Triassic trace fossils from lacustrine shoreline deposits of the Passaic Formation, Douglassville, Pennsylvania

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Lake-margin deposits of the Late Triassic Passaic Formation, Douglassville, Pennsylvania, have yielded a moderate variety of trace fossils. The greatest diversity and abundance of trace fossils occurs on the sole of a thin gray claystone overlain and underlain by gray siltstones. Specimens of Cochlichnus anguineus, Helminthoidichnites tenuis, Helminthopsis hieroglyphica, Treptichnus pollardi, and paired trails reflect simple, unspecialized, horizontal grazing as well as feeding traces under very shallow water lacustrine conditions. The lack of commonly associated Scoyenia burrows in these deposits may possibly be related to the degree of maturation of the organic debris available. The traces, as well as a lack of meniscate burrows, compare favorably to the Mermia ichnofacies, except Passaic deposits exhibit evidence of subaerial exposure. This unit most likely records a lacustrine expansion whereby grazing trails were emplaced under fully subaqueous conditions. Subsequent shallowing and desiccation, followed by sediment influx during rainstorms, favored preservation of these traces. In contrast, the Scoyenia ichnofacies consists of feeding burrows of Scoyenia gracilis and Spongeliomorpha milfordensis within reddish brown siltstones and mudstones (redbed sequence). The Scoyenia ichnofacies records limited exploitation, by opportunistic infaunal deposit feeders, of lake-margin nutrients carried in during occasional rainstorms that punctuated otherwise extended periods of aridity.

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