PALEOHYDRAULIC CHARACTERISTICS OF LATEST PLEISTOCENE CHANNEL NETWORKS ON THE NEW JERSEY SHELF

High-resolution seismic profiling on the outer continental shelf off New Jersey as part of ONR’s Geoclutter program has revealed two dendritic drainage systems buried just meters below the modern seafloor in 70-100 m water depths (Area 1). These remarkably well-preserved features have been mapped in sufficient detail to facilitate an analysis of paleohydraulic properties on basis of channel form and pattern. Channel networks are composed of three- and four-order segments that are not contiguous with drainages of the modern coastal plain and which developed under regional slopes of less than 0.001. Although the nature (fluvial vs. tidal) and timing of channel cutting is unclear, radiocarbon dating at two sites indicates that they backfilled rapidly during 15.5-12 yr B.P. as the shoreline transgressed across the shelf. Morphometric and sedimentological data suggest that the channels comprised overland drainage systems that were likely tidal and possibly estuarine during their final evolutionary stage. Following empirical relationships derived from the hydraulic geometry of modern systems, paleotidal discharges (bank-full, spring tide) would have been on the order of 1-5x10^3 m^3/s with corresponding peak velocities of 0.3-1.5 m/s. These flows would have been sufficient to transport particles up to 10 mm in diameter as bedload and finer-grained material in suspension. This estimate is consistent with the grain-size distribution of the channel-fill successions, which on the basis of two drillcores ranges from medium sand to mud, with a lesser quantity of coarser material at the channel floor. Significantly, the mosaic of data suggests that these particular drainages may have evolved in response to precipitation runoff combined with tidal hydrodynamics, rather than through the catastrophic flows that impacted the shelf during the latest Pleistocene.