A Synopsis of the Bahamas Drilling Project: Results from Two Deep Core Borings Drilled on the Great Bahama Bank

Abstract

Two continuous cores (Unda and Clino) drilled during the initial phase of the Bahamas Drilling Project on top of the western Great Bahama Bank (GBB) penetrated proximal portions of prograding seismic sequences. As such, these cores provide the shallow-water record of sea-level changes and fluid flow of the Bahamas Transect that was completed with the deeper water sites of Ocean Drilling Program (ODP) Leg 166 in the Straits of Florida. The record of several hierarchies of sea-level fluctuations is identified in the lithology and log signature of two core borings (Unda and Clino), and the nature of fluids responsible for diagenetic alteration is interpreted from formation waters and the stable isotope signal of the sediments and rocks. Facies successions document that several hierarchies of changes in relative sea level are responsible for pulses of progradation. These pulses are seen on seismic data as seismic sequences and in the cores as depositional successions. On the platform, the boundaries of the depositional successions are indicated by subaerial exposure, changes in facies, and diagenetic overprint. On the slope, the sequence boundaries are marked by major discontinuity surfaces within the depositional successions consisting mainly of fine-grained skeletal and nonskeletal sediments. These discontinuity surfaces are characterized by hardgrounds that are overlain by 7-and 28-m-thick, coarser grained packages containing sand-sized blackened lithoclasts, planktonic foraminifers and minor amounts of platform-derived grains. The coarser grained intervals are interpreted as deposits during relative sea-level lowstands, while the fine-grained sediments are interpreted as highstand deposits. Higher order sea-level changes are recorded in the rocks and in the geophysical logs. On the platform top, these changes are recorded in shallowing-upward cycles bounded by exposure horizons. On the slopes, higher order sea-level changes are recognized by facies variations, whereby intervals of coarser grained sediments in the periplatform ooze indicate sea-level falls. The change in sedimentation rate and hydrology during these intervals results in the formation of firmgrounds. The intervals are well recognized as sharp peaks on the gamma-ray and velocity logs. The lower permeability on top of these intervals is likely to separate the fluid flow into several levels within each sequence and influence later patterns of diagenesis. The next higher order of cyclicity is represented by alternations (0.3-1 m) of coarser and finer grained beds within the coarse-grained intervals. Because of their relatively thin nature and low contrast in rock properties, these high-frequency cycles are not recorded in the logs. The slope portions of Unda and Clino yield several age diagnostic foraminifers and nannofossil marker species. Although low in abundance, these microfossils are good indicators of depositional age and provide the base for age determination. By combining micropaleontology, strontium-isotope stratigraphy, and magnetostratigrap...

Compositional variations in calciturbidites and calcidebrites in response to sea-level fluctuations (Exuma Sound, Bahamas)

“…Facies (2012) 58:493-507 501 Discussion Calciturbidites Sedimentation in Exuma Sound is clearly dominated by increased sediment input during highstands in sea level (Reijmer et al 1988), ''highstand shedding'' sensu Droxler et al (1983), Mullins et al (1984), and Schlager et al (1994). Carbonate production is high when the shallowwater platform is flooded and the majority of the sediments produced are exported towards the surrounding basins (e.g., Mullins 1983;Droxler et al 1983;Reijmer et al 1988;Eberli et al 1997;Rendle et al 2000;Reijmer 2004, 2005;Lantzsch et al 2007). The glacial periods with low sea level show sharply reduced carbonate production and thus a relative increase in basinal input while the majority of the shallow-water areas are exposed.…”

A Re‐Evaluation of Facies on Great Bahama Bank II: Variations in the δ 13 C, δ 18 O and Mineralogy of Surface Sediments

“…The elevated  13 C values are a result of a combination of isotopic fractionation of the DIC during photosynthesis and the natural enrichment of aragonite relative to LMC (see above). These sediments are exported from the platform and form periplatform sediments (Schlager & Ginsburg, 1981;Eberli et al, 1997;Eberli, 2000) that drape the fl anks of the platform and extend into the pelagic realm. It has been shown that the  13 C of such sediments can be correlated between units of equivalent age, but that these  13 C values are not related to pelagic  13 C values (Swart & Eberli, 2005).…”

Fluid flow through carbonate platforms: constraints from 234U/238U and Cl− in Bahamas pore-waters

“…Improving our understanding of the driving mechanism and the rate of fluid flow for margin sediments was a major objective of two recent drilling programs in the Bahamas -The Bahamas Drilling Project [6] and ODP Leg 166 [1,7]. These programs uncovered clear evidence for fluid flow from porewater chemistry and from temperature measurements [1,7].…”