economy, the principal mineral products in order of value are phosphate rock, 

 crude petroleum, and limestone. 



The ten county area of Southwest Florida is prominent in fuel and nonfuel 

 mineral production. The nonfuel minerals produced are phosphate, limestone, 

 cement, sand and gravel, and oyster shell. Phosphate is by far of greatest 

 value. DeSoto, Hillsborough, and Manatee Counties are estimated to contain 

 442 million metric tons of phosphate reserves, which is about 13% of all known 

 reserves in North America. About one- third of the phosphate rock is shipped 

 out of the State, either to foreign or domestic destinations, for further pro- 

 cessing into final products. In 1978, the leading export from Tampa Port (the 

 third ranked port in the nation in terms of export tonnage) was raw phosphate 

 rock to be used for fertilizer. 



REGIONAL GEOLOGY 



INLAND 



2 

 Florida is the second largest (58,600 mi ) State in the southeast. It 



lies entirely within the coastal plain province, a major physiographic divi- 

 sion of the United States. It is underlain by sedimentary rock with a thick- 

 ness of more than 1,200 m (4,000 ft). The surface mantle over much of the 

 State is composed of soils and sands up to 61 m (200 ft) deep (Calver 1957). 

 The State has a variety of mineral resources and industries (Figures 1 and 2). 



The counties of Southwest Florida are underlain by a thick sequence of 

 sedimentary layers. Beds older than Late Miocene do not crop out in Southwest 

 Florida. The Late Miocene is represented by the Tamiami Formation of undeter- 

 mined thickness. Pliocene deposits are unknown and the Pleistocene is repre- 

 sented by the Caloosahatchee Marl and the overlying Fort Thompson Fomiation. 

 Average thickness of the Pleistocene formation is between 8 m (25 ft) and 23 n 

 or 75 ft (DuBar 1962). Below those formations, which tend to be exposed, are 

 formations of the earlier Hawthorn and Tampa Miocenes. Series below the Mio- 

 cene include the Oligocene, represented by the Ocala Limestone, Avon Park, 

 Lake City, and Oldsmar formations (Florida Department of Administration 1978). 



All of Southwest Florida lies within the Southern or Distal Zone of the 

 Coastal Plain Province (White 1970). The basic geology of the region consists 

 of relatively young sedimentary formation, with most of the coastal lowlands 

 covered by unconsolidated marine and estuarine terrace deposits of the Pleis- 

 tocene or more recent age. Most of the region is covered by sand and clay 

 that have relatively limited economic value. Large areas in the southwest are 

 covered by valuable phosphate and peat reserves. 



The mineral industry in Southwest Florida is supported largely by phos- 

 phate production and modest amounts of petroleum. Onshore oil and gas produc- 

 tion is confined largely to the Sunniland Field in Collier County. Oil and 

 gas production in the southwest may be attributed to lower crustaceous lime- 

 stone (Trinity Age) from the Mesozoic Era (Babcock 1964). The producing zone 

 of this formation has been named Sunniland and is at a depth of about 3,505 m 

 (11,500 ft) (Calver 1957). 



126 



