Gravity Anomalies and Models of the Diabase Sills in the Newark Basin in Eastern Pennsylvania

As part of a project to create a gravity map and database of Pennsylvania, we have been compiling gravity data from various sources including the National Geophysical Data Center and other academic sources. In addition, we have collected new gravity data in eastern Pennsylvania as part of various research and thesis projects. The current endeavor focuses on obtaining a detailed and well-distributed set of gravity data over the Pennsylvania portion of the Newark basin, one of a series of rift basins along the eastern margin of North America created by the rifting of Pangaea during the Late Triassic and Early Jurassic. These basins were filled with cyclic, continental lacustrine sediments and intruded by a series of tholeiitic diabase sills. One application of these data is to map and model the diabase intrusions within this portion of the basin. Regional and residual analysis of the contoured gravity data demonstrate a remarkable special coincidence of a 5 to 10 mgal positive anomaly with the known outcrop pattern of the sills. There are also some areas where the sill is observed to outcrop, but where the gravity signature is minimal or does not exist. This may indicate that the sills were originally very thin in those areas or have subsequently been thinned by erosional processes. We have also constructed 2-dimensional density models from gravity data collected along traverses across the diabase sills. The density models of the sills range in thickness from .3 km to almost 1 km and generally increase in structural thickness from east to west. This implies that the source of the intrusion may be towards the west-central portion in this part of the basin. The thickness information is also useful for thermal history models of the basin.