(5) Type M Sediment. The stratigraphically lowest lithologic unit sampled in the 

 survey area is a brown and silty sand deposit identified as type M sediment. Views of the 

 sediment in reflected light and in polarized light are shown in Figure 26. The sand grains are 

 chiefly quartz and phosphorite. Silt component in most cores is composed of quartz, 

 dolomite, and amorphous aggregates of an unidentified mineral. About one-fourth of the 

 cores recovering type M sediment contained a matrix of finely particulated organic matter 

 and little or no dolomite silt. In cores 181 and 196 both facies occurred and in both 

 instances the organic facies was stratigraphically lower than the dolomite silt facies. 



Silt -sized dolomite grains occur in a rhombohedral shape; most have sharp edges and lack 

 the abrasional features that would indicate a significant transport history. In the shelf survey 

 area off Jacksonville several cores contain evidence of upward chemical degradation of 

 dolomite grains (Sec. IV). The grains gradually disappear upward along with a noted change 

 in grain shape from rhombic to semiangular and a decrease in grain transparency in 

 transmitted light, the latter indicating degradation in crystal structure through chemical 

 weathering. 



In type M sediment the only macrofossUs recovered were a few small fragments of a 

 thin-shelled pelecypod, probably Amusium mortoni (Ravenel). Most type M samples 

 contained no microfossils; however, a few contained abundant tests of a varied assemblage. 



The assemblage in most samples is dominated by Cibicides spp. and usually contains 

 sparse to abundant tests of planktonic types. Cassidulina laevigata (d'Orbigny), Bulimina 

 gracilis (Cushman), and Buccella spp. are generally abundant and appear to be characteristic 

 of this sediment; in places, species of Guttulina are also important. The assemblage 

 resembles type L sediment in its dominant constituents, but there appears to be some 

 significant differences of type and abundance. The foraminifera in type M sediment suggest 

 that it is of Tertiary age and was deposited in water depths greater than 150 feet. 



Because of the stratigrapliic import of types L and M sediments, faunal lists of 

 foraminifera in these two Tertiary sediments are included in Appendix C. 



(6) Unclassified Sediments. A number of sediments with characteristics different 

 from the commonly occurring types discussed was encountered in cores from the study 

 area. Generally, these sediments occur in only a few cores and have not been categorized as 

 types but are grouped as unclassified (U). Because cores are widely separated, especially in 

 the reconnaissance area south of St. Augustine, some of these sediments may occur over a 

 large area. 



Clays are the most common unclassified sediments. These include gray to nearly black, 

 sandy clay which is apparently associated with the silty fine sand (Type F) of the shoreface 

 zone since the strata in which it is found are nearly always directly beneath a deposit of 

 type F material. This appears to be an extensive deposit but probably discontinuous. 



OUve to light grayish-green clay, in some places massive, and in others fissile, and 

 containing varying amounts of included sand, occurs mostly south of St. Augustine. 



54 



