Exploration for sand resources for beach nourishment has led to an increase in the amount of geologic data available from areas offshore Delaware's Atlantic Coast. These data are in the form of cores, core logs, and seismic reflection profiles. In order to provide a geologic context for these offshore data, this cross section has been constructed from well and borehole data along Delaware's Atlantic coastline from Cape Henlopen to Fenwick Island. Placing the offshore data in geologic context is important for developing stratigraphic and geographic models for predicting the location of stratigraphic units found offshore that may yield sand suitable for beach nourishment. The units recognized onshore likely extend offshore to where they are truncated by younger units or by the present seafloor.
RI75 Stratigraphy and Correlation of the Oligocene to Pleistocene Section at Bethany Beach, Delaware
The Bethany Beach borehole (Qj32-27) provides a nearly continuous record of the Oligocene to Pleistocene formations of eastern Sussex County, Delaware. This 1470-ft-deep, continuously cored hole penetrated Oligocene, Miocene, and Pleistocene stratigraphic units that contain important water-bearing intervals. The resulting detailed data on lithology, ages, and environments make this site an important reference section for the subsurface geology of the region.
This report supplements the map "Geology of the Seaford Area, Delaware" (Andres and Ramsey, 1995). The map portrays surficial and shallow subsurface stratigraphy and geology in and around the Seaford East and Delaware portion of the Seaford West quadrangles. The Quaternary Nanticoke deposits and Pliocene Beaverdam Formation are the primary lithostratigraphic units covering upland surfaces in the map area. Recent swamp, alluvial, and marsh deposits cover most of the floodplains of modern streams and creeks. The Miocene Choptank, St. Marys, and Manokin formations occur in the shallow subsurface within 300 ft of land surface. The Choptank, St. Marys, and Manokin formations were deposited in progressively shallower water marine environments. The Beaverdam Formation records incision of underlying units and progradation of a fluvial-deltaic system into the map area. The geologic history of the Quaternary is marked by weathering and erosion of the surface of the Beaverdam and deposition of the Nanticoke deposits by the ancestral Nanticoke River. Depositional environments in the Nanticoke deposits include fresh water streams and ponds, estuarine streams and lagoons, and subaerial dunes.
Heterogeneous unit ranging from very coarse sand with pebbles to silty clay. Predominant lithologies at land surface are white to mottled light-gray and reddish-brown, silty to clayey, fine to coarse sand. Laminae and beds of very coarse sand with pebbles to gravel are common. Laminae and beds of bluish-gray to light-gray silty clay are also common. In a few places near land surface, but more commonly in the subsurface, beds ranging from 2 to 20-ft thick of finely laminated, very fine sand and silty clay are present. The sands of the Beaverdam Formation commonly have a white silt matrix that gives drill cuttings a milky appearance (Ramsey, 2001, 2007). This white silt matrix is the most distinguishing characteristic of the unit and readily differentiates the Beaverdam Formation from the adjacent clean sands of the Turtle Branch Formation. Interpreted to be a fluvial to estuarine deposit of late Pliocene age on the basis of pollen assemblages and regional stratigraphic relationships (Andres and Ramsey, 1995, 1996; Groot and Jordan, 1999; Groot et al., 1990). Ranges from 50 to 120 ft thick in the Georgetown Quadrangle.
The microflora of the Bethany formation and the lower part of the Beaverdam Formation is characterized by a Quercus-Carya assemblage, very few non-arboreal pollen, and Pterocarya and Sciadopitys as exotic constituents. This assemblage has much in common with that of the Brandywine Formation of Maryland and the Eastover Formation of Virginia which are of late Miocene or early Pliocene age. The environment of deposition of the Bethany was probably deltaic, and that of the lower Beaverdam fluviatile.
