Stratigraphy and correlation of the Oligocene to Pleistocene section at Bethany Beach, Delaware

The Bethany Beach borehole (Qj32-27) provides a nearly continuous record of the Oligocene to Pleistocene formations of eastern Sussex County, Delaware. This 1470-ft-deep, continuously cored hole penetrated Oligocene, Miocene, and Pleistocene stratigraphic units that contain important water-bearing intervals. The resulting detailed data on lithology, ages, and environments make this site an important reference section for the subsurface geology of the region. Dark glauconitic to clayey Oligocene to basal Miocene sediments in the bottom of the hole are overlain by a Miocene section characterized by an overall shallowing-upward succession of shallow- and marginal-marine clastic sediments associated with wave-dominated shorelines. The lower Miocene Calvert Formation is composed of shelfal silts and scattered thick shoreface sands and shell beds. These pass upward into shelly shoreface sands and lesser silts of the lower-to-middle Miocene Choptank Formation. Silts and clays of the middle-to-upper Miocene St. Marys Formation separate the sandy middle Miocene shallow-marine strata from the sandy upper Miocene nearshore to marginal-marine section. The Cat Hill Formation is a coarsening- upward succession, changing from sandy offshore silts at the bottom to cleaner, shallow- or marginal-marine sands at the top. This unit is overlain by an interval of interbedded sands and muds of the upper Miocene Bethany Formation and a predominantly sand section of the upper Miocene (possibly Pliocene) Beaverdam Formation. The Pleistocene strata capping the section include predominantly muddy sediments of the Omar Formation in the lower part and micaceous sands of the Sinepuxent Formation nearest the surface. Fifteen sequences are recognized in the Oligocene to lowermost upper Miocene marine section. Each is commonly characterized by a thin, deepening-upward transgressive systems tract (sometimes absent) and a thicker shallowing-upward highstand systems tract. In addition, several possible sequences are identified in the upper Miocene (and Pliocene?) shallowmarine to non-marine section, and two sequences are identified in the shallow-subsurface Pleistocene strata. The findings at this site help delineate the correlation of aquifer-quality sands. Highstand-systems-tract sands in the lower -to-middle Miocene section are stratigraphically equivalent to the Cheswold, Federalsburg, Frederica, and Milford confined aquifers, important ground-water sources further north. Likely uppermost Miocene sands referred to the Manokin and Pocomoke aquifers (undifferentiated) are part of an interfingering complex of nearshore to estuarine deposits and do not appear to be consistently distinct strata in eastern Sussex County. Regional correlation reflects the location of Delaware between the sandy, deltaic Kirkwood-Cohansey system of New Jersey and the shelfal setting Calvert-Choptank succession of Calvert Cliffs, Maryland. The boundary between the Calvert and Choptank Formations appears to be time transgressive, occurring earlier in Delaware than at Calvert Cliffs.