Compared to the Marquesas Keys, 

 corals play a much more significant 

 role in the composition of recent 

 sediments in the Dry Tortugas. The 

 Dry Tortugas represent a drowned 

 Pleistocene platform 17 to 21 m (56 

 to 69 ft) below sea level which is 

 comprised of three biogenic buildups 

 (facies): (1) detrital lagoonal 



bank, (2) Montastrea reef bank, and 

 (3) Acropora palmata reef (Jindrich 

 1972). These facies lie adjacent to 

 one another and are also present in 

 vertical succession as individual 

 growth stages that vary in thickness 

 and lateral extent. A zone of 



Acropora cervicornis develops as a 

 transition between the Montastrea 

 and A . palmata growth stages. The 

 present reef assemblages and bottom 

 topography have been strongly influ- 

 enced by cumulative storm effects 

 that are linked to the slow sea 

 level rise over the past several 

 millenia (Jindrich 1972). 



Storm degradation is manifes- 

 ted by (1) continuous removal of 

 A. palmata and its replacement by 

 storm-resistant coralline algae and 

 Millepora sp. to produce truncated 

 rocky surfaces, (2) abundant reef 

 rubble, (3) erosion of spui — grooves, 

 and (4) development of intertidal 

 rubbly reef flats. 



Noncoralline sediments range in 

 size from cobble-sized algae to 

 foraminifera. Variations in texture 

 and particle composition are mainly 

 a result of sediment transport and 

 grain shape. In general, three 



modes of sediment transport produce 

 three characteristic assemblages of 

 constituent particles: 



(1) a gravel sized population; 



(2) a sand sized population; 

 and 



(3) a fine sand to silt sized 

 population. 



Strong mixing occurs between the 

 gravel and sand population on the 

 storm degraded shoals, and between 

 the sand and silt population on the 

 lagoon bottom. Sands generally 



flank the reefs and reef banks and 

 show minimum mixing. Lagoonal bank 

 sediments, stabilized by seagrass 

 and coral growth, are composed of an 

 incongruous mixture of the in-place 

 fraction and varying proportions of 

 transported sediments. 



During a low sea-level stand 

 created by a period of Pleistocene 

 glaciation, organic acid solutions 

 created numerous pits or holes in 

 the surface of the Keys 1 limestone 

 (Krawiec 1963). These pits (geo- 

 logically referred to as breccias) 

 became, and still act as, storage 

 tanks for coral debris, organic 

 soils from mangrove swamps and hard- 

 wood hammocks, and loose material of 

 all available types generated by the 

 hurricanes and lesser tropical 

 storms. These result today in an 

 exposed karst topography over the 

 Florida Keys (Kissling 1968, Seimers 

 and Dodd 1969, Dodd and Seimers 

 1971, Multer 1977), Florida Bay 

 (Gray 1974), and the seaward lagoon- 

 al environment (Turmel and Swanson 

 1964). The karst topography affects 

 the localized drainage and concen- 

 tration of soils on the Keys. This 

 concentration in turn influences the 

 terrestrial floral and faunal dis- 

 tribution (Multer 1977). Immediate- 

 ly over the bedrock in the Keys are 

 laminated crusts which are either 

 exposed, covered by thin soils, or 

 covered, in the case of solution 

 holes, by thicker miscellaneous 

 debris described above. Multer and 

 Hoffmeister (1968) describe three 

 types of indurated crusts coating 

 the Florida Keys: (1) porous lamina- 

 ted crust characterized by horizon- 

 tal root tubes; (2) dense laminated 



68 



