TRIASSIC PARK: RECONSTRUCTING A FLUVIAL PALEO-LANDSCAPE, BUCKS COUNTY, PENNSYLVANIA

The Upper Triassic Stockton Formation is exposed as a riparian succession along the Neshaminy Creek, Bucks County, Pennsylvania. In this NNE-dipping section of the Newark Basin, an interbedded sequence of siltstones and paleosols is capped by a coarse-grained sandstone channel cut-and-fill unit. Numerous burrows of Scoyenia gracilis and absence of additional traces are indicative of an ancient low-biodiversity floodplain ecosystem (Scoyenia ichnofacies). Tunnels of S. gracilis exhibit longitudinally striated surficial morphology and meniscate burrow fill that indicates direction of movement. The azimuth trend is NNW-SSE (>75% of orientation data clustered within 60^o) and several specimens with well-preserved menisci indicate the preferred tracemaker movement in a SSE direction. Such clustering suggests some ecological or landscape control. Shear stress and paleo-channel velocity were calculated to assess the dynamics of the ancient stream. The d₉₀ of the largest rip-up clasts of mudstone and quartz from the basal channel sections provided the means for quantifying stream competency and yielded flow velocity of up to 250 cm/s. Thin section analysis suggests that variably colored paleo-(verti)sol horizons may reflect seasonality and water table fluctuations. Our findings indicate an oxidizing fluvial paleoenvironment with localized incision by a high-velocity stream and a geomorphic control on substrate exploitation by a monospecific arthropod assemblage.