Ocean Drilling Program, Sequences, and Global Sea-Level Change: Comparison of Icehouse vs. Greenhouse Eustatic Changes

Kenneth G. Miller, James V. Browning, Michelle A. Kominz, James D. Wright, Peter J. Sugarman, Gregory S. Mountain, John C. Hernández, Richard K. Olsson, and Mark D. Feigenson: Department of Geological Sciences, Rutgers University, Piscataway, NJ; and William Van Sickel: Western Michigan University, Kalamazoo, MI

Abstract

Understanding eustatic (global sea-level) changes and their effects on the geological record is an important but difficult task because eustatic effects are complexly intertwined with basin subsidence and changes in sediment supply. Led by Peter Vail, researchers at EPR reconstructed a eustatic history by applying sequence stratigraphy to a global array of proprietary seismic profiles, industry wells, and outcrops. This EPR eustatic record has been controversial owing to methodological concerns and reliance on largely unpublished data. The Ocean Drilling Program (ODP) has focused on drilling the New Jersey, Bahamas, and Australian margins for sea-level studies and has accomplished the following:

1. Validated a transect approach of drilling passive continental margins and carbonate platforms (onshore, shelf, slope);
2. Tested and validated the assumption that the primary cause of impedance contrasts producing seismic reflections on continental margins are stratal surfaces including unconformities;
3. Proved that the ages of sequence boundaries on margins can be determined to better than ±0.5 m.y. and provided a chronology of eustatic lowering for the past 100 m.y. ;
4. Achieved orbital-scale (perhaps suborbital) stratigraphic resolution on continental slopes and carbonate platforms;
5. Showed that siliciclastic and carbonate margins yield correlatable and in some cases comparable records of sea-level change;
6. Evaluated the sedimentary response of both tropical and cool-water carbonate platforms to eustatic changes;
7. Begun to constrain the amplitude and cause of eustatic change for both the Icehouse World of the past 42 m.y. and the Greenhouse World of 250-42 Ma, as outlined below.

Miocene eustatic estimates

Onshore (ODP Legs 150X and 174AX) and offshore (Legs 150 and 174A) drilling in New Jersey has shown that late middle Eocene to Miocene sequence boundaries are linked to glacioeustatic lowerings for the past 42 Ma. We present a new Miocene eustatic record by integrating results from backstripping of onshore New Jersey boreholes (ODP Legs 150X and 174AX) with glacioeustatic estimates derived from deep-sea oxygen isotopic studies. The backstripped results among several onshore boreholes yield similar eustatic estimates (R2), indicating that we have successfully removed the effects of thermal subsidence, loading, and water-depth variations. However, the R2 records are quite discontinuous due to hiatuses (with lowstands generally not represented). The oxygen isotopic record provides a continuous estimate of changes in temperature and ice volume, though these effects can only be evaluated by making certain assumptions about thermal history. Our oxygen isotopic-based glacioeustatic estimates for the Miocene are remarkably similar to the R2 estimates, with 10-35 m third order eustatic lowerings. Convergence of estimates from these two methods provides confidence that we have isolated the eustatic component. Although onshore and offshore results agree with the general number and age of Oligocene to middle Miocene eustatic events published by Exxon Production Research (EPR), backstripped amplitudes obtained from the onshore sites and ice volume estimates of amplitudes are a factor of two to four lower than EPR estimates.

Cretaceous eustatic estimates

We provide a chronology and amplitude estimates for Late Cretaceous global sea-level (eustatic) variations. We identify 12 Upper Cretaceous-lowermost Tertiary sequences from continuously cored boreholes (Bass River and Ancora; ODP Leg 174AX) in the New Jersey coastal plain. We develop a testable chronology for these sequences (resolution ~±0.5 m.y.) by integrating Sr-isotopic and biostratigraphy with an improved time scale, providing a template that can be compared with records from other margins and with other sea-level proxies. Though backstripping of the New Jersey records indicates eustatic amplitudes that are generally lower than those reported by EPR; the number and timing of New Jersey sequence boundaries generally correspond with EPR sea-level lowerings, suggesting a global cause. A New Jersey sequence boundary at ca. 71 Ma correlates with global benthic and planktonic foraminiferal oxygen isotopic increases, suggesting glacioeustatic lowering; sequence boundaries at ca. 76, 84, 92, and 96 Ma, correlate with benthic foraminiferal oxygen isotopic increases. Based on preliminary comparison of backstripped eustatic estimates with oxygen isotopic increases, we suggest that the growth and decay of continental ice sheets may have caused glacioeustatic variations of 10-50 m during the warm Greenhouse World of the Late Cretaceous. This speculation requires testing by dating sequences from margins throughout the world and further detailed stable isotopic studies of deep-sea sections.

The Future

Although ODP has drilled the New Jersey, Bahamas, and Australian margins, a lack of shallow-water drilling capability continues to limit understanding of global sea-level change and sedimentary architecture. As a result, there are fundamental uncertainties in the rates/ amplitudes, mechanisms, for and response to eustatic change.

Amplitudes

Despite success outlined here, the amplitude of sea-level change remains one of the thorniest problems in reconstructing past boundary conditions of the Earth. However, even in the Miocene, an interval of focus for 6 legs, amplitudes remain uncertain, ranging from 20-80 m to 85±30 m. Drilling arrays of boreholes on different margins in different settings will allow 2- and 3-D backstripping that will provide firmer constraints on the amplitudes of eustatic change.

Mechanism

While ODP has confirmed the importance of glacioeustasy since 42 Ma, other mechanisms (e.g., intra-plane stress) were undoubtedly operating. Though we have pushed back the age of inception of continental glaciation to at least 42 Ma, we do not understand the role of glacioeustasy on the older record. Additional potential mechanisms can be evaluated by comparing estimates of Cretaceous-early Eocene sea-level variations with near- and far-field tectonic events, global oxygen isotopic records, and other possible causal events (e.g., basin dessication, large igneous province formation).

Response

Facies models developed by EPR and others potentially provide a means of predicting the distribution of sediments within sequences and resources (oil, water). To develop and test predictive facies models, sequences and facies within sequences must be sampled on carbonate, mixed carbonate-siliciclastic, and siliciclastic margins across a wide spectrum of sediment inputs.

Sea level and IODP

Continuing efforts to understand global sea-level changes and their effects on the stratigraphic record require innovation and use of new drilling tools. Despite valiant efforts, the JOIDES Resolution is not a suitable platform for drilling in shallow water and setting long casing strings. Supplementary platforms (jack-up rigs, semi-submersibles, anchored geotechnical drillships) provide the right tools for this job. As we move into the Integrated ODP (IODP), the successor to ODP, these supplementary platforms will complement drilling by the successor to the JOIDES Resolution and the OD21 drillship.