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Aspects of Sedimentation and Development of a Carbonate Bank in the Barracuda Keys, South Florida
Paul B. Basan
An extensive carbonate bank in the Barracuda Keys, Florida, was studied to ascertain those factors influencing its growth and present configuration. Five hydrodynamically or biologically controlled sedimentary subenvironments were distinguished: tidal channels, unstable banks, stable banks (including bare-sand, Thalassia, and mangrove island) and silty lagoons.
The bank is a closed system wherein local biological production of sediment is in equilibrium with physical dispersal of sediment. Small amounts of fine grained sediment are derived from the Gulf of Mexico, but this material is insignificant relative to continued bank development. Sediment is generally of uniform size, and responds to current flow more as unit "sheets" than as individual particles, thereby permitting a maximum amount of sediment transport. The major constructional process is the flood tide current, which transports sediment by traction, saltation, and to a lesser extent, suspension and flotation. Steady southeasterly wind-waves cause cross-bank transportation but are subordinate to tides as an agent of bank construction.
The basin-shaped Pleistocene bedrock surface exerted principal control on localization of the overlying bank. A resistant limestone ridge on the northern margin of the study area is a barrier to the dispersal of sediment by waves.
Development of this bank may be summarized as follows: preferential accumulation of fine sediment in sink holes, forming coalescing silty banks; contemporaneous colonization of these banks by calcareous algae and marine grasses; entrapment and accumulation of coarse sediment by these marine plants, forming a single, contiguous sand bank; and continued growth by accretion of sediment over avalanche slopes.
The bank is probably extending itself into the adjoining lagoon by a process of differential growth. This process is dependent upon stabilization of one part of the bank, while growth continues in another.
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