Abstract
We have evaluated the Neogene stratigraphic succession beneath the continental shelf off New Jersey using high-quality seismic and well data to (1) construct geologic cross sections that relate seismic geometries to lithofacies in a prograding, tectonically quiescent setting; (2) apply a sequence-stratigraphic approach to subdivide the stratigraphic section and relate the characteristics of the depositional observed sequences to the published Exxon model; and (3) determine the resolution of the available biostratigraphy in the correlation of the offshore New Jersey depositional sequences to the published eustatic cycle chart. In the offshore New Jersey area, the well-developed progradational geometries in the Neogene section, the high-quality seismic data, and the relatively abundant lithologic information from wells allow us to use the results of this study to predict lithofacies distribution in similar, but less geologically constrained, settings.
Seismic sequence analysis and interpretation of stratal stacking patterns in well logs indicate that the observed depositional sequences can be subdivided into lowstand (lowstand fan and lowstand wedge) and highstand systems tracts. Lowstand fans of lowstand systems tracts are characterized by thick, aggradational sandstones that exhibit a "blocky" to fining-upward well-log signature. Lowstand wedges of lowstand systems tracts generally display upward-coarsening, but dominantly shale-prone, lithofacies. Transgressive systems tracts, if present, are very thin. Highstand systems tracts consist of strata that shoal from prodelta mudstone to thick, relatively homogeneous sandstone.
Sandstone beds occur in association with two distinct seismic facies. The first type compose the topset and toplapping beds of prograding depositional systems, typically highstand systems tracts. These sandstone beds thicken to as much as 150 m towards the depocenters of each sequence and thin along strike, away from the depocenters. The second type of sandstone is present at the toes of depositional slopes, where it onlaps against prograding deltaic wedges.
The observed geometry and lithofacies distribution of the Neogene depositional sequences are compared to the "standard" Exxon sequence stratigraphic model. Depositional sequences recognized in this study are characterized by similar overall stacking patterns within component systems tracts; however, the New Jersey sequences are different in several ways. These differences include the lack of type-2 unconformities, an absence of leveed channel deposits within lowstand wedges, very poorly developed transgressive systems tracts, and sandstone-rich, highly regressive highstand systems tracts. These differences are explained in terms of the limited Neogene accommodation on the New Jersey shelf that resulted from slow subsidence rates and a second-order Miocene eustatic fall.
Paleontologic data bracket the ages of the individual sequences but are not detailed enough at present to correlate precisely to the time scale and inferred global sea-level records. Based on the available biostratigraphy, however, Neogene depositional sequences from offshore New Jersey have a cyclicity consistent with interpreted third-order eustatic fluctuations on the global cycle chart.
