Genesis and growth of a carbonate Holocene wedge on the northern Little Bahama Bank
Résumé
The study focuses on the sedimentary dynamics at the transition between the modern Bahamas carbonate platform and related adjacent slopes and deeper environments. A Holocene carbonate sedimentary wedge is developed at the edge of the carbonate platform extending along the northern Little Bahamas Bank (LBB; Bahamas). The transition area between the platform and the wedge shows superposed terraces. The Holocene wedge thickness varies from 0 to 35 m. Its surface is dominated by homogeneous, fine-grained, soft sediments. It lies on a Pleistocene substratum and fills depressions corresponding to lowstand erosional surfaces. On the uppermost slope, this wedge represents the main depocenter of fine-grained bank-derived sediments since the last bank flooding.
Twelve gravity cores were sampled from this wedge and allow the identification of both the processes that can supply this wedge and the wedge evolution based on content analyses and 19 radiocarbon ages.
At present, particle export from the LBB margins mostly occurs during episodes of winter cold fronts but also occurs during hurricanes. Evidence of hurricane impact in the sedimentary record is suggested by the recovery of two laminated sequences showing both decreasing sedimentation rates and varying velocities during platform export. The local morphology of the platform edge allows the better sorting and export of sediments according to the presence of tidal outlets and sediment sorting on the wedge. However, because no ooids from tidal deltas located on the platform edge were found in cores, tidal currents are probably not energetic enough to both mobilize and export sediment, even if tide–induced currents may help the export of fine-grained carbonate particles remobilized by either cold fronts or hurricanes on the platform and upper slope.
The formation of the wedge started at 13.6 ± 3.5 kyr cal BP following the meltwater pulse 1 A. It was supplied by shallow environment-derived particles produced on four narrow terraces between 60 and 20 mbsl after meltwater pulses. Since the LGM (Last Glacial Maximum), terrace formation seems to correspond to periods of sea-level stillstands and wedge formation, whilst escarpments correspond to periods of high rates of rising sea-level due to meltwater pulses. The main period of wedge growth started around 6.5 ± 0.9 kyr cal BP, when sea level finally reached the platform margins. The maximum flooding period around 4 kyr cal BP is characterized by the highest contents of aragonite needles exported from the platform.