Geomorphology of carbonate platform‐marginal uppermost slopes: Insights from a Holocene analogue, Little Bahama Bank, Bahamas

Abstract

As the product of a variety of sediment sources and sedimentation (and re‐sedimentation) and erosion processes, the geomorphology and sedimentology of carbonate slopes are highly variable. The purpose of this study is to describe sub‐bottom profiles and side‐scan sonar, multibeam and optical data acquired by an autonomous underwater vehicle to explore variability in geomorphological and sedimentological character of the present‐day platform‐marginal, uppermost slope environments (< 240 m water depth) on the north, open‐ocean facing flank of Little Bahama Bank, Bahamas. Although at time scales of greater than 100 ka this margin is progradational, the data illustrate a complex juxtaposition of erosional and depositional processes and features. Erosion is evidenced by two prominent escarpments (70 m and 120 m) that expose eroded, bedded rocky outcrops. These escarpments are interpreted to represent relict features, related to past sea‐level positions, although they still may be shedding debris. Aside from erosional remnants, sedimentation and active transport is indicated by several features, including active bedforms (especially above the 70 m escarpment, but ripples occur to depths of ca 200 m), several mass transport complexes that overlie and cover the lower escarpment, gravity flow deposits and rare slump features. Similarly, a thick (up to 20 m) onlapping sediment wedge, interpreted to be Holocene in age, suggests lateral accretion of the slope by more than 75 m in this period. Data illustrate that this open‐ocean margin is distinct from windward margins in the Bahamas, which typically include near‐vertical walls of erosion or bypass, flanked downdip by rubble and talus, and leeward margins, which have onlapping muddy wedges, but that lack marked terraces or escarpments. Collectively, the results provide perspectives into the nature and controls on complex geomorphological patterns of erosion and deposition in Holocene uppermost slope systems, concepts potentially applicable to ancient analogues.

Mapping bathymetry and depositional facies on Great Bahama Bank

“…The tidal range on GBB varies from ca 1 m at the margin of the platform (Wilson & Roberts, , ) decreasing to 0·3 m at the Three Creeks portion of the Andros tidal flats (Hardie & Garrett, ). For northern Little Bahama Bank, Reeder & Rankey (), Rankey & Reeder () and Rankey & Doolittle () noted the tides to vary between ca 0·75 m during neap tides and 1 m during spring tides. Flow velocities associated with these tides range from 40 to 80 cm s −1 within different ooid shoal complexes of Little Bahama Bank (Rankey et al ., ; Reeder & Rankey, ; Rankey & Reeder, ), and similar velocities were measured on GBB in a tidal inlet of the Exumas (Gonzalez & Eberli, ) and a tidal channel of the Schooners Cays area (Rankey et al ., ).…”

Margin collapse and slope failure along southwestern Great Bahama Bank

“…They all display a similar margin-slope morphology. A near vertical slope segment of approximately 100 m height develops around all Bahamian carbonate platforms on both the windward and the leeward sides (Palmer, 1979;Ginsburg et al, 1991;Grammer et al, 1993;Rankey and Doolittle, 2012). These near vertical well-cemented escarpmentlike margins accrete during the Holocene due to rapid early-marine cementation of sediment combined with organic growth of sponges, coralline algae, and microbial communities.…”

Recent morphology and sedimentary processes along the western slope of Great Bahama Bank (Bahamas)

“…This low acoustic facies, present along most of the upper to middle slope, corresponds to unconsolidated silty mud wackestone (Table ) and forms a sedimentary wedge commonly referred to as the ‘Holocene wedge’ (Wilber et al ., ; Schlager et al ., ; Roth & Reijmer, , ). It is also found along the uppermost slope of the Little Bahama Bank (Rankey & Doolittle, ; Mulder et al ., ). This thinning‐downslope sedimentary wedge consists predominantly of aragonite mud‐size particles derived from the bank since the late bank‐top Holocene flooding (Wilber et al ., ).…”