Controls on reef development and the terrigenous-carbonate interface on a shallow shelf,Nicaragua (Central America) |
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Authors: | H H Roberts S P Murray |
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Institution: | (1) Coastal Studies Institute, Louisiana State University, 70803 Baton Rouge, Louisiana, USA |
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Abstract: | Marine geology and physical oceanographic data collected during two field projects (∼4 months) on the Caribbean shelf of Nicaragua
indicate a surprising dominance of carbonate deposition and reef growth on a shelf that is receiving an abnormally large volume
of terrigenous sediments. High rainfall rates (∼400–500 cm/year), coupled with a warm tropical climate, encourage rapid denudation
of the country’s central volcanic highland and transport of large volumes of terrigenous sediment and fresh water to the coast.
Estimates suggest that three times more fresh water and fifteen times more sediment are introduced per unit length of coastline
than on the east coast of the United States. Distribution of the terrigenous facies, development of carbonate sediment suites,
and the location and quality of viable reefs are strongly controlled by the dynamic interaction near the coasts of highly
turbid fresh to brackish water effluents from thirteen rivers with clear marine waters of the shelf. Oceanic water from the
central Caribbean drift current intersects the shelf and moves slowely in a dominant northwest direction toward the Yucatan
Channel. A sluggish secondary gyre moves to the south toward Costa Rica. In contrast, the turbid coastal water is deflected
to the south in response to density gradients, surface water slopes, and momentum supplied by the steady northeast trade winds.
A distinct two-layered flow is commonly present in the sediment-rich coastal boundary zone, which is typically 10–20 km wide.
The low-salinity upper layer is frictionally uncoupled from the ambient shelf water and therefore can expand out of the normally
coherent coastal boundary zone during periods of abnormal flooding or times when instability is introduced into the northeast
trades. Reef distribution, abruptness of the terrigenous-carbonate interface, and general shelf morphology reflect the long-term
dynamic structure of the shelf waters. A smooth-bottomed ramp of siliciclastic sands to silts and clays mantles the inner
shelf floor in a linear belt paralleling the coast. This belt generally corresponds to the western flank of the coastal boundary
zone. Occurrence of reefs is generally confined to areas outside this zone. Terrigenous clays and silts of the inner shelf
are abruptly (<20 km from the coast) replaced byHalimeda-rich sediment of the middle and outer shelf. Within the carbonate facies belt, reef complexes thrive as small, isolated masses;
large, reef-capped platforms; reef fringes around islands; and shelfedge structures with vertical relief that can exceed 25
m. In general, the frequency and proliferation of reefs increase away from the turbid coastal boundary layer and toward the
cooler and saltier water that upwells at the shelf margin. |
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