Preliminary facies studies in the Great Marsh, on the southwestern shore of Delaware Bay, show a complex history of marsh-lagoonal facies forming the leading edge of the late Holocene marine transgression. A molluscan fauna (Crassostrea virginica, Macoma balthica, and Nassarius obsoletus) found in a dark-gray sand-mud, indicates a tidal creek-shallow coastal lagoon environment surrounded below, landward, and above by medium-dark-gray organic mud with variable amounts of Spartina grass and peat. Shallow cores, supplemented by deeper auger borings and reflection seismic surveys, indicate that the morphology of the Holocene transgression wedge and the present drainage patterns are a reflection of topography on a deeply eroded pre-transgression unconformity incised into Pleis ocene sediments.

Late Holocene geologic history and the resulting sediment facies patterns therefore are associated closely with fluvial drainage patterns on the pretransgression surface. A relative sea-level-rise curve has been used to place facies patterns and vertical sequences into a time-space perspective showing the migration of environments landward and upward over this dendritic erosion surface. The initial transgression of the fringing marsh and tidal creeks, and their widening into lagoons were followed by an increased sedimentation rate. This increase led to infilling of lagoons and formation of the present tidal creek-marsh system.

The transgressive sequence, because of continuing sea-level rise, is being partly obliterated and buried by a rapidly advancing washover barrier and shallow marine-estuary complex (Delaware Bay).

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