INVERTEBRATE TRACE FOSSILS FROM THE UPPER MIDDLE DEVONIAN MAHANTANGO FORMATION, PIKE COUNTY, PENNSYLVANIA

The Upper Middle Devonian Mahantango Formation, up to 400 m thick, consists of largely fossiliferous mudstones and sandstones representing shallow marine deposits arranged in coarsening-upward cycles. The formation represents prograding clastic wedges that were associated with the Devonian Acadian Orogeny. Recent studies of the Mahantango suggest that the cyclicity could be explained due to frequent progradation and retreat of what was basically a straight, tide-influenced shoreline onto a storm-dominated marine shelf. Thick deposits of mudstones and siltstones, representing the middle Mahantango Formation, are exposed in a series of very steep, cliff exposures along Route 209 between Milford and Bushkill, Pennsylvania (within the Delaware Water Gap National Recreation Area). Extensive fragmentation along bedding and cleavage intersections as well as weathering along vertical joint faces has resulted in an apron of small shale-chip rubble representing an assortment of upper, middle, and lowermost portions of the exposure. An exception occurred when large clasts slumped after an extensive rainstorm. Close inspection revealed that the clasts came from an isolated lowermost exposure of the Mahantango Formation. Due to the distinctive freshness of the sheared face, the clasts were estimated to represent a thickness of 6 m. The rocks, dark-gray mudstones, weathering brownish-gray, olive-gray, pale brown, and grayish red-purple, revealed the trace fossils Cruziana isp., Dactylophycus quadripartitum, Diplichnites isp., Helminthoidichnites tenuis, Helminthopsis hieroglyphica, Planolites annularis, Planolites beverleyensis, Protovirgularia dichotoma, ?Psammichnites isp., Treptichnus bifurcus, and Treptichnus pollardi. The assemblage is considered to belong to the Cruziana ichnofacies. Associated fossils include cephalopods and bivalvia. In addition, there is evidence of fracture cleavage. This facies portion of the Mahantango Formation (dark-gray mudstones), represnts mid- to outer shelf (= offshore) hemipelagic deposits in which trace-making, largely by deposit-feeding organisms taking advantage of organic-rich sediments, occurred during relatively low energy levels.