AQUIFER GEOLOGY OF KENT COUNTY, DELAWARE: RESOLVING AQUIFER ISSUES IN THE DELAWARE COASTAL PLAIN USING SEQUENCE STRATIGRAPHIC CONCEPTS

Sequence stratigraphy allows reassessment of the correlation and geologic characterization of seven confined aquifers in Kent County, Delaware. The aquifers generally dip in a south-southeastward direction. The stratigraphically lowest aquifer, the Mount Laurel, is an Upper Cretaceous marine shelf deposit interpreted as a highstand systems tract capped by a significant unconformity. It is composed of glauconitic quartz sands; southward, these pass into fine-grained, non-aquifer facies. The Rancocas aquifer consists of upper Paleocene to lower Eocene glauconite-rich shelf sands with shells and cemented layers. It is developed in shallower-water facies deposited mainly as transgressive and highstand systems tracts; these pass southward into muddier non-aquifer facies. The Piney Point aquifer is a coarsening-upward succession of shelly, glauconitic, quartz sands deposited in a shelf environment. Most of the unit represents middle (to upper?) Eocene highstand deposits. The top of the aquifer is a lower Miocene transgressive lag deposited on a basal Miocene unconformity that progressively truncates the aquifer to the north-northwest.

The Miocene section is characterized by alternating sands and muds and includes four aquifer sands: Cheswold, Federalsburg, Frederica, and Milford. Each represents highstand deposits, capped by a sequence boundary, at the culmination of a shallowing-upward succession of shallow-marine to coastal deposits on a prograding wave-dominated coastline. Facies typically change from estuarine to near-shore in the northern extent of each aquifer to shoreface in the south; on wireline logs, this change is reflected in the sharp bases of sands to the north versus more gradual coarsening in the south. Overall progradation of the Miocene section results in equivalent facies occurring further south in each successive aquifer. Thin fine-grained confining layers that separate the aquifers in most areas are locally too thin or coarse grained to be effective flow barriers.

The results of this study highlight the value of sequence stratigraphy in understanding how global sea-level controls, influenced by local sediment supply and subsidence variations, created a unique suite of aquifers in this study area.