Distribution of cores containing late Tertiary sediments (Types L and M) and their 

 relation to the depth of penetration to the first mappable sonic reflector is shown in Figure 

 29. There is a subtle suggestion of parallel alinement of the coast and shape of the shelf 

 sediment lying over the mapped horizon. Cores from the gridded areas correlate well with 

 the structure; cores from areas showing an overburden of 10 feet or less on top of the 

 uppermost acoustic reflector often contain Phocene-age foraminiferal sands or dolomite 

 silts. Cores collected from areas having greater than 10 feet of sediment consistently contain 

 only fine to medium quartzose sands. 



The influence of the shallow structure on sediment distribution is further demonstrated 

 in the cross-sectional profiles shown in Figures 30 and 31. These profiles document the 

 relatively close association between acoustic and Uthologic data and demonstrate that 

 post- Tertiary sediments are often quite tliin on the north Florida shelf, and are even absent 

 in some places. 



Within the Jacksonville grid at shallow subsurface depths and atop the surface of an older 

 (Type M) underlying unit, are found what is interpreted to be the remnants of a weathered 

 surface (soil or ground water profile). This interpretation is based on the presence of 

 iron-stained and organic-coated quartzose sands several feet down in eight cores, all within a 

 well defined area. The Uthologic boundaries correlate with the red (R) acoustic reflector. 

 Core location, depth of discolored sands and extent of the soU horizon are shown in Figure 

 32. Outside the soil horizon area (Fig. 22) but at the same subsea depths, dolomite silts 

 (Type M) are encountered in cores 59, 61, 63, and 184. Individual dolomite grains show a 

 progressive upward degradation towards the soil horizon in both grain shape and crystal 

 structure, suggesting effects of chemical deterioration through exposure to subaerial 

 processes. Overlying the soil horizon in cores 46 and 47, are organic-rich muds and peat. The 

 peat sample in core 46 at —60 feet MLW has a radiocarbon age of 9,625 years (B.P.) and 

 represents initial accumulation upon the exposed surface as the Holocene sea transgressed to 

 that elevation. No other distinct, laterally continuous, soil horizons are apparent within the 

 survey region. However, isolated cores offshore from Fernandina and St. Augustine contain 

 similar lithologic transitions wliich probably record a partially obliterated erosional surface. 

 The same criteria of upward degradation and depletion of carbonate grains towards a 

 stained, quartzose sand deposit is present. Furthermore, the depths of these transitions 

 correlate with depths of strong acoustic reflectors wliich He near the surface and correlate 

 with marked lithologic transitions to Tertiary sediments. 



(2) Jacksonville Beach to St. Augustine. South of the Jacksonville survey area overall 

 sediment character is similar to that from Jacksonville north (Fig. 33). However, the relative 

 distribution of the different sediment types changes significantly both laterally and 

 vertically. Fine to medium quartz sands (Type A) are thicker and more laterally extensive; 

 pre-Pleistocene dolomite silts and foraminiferal sands are less abundant and more restricted 

 in lateral extent. Fine, poorly sorted quartz sands (Type F) characterize the shoreface 

 region, as they do in the northern sector. 



61 



