MIOCENE LITHOSTRATIGRAPHY, SEQUENCE STRATIGRAPHY, AND AQUIFER MAPPING IN THE CENTRAL DELMARVA PENINSULA (Invited Presentation)

Subsurface geological mapping of the Miocene aquifers of the central Delmarva Peninsula using geophysical logs from more than 200 boreholes, including relatively continuous core records from two sites, allows reassessment of regional lithostratigraphic correlation and sequence stratigraphy. This study focuses on three lower to middle Miocene formations: Calvert, Choptank, and St Marys. The Calvert and Choptank Formations are characterized by alternating shallow-marine, and lesser estuarine, sands and muds in central Delmarva. The thicker sand units form important aquifers, each representing the culmination of a shallowing-upward succession deposited by a prograding wave-dominated coastline. The overlying St Marys Formation is a predominantly muddy interval in central Delmarva and acts as a regional confining unit.

The total thickness of the lower to middle Miocene increases basinward to the east and south. Individual aquifer sands generally thicken along this trend, but the majority of the added thickness occurs in the finer-grained intervals between the sands. The lithofacies also show a general trend of deeper environments basinward; in the aquifer sands, this is expressed as a transition from estuarine and nearshore deposits to shoreface deposits. Overall progradation of the Calvert-Choptank section results in equivalent facies occurring further south in each successive aquifer.

The Calvert-Choptank interval is a stack of highstand-dominated stratigraphic sequences in this area. In most sequences, a basal sequence boundary (SB) is overlain by a thin transgressive systems tract (TST), a thicker highstand systems tract (HST) consisting of a muddy lower part and a sandy upper part, and a SB at the top. However, up-basin sequences are mostly sand-rich HSTs with more poorly developed mud intervals. This up-basin change in character reflects shallower environments in the HSTs and more time represented in SBs, likely including shaving of upper HST deposits. The up-basin/down-basin contrast has a significant influence on the hydrological system. In down-basin areas, individual aquifers can be differentiated in the Calvert and Choptank Formations; but up-basin, mud intervals become thinner and individual sand units become amalgamated, forming a composite aquifer system.