Abstract

The Quaternary evolution of the coastal zone along the southern Delmarva Peninsula of Virginia was investigated by means of a high-resolution seismic survey and supplemental shallow core borings. The seismic stratigraphic framework of the area is composed of three depositional sequences separated by two prominent unconformities. In ascending order, the three seismic sequences represent a late Tertiary substrate, Pleistocene deposits of post-Illinoian(?) age, and Holocene deposits. Stratigraphic analysis indicates that the evolutionary development of the area was controlled mainly by glacio-eustatism and paleotopographic features.

The late Tertiary substrate was dissected by southeasterly flowing streams of the ancestral Susquehanna fluvial system, resulting in a highly furrowed surface consisting of valleys and interfluves that exhibit as much as 46 m of local relief. The erosional surface probably represents a mature multicyclic landscape of pre-Sangamonian(?) age. The unconformably overlying Pleistocene deposits attain a maximum thickness of 41 m. Pleistocene sedimentation was closely controlled by the paleotopography, with relatively thicker deposits accumulating within ancestral valleys. In contrast, thinner deposits accumulated over the ancestral interfluves, which did not become active sites of sedimentation until after the topographic relief had been substantially subdued by valley infilling. Pleistocene deposits are characterized by complex fades relationships, multiple generations of fluvial channeling, and the presence of a variety of coastal geomorphic features. Fluvial channel zones largely overlie the thalwegs of major southeasterly trending ancestral valleys, indicating a paleotopographic control of Pleistocene drainage routes. The youngest Pleistocene geomorphic feature, which constitutes a distinctive sand lithosome, is a coastal barrier ridge that appears to have developed during a Sangamonian interglacial or mid-Wisconsinan interstadial high stand of sea level. During the late Wisconsin regression, Pleistocene deposits were eroded, resulting in the development of a relatively low-gradient subdued landscape compared to the highly furrowed pre-Sangamonian landscape.

Holocene deposits unconformably overlie the Pleistocene sequence and attain a maximum thickness of 19 m. Sedimentation during the Holocene transgression was primarily influenced by late Wisconsinan paleotopography, with relatively thicker deposits accumulating within paleochannels and along the modern shoreface. Holocene deposits also exhibit complex facies relationships, extensive channeling, and coastal geomorphic features. The Holocene sediments consist of two distinct lithosomes that are representative of modern barrier-island and lagoon–marginal-lagoon subenvironments. Microfossil assemblages indicate paleosalinities ranging from upper estuarine to marginal marine–inlet conditions. Vertical environmental transitions during the late Holocene transgression reflect a progressive restriction of the southern Delmarva coastal area by barrier-island development, with subsequent lagoonal infilling. The modern barrier islands are retrograding, and lagoonal infilling has progressed to varying degrees, having been influenced by both back-barrier sedimentation rates and pre-Holocene topography.

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