Governor Lawton Chiles, Chairman

Florida Department of Environmental Protection

Tallahassee, FL 32301
 
 

Dear Governor Chiles:

The Florida Geological Survey, Division of Resource Management, Department of Environmental Protection, is publishing as Special Publication No. 35, Florida’s Geological History and Geological Resources, prepared by staff geologist Ed Lane. This publication presents the geological history of Florida and its natural resources. As such, it is a timely report that will be useful to the general public, teachers, planners, and governmental officials who need to know the important aspects of Florida’s geology.
 
 

Respectfully,
 
 
 
 
 
 

Walter Schmidt, Ph.D., P.G.

State Geologist and Chief

Page

Conversion factors and abbreviations

Acknowledgements

Chapter 1

Rocks: igneous, metamorphic and sedimentary

Geologic time and dating techniques

Correlation of rocks

Chapter 2

The Earth’s moving plates

Chapter 3

Florida’s global wandering through the geological eras

Florida basement rocks

Precambrian, Paleozoic and Mesozoic Eras

Cenozoic Era

Quaternary Period: Pleistocene and Recent Series

Chapter 4

Geology and man

Early man and his environment

Modern man

Economic minerals

Cement

Clay

Heavy minerals

Peat

Phosphate

Sand and gravel

Crushed stone

Chapter 5

Oil and gas

Chapter 6

Water resources

Chapter 7

Geologic hazards

Karst terrain

Flooding

Unstable soils

Earthquakes

Global warming and sea level rise

Chapter 8

Waste disposal

Chapter 9

Environmental geology and Florida’s future

References

Glossary of selected geological terms
 

FIGURE

1. Relative age dating and correlation between rock units

2. Standard geologic column and time scale

3. Geologic map of Florida

4. Radioactive age dating and the age of the Earth

5. Continental plates and spreading centers

6a. Pattern of ages of North Atlantic oceanic crust

6b. Cross section showing sea-floor spreading

7. Rock types of Florida basement

8. Florida’s continental drifting through time

9. Cenozoic stratigraphic column for Florida

10. Sea level changes during the Cenozoic Era

11. Oblique view of the Florida Platform

12. Gulf Trough isolated the Florida Platform during the Oligocene

13. Gulf Trough filled by Miocene time

14. Map of occurrence of Hawthorn Group sediments near the surface

15. Major geologic structural elements of Florida

16. Pleistocene shorelines in Florida

17. Major topographic features formed by Pleistocene seas

18. Pleistocene mammoth

19. Florida saber-tooth tiger and Pleistocene horses

20. Pleistocene mastodon

21. Giant sloth and glyptodont

22. Map of mineral mining areas in Florida

23. Suction dredge mining sand and gravel

24. Open pit limestone quarry

25. Cross section showing south Florida oil field

26a. South Florida oil field location map

26b. Generalized stratigraphic column for south Florida

27a. North Florida oil field location map

27b. Generalized stratigraphic column for north Florida

28. Graph of historical trend of oil and gas production

29. Hydrologic cycle

30. Correlation chart for aquifers and confining units

31. Evolution of karst landscape

32. Aerial photograph of the Pitt Landslide, Gadsden County

33. Photograph of the scarp of the Pitt Landslide

34. Water well record of 1979 Colombian earthquake

35. Water well record of 1990 Philippine earthquake

36. Water well record of 1964 Alaskan earthquake

37. Coastal changes in Florida due to sea level rise

38. Early method of landfilling for waste disposal

39. Marion County landfill, showing new cell

40. Marion County landfill, cross section of new cell
 

1. Florida oil field discovery well data

2. List of known earthquakes felt in Florida
 

This conversion table is for the convenience of readers who may prefer to use metric units instead of the English units given in this report.
 

