166 Prospectus


Understanding global changes in sea level has been recognized as a first-order priority in global studies by several planning committees of the Ocean Drilling Program (Conference on Scientific Ocean Drilling II, Joint Oceanographic Institutions, Inc./United States Science Advisory Committee Workshop El Paso, and Sea Level Working Group [SL-WG]). To gather the necessary data to address questions surrounding sea-level changes, a global coverage is needed in a variety of tectonic and sedimentary settings. Analysis of material from the deep sea, carbonate platforms and atolls, and continental margins provides three independent ways to measure sea-level changes:

1. The deep-sea sediments provide a proxy for glacio-eustasy through variations in foraminiferal delta 18O.

2. Aggradational packages separated by exposure horizons on atolls and platforms record variations in sea-level-like dipsticks.

3. Continental margin sediments preserve the changes of sea-level variations by means of unconformities and stratigraphic patterns within the sediments.

To completely understand changes in sea level, the drilling objectives of transects across the continental margins should address four major issues: (1) dating sea level-related stratigraphic events; (2) establishing the stratigraphic response to sea-level oscillations; (3) estimating the magnitudes and rates of sea-level changes through time; and (4) understanding the mechanisms of sea-level change.

As a short term strategy, the SL-WG recommended testing the synchrony of stratigraphic events in the Neogene, where optimum age control and a calibrated signature of sea level are best constrained. Leg 150 on the siliciclastic New Jersey margin was the first transect in this Ocean Drilling Program (ODP) drilling effort. Drilling the Neogene of the Bahamas carbonate province is the appropriate next step because it offers a geographically close test of the ability to correlate stratigraphic events between two areas of contrasting sedimentary settings.

The Potential of the Bahamas Transect to Investigate Global Changes in Sea Level

A major requirement for studying the sedimentary record of sea-level changes are transects from a marginal marine to deep-basin environment. As such, alternative platforms for near-shore sites are necessary. In the case of the Bahamas Transect (Figs. 1, 2), these shallow-water drill sites are in hand. These cores were drilled from a self-propelled workover barge and provide the record at the proximal parts of the sequences (Figs. 2, 3). A second requirement, also met at the proposed drilling area, is a large set of seismic lines that provide good stratigraphic resolution and optimal locations for drill sites.

Probably the most important advantage of the proposed Bahamas Transect is its potential to combine all three independent methods of measuring sea-level changes. The Great Bahama Bank (GBB) is a flat-topped platform on a passive continental margin. Its flat top records sea level much as a dipstick, the prograding sequences record sea-level changes in their stratigraphic pattern, and the correlative deep-water deposits encode the delta 18O proxy of sea-level changes in their foraminiferal assemblages. The correlation of the oxygen isotopic record with the sequence stratigraphic pattern will, for the first time, document a causal link between glacio-eustasy and stratal pattern. This correlation also will provide insights in how high-frequency sea-level fluctuations are recorded in the sediments and how the stacking of these high-frequency cycles produces the lower-order seismic sequences.

The transect proposed in this prospectus is designed to document the facies variations related to sea-level oscillations from shallow water (about 300 m) to a water depth of approximately 700 m. This range allows us to fully assess the sedimentary response of carbonates to sea-level changes. Carbonate depositional sequences are, like their siliciclastic counterparts, unconformity-bounded depositional packages, but they record changes of climate and relative change of sea level in their own characteristic way, resulting in a system-specific depositional sequence architecture (Sarg, 1988; Eberli and Ginsburg, 1989; Schlager, 1991). For example, flat-topped carbonate platforms and shelves produce and export more sediment during sea-level highstands. This highstand shedding puts the carbonate environment out of phase with the siliciclastic environment in which most of the sediment is exported into deeper water during sea-level lowstands (Mullins, 1983; Droxler and Schlager, 1985; Schlager, 1991). The highstand shedding is most pronounced along steep-sided platforms such as the modern GBB. However, throughout most of the Neogene, the western margin of GBB had a ramplike profile. Consequently, the Bahamas Transect offers the opportunity to document the response of the carbonate environment to sea-level changes at different margin profiles. Finally, the sediments recovered along the transect can be compared with the siliciclastic sequences of the New Jersey Sea Level/Mid-Atlantic Transect (NJ/MAT) (ODP Leg 150) and sequence stratigraphic models (Posamentier and Vail, 1988) to assess the difference in sedimentary response for siliciclastic and carbonate margins.

Timing of unconformities along prograding carbonate platform margins can be achieved by an integrated age-dating effort that includes the incorporation of planktonic foraminiferal biostratigraphy, nannofossil biostratigraphy, strontium isotope stratigraphy, and magnetostratigraphy. In the proximal parts of the Bahamas Drilling Project (BDP) transect, dating was not an easy undertaking as a result of pulses of platform-derived sediments that dilute microfossil abundance. However, based on the results from ODP Leg 101 along the slopes of the Little Bahama Bank and in Exuma Sound, biostratigraphic dating will not be a problem in the slope settings (Melillo, 1988). Thus, precise age constraints on sequence boundaries, as well as demonstrating that the seismic reflections are synchronous, can be expected in the deep-water sites of the Bahamas Transect.

The GBB has a relatively predictable subsidence history (Williams et al., 1988) and is a suitable candidate for the evaluation of the amplitude of sea-level changes. In addition, the light-dependent sediment production of the carbonate environment provides carbonate platforms with an accurate paleobathymetric indicator. Because carbonate production in low latitudes is an order of magnitude higher than most sea-level changes, carbonate platforms and reefs are able to keep or catch up with sea-level rises and maintain a relatively flat platform top (Kendall and Schlager, 1981; Schlager, 1981). Sea-level falls usually expose the platform top, which results in the development of a suite of characteristic features that are easily recognized in the rock record (e.g., karst, red soils, caliche horizons, black pebble horizons, etc.) or of diagenetic zones with a typical petrologic and stable isotopic signal (e.g., Halley and Matthews, 1987). On the platform tops, sea-level highstands are recorded in the sediment between these exposure horizons. With the completion of the chronostratigraphy of the shallow drill sites, Unda and Clino, and of three other core borings from farther in the platform (McNeill et al., 1988; McNeill, 1989), recalculation of the subsidence curve of the GBB is possible. Thus, a relatively accurate measurement of the amplitude of sea-level changes can be achieved.

In summary, the Bahamas Transect offers the opportunity to address several fundamental questions regarding sea-level changes and to begin to acquire the global database needed for evaluating the timing and amplitudes of these changes. The proposed transect will provide the sedimentary record of sea-level changes in a carbonate environment on a passive continental margin. In addition, the delta 18O proxy in the foraminiferal assemblages of the basinal deposits can be correlated directly to the sedimentary record of sea-level change.

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