Geomorphologic comparisons of shallowly buried, dendritic drainage systems on the outer New Jersey shelf with modern fluvial and estuarine analogs

An investigation of the morphometry of shallowly-buried channels imaged beneath the middle and outer New Jersey shelf is being conducted in order to determine the hydrologic regime at the time these channels were active, during the last sea level lowstand and transgression ( ∼22-8 ka). These features have been mapped using 1-4 kHz and 1-15 kHz, deep-towed chirp sonar. Profile spacings average ∼200 m over an area of ∼600 sq km. Our analysis measures several parameters associated with the geomorphology of the channel systems. We compare mapped and analyzed shelf drainage patterns with a representative group of modern fluvial and estuarine systems. The following drainage basin and hydraulic parameters were quantified: (1) channel width/depth ratio and cross-sectional area; (2) longitudinal profile, density, and patterns of channel networks; and (3) sinuosity and angles of tributary junctions. These parameters were used to constrain paleohydrologic conditions such as water discharge, bed stress, sediment transport, and the extent of bedload versus suspended load. Calculated width/depth ratios of observed drainage patterns are typically ∼10 for smaller tributaries (1st order) and >100 for trunk or main channels (4th-5th order). High aspect ratios suggest the existence of bed load channels with limited carrying capacities, whereas low aspect ratios reflect more suspended load drainage. The New Jersey shelf drainages suggest partial replacement of a fluvial system by a tidal river and (or) estuarine environment. If the paleodrainage systems were created during the last transgression as opposed to having formed at the last glacial maximum (available radiocarbon dates of ∼12-13 ka for the channel fill suggest that to be the case), then relative Holocene sea level rise must have slowed down or even been in minor stillstand during their formation. Based on available sea-level proxy records (e.g., ice-, deep-ocean cores, etc.), we suggest that these drainages formed during the Younger Dryas at ∼12 ka. Additional support for a Younger Dryas time of formation comes from the location of these drainage systems landward of the ∼90 m isobath, as sea-level was in equilibrium at the ∼80 m isobath at Younger Dryas time.