MULTIPLY	BY	TO OBTAIN
inch (in)	25.4	millimeter (mm)
foot (ft)	0.3048	meter (m)
mile (mi)	1.609	kilometer (km)
gallon (gal)	3.785	liter (L)
gallon (gal)	0.003785	cubic meter (m3)
gallons per minute (gpm)	0.06308	liters per second (L/s)
gallons per minute (gpm)	0.0022	cubic feet per second (cfs)
gallons per minute per foot [(gal/min)/ft]	0.207	liter per second per meter [(L/s)/m]
cubic feet per second (cfs)	449	gallons per minute (gpm)
pound avoirdupois (lb)	0.4536	kilogram (kg)
ton, short	0.9072	megagram (Mg)

Chemical concentrations and water temperatures are given in metric units. Chemical concentration is given in milligrams per liter (mg/L) or micrograms per liter (ug/L). Milligrams per liter is a unit expressing the concentration of chemical constituents in solution as weight (milligrams) of solute per unit volume (liter) of water. One thousand micrograms per liter is equivalent to one milligram per liter. For concentrations less than 7,000 mg/L, the numerical value is the same as for concentrations in parts per million.

Water temperature is given in degrees Celsius (°C), which can be converted to degrees Fahrenheit (°F) by the following equation:

°F = 1.8 (°C) + 32.

The editor wishes to thank the following staff of the Florida Geological Survey who were contributing authors for various sections of the manuscript: Jonathan D. Arthur (basement rocks, and Precambrian and Paleozoic eras), Paulette Bond (Mesozoic era and carbonate platforms), Kenneth M. Campbell (economic minerals, water resources, and waste disposal), Jacqueline M. Lloyd (oil and gas), Frank R. Rupert (Cenozoic Era, early man and his environment), and Thomas M. Scott, Walter Schmidt, and James Jones. Cindy Collier also assisted in preparing the manuscript. All illustrations were drawn or compiled by Ed Lane, except as noted.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Periarchus lyelli, an extinct echinoid (sand dollar), found in the Avon Park Formation (Eocene), x1.
 

This book is a synthesis of geological facts and the manner in which they have been used to reconstruct the geological history of Florida. Basic geological concepts and techniques are presented in order to provide background information that is necessary for discussions of the topics. It is intended to be succinct, yet broad enough in scope to cover the important aspects of Florida geology. As such, it will be suitable as a teaching aid, and as a source book for those who wish to dig deeper into Florida’s geological history.

GEOLOGY means "the science of the Earth." Geology is the branch of natural science that studies the Earth, its rocks and minerals and the changes it has undergone or is undergoing. Some specialized fields of geological study include: hydrology (water resources), paleontology (fossils and ancient life), stratigraphy (the formation, composition, sequence and correlation of rocks), geomorphology (the form of the Earth’s surface and changes that take place), petrology (history of rocks, their origins, changes, and decay), and engineering, mining, and petroleum geology.

Words in bold type are defined in the glossary at the end of the book.
 

The Earth’s crust is not uniform. Its surface and interior are made of an almost endless variety of rocks, each having its own distinctive characteristics, such as minerals, color, density, porosity, and hardness. Geologists classify rocks according to their origin.
 

Igneous rocks (from the Latin word for "fire") are rocks that are formed deep within the Earth’s molten interior. Sometimes they are forced out of the Earth’s interior through volcanoes and appear on the surface as lava. Examples of igneous rocks are granites, basalts, obsidian (volcanic glass), and pumice (the porous, bubble-filled lava that floats on water). There are no igneous rocks exposed at the surface in Florida, although they have been found several thousand feet below the land surface in deep oil wells.
 

Metamorphic rocks (from the Greek words for "changed in form") are formed deep beneath the Earth’s surface. Originally, they were igneous or sedimentary rocks that were transformed by the tremendous heat, pressure, and chemically active fluids to which they were subjected after burial in the Earth. Examples of metamorphic rocks are slate (metamorphosed shale), marble (metamorphosed limestone), and quartzite (metamorphosed sandstone). There are no metamorphic rocks exposed at the surface in Florida, although some have been found in wells at depths of several thousand feet.
 

