CHANGING DEPOSITIONAL STYLES IN RESPONSE TO CLIMATE, SEA LEVEL, AND MANTLE DYNAMICS: EOCENE SEQUENCES ON THE NEW JERSEY COASTAL PLAIN

We map the spatial and temporal distribution, and depositional environments of New Jersey coastal plain Eocene sequences and formations using an array of coreholes and gamma logs and evaluate primary controls on deposition using a forward stratigraphic model. Eocene depositional systems reflect a change from prograding earliest Eocene mud lobes, to Early to Middle Eocene hemipelagic ramp, and finally to late Middle Eocene prograding sandy sequences. Sequence boundaries that bracket Eocene sequences are associated with global sea level falls. The Marlboro Clay containing the Paleocene/Eocene Thermal Maximum (PETM) was deposited as prograding fluid mud during times of high global temperatures; it is found in northern and southern lobes but is absent from the central coastal plain. Lower and lower Middle Eocene sediments consist of carbonate rich-clays (“marls”) deposited in middle to outer neritic (50-150 m) paleodepths on a hemipelagic ramp during a peak in global sea level. Exceptionally deep water depths compared to other coeval regions are attributed to mantle dynamic topography. The upper Middle to Upper Eocene consists of three prograding lithologic units found in parallel belts with coarse-grained sediments in the most updip positions and fine-grained sediments found in the most downdip positions; the lithologic units transgress time and sequences. We date the change from ramp to prograding sequences to the late Middle Eocene (ca. 41.5 Ma). Our forward stratigraphic model shows that the appearance of prograding sands and silts in the Middle Eocene is a response primarily to changes in siliciclastic input, presumably due to climate or tectonics in the hinterland. The shift to increased siliciclastic input (the “siliciclastic switch”) is time transgressive, occurring earlier on the coastal plain (ca. 41.5 Ma) than it does on the adjacent continental slope (Eocene/Oligocene boundary; ca. 34 Ma). Our study of the NJ Eocene shows that by integrating stratigraphic and chronostratigraphic data with an independent estimate of global mean sea level, our forward model was able to disentangle the effects of sea-level, sediment supply, and mantle dynamic topography on the stratigraphic record.