--> Morphometric Analysis of Plunge Pools and Sediment Wave Fields Along the Slopes of Great Bahama Bank

AAPG ACE 2018

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Morphometric Analysis of Plunge Pools and Sediment Wave Fields Along the Slopes of Great Bahama Bank

Abstract

Sediment deposition and redistribution along western Great Bahama Bank are influenced by platform parallel bottom currents, downslope facing cascading density currents and the interaction of the current systems with the slope morphology. Multibeam bathymetric data was used to investigate the slope morphology, plunge pools and sediment wave fields that stretch over an area of 2000km2 and are indicative for these current systems.

The slope consists of a steep upper and lower escarpment (25°-70°), divided by a terrace like step of ~20° inclination in 100-140 mbsl, and incised by regularly spaced grooves (350 m spacing), presumably acquired during the Last Glacial Maximum by karst erosion. A Holocene sediment wedge onlaps onto the escarpment with an inclination of ~9° on the middle and 2-3° on the lower slope, respectively.

Plunge pools and sediment wave fields are located where sudden changes in inclination along the slope profile are observed. These knickpoints create hydraulic jumps where the current undergoes a transition in flow regime from super- to subcritical, resulting in changed depositional and erosional potential responsible for the bedform generation. The currents are directionally confined through the grooves on the escarpment, which enhances their erosional potential. At the base of the escarpment an average change in inclination of 16° is measured, sufficient for sediment wedge erosion at the base of the plunge pool and subsequent sediment deposition, confining the plunge pools downslope by a coarse-grained peloidal sediment wedge. The pools, on average 22 m deep, are between 100 and 650 m long, 80 and 360 m wide, and aligned with the grooves on the escarpment.

Large sediment wave fields occur below the knickpoint between middle and lower slope, where inclination changes from 9° to 2-3° on average. The upslope propagating waves have continuous wave crests, orthogonal to the density current flow direction, are ~4 m high and 100 m long, with a gentle lee side and a steeper stoss side slope. The sediment wave fields are associated with fine-grained peloidal packstone and end abruptly at the transition to foraminiferal grainstone, where bottom currents are responsible for the erosion of finer grained sediment facies. Bedform distribution along the slope can be useful for the prediction of facies changes in the subsurface and therefore yield valuable insight into paleomorphologies of slopes and dominant current systems.