Sedimentary rocks are those that were formed at the Earth’s surface, either by accumulation and cementation of fragments of rocks, minerals, and organisms, or as precipitates from sea water, surface water or ground water. Debris from erosion and weathering commonly form sedimentary rocks. For example, a sandstone and a conglomerate are rocks that are the cemented counterparts of loose sand or loose gravel deposits, respectively. One group of sedimentary rocks found throughout Florida are limestones, which are predominantly derived from the calcium carbonate tests of marine organisms and algae. A common feature of these rocks which indicates their marine origin is the presence of fossils of marine organisms. Some limestones, called coquina, are composed almost entirely of shells of marine animals that became cemented together after the animals died. Many of the sand and clay deposits that cover Florida were transported and deposited into sea water by streams. Some were then reworked by coastal and marine processes, such as shoreline erosion and accretion.
 

The Earth is very old -- over four and a half billion years -- 4,500,000,000 years! This length of time is nearly impossible to comprehend in terms of human events or even lifetimes. How Earth scientists determine geologic time forms the basis for many of the key principles that have helped to explain the mysteries of our planet’s and Florida’s geologic histories.

The secrets of Earth’s age are hidden in its rocks. Interpretation of these secrets may be difficult because rocks can, and often do, vary greatly in age from place to place; and sometimes there are gaps in the rock-record, with layers missing.

Geologic time is measured in two ways: a relative time scale, based on the sequence of layering of rocks; and an absolute (or atomic) time scale, based on the rate of radioactive decay of certain elements in rocks.

One fundamental principle of relative age dating is the Law of Superposition, which states that: in any sequence of sedimentary strata that has not been disturbed by folding or overturning since accumulation, the youngest stratum is at the top and the oldest is at the bottom. Relative age dating also is done by using a second basic principle of geological correlation, namely: that distinctive marker fossils are found only in rocks of certain ages. Chronologic correlation, as used by geologists, means the determination of the approximate equivalence in geologic age and stratigraphic position of two rock strata that occur in different areas of the world (Figure 1).
 

Paleontological studies of fossils around the world have shown that, throughout geological time, countless species of animals and plants have appeared, flourished for millions of years and, then, either died out (became extinct) or slowly changed (evolved) into significantly different plants or animals. In geological terms, this life-span of a distinctive species is its age range.

Another aspect of studying fossils is the detemination of their geographic distribution; in other words, "Where in the world did they live?" As is true with plants and animals today, some fossil species have been found to have had worldwide distribution, while others have only been found in restricted areas or regions. This can best be illustrated by considering the relationship of any animal or plant to its environment. The physical characteristics of every plant or animal requires that it live in certain, and often restricted, environments. Oysters, for example, are restricted to living on the bottom of bodies of brackish water. Therefore, if one found accumulations of fossil oyster shells in a stratum of rock, it could be reasonably assumed that the rock’s constituents had been deposited in a body of brackish water.

These two principles have enabled geologists to identify rocks of the same general age wherever they are found. The actual age of the rocks, in terms of years, however, was not known. The rock units could only be placed in a relative sequence -- some strata were either older or younger than other adjacent rocks. On the basis of such relative age dating in Europe, the standard geologic column was constructed during the 1800s ((Figure 2). Figure 3 is a geologic map and a geologic column for Florida showing rocks that occur at or near the land surface. Also given on the standard geologic column in Figure 2 are approximate ages that are derived from radioactivity studies, summarized in Figure 4.
 

How did Florida get where it is and in its present shape? The answer to this question lies in the restless nature of Earth’s crust. Earth’s surface is active and mobile: mountains rise -- sea floors are created -- and continents move. Until a few decades ago such ideas bordered on science fiction, even to geologists. However, evidence has been accumulating for the past 30 years that supports the basic theories of how the present shapes and arrangements of Earth’s continents came to be. These early theories of sea-floor spreading and continental drift led to our current understanding of plate tectonics.

