THE POTOMAC FORMATION OF NORTHERN DELAWARE: RESOLVING AQUIFER ISSUES IN THE DELAWARE COASTAL PLAIN USING SEQUENCE STRATIGRAPHIC CONCEPTS

Aquifer sands of the subsurface Potomac Formation are major sources of drinking water for the rapidly growing population of northern Delaware. This highly heterogeneous, Cretaceous (Aptian to Cenomanian) formation is characterized by permeable fluvial sands that lie within a matrix of predominantly fine-grained sediments. Connectivity of permeable fluvial units is poorly constrained.

Our recent work has established a new time-stratigraphic framework for the Potomac Formation in the Coastal Plain of New Castle County, Delaware, part of adjacent Cecil County, Maryland, and nearby areas of New Jersey. The framework is based on a geophysical well-log correlation datum that approximates the contact between Upper and Lower Cretaceous sediments. The strata dip gently to the southeast, with generally sandy fluvial facies at the base of the formation lapping onto a south-dipping basement unconformity. The top of the formation is marked by an erosional unconformity that truncates successively older Potomac strata updip; at its northern limit, the Potomac subcrops under Quaternary sands. The correlation datum is constrained by age determinations based on published and unpublished results of studies of fossil pollen and spores in samples of sediment cores from boreholes in the study area. Geophysical log correlation lines established above and below the datum approximate additional chronostratigraphic surfaces. The chronostratigraphic units thus defined are not correlated parallel to the basement unconformity, as in previous practice, but instead onlap it in an updip direction.

Although unconformity-bounded sequences cannot be definitively defined in this section, the chronostratigraphic approach inherent in sequence stratigraphy is the key to delineation of genetically related contemporaneous stratigraphic packages. It provides a chronostratigraphic context within which the complex mosaic of sedimentary facies of the Potomac Formation can be mapped and analyzed. This new stratigraphic framework is in use in an aquifer modeling study being conducted by the US Army Corps of Engineers. It also provides a framework for future efforts aimed at delineating the degree of lateral connection between potential aquifer sands, thus enhancing understanding of aquifer architecture.