QUATERNARY GEOLOGY OF THE DELAWARE ESTUARY

The Delaware is rare among Atlantic streams in flowing from glaciated terrain across unglaciated Piedmont and Coastal Plain. The Delaware estuary between Trenton, NJ and Wilmington, DE thus contains a stratigraphic and geomorphic record of repeated Pleistocene glaciofluvial and periglacial deposition and interglacial estuarine deposition. The Pleistocene deposits are set within a preglacial fluvial valley of Pliocene age, which contains a sandy fluvial braidplain (Pensauken Formation) laid down during the mid-Pliocene highstand by a river flowing from the New York City area. Pre-Illinoian glaciation ~2 Ma diverted this river, and the valley segment below Trenton became the lower Delaware. Incision during the early Pleistocene was followed by estuarine deposition (Cape May Formation) during peak highstands in the middle and late Pleistocene, primarily OIS 11? (Cape May 1) and OIS 5 (Cape May 2 and 3). Nonglacial fluvial terrace deposits were laid down during OIS 6 (upper terrace) and OIS 3 and 2 (lower terrace) in valleys inset in the highstand fills. The late Wisconsinan (OIS 2) glaciofluvial deposit (Trenton Gravel) is inset into the lower terrace deposit. The lower terrace and Trenton Gravel are onlapped and covered by Holocene estuarine deposits. The geometry of these deposits indicates that repeated glacial cycles in the middle and late Pleistocene produced progressive narrowing, deepening, and headward extension of the estuary. The deepening and extension have led to progressively finer-textured highstand fills. Incision following highstands selectively keyed into the fine fill, further sharpening estuarine geometry.