The Geology of Delaware

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Delaware Bay Group

Qdb
Geologic Time Period: 
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 Delaware Bay Group consists of transgressive deposits that were laid down along the margins of ancestral Delaware Bay estuaries during middle to late Pleistocene rises and highstands of sea level. The Delaware Bay Group was described in detail by Ramsey (1997).

The Delaware Bay Group deposits consist of light reddish-brown to gray, medium to medium-to-coarse sands with common beds of fine to medium sand and very fine to fine sand and very fine to fine sandy silt. Also present are beds of gray clayey silt and brown, organic-rich clayey silt that are commonly found in lensoid channel-fill bodies. Beds of gray, fine to very fine clayey sand to clayey silt with shell are found in its eastern extent near Rehoboth Beach. The sands are quartzose with varying amounts of feldspar, slightly less than quantities of feldspar found in the Columbia Formation. The deposits are heterogeneous both vertically and laterally. The general trend within the formations is a fining upwards of sediment textures. Geomorphology: The Delaware Bay Group deposits are found beneath terraces that have scarps roughly parallel to the Delaware River and Bay tributaries, and relatively flat treads that slope gently toward the modern Delaware Bay.

The Delaware Bay Group includes transgressive deposits consisting of stream, swamp, marsh, estuarine barrier and beach, tidal flat, lagoon, and shallow offshore estuary environments (Ramsey, 1997).

The Delaware Bay Group is comprised of the Lynch Heights Formation, the Scotts Corners Formation, and the Cape May Formation (undivided) in New Jersey. Ramsey (1997) suggested that the Pleistocene interglacial deposits on the New Jersey side of Delaware Bay be included in the Delaware Bay Group. The Cape May Formation has similar geomorphic characteristics, ages, and depositional environments (O’Neal and McGeary, 2002; Newell et al., 2001) to the Delaware Bay Group.

Reference(s): 

Newell, W.L., Powars, D.S., Owens, J.P., and Schindler, J.S. 2001, Surficial geologic map of central and southern New Jersey: U.S. Geological Survey Miscellaneous Investigation Series Map I-2540-D, scale 1:100,000.

O’Neal, M.L., and McGeary, S., 2002, Late Quaternary stratigraphy and sea-level history of the northern Delaware Bay margin, southern New Jersey, USA: a ground penetrating radar analysis of composite Quaternary coastal terraces: Quaternary Science Reviews, v. 21, p. 929-946.

Ramsey, K. W., 1997, Geology of the Milford and Mispillion River quadrangles: Delaware Geological Survey Report of Investigations No. 55, 40 p.