GEOLOGY OF A BAYHEAD DELTA WITHIN A POTOMAC RIVER TIDAL-FRESHWATER ESTUARY: CHESAPEAKE BAY, VIRGINIA

Chesapeake Bay is the largest estuary in the United States. Numerous tidal freshwater subestuaries ring the upper reaches of Chesapeake Bay and its tributaries, which are sites of active sedimentation and bayhead delta development. One such bayhead delta is at the mouth of Pohick Creek and has prograded into the apex of Pohick Bay. Pohick Bay was investigated to document the geologic and geomorphic evolution of a tidal-freshwater bayhead delta.

A series of historic aerial photographs show that the delta is actively prograding about 98m/year. Seventeen vibracores were collected throughout the bayhead delta. The vibracores were visually described, subsampled for grain-size analysis, photographed, and x-rayed. From these data, three strike-oriented and two dip-oriented cross-sections were constructed. The entire dataset was used to identify 11 distinct facies, and ten depositional environments were interpreted. The lithologic and sequence stratigraphic analysis of the Pohick bayhead delta sediments were organized into a six-stage paleo-evolutionary model.

The six-stage evolutionary model depicts the sedimentary response in Pohick Bay to fluctuating eustatic sea-level changes from the Cretaceous to Holocene. The model begins with deposition of the Cretaceous Potomac and Tertiary Brightseat formations. These sediments were later incised due to falling eustatic sea levels. Numerous transgressions and regressions during the Pleistocene resulted in deposition of fluvial-deltaic facies on the previously eroded Potomac Formation. These facies were last incised due to a drop in eustatic sea level about 18 kyrs that created a major erosional unconformity (Type 1 sequence boundary). This unconformity was later reworked as the paleo-Pohick Valley was flooded during the post-glacial eustatic rise in sea-level, which resulted in landward shoreline migration. As sea-level rise slowed, the net rate of sedimentation dominated over the eustatic signal resulting in the well-developed Pohick bayhead delta.