Sedimentary processes determining the modern carbonate periplatform drift of Little Bahama Bank
Résumé
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 424 ka. The LBB northwestern slope is divided in (i) a plateau-like structure (margin) at similar to 40 m water depth over at least 4 km parallel to the edge of the LBB; (ii) the uppermost slope with a mean slope angle of similar to 1.15 degrees from 40 to 300 m water depth; (iii) the upper, slope with slope angle of similar to 0.7 from 300 to 650 m water depth, (iv) the middle slope with slope angle of similar to 1.2, from 650 to 800 m water depth, and (v) the lower slope with slope angle of -6 m) that occurred during interglacial periods, the highest sedimentation rates (10-30 cm/ka) and the finest sediment fades were found on the slope. During interglacial periods when RSL < -6 m, LBB was emerged but bank margins were still flooded and correspond to intermediate sedimentation rates (a few to 10 cm/ka) on the slope. Finally, during glacial periods (RSL < -90 m), 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 424 ka: marine isotopic stages 1, 5e and 11. During sea level lowstands, shallow carbonate production was very low but could develop over a 4 km-wide plateau-like structure when RSL was above -40 m. 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 (300-800 m). 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 (similar to 400 m water depth). It also encouraged bi-gradational sequences showing a coarsening-up unit followed by a fining-up unit along the middle slope (similar to 800 m 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. (C) 2015 Elsevier B.V. All rights reserved.