Unconsolidated sediments lying above the blue reflector may have 

 originated largely outside the area of deposition or have been largely 

 derived from local sources. Since no streams discharge directly on the 

 Florida coast south of Jacksonville Beach, this common source of terrig- 

 enous sediment is not considered active here. Pleistocene drainage may 

 have played some part as a previous source, but no evidence of important 

 contributions from this source is available. 



A possible source of sediment from outside the grid area is littoral 

 drift alongshore coupled with offshore transport to the shelf area. Exist- 

 ing beach sediments north of Cape Kennedy are considerably finer and more 

 quartzose than those of the study area, thus could be contributing only 

 minor amounts of material presently on the beach and shelf in the Fort 

 Pierce grid. Net littoral transport in southeast Florida is southward 

 (Watts, 1953); thus, drift from the south is also an improbable source 

 of beach sand near Fort Pierce. The most likely sources of beach sand 

 are coastline erosion and local shell production. Shelly quartzose sand 

 similar to that of the beaches is available in preserved Pamlico Age sands 

 of the -coastal upland. Storm wave erosion of this source could play a 

 significant role in littoral sedimentation. Shell production is high, 

 especially in the vicinity of the inlets. Additional shell and quartz 

 debris is probably derived locally from erosion of coquina outcrops 

 reported in the shallow waters close inshore (USCE, 1967 and 1968); 

 although some of these outcrops may in fact be sabellariid reefs 

 (Kirtley and Tanner, 1968). 



Some cores from the shoreface area contain quartzose sand similar to 

 the beach sands. Elsewhere on the shelf, good evidence of beach-derived 

 sand is lacking. The coarser texture of shelf surface sediment compared 

 to the beach, and progressive diminution of sand size transported from 

 shore into the sublittoral shoreface as evidenced by the size data of 

 Figure 22, tends to indicate that large quantities of littoral sand are 

 not reaching the shelf proper at the present time. In addition, the 

 rounded and polished character of shell fragments in beach sand, though 

 present in shelf surficial sediments, is not characteristic. Difference 

 in quartz- carbonate ratios between the beach and offshore surficial sedi- 

 ments is substantial. If sand does reach the shelf from onshore deposits, 

 it is greatly diluted by carbonate material derived from other sources. 



If the sediment were derived from outside the area, the most probable 

 sources are the adjacent shelf areas to north and south. Characteristics 

 of the shelf to the south of the Fort Pierce grid to north of Palm Beach 

 are not well known, but south of Palm Beach, reliable information is 

 available. Between Miami and Boca Raton (24°45'N to 26°20'N) the near- 

 shore shelf contains white to gray calcareous sand alternating with rocky 

 ridges and flats (Duane and Meisburger, 1969) . Examination of repre- 

 sentative samples from this shelf shows that the major constituents are 

 Halimeda segments and mollusk shells. 



Halimeda and other common constituents such as alcyonarian sclerites 

 and large peneroplid foraminifers are missing in Fort Pierce grid samples. 



47 



