The geological history of the surficial units of the Clayton, Smyrna, and the Delaware portion of the Millington Quadrangles are the result of deposition of the Beaverdam Formation and its modification by erosion and deposition of the Columbia Formation during the early Pleistocene. These units were then modified by the Lynch Heights and Scotts Corners Formations as a result of sea-level fluctuations during the middle to late Pleistocene. The geology is further complicated by periglacial activity that produced Carolina Bay deposits in the map area, which modified the land surface.
Open-File Report 50 presents and describes a database of geochronological information for coastal deposits of the US Atlantic and Pacific coasts, as well as for sites from the Pacific coast of South America. This database represents a synthesis of nearly forty years of study conducted by John F. Wehmiller and students in the Department of Geological Sciences, University of Delaware, as well as many collaborating colleagues.
The complex geologic history of the surficial units of the Harbeson Quadrangle is one of deposition of the Beaverdam Formation and its subsequent modification by erosion and deposition related to sea-level fluctuations during the Pleistocene. The geology is further complicated by periglacial activity that produced dune deposits and Carolina Bays scattered throughout the map area.
The Nanticoke River Group consists of the Turtle Branch and Kent Island Formations. The Nanticoke River Group consists of heterogeneous units of interbedded fine to coarse sand, clayey silt, sandy silt, and silty clay. Where the units are muddy, downstream of Seaford, the sequence consists of a lower fluvial to estuarine swamp to tidal stream deposits (coarse sand to gravelly sand with scattered organic-rich muddy beds) overlain by estuarine clayey silts and silty clays that contain rare to common Crassostrea (oyster) bioherms. The silts and clays are overlain by sands with clay laminae, to fine to coarse well-sorted, clean sand that are estuarne beach and eolian in origin. Upstream, the mud beds are rarer and restricted to the west side of streams and consist of organic rich clayey silt. Most of the stratigraphic section is dominated by clean, well-sorted sands.
The Assawoman Bay Group consists of the well-sorted sands, silts, and clays of the Omar, Ironshire, and Sinepuxent Formations found adjacent to and inland of the Atlantic Coast of Delaware and Maryland. These deposits in Delaware and Maryland were named from oldest to youngest: the Omar Formation (Jordan, 1962, 1964), the Ironshire Formation (Owens and Denny, 1979a), and the Sinepuxent Formation (Owens and Denny, 1979a).
Owens and Denny (1979) named the Kent Island Formation for deposits bordering the Chesapeake Bay found underneath lowlands that ranged in elevation from 0 to 25 feet in elevation but most of the land surface area is less than 10 feet in elevation. These lowlands are bordered by a scarp with at toe at approximately 25 feet. In its type area, the Kent Island Formation was described as consisting of thick beds of loose, light colored, cross-stratified sand overlying dark-colored massive to thinly laminated clay-silt. Pebbles as much as 10 cm (4 in.) in diameter occur in thin beds with the sand or as scattered clasts in both the sand and clay-silt. Locally, large tree stumps in growth position are encased in the clay-silt. Maximum thickness of the Kent Island was about 12 m (40 feet).
Owens and Denny (1979) described the Sinepuxent Formation in Maryland as dark, poorly sorted, silty fine to medium sand with the lower part of the unit being fine grained with thin beds of black clay. The Sinepuxent Formation is described as being lithically distinct from the Omar and Ironshire Formations due to the presence gray, laminated, silty very fine to fine, quartzose, micaceous, sand to sandy silt. The base of the unit is typically a bluishgray to dark-gray clayey silt to silty clay. There are a few shelly zones within the Sinepuxent Formation in the vicinity of Bethany Beach (McDonald, 1981; McLaughlin et al., 2008). The Sinepuxent Formation is up to 40 feet thick.