SECT. 3] 



SHALLOW-WATER CARBONATE SEDIMENTS 



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Fig. 10. Grain size and constituent comiDosition, Bikini Atoll. (After Emerj-, Tracey and 

 Ladd, 1954.) 



quantity of Halimeda in the sediments around Johnston Island, a shallow, 

 flat-topped platform without a windward barrier reef. 



D. Florida Reef Tract 



The Florida reef tract includes a protected area behind the outer reefs that 

 is in many respects analogous to the atoll lagoons of the Pacific. It is much 

 shallower, as seen in Figs. 6 and 11, and less removed from connection with the 

 adjacent deeper water, but it does appear to have a similar physical environ- 

 ment. The sediments of the Florida back reef, like those of the atoll lagoons, 

 are dominated by Hali7neda (Fig. 11). The thin reef sands, on the other hand, 

 are characterized by the presence of fragments of coral and coralline algae, the 

 reef builders, and the fore-reef sediments by coralline algae and Foraminifera. 

 These associations of particles coincide with the distributions of the zones of 

 maximum abundance of their parent organisms. It should be noted that the 

 composition of the reef sands is biased in favor of the fragile sediment-producing 

 organisms because most of the reef builders, massive corals and encrusting 

 algae, remain as cemented reef rock. 



