Section II. HYDROGRAPHY AND GEOLOGY OF STUDY AREA 



1 . Hydrography 



The northern Straits of Florida is a passage through which the 

 Gulf Stream passes northward into the Atlantic Ocean. Flanking the 

 Straits to the east is the Great Bahama Bank surmounted by the cays and 

 islands of the Bahama Group; westward lies the mainland of Southeastern 

 Florida (Figure 2) . 



The thalweg of the passage is broad and lies in the central part of 

 the northern straits. The rise of the east side of the "valley" toward 

 the Bahama Banks is relatively steep with slopes averaging 9 percent; 

 overall the western slope of the Florida Strait is more gentle than the 

 eastern with slopes averaging 4 to 8 percent. In the area of study the 

 western slope is interrupted by a broad terrace at depths from about 720 

 to 1,200 feet (Siegel, 1959; Hurley, 1962). Shoreward of this terrace 

 the slope again steepens and rises to the shelf which extends from 

 approximately 70-foot depths to shore. The seaward edge of this near- 

 shore shelf is marked by a drowned reef -like feature with an irregular 

 crest which generally lies at 40- to 55-foot depths. 



South of approximately 26°20' N the surface of this shelf rises 

 from the outermost reef to shore in a series of step-like linear flats 

 separated by rocky irregular slopes and ridges. North of 26°20' N, the 

 step-like character of the topography gives way to a more or less con- 

 stant sediment slope extending from shore to near the outer reef line 

 (Figure 4) . 



Sediments on the shelf can be divided roughly into two distinct 

 t>"pes . Southward of 26°20' N the dominant sediments are white to gray 

 calcareous skeletal sands and gravel (Figure 5). The acid-soluble con- 

 tent of this sediment is generally over 80 percent. North of 26°20' N to 

 the limits of this study area the dominant sediment type is a homogeneous 

 fine to medium-grained gray sand composed of about 60 percent clear sub- 

 ar.gular and subrounded quartz grains and 40 percent brown, gray, or black 

 calcareous skeletal fragments (Figure 5). 



2. Geologic Setting 



a. Stratigraphy and Geologic History 



Strata cropping out or present in the shallow subsurface of 

 southeastern Florida are summarized in the stratigraphic column of Table 

 I . Along most of the east coast of Florida, rocks of the Pleistocene 

 .■ijiastasia Formation form the main coastal bedrock outcrop (Cooke, 1945). 

 Locally, the Anastasia Formation is exposed in low cuts and benches • 

 along the shore. A submerged rocky platform bordering the shore in 

 many places is probably formed on the Anastasia Formation. 



