eler.ents of the Pleistocene Miami and Anastasia Formation; both highly 

 calcareous and rich in biogenous material. Bordering the shelf to seaward 

 beyond the shelf break at around -70 feet MLW is the western slope of the 

 northern Straits of Florida. 



South of Boca Raton (26°20' N) the shelf is step-like in profile, 

 consisting of two or three linear flats separated by low reef-like ridges. 

 Tne two outer flats are formed by the underlying bedrock surface and the 

 outer ridges which create trough-like linear depressions partially filled 

 with sediment. The inner flat is predominantly rocky with thin and dis- 

 continuous patches of sediment throughout. Characteristically, sediments 

 conprising the southern part of the shelf in the study area are composed 

 of fragments of the biota, poorly sorted, and ranging in size from silt to 

 very coarse sand. In the outer trough-like flats there are accumulations 

 of about 5 to IS feet of sediment. The deposits on the broad inner flat 

 rarely exceed 5 feet in thickness and are generally much thinner tha.n this. 



The total volume of sediment in the tvi^o outer (second and third) flats 

 is estimated to be 160 million cubic yards. About 100 million cubic yards 

 is located in the second flat at around -35 to -50 feet and is therefore 

 more readily accessible to existing dredging equipment. Sand accumulation 

 on the inner flat may aggregate approximately 20 million cubic yards, but 

 the location of most of this material and the nature of the deposit are 

 not favorable from the standpoint of recovery. South of Government Cut at 

 Miani, the shoreface terrace contains an estimated 16 million cubic yards ■ 

 of sediment but the removal of any substantial amount of this material 

 may have an unfavorable effect on the shoreline. 



In terms of accessibility and lateral continuity of the deposits, the 

 most readily available supply of sand lies in the linear second flat at 

 depth of -55 to -50 feet MLW, and from 1 to 3 miles offshore. The size 

 characteristics of the material when compared to existing beach sediments 

 on the adjacent coast are such that much of it would be usable for local 

 beach restoration and nourishment. There is a difference, however, in the 

 character of the constituent particles in that the offshore material con- 

 tains a substantial amount of delicate material which may become mechani- 

 cally degraded in the turbulent littoral zone. Further study and tests 

 or the material are therefore needed to fully appraise its suitability. 



North of Boca Raton (26°20' N) to the northern limit of the study 

 area the shelf topography and sediments change dramatically. Most of 

 the shelf here is covered by a blanket of homogenous 'fine to medium gray 

 quartzose sand which produces a gently dipping relatively smooth shelf 

 surface topography. Near the shore this sand blanket may reach a thickness 

 of around 40 feet, thence it thins progressively seaward to a feather edge 

 in the vicinity of the shelf break, a distance of approximately 1,500 yards 

 from shore. The total volume of sand available in the northern shelf 

 segment is estimated to be 380 million cubic yards. 



In general, this sand is considerably finer than most sand presently 

 found on southeastern Florida beaches and, therefore, of doubtful value 

 for local beach nourishment projects. 



44 



