outer-shelf zone. These features are discontinuous and highly irregular, 

 but there is evidence of fairly persistent ridges lying in depths of 70 

 to 90 feet MLW. 



Separating the features of positive relief are areas of reduced 

 gradient in the general overall seaward inclination of the bottom, re- 

 ferred to as "flats". Outer-shelf flats are not necessarily flat in 

 terms of surface detail. Descriptions of reefs or "ridges" at depths 

 consistent with the flats (Moe, 1963) indicate that they may be highly 

 irregular. The flats develop to their greatest extent northward of Fort 

 Pierce; southward the flats either pinch out or continue only as narrow 

 features as the outer- she If zone becomes progressively narrower until 

 at about 27°00'N it can no longer be divided into discrete sub-units 

 (Figure 5) . 



Very little direct data on the outer-shelf bottom composition is 

 available. The few cores within the Fort Pierce grid recovered from this 

 zone contain both consolidated and unconsolidated white calcareous sedi- 

 ment (Type E material, described later) either for the total depth of the 

 core or commencing 1 foot or so be.low the bottom. Descriptions of the 

 area hy fishermen generally characterize the bottom as "coral" or "reef 

 rock", or hard sand, shell and gravel (Moe, 1963). Such descriptions are 

 not inconsistent with the characteristic "E" type sediment contained in 

 CERC cores from this region. 



2. Shallow Subbottom Structure 



a. General - Information about sediment thi<;kness is from chart nota- 

 tions, core samples and continuous seismic profiles. Figure 6 shows a 

 dual-channel seismic reflection record typical of the profiles in Fort 

 Pierce grid area. Cross-section profiles along survey tracklines are pre- 

 sented in Appendix A. These profiles are characteristic of the study area, 

 and show the position and alignment of the bottom-water interface and 

 subbottom acoustic interfaces within sediment and rock masses. 



Seismic reflection profiles do not provide direct evidence of the 

 character of bottom and subbottom materials. Normally, direct evidence 

 must 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, in some cases, it is possible to con- 

 tinuously define the strata identified in the core. Nevertheless, even 

 where good acoustic definition is available, considerable error is found 

 where lateral changes in sediment or rock character occur within the same 

 bounding acoustic interfaces. 



Delineation of acoustic interfaces by seismic-reflection profiling is 

 a reasonably accurate and straightforward procedure where reflections are 

 continuous or interconnected by survey tracklines. Interpolations between 

 parallel survey lines or between gaps in a line must be based on an assump- 

 tion of continuity of slope or elevation, or on an assumed configuration 



