While Quaternary incised valleys have been abundantly studied along passive continental margins, little work has addressed the development, reoccupation, and infilling of systems with small drainage basins that originate near the coastal-prism apex, despite their abundance on the inner shelf and influence on Holocene coastal evolution and seafloor morphodynamics. This paper documents the evolution of several incised tributary valleys to the ancestral Delaware River. It is based on insights from over 450 offshore sediment cores, approximately 500 km of digital seismic-reflection data, and a detailed geochronology of nearly 50 radiocarbon ages.
Paleovalleys of the Delaware shelf are incised into Pleistocene lagoon deposits and late Tertiary fluvio-deltaic sediments. They are completely infilled with Pleistocene and/or Holocene muds on the shelf. While evidence in the coastal plain indicates that corresponding watersheds (< 250 km2 in size) have persisted over multiple glacio-eustatic cycles, seismic reflection records and fill architectures reveal no evidence of valley reoccupation on the shelf by late Pleistocene systems. It is likely that the marine transgression leading up to the last interglacial helped reset drainage patterns during the subsequent base-level lowering, a process aided by shelf substrate variations. Valley interfluves, composed of easily erodible sands, offered more suitable avenues for incision than stiff estuarine clays of truncated mid-Pleistocene valley fills. Deep valley incision of the late Pleistocene drainage is attributed to the Delaware River's proximity at this time, which facilitated glacial meltwater-induced tributary-valley downcutting to depths atypical for systems of comparable drainage-basin size.