Paper No. 9
Presentation Time: 8:00 AM-12:00 PM
UPPER TRIASSIC PERKASIE MEMBER (PASSAIC FORMATION) TRACE FOSSILS FROM MILFORD, NEW JERSEY
Investigation of largely massive dark-gray to black, minor reddish brown sandstone, siltstone and mudstone lake-margin deposits of the Perkasie Member of the Upper Triassic Passaic Formation at the Smith Clark Quarry, Milford, New Jersey, the type locality for Spongeliomorpha milfordensis, has yielded additional trace fossils. Gray to black beds provided specimens of Circulichnis montanus, Cochlichnus anguineus, Conichnus isp., Helminthoidichnites tenuis, Lockeia siliquaria, Palaeophycus tubularis, Planolites montanus, Scoyenia gracilis, Taenidium barretti, Treptichnus bifurcus, T. pollardi, and an insect trail which are associated with ripple marks, desiccation cracks, tool marks (e.g., drag, chevron), raindrop impressions, and rain impact ripples, as well as classic footprints and plant forms. The traces compare favorably to a Scoyenia ichnofacies. Interestingly, several of the ichnotaxons also characterize the Mermia ichnofacies. Examples include Circulichnus, Cochlichnus, Helminthoidichnites, Lockeia, Palaeophycus, Planolites, and Treptichnus. Thus, a transition occurred between the more fully aquatic Mermia ichnofauna and the Scoyenia ichnofauna as lake regression proceeded. Dark-gray to black sediments, domination by burrowing forms, and examples of deeply-fissured, large desiccation polygons point to closely-spaced wet and dry cycles. Field evidence shows burrows both paralleling and crossing earlier formed desiccation cracks and footprints. Lake-margin surface waters, retained as such until desiccation, maintained an organic-rich environment for the tracemakers. Subsequent sediment influx, likely during rainstorms, covered the traces. Upsection, thin reddish brown siltstones and mudstones yielded only Scoyenia and Spongeliomorpha. The lower diversity records limited exploitation by opportunistic deposit feeders, of nutrients brought in during sporadic rainfall that interrupted otherwise extended periods of aridity. In contrast, these reddish brown siltstone and mudstone deposits are cut across by thin gray conglomerates. Field evidence points to local, intense rainstorms which carried in coarser gravel material, forming scour-and-fill structures along the backshore.