TRACE FOSSILS FROM LACUSTRINE SHORELINE DEPOSITS OF THE LATE TRIASSIC PASSAIC FORMATION (NEWARK SUPERGROUP), MILFORD, NEW JERSEY

Lacustrine shoreline deposits of the Late Triassic Perkasie Member (Passaic Formation), Smith Clark Quarry, Milford, New Jersey, have yielded a moderately diverse invertebrate ichnofauna. Dark-gray to black sandstones, siltstones, and mudstones representing the lower interval of the Perkasie Member produced the specimens Bergaueria hemispherica, Circulichnus montanus, Cochlichnus anguineus, Helminthoidichnites tenuis, Lockeia siliquaria, Palaeophycus tubularis, Planolites montanus, Scoyenia gracilis, Taenidium barretti, Treptichnus bifurcus, T. pollardi, an insect trail, as well as the previously described Spongeliomorpha milfordensis. In general, domination by burrowing forms, associated sedimentary structures, and vertebrate footprints and plant remains allow the assemblage to be assigned to the Scoyenia ichnofacies. However, seven of the ichnotaxa also characterize the Mermia ichnofacies. The dark strata, preponderance of burrowing forms, and deeply-fissured, large desiccation polygons imply infrequent drying events. Thus, Mermia associated tracemaking was probably formed under initial shallow aqueous lacustrine shoreline conditions. Subsequently, during transient periods of lake regression, organisms produced traces characteristic of both the Scoyenia and Mermia ichnofacies. Nevertheless, as regression continued, deeper burrowing by striated forms occurred in a firmer lake-margin substrate. This brief pause was soon followed by lake regression.

In contrast, up section, reddish brown siltstones and mudstones mark a sustained period of lake regression such that most Scoyenia and all Mermia associated traces were eliminated. There is evidence, however, of sporadic, widely-spaced rainstorms which carried nutrient-rich deposits into the environment. Opportunistic organisms likely took advantage of such, and in turn produced sharply-defined burrows of Scoyenia and Spongeliomorpha within a firmer substrate.