Influence of antecedent geology on stratigraphic preservation potential and evolution of Delaware's barrier systems

Abstract

Delaware's Atlantic coast is one of the most studied (in three dimensions) in the world. Recently acquired shallow shoreface vibracores have added a new facet to that knowledge, allowing complete profiles from pre-Holocene headland to marsh, lagoon, barrier, and into shoreface environments. The Delaware coast is dominantly transgressive, acting in response to rising local relative sea level. Pre-Holocene geology plays a major role in coastal evolution as it is overridden and intersected by the ongoing transgression. The pre-Holocene surface is an erosional trellis-dendritic drainage system of tributaries to the ancestral Delaware River, cut during a time of lower Pleistocene sea level and increased flow which was enhanced by glacial meltwater. This erosional surface forms the basal unconformity. As Holocene coastal lithosomes migrate over this surface, they fill pre-Holocene valleys first, and then continue to thicken. The shoreface forms a second erosional surface — the ravinement unconformity. Spatial relationships between these two unconformities determine degree of preservation of the Holocene section. Deeper portions of the Holocene stratigraphic column, especially in antecedent valleys, provide the only chance for preservation in the shoreface under prevailing conditions of slow relative sea-level rise and 10 m deep shoreface scour. In nearby Pleistocene sediments of similar origin, the entire column has been preserved, because sea-level peaked, and then fell. Distribution of Holocene sediments in the shoreface is patchy, with little or no Holocene shoreface sediments found off headlands. A second role the antecedent geology plays is through sediment supply to the long-shore drift system. Coarse sediments, which make up a large fraction of the headlands, are transferred to littoral drift. As sea-level rise continues, rather than dominantly off-shore loss there appears to be a loss of sediments alongshore to Cape Henlopen and Hen and Chickens shoal, at the downdrift terminus. Much of the Delaware coast evolves by continuous retreat along a linear shoreline. Positions of baymouth barriers and headland beaches are determined by the antecedent topography. Barriers then are affected by overwash and inlet processes, which are not found on headland beaches.