NEW PERSPECTIVES ON CORRELATION OF NONMARINE DEPOSITIONAL PACKAGES IN THE CRETACEOUS POTOMAC FORMATION, DELAWARE COASTAL PLAIN

Nonmarine sands of the Potomac Formation (Barremian-Aptian to Cenomanian) are major sources of ground water beneath the inner Coastal Plain of Delaware. We have established a new stratigraphic framework for the Potomac Formation that provides an improved understanding for the correlation of aquifer units. The basis for this framework is the recognition of a geophysical well-log correlation datum near the Albian-Cenomanian boundary within the Potomac Formation. Previous aquifer modeling work assumed basement-parallel stratigraphic correlations. The depositional packages carried in this revised framework exhibit a gentle southward dip. The base of the formation is characterized by onlap of generally sandy fluvial facies onto a south-dipping basement unconformity. In downdip areas, the top of the formation is marked by a Cretaceous erosional unconformity that truncates successively older Potomac strata in an updip direction. Turonian and Coniacian strata are absent. The unconformity may have erosional relief and is onlapped by marginal-marine Santonian strata of the Magothy Formation or the overlying marine Merchantville Formation. In the updip part of the study area the Potomac Formation is unconformably overlain by Quaternary deposits.

The proposed correlation datum is clearly recognizable on geophysical logs and may represent a regional change in the depositional systems. At two key reference wells near Delaware City, Delaware, this surface is expressed as the sharp basal contact of a 25-30-m-thick sand section over a prominent silt-clay interval. This same general character is recognized in correlations over much of the study area and is corroborated by previous palynological analyses of core samples from the two Delaware City wells and our new palynological analyses of a continuously cored hole near New Castle, Delaware. Samples from below the datum yielded taxa that are restricted to or more typical of the upper part of Potomac spore-pollen Zone II (uppermost Albian?). Samples from above the datum yielded taxa that appear at the base of Potomac spore-pollen Zone III (lower Cenomanian?).