Stratigraphic Analysis of New Jersey Mid-Outer Continental Shelf Surficial Stratigraphy: Implications for Deposition, Erosion and Preservation during the Last Eustatic Lowstand and Holocene Transgression
Sylvia Nordfjord1, James A. Austin1, and John A. Goff2
1 University of Texas at Austin, Austin, TX
2 University of Texas Institute for Geophysics, Austin, TX
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 channel systems truncated by a transgressive ravinement. These channels were presumably fluvial systems developed on a subaerial shelf during the Last Glacial Maximum, ~22-20 ka. During ensuing Holocene sea-level rise, these fluvial channels became estuarine/tidal, before erosion and final burial. Mean paleo-flow estimates calculated for these systems suggest an inconsistency. If tidal flow velocities of 1.0-1.5 m/s (sufficient to initiate transport of grains 2-8 mm in diameter) are assumed, observed drainages are consistent with modern tidal creeks of the same dimensions. However, calculated paleo-flow estimates, assuming a fluvial environment characterized by velocities of 1.1-2.0 m/s, (sufficient to transport particles up to ~15 mm in diameter) are too high for a non-tidal discharge system on an emerged coastal plain. We suggest that either these fluvial drainages never reached equilibrium during high-discharge flows, 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, 2) estuarine mixed sand and muds, (3) estuary central bay muds, and 4) redistributed estuary mouth sands. Three intra-fill transgressive surfaces are interpreted as bay flooding surface, intermediate flooding surface and tidal ravinement, respectively. These fill units are truncated by a morphologically irregular, transgressive wave ravinement surface, termed the T-horizon, which, in turn, is overlain by Holocene marine sand deposits now experiencing post-transgressive erosion.