Print this pageLithology and Log Signature of the Neogene Seisnuc Sequences of Western Great Bahama Bank - Results of a Seven Hole Transect
EBERLI, GREGOR P., ANSELMETTI, F. S., CARLOS PIRMEZ, and LEG 166 SHIPBOARD PARTY
Cores from five sites drilled during Ocean Drilling (ODP) Leg 166 on a transect from the Great Bahama Bank to the adjacent basin provide, in conjunction with two core borings drilled on top of bank, for the first time a calibration of the lithology and log signature from the proximal to the distal portions of Neogene carbonate sequences. In addition, biostratigraphic ages yielded an excellent correlation between sites, documenting the age consistency of the sequence boundaries and chronostratigraphic significance of the seismic reflections.
Cores from these holes corroborate that global sea level changes control the architecture of the sedimentary deposits. We found 17 depositional units that together build a sedimentary pile that is over 1300 m thick. Facies successions within the cores contain indications of sea-level changes on two different scales. First, 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-deprived material coincide with prograding pulses that are seen on the seismic data as sequences. Second, 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 hetween 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. The compositional variations are also reflected in the log data, allowing a sequence stratigraphic interpretation based on the log data.
The abundance ot biostratigraphic markers allowed to define the ages of 16 Neogne seismic sequence boundaries. The ages yielded an excellent correlation between sites, documenting that the seismic reflections are indeed time lines and have a chronostratigrapic significance.