MIOCENE TO PLEISTOCENE PALEOCHANNELS OF THE MID-ATLANTIC COASTAL PLAIN: REGRESSIVE LOWSTAND CONDUITS AND TRANSGRESSIVE HIGHSTAND INFILLING

Recent core and auger drilling in the western part of the Delmarva Peninsula reveals prominent differences between uplands (12-22 m elev.) and lowlands (<8 m elev.). Uplands are riddled with buried broad (1.6-2.2 km), shallow (up to 15-m-thick) Pliocene paleochannels (Pensauken and Beaverdam Fms.). These channels are part of a paleo-Hudson/Delaware river system that is filled with arkosic sand and gravel. Lowlands have similarly broad (~1.5-2.5 km) but deeper (up to 70-m-thick) Pleistocene paleochannels. These channels were cut by a paleo-Susquehanna system initially, filled diachronously by regressive fluvial sand and gravel, and then overlain by transgessive-highstand estuarine clay, silt, and sand. A 50-m-thick section of the Exmore paleochannel, cored on Kent Island, includes a 20-m-thick marine section (-21 to -41 m elev.) with Pleistocene dinoflagellates, diatoms, and silicoflagellates.

Cores and seismic surveys have led to the recognition of at least six major Miocene to Pleistocene paleochannel systems in the mid-Atlantic Coastal Plain. These paleochannels were cut diachronously during lowstands and filled during highstands in response to eustatic and tectonic base-level changes. During lowstands, they served as conduits for large amounts of sediment that bypassed the emergent coastal plain and most of the shelf to be deposited in the depocenters on the shelf edge, slope and rise. Fluvial-deltaic regressive-progradational deposits filled the inner coastal plain parts of these paleochannels. Some eustatic events cut new drainage pathways; others reoccupied previous unfilled paleochannels. Local structural influences contributed to southward shifting of these fluvial-deltaic systems.

Mapping of paleochannels shows that the early Miocene was dominated by a paleo-Hudson channel system; the middle Miocene was dominated by paleo-Hudson, paleo-Susquehanna, paleo-Potomac, and paleo-James systems; and the late Miocene was dominated by a combined paleo-Hudson/Delaware system and a paleo-Potomac system. The Pliocene and early Pleistocene were similar to the late Miocene but with the addition of the paleo-James. Mid-late Pleistocene deposition was dominated by separate paleo-Hudson and paleo-Delaware systems, as well as paleo-Susquehanna, paleo-Potomac and paleo-James systems.