The Bahamas Transect was designed to assess the response of the carbonate depositional system to sea-level changes and to evaluate the relationship between the sedimentary and oxygen isotope records of Neogene sea-level changes in order to establish a causal link between glacio-eustasy and the stratigraphic pattern. In addition, the timing of unconformity-bounded sequences and the question of chronostratigraphic significance of seismic sequence boundaries were of special interest. Cores from seven sites drilled during Leg 166 along the western Great Bahama Bank retrieved the sedimentary record and the timing of high- and low-frequency sea-level changes throughout the Neogene. Facies successions within the cores contain indications of sea-level changes on two different scales. First, there are high-frequency alternations between meter-thick layers with platform-derived material and thin layers with more pelagic sediments. Carbonate-rich intervals are interpreted to reflect periods of high sea level, whereas the thin intervals correspond to times of increased pelagic and siliciclastic input during sea-level lowstands. The duration of these alternations (20-40 k.y.) correlates with orbitally induced high-frequency climate and sea-level changes. Second, longer term sea-level changes with a duration of 0.5-2.0 m.y. are also recorded in slope deposits along the Great Bahama Bank. Alternating high (up to 20 cm/k.y.) and low (<2 cm/k.y.) sedimentation rates record a long-term pattern of bank flooding with concomitant shedding to the slopes as well as periods of bank exposure with reduced shallow-water carbonate production, upper slope erosion, and largely pelagic sedimentation in the basin. The longer term changes coincide with progradation pulses that are imaged on the seismic data as depositional sequences. The internal facies architecture of these carbonate depositional sequences displays five major elements. In the undathem, or the platform top, the sediments are arranged in shallow-water packages separated by exposure horizons. Thick bulges of the prograding pulses are characterized by fine-grained platform-derived material that accumulates on the upper slope. The middle to lower slope has a variable facies assemblage consisting of periplatform, pelagic, and redeposited carbonates. Small-scale channeling and lobes of turbidites produce irregular depositional surfaces. At the toe-of-slope, redeposited carbonates accumulate during both sea-level highstands and lowstands. These carbonate turbidite series are arranged in mounded lobes with feeder channels. The distal portion of the sequences is dominated by cyclic marl/limestone alternations with few turbidites. The ages of the 17 observed Neogene seismic sequence boundaries yielded an excellent correlation between sites, documenting the age consistency of the sequence boundaries and chronostratigraphic significance of the seismic reflections. The ages of the sequences along the Bahamas Transect provide a data set that, in conjunction with data sets from other margins, will eventually solve the question about global synchronous longer term sea-level changes. A comparison between the sedimentary and isotope records reveals a discrepancy in the frequency of sea-level changes. Oxygen isotopes record sea-level changes at an obliquity frequency, whereas resistivity and gamma-ray values record sea-level changes dominated by orbital precession. These precessional cycles are packaged into longer term cycles of eccentricity (100, 400, and 2000 k.y.).
|Original language||English (US)|
|Number of pages||11|
|Journal||Proceedings of the Ocean Drilling Program: Scientific Results|
|State||Published - 2000|
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