Section III. DISCUSSION 



1. Sediment Distribution and Origin 



Distribution of unconsolidated sediment on the shelf in Section 

 A is largely controlled by configuration of the bedrock surface. Depres- 

 sions such as the linear troughs between reef lines, shallow swales and 

 bowls on the inner flat, and areas in the lee of topographic highs are 

 favored sites of deposition. The effect of waves and currents on shelf 

 sedimentation processes in this area are not known in detail. 



Available data indicate there is little if any sediment transported 

 into the shelf area from the north (Section B) v\fhere shelf sediments are 

 finer and much higher in quartz than those of Section A. Introduction 

 of sediment from the slope is also highly unlikely since it would require 

 migration upslope and across the outer reef line. 



No significant quantities of the material presently comprising the 

 beach were found in the cores obtained offshore. Further, the general 

 median sand size seaward from the beach shows a decrease to depths of 

 -12 to -IS feet MLW CU. S. Army Engineer District, Jacksonville, 1956, 

 1960, 1961, 1963, 1965). Beyond this depth, the inner flat Cessentially 

 rocky) separates the nearshore zone from the shelf proper. Thus, there is 

 a zone of relatively fine sand and rock separating two zones of coarser and 

 compositionally dissimilar materials on the beach and shelf. Significant 

 interchange of material between the beach zone and the shelf either in a 

 landward or seav/ard direction within the study area is judged improbable. 



Shelf sediments of Section A are judged to be produced more or less 

 in situ from organisms presently comprising the biota of the shelf bottom, 

 particularly that biota along the reef and slope lines. Sedimentary ma- 

 terial thus produced could subsequently be swept into the adjacent troughs 

 by wave or current action. Sediment produced under shallow water condi- 

 tions extant during lower relative stands of sea level associated with 

 late Wisconsin glaciation may account for some quantity of the trough 

 sediments. However, as no age dating of these sediments has yet been 

 undertaken, it is not possible to determine relative quantities of 

 sediment contributed during Wisconsin or Holocene time. 



Sediments comprising the beach and shoreface zone in Section A are 

 believed to be a combined product of littoral drift from the north and 

 south, local shell production, shoreward transfer of material eroded from 

 the inner flat, and erosion of the shore. Impoundment of sediment at inlet 

 jetties and other coastal engineering works is evidence of net drift from 

 the north. 



Nkich sand has been lost from the littoral zone during recent years; a 

 conservative estimate is that a net loss of 10 to 15 million cubic yards 

 has occurred in the past 30 years (Watts, 1962). This material is probably 

 transported to the deeper water of the shelf or slope. If littoral sedi- 

 ment moving southward is lost to the littoral stream through movement into 



