MODES OF INCISED-VALLEY FORMATION REVEALED FOR RIVERS WITH ALMOST NO DRAINAGE BASIN

Previous work on late Quaternary paleovalleys along the northern Gulf of Mexico and the U.S. mid-Atlantic margins highlights the importance of discharge in governing valley size and shape at the highstand shoreline. Valley width and depth scale to drainage-basin size, a proxy for discharge. However, it remains unclear what happens to this relationship at the low end of the scale as drainage-basin size approaches zero.

Three small late Quaternary systems from the North Carolina lower coastal plain are investigated including: 1) the Newport, whose 210 km² watershed extends landward of the last interglacial shoreline, 2) the North, which lacks a measurable drainage basin, and 3) the Scuppernong, a tributary system whose 400 km²-catchment is positioned seaward of the last interglacial shoreline.

Studied estuaries are similar in size and function today; however, cores and seismic data reveal differences in valley morphology and fill architecture attributable to varying modes of formation. Unlike the North and Scuppernong, the Newport valley exhibits terracing and contains basal fluvial deposits. The North and Scuppernong valleys are V-shaped and contain fill sequences dominated by bay sediments. In addition, the Scuppernong valley is defined by abrupt seaward widening and deepening at the estuarine mouth, associated with an increase in bay sediment thickness from 0.5 m landward to 2.5 m bayward over a linear distance of 200 m. This transition coincides with the upstream limit of a pronounced knickzone that formed at the system’s juncture with a larger valley.

Our data suggest three primary modes of formation for seemingly similar estuaries: Fluvial incision for the Newport, gully incision for the North, and tributary effects for the Scuppernong, including fluvial scouring and transgressive ravinement concentrated at the juncture.