Lower to middle Eocene sequences of the New Jersey coastal plain and their significance for global climate change

Abstract

Boreholes from Island Beach, Allaire, Atlantic City, and Mays Landing, NJ provide an excellent chronology of lower to middle Eocene passive margin sequences and allow analysis of long-term sea-level changes and sedimentation patterns. These New Jersey sequences are tied directly to the geomagnetic polarity time scale through magnetostratigraphy. Integrated stratigraphy (including magnetostratigraphy, lithostratigraphy, plankton biostratigraphy, and benthic foraminiferal biofacies studies) reveals that these sections contain a relatively continuous record of lowermost Eocene to middle Eocene deposition interrupted by short hiatuses. A sequence boundary at the base of the section spans the Paleocene/Eocene boundary, representing a hiatus from 55.8 to 54.7 Ma. A second sequence boundary is associated with a hiatus from 54.0 to 53.4 Ma. A third sequence boundary is associated with a hiatus from 52.9 to 52.3 Ma. Two major unconformities occur near the lower/middle Eocene boundary at all three boreholes. The first represents a hiatus between 50.9 and ~50.5 to 49.9 Ma; the second, between ~49.6 and 48.6 Ma. Early Eocene water depths inferred from benthic foraminiferal biofacies analysis varied from 150 to 200 m and reached their maximum depth between 53.4 and 53.2 Ma. In the middle Eocene, the sedimentation record is less continuous with hiatuses from 48.3 to 47.9 Ma, 46.9 to 44.5 Ma, and 43.4 to ~41.2 Ma. A major lithofacies change from carbonate dominated to siliciclastic dominated occurred by 41.2 Ma. Of the 14 sequences inferred by Exxon for the early to middle Eocene, nine are resolvable on the New Jersey Margin and the remaining five appear to be combined with others. Thus the New Jersey record is consistent with the Exxon record for the early to middle Eocene, although the New Jersey record is better dated. Comparison of the record of sedimentation with the global δ 18 O record shows interesting parallels. It is unlikely that early Eocene sequence boundaries are the result of glacioeustasy because hiatuses from the New Jersey Coastal Plain in this interval do not match increases in the δ 18 O record. In the late middle Eocene, concomitant increases in the planktonic and benthic δ 18 O records coincide with the timing of hiatuses on the New Jersey Coastal Plain. Increases in the δ 18 O records between 43 and 41 Ma coincide with a New Jersey hiatus and a corresponding change in sedimentation type from carbonate dominated to siliciclastic dominated. We conclude that these represent evidence of the first Antarctic ice cap and the beginning of the “Icehouse” world.