Sedimentary processes determining the modern carbonate periplatform drift of Little Bahama Bank

Abstract

This paper presents an analysis of the combined influence of along-slope sediment transport and off-bank sediment export from the Little Bahama Bank (LBB) to the periplatform sediment wedge of the northwestern part of the slope over the last 424ka. The LBB northwestern slope is divided in (i) a plateau-like structure (margin) at ~40m water depth over at least 4km parallel to the edge of the LBB; (ii) the uppermost slope with a mean slope angle of ~1.15° from 40 to 300m water depth; (iii) the upper slope with slope angle of ~0.7 from 300 to 650m water depth, (iv) the middle slope with slope angle of ~1.2, from 650 to 800m water depth, and (v) the lower slope with slope angle of <~0.5, from 800 to 900m water depth.The uppermost slope, the upper slope, and the middle slope of the northwestern LBB were characterized by periplatform oozes that became more diluted with pelagic sediment toward the distal part of the slope. This sediment distribution of the northwestern LBB slope varied significantly over times according to the flooded surface of the LBB. The major flooding periods are related to the highest Relative Sea Level (RSL) (>−6m) that occurred during interglacial periods, the highest sedimentation rates (10–30cm/ka) and the finest sediment facies were found on the slope. During interglacial periods when RSL<−6m, LBB was emerged but bank margins were still flooded and correspond to intermediate sedimentation rates (a few to 10cm/ka) on the slope. Finally, during glacial periods (RSL<−90m), LBB was emerged (including its margins), sedimentation rates on the slope dropped to a few mm/ka associated to coarser sediment facies.Off-bank-transported sediment is the main sediment supply during sea-level highstands, occurring preferentially during three major periods of LBB flooding over the last 424ka: marine isotopic stages 1, 5e and 11. During sea-level lowstands, shallow carbonate production was very low but could develop over a 4km-wide plateau-like structure when RSL was above −40m. The regional Antilles Current affected the sea floor along the northwestern LBB slope and influenced coral mound distribution as well as sediment facies and sequences along the upper and middle slopes (300–800m). During glacial periods, the stronger influence of the Antilles Current upon the along-slope sedimentation promoted diagenesis via the development of indurated nodules in the upper slope (~400m water depth). It also encouraged bi-gradational sequences showing a coarsening-up unit followed by a fining-up unit along the middle slope (~800m water depth) that is thoroughly bioturbated. The characteristics of these contourite sequences were similar to those described in siliciclastic environments, but in contrast were condensed with low sedimentation rates over long (glacial) periods.