perkasie Member have yielded specimens of Bergaueria hemispherica, Circulichnus montanus, Cochlichnus anguineus, Helminthoidichnites tenuis, Lockeia siliquaria, Palaeophycus tubularis, Planolites montanus, Scoyenia gracilis, Taenidium barretti, Treptichnus bifurcus, T. pollardi, and an insect trail. Overall, domination by burrowing forms, associated physical sedimentary structures (e.g., desiccation cracks), as well as vertebrate footprints and plant remains allow the assemblage to be characterized as a Scoyenia ichnofacies. Interestingly, however, seven of the ichnotaxa present in these strata also characterize the Mermia ichnofacies. The presence of dark strata, domination by burrowing forms, as well as deeply-fissured, large desiccation polygons point to infrequent drying events. As such, Mermia associated tracemaking was likely emplaced under initial shallow aqueous lacustrine shoreline conditions. During brief periods of lake regression, organisms produced trace evidence characteristic of both the Mermia and Scovenia ichnofacies. As regression continued, however, deeper burrowing by striated forms was produced in a firmer lake-margin substrate. This brief respite was soon followed by lake transgression. In contrast, up section, reddish brown siltstones and mudstones denote a sustained period of lake regression whereby all Mermia and most Scoyenia associated traces were eliminated. Interestingly, there is evidence of occasional, widely-spaced rainstorms, during which opportunistic organisms took advantage of nutrient-rich sediments carried into the environment, and as a result produced sharply-defined burrows of Scoyenia and Spongeliomorpha in a firmer substrate.

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Northeastern Geology and Environmental Sciences

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