New high-quality multibeam and seismic data image the western slope of the Great Bahama Bank and the adjacent fl oor of the Straits of Florida. The extensive survey reveals several unexpected large-and small-scale morphologies. These include bypass areas, channel-leveelobe systems, gullied slopes, and products of slope instabilities at various scales, including long slump scars at the lower slope and mass transport complexes that extend ~30 km into the adjacent basin fl oor. The toe of the slope is irregularly covered with deep-water carbonate mounds. The abundance of the individual morphological features varies from north to south. From 26°00′N to 25°20′N, the slope is dissected by numerous deep canyons that abruptly end southward, where the slope is characterized by a smooth lower portion and small regularly spaced furrows in its upper part. Further south, two long (25-50 km) scars document instability at the lower slope. One of these scars is the source area of a large mass transport complex. In addition to this large-scale feature, several types of gravity-induced sedimentary processes are revealed. Most of the morphologies and inferred processes of this carbonate system are similar to those observed in siliciclastic systems, including mass transport complexes, gravity currents initiated by density cascading, and overspilling channeled turbidity currents. For the fi rst time, a clear asymmetric channel-levee system has been identifi ed along the slope, suggesting similitude in sorting processes between carbonate and siliciclastic systems and enhancing the reservoir-bearing potential of carbonate slopes. Notable differences with siliciclastic systems include: the lack of connection with the shallow and emerged part of the system (i.e., bank top), and the small size of the sedimentary system.
Coeval stratigraphic units of similar petrology occur throughout the northern Bahamas islands. The petrographic composition of these limestones provides clues about regional sea level and climate changes during the late Quaternary. At least eight fossil shoreline units, which are linked to transgressive episodes between the middle Pleistocene and the late Holocene, are recognized in the Bahamas. The petrographic composition of these units is either dominated by ooids and peloids or by bioclasts. Sedimentological observations demonstrate that oolitic-peloidal units were formed when sea level was higher than today, whereas skeletal units were deposited at or below modern ordnance datum. Skeletal units may reflect times of partial, or modest platform flooding, when the bulk of sediments brought to islands originates from bank-margin reefs. In contrast, oolitic-peloidal units correspond to major flooding events and active water circulation on the bank top. Cement fabrics further show that the early diagenesis of oolitic units took place during warm and humid climatic conditions, whereas skeletal rock bodies underwent subaerial diagenesis during drier climatic conditions characterized by marked seasonal changes. This example from the Bahamas suggests that compositional analysis of limestone from fossil carbonate platforms could be used for resolving ancient climate and sea-level changes.
International audienceNew high-quality multibeam data presented here depict the northern slope of the Little Bahama Bank (Bahamas). The survey reveals the details of large- and small-scale morphologies that look like siliciclastic systems at a smaller scale, including large-scale slope failure scars and canyon morphologies, previously interpreted as gullies and creep lobes. The slope exhibits mature turbidite systems built by mass-fl ow events and turbidity currents. The sediment transport processes are probably more complex than expected. Slope failures show sinuous head scarps with various sizes, and most of the scars are fi lled with recent sediment. Canyons have amphitheater-shaped heads resulting from coalescing slump scars, and are fl oored by terraces that are interpreted as slump deposits. Canyons rapidly open on a short channel and a depositional fan-shaped lobe. The entire system extends for ~40 km. The development of these small turbidite systems, similar to siliciclastic systems, is due to the lack of cementation related to alongshore current energy forcing the transport of fi ne particles and fl ow differentiation. Detailed analyses of bathymetric data show that the canyon and failurescar morphology and geometry vary following a west-east trend along the bank slope. The changing parameters are canyon length and width, depth of incision, and canyon and channel sinuosity. Accordingly, failure scars are larger and deeper eastward. These observations are consistent with a westward tectonic tilt of the bank during the Cenozoic
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