FACIES AND DEPOSITIONAL ENVIRONMENTS IN THE NON-MARINE CRETACEOUS DEPOSITS OF THE POTOMAC FORMATION, NORTHERN DELAWARE: IMPLICATIONS FOR AQUIFER SAND CONNECTIVITY (Invited Presentation)

This Potomac Formation is a highly heterogeneous succession of Cretaceous non-marine deposits in the subsurface of the Coastal Plain of the US Middle Atlantic region. As part of a series of multidisciplinary studies of this unit, we obtained nearly continuous wireline core records at five sites in the northern Delaware Coastal Plain that allow us to characterize the lithofacies and depositional environments present which, combined with other borehole and geophysical data, help delineate the geometry and connectivity of Potomac aquifer sands.

The Potomac Formation is predominantly fine-grained floodplain facies in the study area; the highly permeable sand lithologies that serve as important aquifers comprise approximately 20% of the volume of the formation. Five facies are identified in core: 1) amalgamated sands facies, typically 10- to 20-m thick sand intervals, interpreted as amalgamated stacks of multiple fluvial channels; 2) thick sands facies, with 2- to 6-m-thick sands, commonly fining upward near the top, interpreted as individual fluvial channels; 3) thin sands facies, sands < 3 m thick, interpreted as crevasse splay and levee; 4) interlaminated sands and silts facies, with centimeter-to decimeter-scale darker-colored sandy silt, silty sand, and silty clay, interpreted as floodplain lake, abandoned channel, and distal levee deposits; and 5) variegated silts and clays facies, composed of multicolored, mottled or banded silts and clays, mostly red and whitish gray, representing paleosols from weathered floodplain environments.

Examination of cores and/or geophysical logs from closely spaced boreholes indicates that the permeable sand bodies have limited aerial extent, in many cases thinning significantly or disappearing in distances of as little as 200 m. These borehole-based observations confirm recent findings from seismic reflection data that fluvial channel widths are between tens of meters and just over 800 m, with an average width of 170 m. These channel dimensions indicate that the the sand bodies are typically narrow and winding with limited lateral connectivity, contrasting with multi-kilometer scale correlations depicted in some previous studies. The facies associations and channel dimensions suggest the Potomac Formation was deposited by anastomosing fluvial systems.