The notion of continental drift is based on the fact that the so-called "solid" rocks that form the Earth are capable of moving. Plate tectonics theorizes that large chunks (plates) of the Earth’s colder, upper crust are capable of moving slowly (like rafts) on top of deeper, hotter, and more fluid rocks in the mantle (Figure 5, 6a, and 6b)). Geologists have identified seven large plates on the Earth’s surface, with 11 or more smaller plates. The picture that has emerged from recent discoveries in geology is a simple one: the Earth is girdled by numerous linear spreading centers dominated by a continuous 40,000-mile-long system of mid-ocean ridges, such as the Mid-Atlantic Ridge and the East Pacific Rise (Figure 5). These ridges form portions of the plate boundaries and have cracks (rifts) along their crests. Lava from the Earth’s upper mantle is more-or-less continuously forced out from the rifts along the ridge crests, producing new (young) rock. This process of creating new oceanic crust forces adjacent plates apart and is called sea-floor spreading. Sea-floor spreading causes the continents to migrate or "drift." Rates of spreading vary from place to place along the mid-ocean ridges. For example, the north Atlantic basin is growing wider by approximately one inch per year, while the south Atlantic basin is growing wider at the rate of about one and one-half inches per year.

What happens when all these plates move about over the Earth? Spreading in one place will create a collision in another. Plates come together in different ways. They may slide past each other along their margins, along faults, as does the San Andreas Fault in California, which is part of the boundary between the North American and Pacific plates (Figure 5). The many earthquakes of that region are the result of the two plates grinding past each other, in some places at the rate of several inches per year. Or, one plate may slide under another in a geologic process called subduction, in which case two things may happen. A deep oceanic trench may be formed, such as the Aleutian Trench, Alaska (Figure 5). Mountain ranges may be created as the edge of the overriding plate is lifted and buckled up, such as where the Indian continental plate is pushing up the Himalayas, which are on the southern edge of the Eurasian plate. Accompanying all of this trench and mountain building are earthquakes and volcanic activity. Fortunately for Florida, most of the earthquake and volcanic activity takes place along the leading edges of the plates. Because Florida is on a passive interior part of the North American plate, it seldom feels any quakes and has no volcanoes.
 

Rocks of Precambrian, Paleozoic and Mesozoic age occur several thousand feet below the surface of Florida (Figure 7). These older, deeper rocks are either igneous, metamorphic, or sedimentary and may collectively be termed basement rocks. In north-central Florida, these rocks have been penetrated by oil test wells at depths of 3,500 feet below the surface. The distance to these basement rocks gradually increases away from this region, reaching depths of more than two miles below the surface in the western panhandle, and over three miles in south Florida.

During the Late Precambrian, about 700 million years ago (mya), terrain that was to become Florida was part of an ancient supercontinent, which was composed of what is now North and South America, Africa, Europe, and other land masses. More than 600 mya, this supercontinent split apart -- most significantly North America from Africa. Later, in the Paleozoic, the tectonic forces which had split the supercontinent continued to operate, driving the detached land masses to migrate together again in a collision which formed another supercontinent, called Pangea (Figures 8a and 8b). The tectonic cycle continued with Pangea rifting apart again (Figures 8c, 8d, 8e), as had the first supercontinent, and during the next 200 million years the Earth’s plates migrated to their present-day configuration.

