which is geologically reasonable. Subbottom reflecting horizons on the 

 seismic records are occasionally interrupted by absorption or scattering 

 of the signal near the bottom-water interface with consequent partial or 

 total loss of subbottom resolution. It is difficult in these cases to 

 trace the same reflecting horizon with any assurance. 



Acoustic contrasts usually indicate lithologic differences in layers 

 bounding the reflecting surface because of relationships between physical 

 and acoustic properties of rock and sediments. However, acoustic bound- 

 aries can exist without significant lithologic contrast, and conversely, 

 lithologically different beds may be acoustically similar. The working 

 assumption in interpretation of seismic reflection records is that all 

 reflecting surfaces are geologically significant boundaries. 



Many strong and persistent reflectors appear on the seismic reflec- 

 tion records. Three reflectors were selected for mapping in the Fort 

 Pierce grid area because of their extent and pertinence to study objec- 

 tives. Selected profiles (Appendix A) show all strong reflectors in 

 addition to the ones mapped and specifically identified. 



b. Fort Pierce Grid Area - A schematic profile of typical reflectors 

 in the Fort Pierce grid is illustrated by Figure 7. The shallowest mapped 

 reflector (referred to as the "blue" reflector) lies just beneath the 

 Shelf surface and outcrops at -60 to -70 feet MLW (Figure 8) . Almost all 

 cores which reached the blue reflector encountered "rocky" material at 

 the reflector level, usually light gray or white calcarenite or sediment 

 containing calcarenite fragments. 



While the overlying inner-shelf surface is marked by many low ridges, 

 swales and hills, the blue reflector has an even surface and passes without 

 apparent distortion under shelf irregularities. The seaward dip of about 

 4 feet per mile (1 on 1,300 slope) is parallel to the general dip of the 

 surface of the inner-shelf zone. The blue reflector was selected as a base 

 for the isopach map in the Fort Pierce grid (Figure 9) because it is the 

 surface of a stratum which persists over a large area, and because the 

 physical characteristics of the stratum and its depth constitute a horizon 

 below which dredging for beach material is presently considered impractical. 



Lying about 10 to 30 feet below the blue reflector and parallel to it, 

 is a second prominent reflector, called the "yellow" reflector (Figure 10). 

 The yellow reflector is identifiable throughout the Fort Pierce grid, north- 

 ward to Cape Kennedy and possibly beyond. It can be followed south as far 

 as Jupiter Inlet. Because the yellow reflector is of regional extent and 

 generally conforms to the attitude of upper subbottom strata, it is of 

 stratigraphic interest. No cores have penetrated the yellow horizon or 

 below it within the Fort Pierce grid area. Consequently, the nature of 

 the sedimentary material at or below the acoustic horizon is unknown. The 

 seaward dip of the yellow reflector is about 4 feet per mile (1 on 1,300) 

 to -110 feet MLW, thence increases to more than 10 feet per mile (1 on 

 500) . Between the blue and yellow horizons is a layer characterized by 



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