SEDIMENTOLOGY, STRATIGRAPHIC CONTEXT, AND IMPLICATIONS OF MIOCENE INTRASHELF BOTTOMSET DEPOSITS, OFFSHORE NEW JERSEY

We developed a facies model for deposition of bottomset deposits by studying the sedimentology and stratigraphy of offshore New Jersey intrashelf Miocene clinothems. Three IODP Exp. 313 coreholes (M27-29), recovered bottomset deposits and their analysis helps to refine sequence stratigraphic interpretations and process response models for intrashelf clinothems. At Site M29, the most downdip location, chronostratigraphically well-constrained bottomset deposits follow a repeated stratigraphic pattern. Coarse-grained glauconitic quartz sand packages abruptly overlie deeply-burrowed surfaces. Typically, these packages coarsen- then fine-upwards and pass upward into bioturbated siltstones. These coarse sand beds are amalgamated, poorly-sorted, and contain thin-walled shells, benthic foraminifera and extrabasinal clasts, interpreted as debrites. The sedimentology and mounded seismic character of these packages support interpretation as debrite-dominated lobe complexes. Farther updip, at Site M28, the same chronostratigraphic units are amalgamated, with the absence of bioturbated silts pointing to more erosion in proximal locations. Graded sandstones and dune-scale cross-bedding in the younger sequences in Site M28 indicate deposition from turbidity currents and channelization. The sharp base of each package is interpreted as a sequence boundary, with a period of erosion and sediment bypass evidenced by the burrowed surface, and the coarse-grained debris flow and turbidite deposits represent the lowstand systems tract. The overlying fine-grained deposits are interpreted as the combined transgressive and highstand systems tract deposits and contain the deep-water equivalent of the maximum flooding surface. This study demonstrates that large amounts of sediment gravity flow deposits can be generated in relatively shallow (~100-200 m paleodepth) and low gradient (~1-4°) clinothems that prograded across a deep continental shelf/platform. This physiography likely led to the dominance of debris flow deposits due to the short transport distance limiting transformation to low concentration turbidity currents.