So where does Florida fit into this story, especially if it was not part of the "North America" that existed during the early Paleozoic? At one time, scientists believed that the basement rocks of the southeastern United States, including Florida, were a subsurface extension of the igneous, metamorphic, and sedimentary rocks that are exposed in the Appalachian Mountains. However, recent research indicates that the area that was to become what we know as Florida was a part of northwest Africa. In the last 25 years scientists have found distinct similarities between Florida basement rocks and subsurface rocks in northwest Africa. Certain Florida sandstones, siltstones, and shales as well as the fossils which they contain are very similar to the rock sequences and fossil assemblages which occur in northwest Africa. The igneous and metamorphic rocks of Florida are comparable in rock-type and age to those of northwest Africa. Also, when the continents are fit back together in order to envision the layout of Pangea (Figure 8b), the location of the various types of rock in the basement are better understood if they are considered to have been a part of Africa. The Florida basement seems to provide a missing piece of the African puzzle. Various magnetic properties within the Florida Paleozoic rocks also match better with those of Africa than those of North America. The Mesozoic suture -- the boundary between ancient Africa and North America which was formed when they collided to form Pangea -- may be located in the subsurface of southern Georgia. Scientists do not know exactly where the suture zone between North America and the ancient Afro-South American plate is located, but attempts have been made to find it. Using information from deep wells in north Florida and southern Georgia, geologists think they may have found the ancient suture zone. Also, a 1985 seismic survey across southern Georgia indicated that the suture zone may be there. Although indirect in nature, this evidence supports the idea that Florida was once a part of Africa. If the boundary between ancient North America and Africa is now located north of Florida, then the deep Paleozoic rocks of Florida represent a rifted-off portion of Africa.

In every comparison of geologic information, the affinity with Africa becomes more apparent and the resemblance to Paleozoic North American rocks lessens. Because of the current theory that Florida was once a part of northwest Africa, geologists refer to the basement of Florida as an exotic terrain.

The Atlantic Ocean basin began to form in the Late Triassic when Pangea began to split (Figure 8c). By mid-Jurassic time rifting was probably complete. To the east of proto-Florida a spreading center was creating new sea floor for the young Atlantic Ocean; this spreading center is now called the Mid-Atlantic Ridge (Figures 6b). As the new sea floor spread outward to both sides of the ridge, the North American continental plate was forced away from Africa (Figure 8c).

By the late Middle Jurassic the spreading center, which had begun earlier to pour out basaltic lava, gradually shifted its position to the east. The lava flows cooled and hardened, forming new ocean floor or oceanic crust.

Near the margin between the newly-formed oceanic crust and the older continental crust, a basin began to slowly form. At first this sinking (or subsidence) occurred mainly because the basaltic crust shrank as it cooled. As the crust continued to cool and shrink, various types of sediment were carried into the basin. The weight of this accumulating sediment also forced the crust beneath the basin to sink. This extremely gradual sinking was essential in the early development of the carbonate Florida Platform during the Cretaceous.

A carbonate platform is an area where great thicknesses of carbonate rock have accumulated in the past. Carbonate sediments are continuing to accumulate to the present day on the Florida Platform and on modern carbonate platforms, such as the Bahama Banks, east of Florida. Carbonate rocks on the Florida Platform are limestones (calcium carbonate, CaCO₃) and dolostones (calcium-magnesium carbonate, CaMg(CO₃)₂).

The calcium carbonate which makes up the rocks associated with carbonate platforms is produced by various organisms which live in marine environments. When the tiny animals that live in coral reefs die, the reefs (made of calcium carbonate) may be preserved as one type of limestone. Some varieties of seaweed (algae) have the ability to secrete fragile skeletons of calcium carbonate. When the algae die tiny crystals of calcium carbonate fall to the sea floor and form carbonate mud, or lime-mud. This carbonate mud is preserved as another type of limestone. These are only two examples of the sorts of organisms which construct calcium carbonate skeletons as part of their life cycle. If a carbonate platform is to form, these carbonate-producing organisms must be able to grow prolifically. The water in which the organisms live must remain shallow, since some of them require light to survive.

Minor amounts of anhydrite (calcium sulfate, CaSO₄) occur in the thick section of Cretaceous carbonate rocks of south Florida. Anhydrite forms today in very dry climates such as the Persian Gulf. Generally, it seems to form when sea water flows into a shallow basin which is cut off from additional sources of water. In that situation the sea water evaporates until eventually anhydrite is formed by precipitation. The presence of anhydrite layers in the thick carbonate accumulations of south Florida suggests that at sometime in the past the climate may have been hotter and much drier than it is now.

