Sediment response to the present hydraulic regime on the Central New Jersey Shelf

ABSTRACT Petrographic data, from vibracores and grab samples collected on the Central New Jersey Shelf, suggest a substrate still actively responding to the hydraulic regime. Radiocarbon dates of shell material from the ridge and swale topography indicates aggradation of the ridge's crest during the last 500 years and exposure of earlier Holocene material in the deeper troughs of the area. The samples from both the cores and the surficial samples were investigated for heavy mineral percentages and grain size analysis in addition to radiocarbon dating. The concentration of heavy minerals into disseminated bands, as observed in the vibracores, is compatible with sediment transport by sand ripples on the ridge's flanks. The grain size variation was subjectively analyzed by applying a Q-mode facto analysis which produced three distinct groupings of the grain size distribution. Each grouping is found to characterize a particular part of the ridge topography. Fine sand and moderate sorting occurs on the flanks, medium to fine sand and moderate sorting occurs on the crests whereas two populations are found in the troughs; coarse, poorly sorted sands and very fine, well sorted sands. This textural variation supports a hypothesis of up-flank rheologic and suspensive transport of medium and fine sand during intense storms and subsequent down-flank winnowing of fine sand during less intense meterological events. The radiocarbon dates indicate that size fractionation and heavy mineral concentrations are subsequent to isolation from a beach environment.