Abstract:
Cores from five sites drilled during ODP Leg 166 in the Straits of Florida along a transect through prograding sequences of western Great Bahama Bank document the sedimentary record and the timing of 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 layers with more platform-derived material and layers with more pelagic sediments. These alternations are reflected in the ratio between neritic and pelagic components, and mineralogically between aragonite and low Mg-calcite. In the Miocene, the more pelagic layers contain a small amount of clay and silt. Periods of increased platform input are interpreted as deposits during times of high sea level, whereas the pelagic and siliciclastic input is thought to have occurred during sea-level lowstands. In addition, changing sedimentation rates give the sedimentary record of longer-term sea-level fluctuations. Alternating high (up to 20 cm/k.y.) and low sedimentation rates (<2 cm/k.y.) record a long-term pattern of bank flooding (0.5-2 m.y.) with concomitant shedding to the slope, and periods of bank exposure with reduced shallow-water carbonate production and largely pelagic sedimentation. The pulses of bank-derived material coincide with prograding pulses that are seen on the seismic data as sequences. The abundance of biostratigraphic markers allowed to define the ages of 16 Neogene seismic sequence boundaries; the preliminary ages are: 0.1, 0.6, 1.7, 3.1, 3.6, 5.6, 8.7, 8.9, 10.5, 12.1, 12.5, 15.1, 16.0, 18.2, 19.4, 23.2, and 23.7 Ma. The ages yielded an excellent correlation between sites, documenting the age consistency of the sequence boundaries and chronostratigraphic significance of the seismic reflections. A comparison with the global sea-level curve indicates that nearly all major (third-order) sea-level changes are recorded in the seismic sequences along the Bahamas Transect.