The Mesozoic Era, from about 250-million years ago to about 65-million years ago, is popularly known as "The Age of Dinosaurs" because they were the dominant forms of life for over 150-million years. Although dinosaur fossils occur in many places in the world, none have been found in Florida, and a look at Figures 2 and 3 will help to explain why this is so. Dinosaurs became extinct about 65-million years ago, and the oldest rocks that occur at or near the surface in Florida are Middle Eocene in age, about 45-million-years old, deposited some 20-million years after the dinosaurs became extinct. While it is possible that dinosaur fossils may exist in the Cretaceous rocks under Florida, the closest ones would be several thousand feet deep. Florida’s oldest vertebrate fossil was recovered in 1955 during oil test drilling near Lake Okeechobee. A well core brought up a partial skeleton of an aquatic turtle from a depth of 9,210 feet from rocks of Early Cretaceous age. The core hole just happened to be in position to penetrate the rocks where the fossil was embedded.
 

The Cenozoic Era in Florida is represented by sediments that were deposited during the last 65-million years of geologic time (Figure 9). Sea-level fluctuations throughout the Cenozoic played a major role in creating the present configuration of Florida, through the processes of sediment deposition and erosion. In general, the sea level during the early Cenozoic was significantly higher than the present level. Throughout the Cenozoic, sea level fluctuated considerably along a broad general trend of falling sea level since the end of the Cretaceous (Figure 10). This general sea level trend has superimposed upon it many shorter duration fluctuations, both sea level rises and falls. The geologic record of Florida reveals unconformities where sediments are absent due to nondeposition or erosion in response to sea level fluctuations. Geologists believe that the Cenozoic sea levels in Florida have fluctuated from several hundred feet or more above the present level to more than several hundred feet below present sea level.

The Cenozoic of Florida is represented by two groups of sediments: the Paleogene and the Neogene-Quaternary (Figure 9). Carbonate rocks predominate in the rock-record of the Paleogene in Florida while quartz sands, silts, and clays dominate the Neogene-Quaternary. The carbonate rocks are principally limestone and dolostone with varying but generally minor percentages of evaporites. The evaporites present in the Cenozoic rocks are gypsum (CaSO₄.nH₂O) and anhydrite. The evaporites are present as thin-to-thick beds and as pore fillings in the carbonate rocks comprising the lower portion of the Paleocene section. The evaporites formed in response to restricted circulation of the sea water allowing evaporation to concentrate the minerals in solution. The minerals were then deposited along with the carbonate sediments.

The Florida peninsula is the emergent portion of the wide, relatively flat geologic feature called the Florida Platform, which forms a rampart between the deep waters of the Gulf of Mexico and the Atlantic Ocean (Figure 11). The Florida peninsula is located on the eastern side of the platform. The edge of the Florida Platform is arbitrarily defined to be where water depth is 300 feet. The edge of the platform lies over 100 miles west of Tampa, while on the east side of Florida it lies only 3 or 4 miles off the coast from Miami to Palm Beach. Within relatively short distances from the edge of the platform water depths increase more sharply, eventually reaching "abyssal" depths of over 10,000 feet, creating what is known as the Florida Escarpment. Diving expeditions along the escarpment west of Tampa, with the deep submersible Alvin, found the escarpment there consisted of a gigantic limestone cliff that rose over 6,000 feet above the 10,700-feet-deep Gulf floor. Based on evidence from oil exploratory work, it has been estimated that carbonate and evaporitic rocks may underlie south Florida at depths greater than 20,000 feet.

