NJ Coastal Plain

Oligocene sediments are found only in the subsurface and have a patchy distribution. The early to middle Oligocene (Sewell Point Formation) is characterized by glauconite-rich sedimentation that was deposited very slowly.  The entire New Jersey margin was sediment-starved not only of siliciclastics but also of pelagic carbonate.

Sewell Point Formation from the Ocean View (1103-1104 ft) core. The greenish color is from the mineral glauconite. There are also common shell fragments.

In the late Oligocene (approximately 27-25 Ma) sedimentation rates increased and thick prograding sequences developed. Sedimentats changed (Atlantic City Formation) and medium-to-coarse quartz sand appeared as an important constituent, along with glauconite, in the onshore boreholes. This increase in siliciclastic input clearly marks the beginning of increased sediment input from the hinterland.

The transition to the ice-house world began in the late Eocene (Miller, et al., 1991) and was associated with a new phase of deposition on the mid Atlantic margin. Sediments on the shelf shifted from carbonate-rich clays to siliciclastics beginning onshore in the late middle Eocene, and culminating on the modern continental slope at the Eocene/Oligocene boundary (Miller, et al., 1998). The sediments also changed the nature of the shelf from a carbonate ramp to a prograding siliciclastic margin that fully developed by the latest Oligocene (~27 Ma pulse; Miller, et al., 1997) to early Miocene.

Deepest water depths of the Cenozoic were attained in lower to lower middle Eocene sequences. Water depths, determined using benthic foraminiferal biofacies (Browning, et al., 1997), show middle to outer neritic paleodepths.  Sequences consist of very thin glauconitic clays overlain by carbonate-rich clays with layers of porcellanite interspersed. Sequence boundaries are inferred by matching the few surfaces in the cores with biostratigraphic breaks. Above the lower/middle Eocene boundary, there are three, thin, glauconite-rich sequences.  The lower-middle Eocene, although still carbonate rich, is slightly shallower (middle neritic), and sequences can be more easily distinguished lithologically particularly by basal glauconite sands.

Sea level continued to be very high during the Paleocene and the Paleocene section is generally thin. The Hornerstown Formation was deposited in the lower Paleocene. In outcrop, the Hornerstown is glauconite sand ith a conc entration of the fossil Oleneothyris harleni near the top. Down dip, in cores, the Hornerstown is dark greenish gray, very clayey glauconite sand to micaceous, quartzose, glauconite sand. The Vincentown Formation is slightly glauconitic quartz sand and silt. In places the Vincentown is very fossiliferous consisting of a bryozoan bioherm. In addition, there is an unnamed clay member of the Vincentown Formation that spans the Paleocene/Eocene boundary. This kaolinitic clay unit contains evidence for the Paleocene-Eocene thermal maximum (Cramer, et al., 1999).

Hornerstown Formation from the Millville corehole (approx. 970 ft), consisting of silghtly fossiliferous andbioturbated, glauconite sand, clay.