Environmental Relationships of Grain Size and Constituent Particles in Some South Florida Carbonate Sediments

Abstract

In the southern extension of the Florida peninsula variations in the submarine topography, areal geography, and hydrography which control the distribution of sediment-producing organisms are reflected in the grain size and constituent particles of the calcareous sediments being deposited. Two major environments can be recognized: (1) a curving band-shaped reef tract with good water circulation, and (2) Florida Bay, a very shallow triangular area with semi-restricted water circulation. Florida Bay sediments have larger proportions of particles less than 1/8 mm. than the sediments of the reef tract. The constituent particle composition of the fraction larger than 1/8 mm. in Florida Bay is almost exclusively molluscan and foraminiferal, but in the same size fraction of the reef-tract sediments fragments of algae and corals are abundant. Similar distinctions in grain size and constituent particles for comparable environments can be derived from published data for the sediments around Andros Island, Bahamas. In Florida Bay large local variations in physical environment obscure the expected effects of differences in environment from one part of the Bay to another, and no distinct sub-environments could be recognized from the gross grain size and constituent particle composition. However, in the reef tract local variations of environment are smaller, and the gradual but consistent changes in depth and water circulation effect differences in the fauna and flora, and thereby produce sediments which have recognizably different abundances of the major constituent particles as shown in Figure 1. The three sub-environments, back reef, outer reef-arc, and fore reef are indicated by progressive changes in constituent composition, and in less degree by variations in gross grain size. Because the estimates of constituent particle composition of the reef-tract sediments were made by point counts on standard petrographic thin sections this approach can be used to analyze ancient limestones. Fig. 1. Generalized variations in sediment grain size and constituent composition of South Florida sediments. End_Page 2384------------------------------