NORTH 



VERY FEW OB NO SHELLS 

 SLIGHTLY SHELLY 

 MODERATELY TO VERY SHELLY 

 I PEAT 

 It- THALLASSIA GRASS BEDS 



LOCAL RELIEF^ 

 ON ROCK FLOOR 

 IDIAGRAMATICI 



Figure 27. Cross section of Cross Bank in Florida Bay (adapted from Turney 

 and Perkins 1972). 



4.43 FLORIDA KEYS 



Our discussion of Holocene 

 sediments in the Florida Keys is 

 presented in two sections: (1) sedi- 

 ments of the marine environment; and 

 (2) sediments of the terrestrial and 

 freshwater enviroment. Based on 

 surface area alone, it is obvious 

 that the former of the two dominates 

 in sediment contribution to the 

 area. 



Recent marine sediments of the 

 Keys are produced in what amounts to 

 a vast, multifaceted carbonate fac- 

 tory (Enos 1977). These bioclastic 

 sediments are produced by organisms 

 restricted geographically by bottom 

 morphology, circulation, and most 

 importantly by the very substrate 

 the organisms themselves have pro- 

 duced. Figure 28 illustrates and 

 summarizes the Holocene sediments of 

 the south Florida shelf margin in 

 the vicinity of the lower Keys. 



Enos (1977) describes three 

 natural subdivisions of the seaward 

 shelf off of the lower Keys: 



(1) the slightly restricted in- 

 ner shelf margin; 



(2) the outer shelf margin where 

 circulation and turbulence 

 are maximum; and 



(3) the shallow slope seaward of 

 the shelf break. 



The primary controls on sediment 

 distribution patterns are skeletal 

 productivity, mechanical redistri- 

 bution, pre-existing rock topo- 

 graphy, and contemporary sediment 

 topography. 



Sediment accumulations in the 

 inner shelf margin are generally 

 less thick and muddier than those on 

 the outer margin. Wedges of sedi- 

 ment piled against the Pleistocene 

 rock of the Keys reach more than 5 m 

 (15 ft) in thickness. These wedges 

 are elongate parallel to the shelf 

 edge. Patch-reef banks of the inner 



64 



