Structure and regional tectonic setting across the Atlantic Coastal Plain of northeastern Virginia as interpreted from reflection seismic data

Abstract

This study is a geophysical investigation that uses reflection seismic and potential field data to contribute to the development of a structural model of the North American Atlantic Passive Margin beneath the Atlantic Coastal Plain of northeastern Virginia. Specifically, this study focuses between 37.5° and 38.5° north latitude and 75.5° and 77.5° west longitude. The geophysical data include two seismic lines that were reprocessed at the Regional Geophysics Laboratory at Virginia Polytechnic Institute and State University. In addition, gravity modelling is performed in order to test the model developed from the seismic data. Several important results have been achieved from this study. Lower Cretaceous fluvial sediments are less reflective than the overlying marine sequence. This observation is most obvious toward the east, particularly on line CF-1. Reverse faulting, which might be related to movement within the basement, is observed in at least one location on line NAB-11A, near Loretto, VA. Curiously, the dip is in the opposite direction of other reverse faults observed within the coastal plain. The thickness of Triassic strata in the Taylorsville basin is constrained by seismic reflection data and gravity modelling. Results indicate that the basin is approximately 3 km deep. The strata within the basin appear to be poorly reflective except where they locally onlap the bottom of the basin, which is marked by a prominent reflector that is interpreted to be a diabase sill associated with Jurassic magmatism. In addition, the basin appears to be intruded by moderately dipping dikes that were fed by the sill. The occurrence of basaltic material within the basin is confirmed by well log data. Probably the most important result of this study is the tectonic implications of prominent, arcuate potential field anomalies and their relationships to changes in midcrustal reflectivity observed on the east side of line NAB-11A. Gravity modelling confirms the likelihood of a nearvertical, anomalous, mafic mass that extends to the Moho. This observation is supported by the loss of contiguous reflections in this area. A similar observation was made along the southern extension of the same anomaly by Coruh and others (1988) who proposed that this feature is a dike swarm associated with Mesozoic rifting. It is proposed here that this body also could be an ancient Mesozoic magma chamber that collapsed during cooling after the Atlantic margin passed into the drift sequence.