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Significance of Constituent Composition, Texture, and Skeletal Breakdown in Some Recent Carbonate Sediments
Jonathan P. Swinchatt
A study of sediments along a 35-mile length of the Florida reef tract indicates the presence of two sub-environments between the reefs and the Keys. Variational trends of mean phi, standard deviation, percent fines (less than 62 microns), roundness, and constituent composition indicate a distinct change in the sediment at a point about half-way between the reefs and the Keys. Trends show a general increase or decrease in the outer (reefward) one-half, but general uniformity within the inner one-half.
Bottom observations indicate a correlation between the area in which the change in sediment takes place, and the area in which density of growth of marine grass increases. Analysis of the sediments and the processes affecting them indicates that marine grasses not only modify the effect of current and cause the entrapment of fine sediment, but they also exert an indirect control on processes of skeletal breakdown and textural evolution of the sediment. For instance, in the outer (reefward) area, skeletal breakdown is dominantly a mechanical process and the sediment textures at least partially reflect current phenomena. In the inner (Keyward) area, however, skeletal breakdown is dominantly a biological process and the ultimate texture of the sediment reflects a complex of processes inc uding those associated with growth and the life cycle of the organisms, the reaction of the shell carbonate to breakdown, and transport of fine sediment from the outer area. Of particular importance are the relative rates at which these processes take place; the variability of these rates strongly influences texture and constituent composition of the sediment.
Though little quantitative data concerning these processes is yet available, and their relative importance is poorly known, knowledge of their possible effects suggests caution in some aspects of interpretation of carbonate rocks.
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