Seismites of the Late Triassic Lockatong Formation, Pennsylvania, USA: implications for Newark Basin border fault movement

Abstract

Numerous soft-sediment deformation structures (SSDS) within thin-bedded, heterolithic lacustrine shoreline deposits of the Upper Triassic Tumble Falls Member, Lockatong Formation of eastern Pennsylvania, demonstrate liquefaction, fluidization and shearing features processes affecting sediment. This study employs thin-section analysis of SSDS to interpret them as probable seismites from the Triassic-Jurassic Newark Basin. Sedimentary structures include right-lateral offset of desiccation cracks and ichnofossils, thrust, reverse, and normal faults with mm-scale offsets, horsts and grabens, lateral spreads, and single and complex multiple networks of dikes and sills and associated sedimentary volcanoes, thin mudflows on a paleosurface linked to sediment dikes, load and flame structures, ball and pillow structures, recumbent folds, and incipient breccias. Dike fill consists of mudstone, angular sedimentary clasts, sandstone margins with mudstone cores, and plastically deformed sediment fill. The SSDS indicate that Late Triassic lacustrine shoreline sediments were subjected to forces, probably compressive with a minimal shear component reflected in the offset of the dikes and ichnofossils, that induced liquefaction followed by fluidization and vertical movement of fluids. Kinetic forces that altered sediment vertical stability, leading to deformation of the sediment included gravitationally unstable density gradients, vertical shear stress, and gravitational body forces. SSDS can be induced by seismic and aseismic processes. However, this unique combination of forces and resulting sedimentary structures indicate an origin by earthquake activity, likely produced by the nearby, active Newark Basin border fault system. The Lockatong Formation SSDS seismites improve our understanding via thin section analysis of the rheology and preservation potential of SSDS in lacustrine shoreline deposits, critical for the geologic history of Newark Basin, as well as the refinement for paleoseismic reconstruction in general.