shelf margin also tend to be elon- 

 gate parallel to the shelf edge, 

 but most are small, less than 2 km 

 (1.2 mi) long. Tidal deltas, ori- 

 ented perpendicular to the shelf 

 edge, develop where passes between 

 rockfloor highs enter restricted 

 basins of the inner shelf. Most of 

 the sediment in tidal deltas is gen- 

 erally less than 5 m (15 ft) thick. 



Changes in sediment thickness 

 and total volume along the lower 

 Keys are most closely tied to the 

 amount of tidal exchange with the 

 restricted shelf. Where water from 

 the inner shelf and the broad Gulf 

 of Mexico shelf flows across the 

 shelf margin, sediment acumulation 

 is reduced, probably owing to de- 

 creased productivity. Tidal deltas 

 and inner shelf-margin sediment 

 wedges are lacking here, too, be- 

 cause they are dependent on current 

 constriction by the Florida Keys. 



The outer margin, ( > 3 km or 

 2 mi) is the site of the thickest 

 (typically 8 m (26 ft) or more) and 

 most permeable (up to 35 darcys) 

 sediments in the Holocene package. 

 The thickest sediment accumulations 

 lie in belts parallel to the shelf 

 edge. The trends of these belts are 

 quite predictable, but the sediment 

 thickness and physical properties 

 vary along the axis of the coast. 

 The outer reef belt is the most 

 nearly continuous and the thickest. 

 It is 1 to 2 km (about 1 mi) wide 

 and located immediately behind the 

 shelf break. 



The shallow-slope sediment 

 blanket is thickest within a kilo- 

 meter of the shelf break at a water 

 depth of about 30 m (100 ft). Shelf- 

 edge sands form prisms of thick 

 sediment 1 to 2 km (.6 to 1.2 mi) 

 wide, 2 to 3 km (1.2 to 1.9 mi) be- 

 hind the slope break. The largest 

 sand shoal is 40 km (25 mi) long and 



as much as 9 m (30 ft) thick. Where 

 sand shoals are lacking, a belt of 

 discontinuous patch-reef banks may 

 occupy the same position. The 



patch-reef belt is also 1 to 2 km 

 (.6 to 1.2 ft) wide, trends parallel 

 to the shelf, and may be more than 

 8 m (25 ft) thick, but continuity is 

 poor and the sediments are generally 

 muddy. A bed-rock depression with a 

 thin layer of muddy sediment isola- 

 tes the patch reef belt of the lower 

 Keys from the outer reef belt. 



Enos (1977) states that the 

 Holocene sedimentary sequence dif- 

 fers from the Pleistocene in that it 

 contains little quartz or nonskele- 

 tal carbonate, and the distribution 

 of grainstone is less widespread. 

 He attributes these differences to 

 the relatively small degree of 

 submergence in recent time and 

 the low supply of terrigeosus ma- 

 terial from the Florida mainland. 

 Table 12 summarizes the existing 

 sediment producing communities off 

 the lower Keys and their relative 

 productivity. 



Rock or dead reef 



a 

 b 



Mud 



open marine—mainly encrusting and boring organisms 

 restricted circulation — mainly encrusters and borers 



grass covered--turt le grass ( Tnalassia ) , green algae 

 ( Hal imeda , Penicil lus ) mi 1 i ol id foraminifera, browsing 

 gastropods, burrowing pelecypods and shrimp ( Calianassa ) 

 bare— a few green algae, foraminifera 



grass covered— Thalassia , Halimeda . peneroplid 

 foraminifera, browsing gastropods, burrowing pelecypods 

 bare — burrowing echinoids 



Patch reef— head corals 



Outer reef— corals ( Acropora , Montastrea . Oiploria . Porites ) , 

 Mi llepora , Hal imeda opunt ia 



Forereef muddy sand— pelagic foraminifera 



Shoal fringe, restricted— f inger coral ( Porites ). red algae 

 ( Goniolithon ) , Halimeda opunt l a 



8. Reef rubble— few organisms 



Relative skeletal productivity by the habitat communities is estimated 

 as 



5 > 4 > 7 » 2a » 3a > la > lb > 6 » 2b > 3b > 8. 



Table 12. Mappable habitat communi- 

 ties of the south Florida 

 reef and shelf (adapted 

 from Enos 1977). 



4. 

 5. 



e. 



7. 



66 



