SEISMIC IMAGING AND HYDROGEOLOGIC CHARACTERIZATION OF THE POTOMAC FORMATION IN NORTHERN NEW CASTLE COUNTY, DELAWARE

The non-marine Potomac Formation, which includes the most important confined aquifers in the Coastal Plain of Delaware, is characterized by a complex and lithologically heterogeneous stratigraphic framework. Our study integrates seismic reflection and borehole data to delineate the stratigraphic architecture of this thick accumulation of Cretaceous alluvial sediments, with a focus on the sand bodies that provide significant volumes of groundwater to northern Delaware.

A land-streamer system was used to collect 20 km of near-surface, high-resolution seismic reflection data on public roadways. Six lithofacies and their respective geophysical log patterns were identified in core samples and wireline geophysical logs and correlated with the seismic data. Using seismic attribute analysis, seismic facies that correspond to four of the lithofacies were identified and interpreted on six seismic lines: fluvial channel, paleosol, splay/levee, and a frequently flooded lake/abandoned channel and splay/levee combined seismic facies. Correlations for eleven horizons identified in the seismic sections and log cross sections show local changes in thickness and erosional relief within which these facies occur.

The seismic interpretations reveal that the 2D lateral connectivity of the sand bodies of the Potomac Formation is limited to short distances, contrary to correlations in previous studies that have indicated connection of sands at distances of at least 3 km. This suggests more limited connectivity of fluid flow pathways in the Potomac aquifer system than previously thought. This study provides, for the first time, a two-dimensional basis for detailed understanding of the stratigraphy of and depositional styles in the Potomac Formation. Overall, the seismic and borehole data establish that the Potomac Formation appears to be dominated by an anastomosing fluvial style with poorly connected winding channel sands encased in fine-grained overbank sediments that produced a complex, labyrinth-style heterogeneity. Understanding the depositional style of the aquifer sands produced by this fluvial system is important for understanding water availability, distribution, and flow in the aquifer system.