Ichnologic study of the lockatong formation (late Triassic), Newark basin, southeastern Pennsylvania

Abstract

Lake‐margin deposits of the Upper Triassic Lockatong Formation of the Newark Supergroup, Newark Basin, southeastern Pennsylvania, have yielded an assemblage of trace fossils. Dominated by burrows, specimens include: Cochlichnus anguineus, Lockeia siliquaria, Planolites montanus, Scoyenia gracilis, Spongeliomorpha milfordensis, Treptichnus pollardi, and an arthropod trackway. The assemblage is considered to belong to the Scoyenia ichnofacies. The greatest diversity and abundance of trace fossils are present in reddish brown to gray and black siltstones and mudstones formed during lake regression. During this time newly exposed lake‐margin organic‐rich sediments provided an opportunistic time for ‘feeding and trace‐making. Subsequent desiccation of the sediment followed by rapid sediment influx due to flooding favored preservation of these traces. However, largely “redbed”; deposits formed during lake low stand and the beginning of lake transgression yielded evidence of only two forms, Scoyenia and Spongeliomorpha. Longer periods of dryness coupled with limited rainfall appear to have favored the opportunistic organisms responsible for these two traces. Occasionally, minor evidence of small, highly compressed and nondescript tube‐like trace fossils is present in organic‐rich deeper water Lockatong deposits (lake high stand). The paucity of trace fossils in these deposits may be due to low oxygen conditions and/or insufficient sediment cohesiveness.