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GPR investigation of multiple stage-5 sea-level fluctuations on a siliciclastic estuarine shoreline, Delaware Bay, southern New Jersey, USA

Abstract Ground penetrating radar (GPR) surveys were used to identify three distinct sea-level highstands within the oxygen isotope stage (OIS) 5-equivalent sediments of the Quaternary Cape May Formation in southern New Jersey. The three units, QCM1a, QCM1b, and QCM1c, are designated as submembers of the Cape May 1 member. The transgressive ravinement surfaces (TRS) forming the bounding unconformities of these units are recognized as laterally extensive reflections on the radar records and have been corroborated as gravel lag deposits and associated lithological discontinuities in continuous vibracores and hand-augered boreholes. GPR and lithological data have been used to interpret these highstand deposits in a sequence-stratigraphic framework. These units are thin (3 m or less), lithologically similar and present limited geomorphic expression of their individual existence beneath a low terrace (+6 m to mean sea level/MSL) on the northern margin of Delaware Bay. In the upper two units, QCM1b and QCM1a, radar surveys across subtle, bay-margin parallel, sublinear topographic highs of 3 m relief or less in the Jones Island area reveal internal structures consistent with geomorphic interpretation as stranded estuarine shorelines. The newly identified shoreline complexes are located well bayward of, and lower in elevation than, the commonly accepted +5 m to +6 m MSL OIS 5e shoreline. This older shoreline abuts the Cedarville Scarp and is attributed to the sea-level highstand associated with QCM1a. GPR was also used to accurately place vibracores for gathering samples of the three units for optically stimulated luminescence dating. Together, these three highstand deposits record sea-level fluctuations within OIS 5, at time scales shorter than those commonly recognized in the mid-Atlantic region.