174AX Leg Summary: Sequences, Sea Level, Tectonics, and Aquifer Resources: Coastal Plain Drilling¹

Kenneth G. Miller, James V. Browning, Peter J. Sugarman, Peter P. McLaughlin, Michelle A. Kominz, Richard K. Olsson, James D. Wright, Benjamin S. Cramer, Stephen J. Pekar, William Van Sickel²

SUMMARY

This chapter provides the background, objectives, and major scientific accomplishments of Ocean Drilling Program (ODP) Leg 174AX/174AXS, during which four boreholes were drilled onshore in the New Jersey (NJ) and Delaware (DE) Coastal Plains. These boreholes not only targeted the onshore equivalents of Miocene sequences drilled offshore during Leg 174AX with holes at Ocean View, NJ, and Bethany Beach, DE, but also provided an unprecedented sampling of marine Upper Cretaceous-Paleogene onshore sequences with holes at Bass River and Ancora, NJ. Major scientific accomplishments of Leg 174AX include evaluating controls on sea level and sequences and global events in Earth history.

We established that eustasy is the dominant process that determines the template for potential sequences and their general architecture (stacking patterns and preservation of stratal surfaces) on the U.S. Mid-Atlantic margin. Leg 150X and 174AX onshore cores yielded a high-resolution (1-m.y. resolution) chronology of ~30 Cenozoic and 11-14 Late Cretaceous sequences by integrating Sr isotopic stratigraphy and biostratigraphy. Sequence boundaries (from 42 to 8 Ma) correlate with global ¹⁸O increases, linking them with glacioeustatic lowerings. One- and two-dimensional backstripping of mid-Cretaceous to Miocene sequences yields eustatic estimates for the interval from 100 to 8 Ma that are less than one-half of those published by Exxon Production Research (EPR). Oligocene sequences in NJ preserve a remarkably complete record of sea-level change derived from two-dimensional backstripping. Backstripping establishes that sea-level changes were large (>25 m) and rapid (<1 m.y.) even during the greenhouse world of the Late Cretaceous to middle Eocene. The rapidity and amplitude of these changes requires a glacioeustatic control of sea-level variations during the Late Cretaceous, or our understanding of sea-level mechanisms is fundamentally flawed. Comparisons between Late Cretaceous sequence stratigraphy and ¹⁸O records are consistent with the presence of small ice sheets in this alleged greenhouse world.

Comparisons between boreholes in NJ and DE allow evaluation of the effects of tectonics and sediment supply on sequence architecture. We discovered higher-order variability within lower Miocene sequences in NJ and DE embedded within the million-year-scale sequences previously defined onshore; these may be due to lobe switching or unusual preservation of high-frequency sea-level changes in areas with high sedimentation rates. Minor (~10-m scale) tectonic differences determine the preservation of sequences in different parts of the basin. Sediment supply determines paloeoenvironmental setting, regional and local facies, and the expression of stratal surfaces. Our studies show that Miocene facies in NJ are deltaic dominated, whereas those in DE are wave-dominated shorelines. Despite this difference, these areas share a similar Miocene sequence stratigraphic signature. Thin transgressive systems tracts (TSTs) are present at the base of most sequences. Highstand systems tracts (HSTs) can generally be divided into a lower fine-grained unit (silty clay in NJ and silts in DE) and an upper sandy unit. The general absence of lowstand systems tracts (LSTs) is due to bypassing. Understanding of sequence stratigraphic architecture allowed development of an improved hydrostratigraphic framework to be used in evaluating local and regional water resource potential.

The record obtained at these sites allowed us to evaluate the causes and effects of several major global events in Earth history, including:

1. Demonstrating that the latest Cenomanian-Turonian (C/T) ocean anoxic event was unrelated to sea-level change on million-year or 100-k.y. scales.
2. Suggesting that a major cooling spanning the Campanian/Maastrichtian boundary was associated with a sea-level lowering and inferred ice volume increase.
3. Correlating a latest Maastrichtian global warming with Deccan trap volcanism.
4. Linking the marine mass extinctions at the end of the Cretaceous with ballistic ejecta. In addition, we showed that collapse of the vertical isotopic gradient ("Strangelove Oceans") extended to neritic environments and that there was minimal change in sea level associated with the Cretaceous/Tertiary (K/T) boundary.
5. Establishing that low ¹³C and ¹⁸O and high kaolinite values were associated with the Paleocene/Eocene thermal maximum (PETM) in NJ neritic sections and that isotopic values remained low and kaolinite remained high throughout a thick section above the carbon isotope excursion (CIE). This reflects either that warmer and wetter climate persisted for >300-400 k.y. in NJ (unlike deep-sea records that show an exponential return to pre-PETM conditions after ~200 k.y.) or that the extremely rapid deposition of this section occurred in response to a cometary impact.
6. Showing that a large (~60 m), earliest Oligocene drop in sea level was associated with development of an ice sheet equivalent in size to the modern East Antarctic ice sheet, though sea level again rose by nearly 50 m ~1 m.y. later, suggesting near collapse of the ice sheet. The ice sheet subsequently grew and decayed numerous times in the Oligocene-middle Miocene.

Future studies will integrate onshore and offshore drilling with seismic profiles and provide a full sampling of sequences across the margin.

¹Miller, K.G., Browning, J.V., Sugarman, P.J., McLaughlin, P.P., Kominz, M.A., Olsson, R.K., Wright, J.D., Cramer, B.S., Pekar, S.J., and Van Sickel, W., 2003. 174AX leg summary: sequences, sea level, tectonics, and aquifer resources: coastal plain drilling. In Miller, K.G., Sugarman, P.J., Browning, J.V., et al., Proc. ODP, Init. Repts., 174AX (Suppl.): College Station, TX (Ocean Drilling Program), 1-38. doi:10.2973/odp.proc.ir.174axs.104.2003
²Scientific Party addresses.

Ms 174AXSIR-104