Control of terrigenous-carbonate facies transitions by baroclinic coastal currents - nicaragua

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The cross-shelf movement of fine-grained terrigenous sediment has received considerable attention in recent geological literature. Of perhaps greater interest is the spatial relationship with other sediment types and the processes that control facies segregation. Studies of sediment distribution on the shallow and broad shelf off the east coast of Nicaragua have revealed a 20- to 30-km-wide (12- to 19-mi) band of terrigenous sediment confined near the coast. The band grades abruptly into an area of carbonate deposition that is composed principally of the disintegration products of calcareous green algae. Detailed studies of watershed runoff, structure of nearshore currents, and density gradients indicate the existence of a well-organized band of currents that run essentially parallel to the coast. An analytical model is formulated that successfully predicts the strength and location of this coastal boundary current system as a function of wind speed and direction, water depth, and local density gradients. The analytical prediction of the cross-shore current structure agrees well with the field observations in delineating the presence of convergences and divergences that will act to restrict the seaward migration of fine-grained terrigenous particulates and thus preserve the identity of the two contiguous facies. Variations in the model inputs (e.g., rainfall runoff, wind regime) allow estimates of how such systems operated in different geological regimes, and consequently how such facies relations evolved in climatic regimes different from that of the present time. An understanding of the physical processes that control the siliciclastic-carbonate interface on the eastern Nicaragua shelf emphasizes the delicate balance that exists between terrigenous and carbonate dominance over the shelf. © 1988, Elsevier Science & Technology. All rights reserved.

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Developments in Sedimentology

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