The Geology of Delaware

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Nanticoke River Group

Qnrg
Geologic Time Period: 
middle to upper Pleistocene
Coastal Plain - Primarily Surficial Unit

The following description was published in RI76 Stratigraphy, Correlation, and Depositional Environments of the Middle to Late Pleistocene Interglacial Deposits of Southern Delaware, Ramsey, K.W., 2010:

The Nanticoke River Group consists of the Turtle Branch and Kent Island Formations. In Delaware, the Nanticoke River Group extends along the margins of the Nanticoke River and its tributaries. It continues along the Nanticoke River into adjacent Maryland.

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 Nanticoke River Group consists of fluvial to estuarine, fine to coarse sand and estuarine clayey silts to silty clays that were deposited during highstands of sea level during the late Pleistocene. In Delaware, these deposits underlie terraces that flank the margins of the present Nanticoke River and its tributaries. Upstream the terraces become less distinct, and in places the surface of the Nanticoke River Group does not have a distinctive boundary scarp with the adjacent Beaverdam Formation. The Nanticoke River Group sands, however, are distinct and readily discernable from those of the Beaverdam Formation; the Nanticoke River Group sands are more well sorted, less feldspathic, and lack the distinctive white silty matrix of the Beaverdam Formation.

The informal term “Nanticoke deposits” was used by Andres and Ramsey (1995, 1996) for Quaternary sediments along the Nanticoke River in the vicinity of Seaford in western Sussex County. These deposits included estuarine sediments as well as eolian dunes along the margins of the Nanticoke River. More recent mapping in the Georgetown area in 2006 and 2007 (Ramsey, 2010), as well as along the Nanticoke River to the southwest of Seaford in 2005 (unpublished DGS data), has allowed for more detailed analysis of the deposits and for recognition of two stratigraphic units within what was mapped as the Nanticoke deposits.

Reference(s): 

Andres, A.S., and Ramsey, K.W., 1995, Geologic Map of the Seaford area, Delaware: Delaware Geological Survey Geologic Map Series No. 9, scale 1:24,000.

_____, 1996, Geology of the Seaford area, Delaware: Delaware Geological Survey Report of Investigations No. 53, 22 p.