ICHNOLOGY, PALEOPEDOLOGY, AND MINERAL ASSEMBLAGES OF TRIASSIC STOCKTON FM MUDSTONES, SE PENNSYLVANIA: IMPLICATIONS FOR PALEO-LANDSCAPE RECONSTRUCTION

This study examines a recently exposed section of sandstones and fissile mudstones of the Upper Triassic Stockton Formation near Blue Bell, Pennsylvania. These units have few accessible outcrops in this heavily developed region. More than 600 m of section were measured and characterized, containing at least four distinct paleosol profiles. Paleosol horizons were identified mainly on the basis of ped size and shape, with color and grain size differences being secondary. Abundant trace fossils include Scoyenia and Taenidium isp., and other unidentified traces of varying preservation. Taenidium burrows have subvertical orientation, with diameters between 20-16 mm and meniscate burrow fill with a curvature amplitude of ~2.0 mm. At macroscale, evidence of vertisol development was found, including decimeter-scale curvilinear cracks with slickensided surfaces. Micromorphological analysis of thin sections revealed pedogenic carbonate nodules and distinctive x-shaped ped boundaries. The Scoyenia ichnofacies and presence of mm-sized carbonate clasts, as well as films and smaller nodules of pedogenic carbonate, suggest a fluvial system with relatively weak sediment transport that existed under an arid to semi-arid moisture regime. Slickenside features within the soil horizons, as well as the redoximorphic mottling lower in the section, together imply alternating wetting and drying cycles, although the relatively small size of the observed slick faces in outcrop (less than 11 cm) suggests that drying cycles may have been irregular rather than seasonal. These results are consistent with previous paleoenvironmental research of Stockton Formation and support the interpretation as a dryland fluvial depositional system.