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An investigation of the relation between dynamic oceanography and the dispersal of fine-grained sediment has been conducted on the unusually broad (100 to 250 km) and shallow (20 to 30 m) Mosquito Bank (Cayos Miskitos), off the east coast of Nicaragua. Exceedingly high rainfall (500 cm/year) on the coastal watersheds supplies enormous quantities of fresh water and suspended sediment to the nearshore area. A dynamic balance between the density and water-slope pressure-gradient forces, the Coriolis forces, the forces of internal friction, and the spatial acceleration of the water parcels produces a very distinct, turbid, brackish coastal-boundary layer (CBL). The dynamics are such that this CBL is dominated by a coastal jet 20 to 30 km wide, with a velocity of 50 to 70 cm/s c predominantly alongshore. Owing to the steadiness of the local trade winds, the jet appears to be a persistent feature, thereby minimizing large-scale exchanges with the shelf water beyond the CBL. Data on both suspended and bottom sediment clearly show the overriding influence of CBL dynamics on sediment dispersal. Despite the enormous input of terrigenous material brought to the shelf of about 25 × 106 m3/year (five times more than is delivered to the entire U.S. Atlantic Coast), fine-grained sediments are distinctly confined to the vicinity of the CBL.
The vast expanse of this shallow bank outside the CBL, essentially free of terrigenous sedimentation, has developed into a suite of carbonate environments. Environments grade seaward from a relatively flat depositional plane dominated by Halimeda-rich aragonitic muds to a mid-shelf and outer-shelf zone of island-flank platforms and topographic highs associated with coral-algal reefs. Likewise, the bank-edge escarpment displays rough bottom conditions typical of reef development.
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