Comparative morphology and dynamics of Holocene carbonate systems, northwestern Abaco Islands, Bahamas.

摘要

Although the physical processes controlling siliciclastic shallow marine tidal sand bodies are reasonably well-understood, the role of these processes in shaping carbonate sand systems has not been systematically explored. To what extent are these processes important in shaping the geomorphology of such carbonate systems, in which sands can be "born, not made?" The tidal sand bodies of the northwestern Abaco Islands, Bahamas, provide a unique opportunity to explore these processes in a heterogeneous carbonate system.; Tropical cyclones commonly are cited as being influential, or even dominant, controls on the geomorphologic evolution of carbonate systems. An opportunity to explore the effects of storms on platform systems occurred when two strong tropical cyclones (Frances and Jeanne) passed directly over the shoals and reefs of the Abacos, northern Bahamas, in 2004. Observations of the influence of tropical cyclones coupled with hydrodynamic observations and wave models provide insights into the role of these types of events in carbonate platform geomorphic evolution. These results illustrate that these storms were neither sufficient to drive major change nor necessary to explain the geomorphic evolution of this tidally influenced system. Further observations of wind, wave, and tidal forces acting on this carbonate system indicate that these quotidian processes may be an important factor in controlling the geomorphology of the system. A conceptual morphologic model developed from observations and numerical modeling illustrates how feedbacks amongst bathymetry, hydrodynamics, and sedimentology lead to systematic trends in granulometry and geomorphic trends in the carbonate tidal deltas akin to their siliciclastic counterparts.; A closer look at the daily tidal forces effecting the oolitic sand bodies of the northwestern Abacos indicate that links between the tidal currents and bathymetry can set up a circular flow pattern, explaining the presence of the shoals and the oolitic grains creating them. Numerical modeling experiments indicate that flow restriction creating current jets and the presence of a sediment source are two necessary (but perhaps not sufficient) factors controlling the morphodynamics of oolitic tidal deltas.; The results of this study represent a first step in advancing carbonate facies models beyond qualitative concepts towards quantitative, specified environmental and sedimentologic characterizations.