New Jersey Coastal Plain Oligocene sequences

Abstract

We evaluate Oligocene sequences recovered at three Leg 150X boreholes (Island Beach, Atlantic City, and Cape May, NJ) and their correlations to the δ 18 O proxy of glacioeustasy and the inferred eustatic record of Exxon. Sequence boundaries are identified by lithologic, gamma-ray log, and benthic foraminiferal changes and dated by integrated Sr-isotopic stratigraphy, biostratigraphy, and magnetostratigraphy. Three lower (O1, 32.8-32.2 Ma; O2, 30.8-29.9 Ma; and O3, 29.0-28.3 Ma) and three upper Oligocene sequences (O4, 27.5-27.0 Ma; O5, 27.0-25.6 Ma; and O6, 25.1-24.2 Ma) are described. Hiatuses associated with six of seven Leg 150X Oligocene to earliest Miocene sequence boundaries correlate with major deep-sea δ 18 O increases: O1 with Oil (33.5 Ma) and Oila (32.8 Ma), O2 with Oilb (31.7 Ma), 03 approximately with Oi2 (30.0 Ma), O4 approximately with Oi2a (28.3 Ma), O5 with Oi2b (27.1 Ma), and Kw0 with Mil (23.7 Ma). The hiatus associated with the 06 sequence boundary may be correlated with a minor δ 18 O increase. The New Jersey Oligocene sequence boundaries also coincide with six Oligocene eustatic falls of Exxon. However, there are one (04) or two (the Mays Landing unit at ACGS#4 borehole) additional sequences in New Jersey that correlate with inferred glacioeustatic lowerings that are not shown in the Exxon synthesis. We conclude that the excellent correlation between inferred glacioeustatic lowerings (δ 15 O increases) and sequence boundaries implies a causal link. New Jersey Oligocene sequences are dominated by glauconite sands and silts. Transgressive Systems Tracts contain in situ glauconite, and Highstand Systems Tracts contain recycled glauconite. The sequence architecture in New Jersey is different between lower Oligocene (thin, Transgressive Systems Tract dominated, sediment starved) and upper Oligocene sequences (thick, Highstand Systems Tract dominated, Mastic rich). This is may be due to higher amplitude early Oligocene sea-level changes or increased sediment supply in the late Oligocene. Depositional centers during the early Oligocene were located south of Cape May and near Island Beach. However, the midOligocene (∼27 to ∼25 Ma) sequences at Cape May and at Island Beach are either thin or absent, whereas the sequences are thick at Atlantic City. This indicates that the depositional center shifted from near Cape May during the early Oligocene to near Atlantic City during the mid- and late Oligocene. These changes are probably the result of local tectonics and changes in sediment supply.