Stratigraphic and geomorphic framework of uppermost Cenozoic deposits in the southern Delmarva Peninsula, Virginia and Maryland

Abstract

The surface of the southern Delmarva Peninsula is a flat to gently rolling central ridge or upland bordered on both east and west by low plains or terraces that slope very gently toward the Chesapeake Bay and the Atlantic Ocean.Surfaces of the less dissected parts of both the upland and the lowland plains are inferred to be remnants of the original depositional surfaces, which formed during emplacement of the directly underlying marginal-marine and fluvial-estuarine deposits.The surface and shallow subsurface deposits of the study area include (1) a 5-to 200-ft-thick (2-to 60-m-thick) mantle of unconsolidated sand, gravel, silt, clay, and peat of Quaternary age that unconformably overlies (2) several hundred feet of more consolidated upper Tertiary glauconitic sand and clay-silt.The contact between the two sedimentary systems is generally marked by a lag gravel or peat at the base of the Quaternary beds and by differences in color and compaction.The erosion surface at the top of the Tertiary section is commonly gently undulating with about 10-30 ft (3-9 m) of local relief; however, contouring of the erosion surface shows major paleotopographic lows (paleovalleys) trending southeastward across the northem, central, and southern parts of the study area The largest buried paleovalley, which crosses the peninsula near Eastville, Va, was eroded to depths of 165 ft (50 m) below present sea level and is be-.lieved to mark a main drainageway of the ancestral Susquehanna-Potomac river system.The upper Tertiary strata, which form the core of the peninsula, dip gently southeastward toward the Baltimore Canyon trough.The lithology and the fairly diverse molluscan and ostracode assemblages of these beds indicate shallow-shelf depositional environments and enable correlation with the upper Miocene to middle Pliocene Eastover and Yorktown Formations, which crop out west of the Chesapeake Bay.However, the thick, sideritic clay-silts and pebbly sands of the Tunnels MiU Member of the Yorktown Formation are interpreted to be the distal part of a southeastward-prograding deltaThe Quaternary sediments that mantle the mainland part of the peninsula consist largely of marginal-marine and estuarine deposits emplaced during high stands of the sea associated with one or more Gl G2 GEOLOGIC STUDIES IN EMERGED COASTAL PLAIN OF MIDDLE ATLANTIC STATES (the Bell Neck sand-ridge complex).The fining-upward Joynes Neck Sand is a fine to coarse sand and sandy gravel, as much as 30 ft (10 m) thick, interpreted to be a transgressive shelf-plain deposit and thought to be equivalent, in part, to the Nassawadox Formation.The coarsening-upward Wachapreague Formation, as much as 40ft (12m) thick, includes a lower member of fossiliferous, muddy, fine sand of shallow-shelf origin and an upper member of relatively clean, gravelly sand that forms a regressive sequence of at least 11 barrier ridges.Molluscan and ostracode assemblages composed of both coldtemperate and warm-temperate species and pollen assemblages containing abundant spruce and pine indicate that the Wachapreague beds were deposited during cooling climatic conditions.Shell material from the Wachapreague beds yields a radiocarbon age of greater than 33,000 years B.P. and an amino-acid-racemization age estimate of 82,000 years B.P., which are comparable to uranium-thorium and amino-acid-racemization ages from the Butlers Bluff Member of the Nassawadox Formation.These age estimates suggest that the Wachapreague beds represent the regressive phase of a Nassawadox transgressive-regressive cycle rather than a separate major transgression in mid-Wisconsin time.