A DIVERSE ICHNOCOENOSIS ALONG AN UPPER TRIASSIC RIPARIAN MUDSTONE PALEO-SURFACE, BUCKS COUNTY, PENNSYLVANIA

A variably exposed sequence (>3 m) of riparian siltstones and paleosols of the Upper Triassic Stockton Formation crops out along the Neshaminy Creek, Bucks County, Pennsylvania. This NNE-dipping sequence within the Newark Basin is incised and overlain by coarse-grained sandstone and basal conglomerate channel cut-and-fill lithosomes, which results in abrupt lateral and vertical facies changes. Burrows of Scoyenia gracilis dominate in paleosol horizons, which is indicative of an ancient low-biodiversity floodplain ecosystem. Along the top of the riparian sequence, a minimally displaced block exposes the mudstone-paleosol interface revealing a rare snapshot of an intensely bioturbated paleo-surface. Presence of meniscate fill, bifurcating tunnels, lateral chambers, and rimmed craters (some >1 cm in diameter) indicate a diverse tracemaker community, still dominated by Scoyenia isp. Burrow diameters range from 0.15-1.50 cm. Although a number of tunnels show sharp turns and cross-cutting, the azimuth of meniscate-filled burrows clusters within 60° (75% of directions). This suggests a local paleo-topographic or hydrologic control on tracemaker activity. Our findings indicate the need for access to naturally weather paleo-surfaces, often not available in outcrops. The riparian sequences provide a window into a fluvially dominated rift setting, with autochthonous controls on substrate exploitation by a diverse assemblage of Carnian arthropod-like biota dominated by a keystone Scoyenia ichnogenus.