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The ravinement surface is a key marker for examining the effects of the most recent sea-level transgression on development of the latest Pleistocene–Holocene stratigraphic record of the New Jersey middle and outer continental shelf. This surface is marked by a moderate-amplitude reflector, identified as the “T” horizon in chirp seismic data, which caps filled incised valleys formed during or soon after the Last Glacial Maximum (LGM) and eustatic lowstand, and also serves as the base of a Holocene sand sheet. The “T” horizon is clearly erosional; we interpret it as marking landward migration of the shore zone across the coastal plain during the earliest-mid Holocene. We use chirp seismic surveys acquired in 1999 and 2001 to map the geomorphology of the “T” horizon over a ~ 100 by 60 km region. Shallow, bathymetric contour-parallel depressions, ~ 10 km wide and ~ 8 m deep, are present in the horizon at ~ 50–60 m and ~ 70 m water depths; we interpret them as preserved lagoonal/back-barrier topography; portions of filled tidal channels occur in these depressions. Perhaps these depressions represent pauses in the retreat of the shoreline, either as a result of eustatic fluctuations or slope steepening, allowing for formation of back-barrier/lagoonal features substantive enough to resist complete destruction by subsequent ravinement. Alternatively, they may be locations of local flattening in the antecedent topography that forced rapid drowning and preservation of back-barrier morphology. These paleo-estuarine/back-barrier features may also be more susceptible to erosion during transgression, so the presence of these depressions might have focused marine erosion in some way, perhaps by promoting near-bottom turbulence during storm events. Seismic stratigraphy and sediment cores show that the mid-shelf wedge (MSW) is composed of clay-rich latest Pleistocene and sand-rich Holocene components. Latest Pleistocene sediments of the MSW are complex, representing both erosional and depositional processes. In part, the seaward boundary of this part of the MSW is defined by the mid-shelf scarp (MSS), an erosion product of transgressive ravinement ~ 11.4 ka. The Pleistocene MSW appears to predate post-LGM glacial lake collapses to the northwest, which have been suggested as sources for subaerial, lobate deposition north of the Hudson Shelf Valley (HSV). The primary origin of this part of the MSW may instead be subaqueous deltaic formation associated with fluvial drainage, perhaps part of the ancestral Hudson River system, prior to the LGM. In contrast, the Holocene, sandy part of the MSW must have been deposited in a marine setting, because this deposition postdates transgressive ravinement. The Holocene MSW may be derived from Hudson river sediments, or from continuing shallow submarine erosion following landward shore-zone passage. The preexisting Pleistocene MSW provided accommodation and a source of reworked sediment for this deposit at its toe. Published sea-level curves confirm that Holocene deposits postdate known glacial lake collapses behind terminal moraines to the north. Erosion on this margin continues and, as a result, the maximum flooding surface has not yet formed. |
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