GEOLOGIC MAPPING OF MIOCENE TO PLEISTOCENE PALEOCHANNELS ACROSS THE MID-ATLANTIC COASTAL PLAIN REGION: RECORD OF SEA LEVEL, PALEOCLIMATE, AND STRUCTURAL INFLUENCES

Modern geologic mapping of the mid-Atlantic Coastal Plain has revealed a landscape incised by numerous Miocene to Pleistocene paleochannels filled with fluvial, estuarine, swamp, to shallow-marine sediments, and capped by aeolian deposits. Regional compilation of these paleochannels with previously mapped offshore depocenters reveals at least five major Miocene to Pleistocene paleochannel systems. Our mapping 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 channel systems; and the late Miocene was dominated by a combined paleo-Hudson/Delaware system and a paleo-Potomac channel systems. The Pliocene and early Pleistocene were similar to the late Miocene and also included a paleo-James system. During the mid-late Pleistocene all 5 paleochannel systems were active.

Pliocene paleochannels vary from broad (1.6-2.2 km) and shallow (up to 15-m-deep) with outwash or braided river fluvial coarse arkosic sand and gravel fills (Pensauken and Beaverdam Fms.) to broad (~1.5-2.5 km) and deep (up to 70-m) with regressive fluvial sand and gravel fills overlain by transgessive-highstand estuarine-clay, silt, and sand to occasionally open bay to marine deposits with dinoflagellates, diatoms, and silicoflagellates (Kent Island Fm.). The Exmore paleochannel (paleo-Susquehanna) has a cosmogenic burial isochron age (~1.7 Ma) derived from basal gravels. Microflora and fauna provide depth of incision ages.

The age and location of individual paleochannel systems reflects the interplay of the following regional-to-local variables: tectonic influences that affected location of sediment source (uplift-erosion Appalachian Mountains) and drainage patterns (including fluvial meander bends); sea level (tied to climate and tectonics, effects base-level, erosion, deposition); and climate which includes glacioisostatic driven subsidence-deposition across the proposed glacial bulge area and beyond. These paleochannels preserve sea-level and paleoclimate data that provide constraints on the timing and magnitude of glaciations and are essential to unraveling the subsidence and sea level oscillation record of the past and provide potential possible insights into the future.