27. CENOZOIC EVOLUTION OF THE NEW JERSEY COASTAL PLAIN: CHANGES IN SEA LEVEL, TECTONICS, AND SEDIMENT SUPPLY1Kenneth G. Miller,2,3 James V. Browning,2 Stephen F. Pekar,2 Peter J. Sugarman2,4 |
ABSTRACT The New Jersey Coastal Plain Drilling Project continuously cored three boreholes at Island Beach, Atlantic City, and Cape May, New Jersey. Sequence boundaries in the cores are expressed as physical surfaces, lithofacies breaks, and paraconformities (hiatuses) recognized using biostratigraphy and Sr-isotopic stratigraphy. By drilling along dip and strike profiles, we assembled a mosaic of 29 Paleocene to Miocene sequences and dated them using integrated magnetostratigraphy, biostratigraphy, and isotopic stratigraphy. Correlation between major late middle Eocene to middle Miocene (42–10 Ma) onshore sequence boundaries and delta18O increases (inferred glacioeustatic lowerings) indicates that eustasy exerts a primary control on sequence boundaries. Onshore sequence boundaries also correlate with Miocene unconformities on the New Jersey shelf and slope, Oligocene to middle Miocene unconformities in Florida, early Oligocene unconformities in Alabama, and the sequence boundaries of Exxon. Such regional and interregional correlations support a eustatic control. However, early middle Eocene correlations between sequence boundaries and delta18O increases are equivocal, and it is not clear that glacioeustatic changes occurred at this time. In contrast, early Eocene sequence boundaries do not correlate with delta18O change, and we infer ice-free conditions at this time; nevertheless early Eocene sequence boundaries do correlate with those of Exxon, indicating that they may record global sea-level events. However, there is no known mechanism for explaining such large, rapid early Eocene eustatic variations other than glacioeustasy. Paleocene sequence boundaries apparently do not match the Exxon sequence boundaries, and further study of New Jersey Paleocene sequences is warranted. Regional and local tectonics resulted in differential preservation of sequences in the Mid-Atlantic Coastal Plain. For example, lower Miocene marine sequences are well represented in New Jersey, but are less complete in Maryland, whereas the converse is true for upper Miocene marine sequences. The tectonic mechanism responsible for this distributional pattern has not been established. In general, Miocene downdip sections in New Jersey are stratigraphically more complete than updip sections, reflecting a hinged margin with increased subsidence downdip. In contrast, Oligocene sequences have a patchy distribution: lower Oligocene sequences are better preserved updip at Island Beach, and middle Oligocene sequences are better preserved at Atlantic City than they are downdip at Cape May. These differences result from differential subsidence on the order of tens of meters and probably reflect migration of sediment supply and depocenters. Eocene sequences are widely distributed throughout the New Jersey Coastal Plain, reflecting high sea level and deposition in deep (>100 m) water. New Jersey Coastal Plain sequences were also influenced by sediment supply and climate. Low siliciclastic and high pelagic input during the early to middle Eocene resulted in carbonate silty clay deposition in association with warm regional and global climates. By the late Eocene, climatic cooling resulted in dominance by uniform siliciclastic clays onshore, although pelagic carbonate deposition dominated offshore. The entire margin was sediment starved in the early Oligocene. Siliciclastic input increased in the late Oligocene and again in the Miocene with a shift to deltaic sedimentation, perhaps related to hinterland tectonics. Thus, the New Jersey Margin progressively evolved from an early to middle Eocene carbonate ramp, to a late Eocene mixed carbonate-siliciclastic ramp, to a sediment-starved region in the early Oligocene, to a ?late Oligocene to middle Miocene siliciclastic progradational margin with high sedimentation rates. We also note the progressive shallowing onshore from outer neritic (~185 m) environments in the early Eocene, to generally inner to middle neritic environments in the Oligocene, to inner neritic prodelta, marginal marine, and fluvial environments in the Miocene. |
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