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 fluctua tions is identified in the lithology and log signature of two core borings (Unda and Clino), and the nature of fluids responsib le 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 relativ e 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. T he 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 t o 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 cyc les 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 magnetostratigraphy, a chronostratigraphy is obtainable in the prograding margin of GBB. The chronostratigraphy helps assess the stratigraphic evolution of the Great Bahama Bank margin, the timing of sea-level changes observed in the sedimentary record, and the synchroneity of the seismic sequence boundaries. The correlation of the rock with the seismic record not only confirms some of the major assumptions of the sequence stratigraphic concept but also showed some of the limitations when using sequence stratigraphy as a dating tool. The assumption that seismic reflections follow depositional surfaces (i.e., time lines) is indicated by two lines of evidence. The combination of changes in composition and diagenesis produces the necessary impedance contrasts for the imaging depositional unit boundaries as seismic reflections. In addition, chronostratigraphic dating shows no crossing of seismic reflections with time lines. The inability to recognize condensed or expanded sections causes problems when dating sequences solely by correlation to the global cycle chart. For example, the condensed interval in the early Pliocene falls below seismic resolution, whereas the thick Quaternary sequences are of different duration than the ones shown on the global chart of Haq et al. (1987). Geochemical data from the rocks and fluid samples taken during the drilling provide abundant evidence of the existence of fluid flow within the margin of Great Bahama Bank. Despite pervasive alteration and marine cementation, pore water analyses in the upper 100 m show an essentially isochemical profile suggesting the active flushing of seawater through this interval. Below 100 m in both cores, there are large increases in the concentration of nonconservative elements such as Sr, indicating dissolution and cementation processes.