Both the inner flat and the third reef line are more or less contin- 

 uous throughout the study area. North of about 26°20' N, i.e.. Section 

 B, the inner slope, the second plateau, and the second reef (and at least 

 part of the third plateau) are overridden by a body of fine quartzose 

 sand (Figure 7b) . Topographically the shelf surface in Section B between 

 the inner flat and the outer part of the third plateau exhibits in profile 

 a long uniform sediment slope dipping continuously seaward (Figure 7b). 

 IvTiile the shelf profile in Section B exhibits undulations, the relief is 

 not great and the prominent stepped profile characteristics of Section A 

 are no longer evident. 



Information concerning the character of bottom surface sediments in 

 the study area is based upon analyses of cores supplemented by USC^GS 

 boat sheets and other sources. Most of the cores for this study were 

 taken from a rather narrow depth zone, -35 to -48 feet MLW. Information 

 obtained from the boat sheets and the other sources indicates similar 

 characteristics in the surface sediments landward and seaward of this 

 extensively sampled zone. Sediment exposed on the surface in Section A 

 is white or gray, medium to coarse grained, carbonate skeletal sand with 

 an average acid soluble content of more than 80 percent (Figures 8 and 9), 

 Sediment comprising the marine bottom in Section B is characteristically 

 gray, fine, and well sorted calcareous quartzose sand (Figure 10). 



Size parameters and visual descriptions of surface samples, obtained 

 for this report, are contained in Appendixes B and C. 



c. Nearshore Subbottom Morphology and Sediment Characteristics 



(1) Character of Seismic Reflectors . Information concerning 

 sediment thickness on the southeast Florida shelf was gathered from chart 

 notations, core samples and continuous seismic profiles. Cross-sectional 

 profiles along all east-west survey tracklines shown in Figure 2 are 

 contained in Appendix D. These profiles are line drawings showing the 

 position and alignment of the bottom-water interface and subbottom acous- 

 tic interfaces within sediment and rock masses. Figure 11, a photograph 

 of the dual channel seismic reflection record is typical of the east -west 

 profiles south of 20°20' N (Section A). 



Seismic reflection techniques do not provide direct evidence of the 

 character of bottom and subbottom materials. Direct evidence must normally 

 be gathered by drilling or coring into subbottom strata, or by tracing a 

 stratum to an exposure which can be sampled more directly. The correlation 

 of sediment or rock characteristics between data points is made easier by 

 seismic data since it is possible in some cases to continuously define the 

 strata identified in the core. Nevertheless, even where good acoustic 

 definition is available, considerable error is found where lateral changes 

 of sediment or rock character occur within the same bounding acoustic 

 interfaces . 



17 



