SOFT-SEDIMENT DEFORMATION STRUCTURES IN THE LATE TRIASSIC LOCKATONG FORMATION OF THE NEWARK BASIN IN EASTERN PENNSYLVANIA: EVIDENCE FOR SEISMIC ACTIVITY ON NEWARK BASIN BORDER FAULTS

The distribution and morphologies of soft-sediment deformation structures (SSDS) have been employed to recognize past seismic activity on faults. Deep-water lacustrine deposits are particularly sensitive recorders of the seismic history of nearby fault systems, in part because of their continuous record of sedimentation. However, the impact and preservation of seismic disturbances on shallow lacustrine sediments is poorly understood. Recent discoveries in the Upper Triassic Lockatong Formation, a dominantly shallow lacustrine facies of the Newark Supergroup strata often exposed in strandline deposits, have revealed several examples of SSDS that are potential indicators of seismic activity on bounding border faults.

SSDS are developed in thin-bedded, heterolithic mudstones, siltstone and very fine-grained sandstones of the Tumbling Falls Member of the Lockatong Formation. The SSDS are exposed in collected slabs and are best observed in thin sections. Bedding plane structures include: right- and left-lateral offset of desiccation cracks; mm-scale offset thrust faults; grabens; lateral spreads; and multiple, thin mudflows linked to dikes. Vertical slab and thin-section features include: normal faults; reverse faults; grabens; load and flame structures; recumbent folds; sills; incipient breccias; ball and pillow structures; and single and multiple networks of dikes. Dike fill consists of: mudstone; angular, cohesive ‘exotic’ sedimentary clasts; sandstone margins with mudstone cores; and plastically injected sediment layers. The SSDS reported above indicate that the shoreline sediments were subjected to forces that altered their stability, inducing liquefaction and fluidization. Kinetic forces included gravitationally unstable density gradients; vertical shear stress; and gravitational body forces. This unique combination of forces can only be generated by earthquake activity, likely produced by the nearby, potentially active Newark basin border faults. The description and interpretation of the Lockatong SSDS as seismites represent advancements in our understanding of the rheology and preservation potential of deformation in shallow lacustrine deposits, critical for paleoseismic reconstructions.