and more detailed analysis of constituents. Poorly sorted sediments such 

 as Type C could not have been transported as entities from one locale to 

 the other since the transportation processes would have better sorted the 

 material. The similarities between some sediments in the two areas can 

 be attributed as well to common factors in the depositional environment 

 and history as to actual interchange of material. 



The foregoing discussion leads to a conclusion that most particles 

 in the shelf sediments of Fort Pierce grid are locally produced and - 

 at least at present - only relatively small quantities of sediments are 

 entering the grid from adjacent shelf areas or from the littoral stream. 



(3) Rate of Accumulation - If the surface of the stratum con- 

 taining Type E material is indeed pre-Holocene, the overlying sediments 

 represent accumulation during, and subsequent to, passage of the Holocene 

 sea across the shelf platform. Since nearly all non-Type E sediments lie 

 on the inner shelf, deposition in this zone would have commenced when 

 relative sea level rose above -70 feet MLW. Assuming that this region 

 has been structurally stable during the period in question, the onset of 

 transgression across the inner shelf would have occurred at about 8,200 

 years Before Present according to data from Curray (1964) and about 7,200 

 years Before Present based on the sea level curves of Milliman and Emery 

 (1968) . Both curves indicate a rate of rise which would have brought the 

 sea landward across most of the inner shelf (to -40 feet MLW where the 

 slope changes from 1 on 1,300 to 1 on 80) about 1,000 years after the 

 onset. Using an average sediment thickness above the blue reflector esti- 

 mated from the isopach map of Figure 8 to be about 7 feet and a sedimenta- 

 tion time of about 7,000 years, the average rate of accumulation is only 

 about 1 foot in 1,000 years. 



It seems unlikely that sedimentation of the inner shelf has pro- 

 gressed at a steady rate during this period. Increasing depth over the 

 shelf during the last transgression and ancillary variations of conditions 

 affecting local shell production probably also affected accretion rates 

 so that periods of relative increase or decrease in rate of accumulation 

 are likely to have occurred continuously. 



2. Sand Requirements 



In a 1965 appraisal of Florida beach conditions (USCE, 1965), the 

 Jacksonville District, Corps of Engineers, listed about 50 percent of the 

 shoreline within the limits of this study as subject to severe erosion. 

 The beach was found to be generally narrow and low, and dune heights 

 rarely exceed +15 feet. 



Beach Erosion Control studies have been completed for only two of the 

 four counties in the study area: Brevard County (USCE, 1967) and Martin 

 County (USCE, 1968). Sand requirements for beach mourishment summarized 

 earlier (Duane, 1968) are in Table III. The total sand requirements for 

 Martin County and that part of Brevard County south of Canaveral Harbor 



49 



