STRATIGRAPHIC ANALYSIS OF A DEEP CORE RECORD THROUGH MIOCENE-AGE SANDY SHALLOW-MARINE DEPOSITS IN SOUTHERN DELAWARE

The Calvert and Choptank Formations are two aquifer-bearing, Miocene geologic units prominent in the subsurface of southern Delaware. Aquifers in this region are an important source of fresh water for residential, commercial, and agricultural uses. A near-continuous wireline core throughout these Miocene units was recovered in a 630-ft-deep borehole drilled in 2017 near Milford, Delaware. This location is central compared to other studies in the region, and as such, results from this borehole can be correlated to these recent studies on the same units in the subsurface of southern Delaware.

Geophysical logs including gamma and spontaneous potential were taken and lithological analyses were completed. Preliminary findings show interbedded sequences of sands and muds, where sands typically represent aquifers and muds are the confining units. Geophysical logs, along with the sedimentological data, are used to characterize sediments and formation thicknesses, as well as determine stratigraphic sequences and sequence boundaries.

Aquifer sands are generally medium-grained quartz, with some sections containing coarser sands with granular quartz, varying in thickness from 5 to 30 feet. Given the high abundance of shell fragments and whole specimens (bivalves, gastropods), these sands are interpreted to be deposits from intertidal to shoreface environments. Muds, in which sections range from 3 to 45 feet thick, are indicative of deeper shelfal environments, confirmed by the presence of fine shell fragments, common bioturbation, and thin very-fine-grained sands with lenticular cross-laminations. Alternations between these environments can be seen throughout the core, representing sea level rise and fall during the Miocene. Sequence boundaries are generally associated with the tops of shallow marine aquifer sands.

The results from this stratigraphic analysis can be used to correlate this borehole to other sites around the region, resulting in a better understanding of the geology, aquifer distribution, and lithofacies changes in the subsurface of southern Delaware. Future palynomorph assemblage and dinoflagellate analyses of this core will be integrated with the stratigraphic findings to allow for better interpretations of changes in past depositional environments and climates.