SOURCE, TIMING AND DEPOSITIONAL ENVIRONMENTS OF PALEOCHANNEL INCISION AND INFILL ON THE NEW JERSEY SHELF

Sedimentation on the NJ margin is complicated by 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 geophysical systems documented the presence of buried channels within the NJ continental shelf. Drill cores (4.5 to 13.2 m in length), collected with a heave-compensated drill rig, recovered sediments from above, within and below these relict paleochannels, allowing us to determine the paleoenvironmental history of incision and infill. This study examines patterns of deposition and erosion using detailed analyses of sediment grain size and composition, radiocarbon and K-Ar age structure and foraminiferal micropaleontology.

Major geologic features of the NJ shelf include a mobile, surficial sandsheet, shallowly buried paleochannels and a time-transgressive regional unconformity (termed “R”). Three sites were drilled into the middle and outer NJ shelf to examine characteristics and timing 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 deltaic setting, consistent with deposition during Stage 3. At sites 2 and 3 on the midshelf (~80 m of water), muddy sediments into which the channels are 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 infill reflects first estuarine and subsequently nearshore conditions as sea level rose and the channels filled. All three study sites are capped by the thick, mobile Holocene sands. K-Ar dates suggest that the Holocene sand sheet (and much of the channel infill) are from a NJ Highlands source, whereas pre- and post-“R” shelf sediments are a mixture of 75-85% NJ Highlands and 15-25% Cortland Complex sources.