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

The geology of a bayhead delta at the head of Pohick Bay, Virginia is under investigation using seventeen vibracores (up to 6m long), x-ray radiography, grain-size analysis, and ¹⁴C dating. Pohick Bay is a relatively isolated freshwater embayment of the Potomac River estuary that experiences a tidal range of ~0.6m and low wave energy. Pohick Creek has an average discharge of 1.1m3/sec and its watershed covers 8,806 ha of southeastern Fairfax County, VA. As the hydraulic regime shifts from fluvial dominated flow to tidal flow at Pohick Bay, the carrying capacity diminishes and sediment is deposited at the mouth of the creek, creating a 36.2 ha bayhead delta capped by a freshwater marsh. Comparisons of aerial photographs from 1937 and 2000 indicate that the bayhead delta is actively prograding seaward.

A stratigraphic investigation is being performed at Pohick Bay to document the geologic history of the area. The vibracores were collected along three strike sections across the marsh, and the following results are based on one vibracore from the most up-dip cross-section. Five sedimentary facies were identified within the 2.15m core. From bottom to top, facies 1 is 4 cm thick and characterized by a dark brown silty clay interlaminated with fine sand with minor bioturbation. Facies 2 is 100 cm thick and is characterized by a dark brown, highly bioturbated silty clay. Facies 3 is 45 cm thick and is a light brown, interlaminated silty clay and fine quartz sand that is highly bioturbated. Facies 4 is a light gray, highly bioturbated mud that is 30 cm thick. Facies 4 and facies 5 are separated by a distinct erosional surface at 36 cm. Facies 5 36 cm thick and is characterized by a brown, medium to coarse quartz sand.

This preliminary data shows that coarser sediments have prograded over finer sediments (coarsening upward sequence), which is consistent with bayhead-delta sedimentary models.