Beaverdam Branch, the Nanticoke River, Sowbridge Branch, and Stockley Branch drain small basins in the Delaware Coastal Plain that are characterized by similar climate, topography, geology, and land use. Withdrawals of ground water and surface water are very small, there is little urbanization, and other man-made effects, which include minor regulation on Sowbridge Branch and construction of drainage ditches in the Nanticoke basin, probably have had minimal effect on the natural hydrologic regimen. These are virtually natural-flow streams, which, because of similar basin characteristics, have nearly identical rates of evapotranspiration and runoff. During the 10-year period, 1959-68, precipitation averaged 40-42 inches annually, runoff averaged 16-17 inches annually, and evapotranspiration averaged 23-25 inches annually.
This map shows the surficial geology of Kent County, Delaware at a scale of 1:100,000. Maps at this scale are useful for viewing the general geologic framework on a county-wide basis, determining the geology of watersheds, and recognizing the relationship of geology to regional or county-wide environmental or land-use issues. This map, when combined with the subsurface geologic information, provides a basis for locating water supplies, mapping ground-water recharge areas, and protecting ground and surface water. Geologic maps are also used to identify geologic hazards, such as flood-prone areas, to identify sand and gravel resources, and to support state, county, and local land-use and planning decisions.
The surficial geology of the Lewes and Cape Henlopen quadrangles reflects the geologic history of the Delaware Bay estuary and successive high and low stands of sea levels during the Quaternary. The subsurface Beaverdam Formation was deposited as part of a fluvial-estuarine system during the Pliocene, the sediments of which now form the core of the Delmarva Peninsula. Following a period of glacial outwash during the early Pleistocene represented by the Columbia Formation found to the northwest of the map area (Ramsey, 1997), the Delaware River and Estuary developed their current positions. The Lynch Heights and Scotts Corners Formations (Ramsey, 1993, 1997, 2001) represent shoreline and estuarine deposits associated with high stands of sea level during the middle to late Pleistocene on the margins of the Delaware Estuary. In the map area, the Lynch Heights Formation includes relict spit and dune deposits at the ancestral intersection of the Atlantic Coast and Delaware Bay systems, similar in geomorphic position to the modern Cape Henlopen.
The surficial geology of the Ellendale and Milton quadrangles reflects the geologic history of the Delaware Bay estuary and successive high and low sea levels during the Quaternary. Ramsey (1992) interpreted the Beaverdam Formation as deposits of a fluvial-estuarine system during the Pliocene. Sediment supply was high, in part due to geomorphic adjustments in the Appalachians related to the first major Northern Hemisphere glaciations around 2.4 million years ago. The Beaverdam Formation forms the core of the central Delmarva Peninsula around which wrap the Quaternary deposits.
This map shows the distribution of geologic units found at or near land surface. These units support agriculture and development, are mined for sand and gravel resources, and are the surface-to-subsurface pathway for water. Previous maps and reports covering the same of adjacent areas have focused on hydrogeology (Andres, 1994), surficial geology on a regional basis (Jordan, 1964, 1974; Owens and Denny, 1979, 1986; Denny et al., 1979; Ramsey and Schenck, 199), or subsurface geology (Hansen, 1981; Andres, 1986).
These vector and raster data sets contain the rock unit polygons for the surficial geology in the Delaware Coastal Plain covered by DGS Geologic Map No. 11 (Milton-Ellendale area) in ESRI shapefile and TIF format.
These raster and vector datasets contains the rock unit polygons for DGS Geologic Map No. 9 (Seaford). This map shows the distribution of geologic units found at or near land surface.
This data set contains the rock unit polygons for the surficial geology in ESRI shapefile format for DGS Geologic Map No. 14 (Geologic Map of Kent County, Delaware). This map shows the surficial geology of Kent County, Delaware, at a scale of 1:100,000.
These vector and raster data sets contain the rock unit polygons for the surficial geology in ESRI shapefile and TIF format for the Delaware Coastal Plain covered by DGS Geologic Map No. 12 (Lewes-Cape Henlopen area).