DISTRIBUTION OF LATE PLIOCENE AND QUATERNARY DEPOSITS IN THE MIDDLE ATLANTIC COASTAL PLAIN: DELAWARE, MARYLAND, AND VIRGINIA

Factors controlling distribution of late Pliocene and Quaternary deposits in the Middle Atlantic Coastal Plain include long-term tectonic history of the Salisbury Embayment, drainage basin history of the major rivers feeding into the region, sediment supply influenced by glaciation and glacial-interglacial climatic fluctuations, and sea-level rise and fall. A shift of the depocenter of the Salisbury Embayment from north to south occurred from the Miocene to the Pleistocene. Late Pliocene and Quaternary deposits are more widespread and have a stronger marine influence in Virginia than to the north in Maryland and Delaware. Deposition of inner Coastal Plain stratigraphic units was essentially complete by the late Pliocene. This interval was characterized by high sediment supply driven by climate change and establishment of present drainage systems. This resulted in widespread deposition of the Beaverdam Fm. in Delaware and Maryland and the Bacons Castle Fm. in Virginia. In the early Pleistocene, river valleys were carved through the late Pliocene deposits. These valleys became the locus of erosion and deposition of the fluvial-estuarine terrace deposits throughout the rest of the Quaternary. Early Pleistocene deposition in the region was limited to glacial outwash deposits of the Columbia Fm. in Delaware and estuarine to shallow marine deposition in the Windsor Fm. in Va. Seaward-facing terrace deposits and the mega-spit deposits of the southern part of the Delmarva Peninsula represent the redistribution of sediment deposited during late Pliocene and early Pleistocene with some minor to moderate introduction of new sediment. These deposits are the result of the accommodation space in lagoonal and fluvial to estuarine environments produced by sea-level rise during the Quaternary. Eolian deposits related to Quaternary cold climate intervals are found scattered throughout the region.