666 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



submarine limestone reef that formed a ridge. The ridge area was ele- 

 vated in post-Turonian time, and again repeatedly in Eocene and Oligo- 

 cene time. With sedimentation repeatedly burying it, a number of un- 

 conformities occur around and over it. The spatial aspect of the buried 

 ridge is difficult to fit into the structural picture ( Muir, 1936 ) . 



FLORIDA PLATFORM 



Sediments from Recent to Early Cretaceous age are known to overlie a 

 crystalline basement in Florida, and beds are probably present in places 

 between the Lower Cretaceous and the crystalline rocks. The thickness 

 of the sedimentary cover ranges from 4350 feet in southeastern Georgia to 

 more than 11,600 feet in the southern end of the peninsula. The earliest 

 tangible history of Florida is that of the Early Cretaceous, when approxi- 

 mately the western half of the peninsula was submerged and the eastern 

 half was land. A number of drill holes bear out this picture fairly well. 

 If an outer belt of Lower Cretaceous strata in Georgia and the Atlantic 

 Coastal Plain under the Upper Cretaceous and Tertiary beds is related 

 to the land area of Florida, a long peninsula seems to have existed then, 

 as now, only slightly eastward of the present. 



If the isopachs of the Upper Jurassic of the Mississippi embayment 

 region and Arkansas and Texas are projected to Florida where problemati- 

 cal Upper Jurassic has been recognized in just two deep wells (Applin 

 and Applin, 1944), only the southern third of Florida seems to have 

 been under water, and the rest was land. In fact a very broad land 

 projection seems to have existed. See paleotectonic map of Plate 10. 



The cross section and map of Fig. 41.13 show the stratigraphic and 

 structural relations recognized in Florida. The chief structural feature is the 

 Peninsular arch in the north-central part of the peninsula which first ap- 

 peared in the Late Cretaceous. The axis of the arch trends northwestward 

 and is parallel with a deep trough that centered in the Greater Antilles. 

 The arch is also pronounced in the Middle and Upper Eocene beds, but 

 with variations in detail. A flexure developed on the west flank of the 

 Peninsular arch has distinct outcrop expression and is properly called 

 the Ocala uplift, according to the Florida Geological Survey, but the 



large arch itself is commonly called the Ocala. 



According to Applin and Applin (1944) the chief structural features 

 of Florida are: 



(1) An axis extending northwest from about Cape Canaveral on the east 

 coast of Florida to south-central Georgia, upon which are located two large 

 locally high areas; (2) a channel or trough extending southwestward across 

 Georgia through the Tallahassee area of Florida to the Gulf of Mexico, nearly at 

 right angles to the aforementioned axis; (3) an upwarped area in the vicinity o£ 

 Jackson County, Florida, with dips extending away from it toward the southeast, 

 south, and southwest; (4) a structurally low area with an axis extending north- 

 west from the vicinity of Lake Okeechobee toward Tampa, approximately 

 parallel with the axis first mentioned; (5) a possible second north-west-trending 

 upwarped area at the south end of the Peninsula. 



The modern peninsula of Florida is about the emergent third of a 

 broad platform, as may be seen in Fig. 42.1 The shelf on the west 

 side is 100 miles wide and ends in a very steep escarpment which carries 

 down to the abyssal plain of the Gulf of Mexico. This West Florida 

 escarpment has been thought of as a fault scarp (Jordan, 1951), but on 

 hand of a uniform magnetic intensity field over the escarpment and 

 the aseismic nature of the region, Miller and Ewing (1956) believe it is 

 not due to faulting but to processes of sedimentation. The constitution 

 of the crust under the Gulf of Mexico, and the origin of the Gulf will be 

 discussed under a later heading. 



The shelf on the east of the peninsula of Florida is continuous with 

 and supports the Rahama Ranks whose geology will be discussed in 

 the next chapter. The great shelf region is largely one of carbonate de- 

 position today, and as explained, has so been in the southern half 

 since at least Early Cretaceous time. Accumulation has equaled subsid- 

 ence, and the imposing submarine escarpments may be due to the 

 growth of reefs, firm enough to keep the sediments from slumping down 

 to the abyssal plain. Some local magnetic anomalies on the West Florida 

 escarpment may indicate buried volcanic piles (Miller and Ewing, 

 1956). 



A number of wells have penetrated the Mesozoic sedimentary rocks, 

 and maps of the surface are shown in Fig. 41.14. The contour of the 

 surface is that of the dominant Peninsular arch. The outcrop pattern, how- 



