Late Quaternary geologic history of New Jersey middle and outer continental shelf.

摘要

Analysis of high-resolution (1-4 kHz) seismic chirp profiles of the New Jersey mid-outer continental shelf, coupled with sediment samples, reveal the stratigraphic architecture resulting from the last glacio-eustatic cycle: dendritic (fluvial?) channel systems truncated by a transgressive ravinement. Quantitative geomorphologic analysis of these systems provides estimates of paleo-hydrologic parameters needed to link channel morphology to the hydrodynamic setting when these systems were active. These channels were presumably fluvial systems developed on a subaerial shelf during the Last Glacial Maximum (LGM), ∼22-20 ka. During ensuing Holocene sea-level rise, these fluvial channels became estuarine/tidal, before erosion and final burial. Mean tidal paleo-flow estimates for these systems with flow velocities of 1.0-1.5 m/s and shear stresses sufficient to initiate transport of grains 2-8 mm in diameter (i.e., coarse sand and fine gravel) as bed-load, are consistent with modern tidal creeks of the same dimensions. However, paleo-flow estimates, assuming a fluvial environment with velocities of 1.1-2.0 m/s, and retrodicted fluvial discharge and boundary shear stresses would have been sufficient to transport particles up to ∼15 mm in diameter (i.e., gravel) as bed-load. We suggest that either the fluvial drainages never reached equilibrium during high-discharge flows, perhaps due to melt-water pulses following the LGM, or that tidal influences have modified the original fluvial geometry. Imaged and sampled channel-fills reveal a retrogradational shift of four sedimentary facies, in ascending order: (1) fluvial lags, SF1, (2) estuarine mixed sand and muds, SF2, (3) estuary central bay muds, SF3, and (4) redistributed estuary mouth sands, SF4. Three intra-fill transgressive surfaces, B1-3, interpreted as bay flooding surface, intermediate flooding surface and tidal ravinement, respectively, are also either wholly or partly preserved. My study demonstrates that wave- and tide-dominated facies can coexist within channel fills. These fill units are truncated by a morphologic irregular, transgressive wave ravinement surface, the T-horizon, which, in turn, is overlain by Holocene marine sand deposits. A ∼10 m bathymetric rise in T forms both the mid-shelf scarp and the base of a smaller Holocene wedge. The modern seafloor of the New Jersey shelf experiences post-transgressive erosion, which is variously expressed.