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Seismic Facies Analysis of Shallowly Buried Channels,
New Jersey Continental Shelf: Understanding
Late Quaternary Paleoenvironments
During the Last Transgression
By Sylvia Nordfjord
We are investigating the late Quaternary sedimentary record of the New Jersey mid-outer continental shelf using deep-towed chirp sonar (1-4 kHz and 1-15 kHz) profiles, coupled with lithologic and chronostratigraphic control from long sediment cores collected using the DOSECC AHC-800 drilling system. We have seismically mapped extensive, shallowly buried, dendritic drainage systems; individual channels are typically box-shaped in cross-section, with width-depth ratios of >35, suggesting that these are bed-load channels. Observed seismic facies distributions suggest the complex nature of channel fills, and synthetic seismograms derived from MST logs enable us to correlate the chirp data to changes in lithology and physical properties of the cored samples, including channel fills, confirming that fine-grained material is transparent seismically, while interbedded sand and mud produce laminated reflections. Using our new data, with previous interpretations, we suggest that these channels probably formed subaerially during or after the last lowstand, ~22-20 ka. Remnants of fluvial coarse-grained deposits may be represented by chaotic acoustic patterns in the bases of many channel fills. We interpret complex re-excavation surfaces within incised valley system-fills as sequential episodes of cut-and-fill, associated with changes in paleo-flow conditions. A seismic transition upward from chaotic to flat-lying reflections and more transparent facies indicates less depositional energy, suggesting replacement of fluvial systems by tidal/estuarine environments. This has been confirmed by previous vibra-coring of one channel. Our paleo-flow reconstructions also yield velocities in the range of 0.5-1.5 m/s, which are reasonable estimates for flows in estuarine environments.