PALEOCHANNEL INCISION AND INFILL ON THE NEW JERSEY SHELF: TIMING, CHARACTER AND DEPOSITIONAL ENVIRONMENT

Sedimentation on the NJ margin is complicated by the repeated cycles of erosion and deposition associated with glacially forced sea level rise and fall which often remove the sedimentary record of the previous cycles. Recent investigations using high-resolution CHIRP systems documented the presence of buried, infilled channels within the middle and outer NJ continental shelf. Drill cores (4.5 to 13.2 m in length), collected with the DOSECC heave-compensated AHC-800, recovered an unbroken sequence of sediments from above, within and below these relict paleochannels, allowing us to determine the paleoenvironmental history of incision and infill. Our study attempts to unravel the patterns of deposition and erosion using detailed analyses of sediment grain size and composition, radiocarbon age structure and foraminiferal micropaleontology.

Major geologic features of the NJ shelf include a mobile, sandy surface layer, numerous shallowly buried paleochannels with associated infill and a time-transgressive regional unconformity (previously termed “R”). Three sites were drilled into the middle and outer NJ shelf to examine patterns of incision and infill. Site 1, in 130 m of water and penetrating the outer shelf wedge, shows that the upper ~1 m of the seafloor is actively being reworked (with C-14 ages ranging from modern to ~7kyrs). Age control indicates that sediments overlying R on the outer shelf wedge are ~30 to 40 K cal y BP and foraminiferal as well as sedimentological indicators suggest a marine environment, consistent with deposition during Stage 3. At sites 2 and 3 on the midshelf (~80 m of water), muddy sediments into which the channel is incised, whether above or below R are old, ~35-45K cal y BP. The timing of infilling is much later, ~14K cal y BP, constraining downcutting to between 35–45K cal y BP and 14K cal y BP. This is consistent with a recent study indicating regional continental downcutting (Reusser et al., 2004) between 35 and 13-14 Ka. Foraminiferal and sedimentological evidence suggest that infilling with sea level rise was not a simple deepening depositional sequence from estuarine to open ocean conditions, but can be shown to have been a series of repeated depositional sequences, alternating between restricted, estuarine and open, inner-shelf environments.