Offbank Transport of Carbonate Sands Along Open, Leeward Bank Margins: Northern Bahamas

Abstract

Hine, A.C., Wilber, R.J., Bane, J.M., Neumann, A.C. and Lorenson, K.R., 1981. Offbank transport of carbonate sands along open, leeward bank margins: northern Bahamas. Mar. Geol., 42: 327-348. Vigorous offbank transport of carbonate sands occurs along the west-facing, leeward, open margins of Little and Great Bahama Bank. Large, offbank-oriented sand waves, thick sand bodies covering reefs, and shallow-water sands (mostly non-skeletal) obtained from adjacent deep flanks, all demonstrate the existence of this shallow-to-deep pathway of sediment transport. A variety of data suggests that transport occurs during storms and not during normal tidal-current fluctuations. An abundance of composite grains as well as vertical sequences of submarine cemented horizons (seen via seismic profiling within the marginal sand bodies) indicate cyclic periods of relative quiescence followed by moments of intense sediment movement. Current-meter data obtained during six weeks of low wind activity show that critical threshold velocities for sand transport are only barely exceeded during portions of the tidal cycle. These flows coupled with higher-frequency bottom currents (generated by surface gravity waves) winnow the sands but do not provide significant net offbank transport. Offbank sand transport provides material to the shallow and deep bank margins enabling them to expand during sea-level highstands. Each sea-level fluctuation is likely to produce within the shallow margins a vertical sequence of basal reefs, covered by a thick non-skeletal calcarenite, capped by a subaerially exposed, diagenetically altered surface. Shallow-water sands carried off the marginal escarpment help to construct deep flanks and large sediment drifts.