d. Miami Grid Area 



Tracklines were surveyed on a grid pattern in an approximate 

 5- by "-r-ile area enconipassing the shelf and upper slope off south Miami, 

 Key Bi5ca;.Tie and Virginia Key (Figure 3) . A total of 10 cores were taken 

 on the shelf portion of the grid area. Analyses of the cores, geophysical 

 profiles, and USC&GS boat sheets are the bases of Figures 12, 15, and 16 

 which show the bedrock surface, bottom morphology, and sediment thickness,' 

 respectively, in the Miami grid area. 



The nultiple-stepped bottom morphology described previously for 

 Section A is typically developed in the Miami grid area and is depicted 

 in profile in Figures 7 and 11. Adjacent to the shore is a well-developed 

 terrace with the surface at to -6 feet MLW. The slope terminates on 

 the inner flat at approximately -12 to -18 feet MLW. 



Relief of the inner flat is low. A broad irregular rocky ridge ris- 

 ing in places to -15 feet MLW rims the outer part of the plateau throughout 

 most of the grid area, becoming less distinct toward the southern end of 

 the area. Parallel to the shoreward edge of this ridge a broad shallow 

 swale with central depths of -28 to -30 feet MLW extends through the 

 northern half of the grid area. 



The seaward slope of the ridge marking the outer edge of the inner 

 flat drops to a linear flat at a characteristic depth of -35 to -40 feet 

 MLVi". This second flat, which is continuous with the second flat to the 

 north, averages about 800 yards wide. Bordering the flat to seaward in 

 the northern half of the grid area a reef-like rocky ridge cresting at -30 

 to -40 feet MLW marks the edge of the shelf. In the south, this reef is 

 discontinuous, and where absent, the shelf break occurs at around -45 feet 

 MLVi". Fron -45 feet MLW to about -100 feet, the upper part of the continen- 

 tal slope is rocky or comprised of coarse reef debris. Below 100 feet, 

 geophysical profiles show a thick section of sediments with seaward dipping 

 bedding planes to the maximum depth of survey which is about -350 feet MLW. 

 On the basis of depth and the configuration of the bedding, these sediments 

 are pres'-ized to be Pleistocene and Holocene shoreface deposits overlain by 

 a thin blanket of more recent slope sediments. This assumption has not 

 been confirmed. 



Ov'er zost of the inner flat the bedrock surface is either exposed or 

 close t3 the surface, and sediments in the grid area are thin and dis- 

 continuous. Linear troughlike depressions in the bedrock of the 40-foot 

 flat contain a 5- to 15-foot accumulation of sediment (Figure 16) . Rock 

 is either exposed or covered by a thin veneer along most of the remainder 

 of the shelf edge and upper slope to 100-foot depths. The only other 

 extensive accumulation delineated by this sand inventory program lies in 

 the southernmost part of the inner flat zone where between 5 and 10 feet 

 of sedir.er.t have accumulated. 



28 