During the Paleogene the Florida Platform was very much like the present-day Bahama Banks, with carbonate sediments forming over a large area. The carbonate sediments formed due to biological activity and, for the most part, are made up of whole or broken fossils. These fossils include foraminifera, bryozoa, mollusks, corals, and other forms of marine life.

Very little siliciclastic material was able to reach the Florida Platform due to the presence of a marine current running through the Gulf Trough (Figure 12) which transported these sediments away from the platform. This current was similar to the Gulf Stream today. Another factor was that the Appalachian Mountains, the primary source for the siliciclastic sediments, had been eroding for millions of years through the Mesozoic and early Cenozoic. As the mountains were reduced by erosion, limited amounts of siliciclastics were produced and carried by streams and rivers to the ocean where currents carried the sediments away from the Florida Platform.

In the mid-Cenozoic, late Paleogene, the Appalachians were uplifted and erosional rates increased greatly, providing a flood of siliciclastic sediments which eventually filled the Gulf Trough. With the filling of the trough, the siliciclastic sediments encroached upon the carbonate-depositing environments, replacing them with sands, silts, and clays (Figure 13). In northern Florida, the siliciclastic sediments appear very early in the Miocene while in southern Florida carbonates continued to be deposited until at least mid-Miocene. The siliciclastic sediments spread southward most rapidly along the east coast of Florida in response to the more vigorous coastal conditions on the Atlantic coastline.

The sediments deposited during the Neogene are primarily quartz sands, silts, and clays with varying amounts of limestone, dolostone, and shell. With the exception of the Pliocene Tamiami Formation in southwestern Florida, the Neogene carbonates occur as thin beds and lenses disseminated in the siliciclastic sediments. Deposits composed primarily of shells with subordinate amounts of sands and clays become very common in the Pliocene over much of Florida.

The beginning of the Neogene not only marked a distinct change in sedimentation but also the initiation of phosphate deposition in Florida. The conditions leading to the deposition of marine phosphates are variable but specific conditions are thought to be required. One of the most important factors is the upwelling of cold, nutrient-rich, phosphorus-laden water from the deep ocean basins. The increased phosphorus supply allows the rapid development of large populations of marine organisms such as plankton. As these organisms die and settle to the bottom, large amounts of organic material accumulate, mix with the sediments, and are buried. It is thought that reactions within the sediments cause the formation of the phosphate mineral francolite. The subsequent development of economically significant phosphate deposits results from the reworking of the phosphatic sediments and the concentration of the phosphate by current and wave action.

Sediments of the Miocene-Pliocene age Hawthorn Group contain large quantities of phosphate, some of which occurs in economically important concentrations. Current mining operations can be seen in Polk, Hillsborough, and Hardee Counties in central Florida, and in Hamilton County in northern Florida. Much of the phosphate mined in Florida is processed to form various types of fertilizers.

The Neogene phosphates in Florida contain varying amounts of uranium incorporated in the mineral francolite. The percentages of uranium present range from hundredths to tenths of a percent of the total mineral. The uranium isotope U²³⁸ is the most abundant form of uranium present in Florida’s phosphates. As U²³⁸ decays radioactively, radon (Rn²²²) eventually forms as one part of the decay series. Radon, a short-lived radioactive isotope, occurs as a colorless, odorless gas which may accumulate in buildings, causing potential health problems. Wherever the Hawthorn Group phosphatic sediments are present near the surface, the possibility of radon problems exist (Figure 14).

The early Cenozoic rocks of Florida are not flat lying but form a series of highs (platforms) and lows (basins). These geologic features are known as structures. Figure 15, a geologic structure map of Florida, shows these features. The later Cenozoic sediments are thinnest over the highs and thickest in the lows. The oldest sediments exposed in the state are exposed on the crest of the Ocala Platform, a major high feature in west-central Florida. Other prominent highs include the Chattahoochee Anticline, Sanford High, Brevard Platform, and the St. Johns Platform. The lows include the Okeechobee Basin, Osceola Low, Jacksonville Basin, the Gulf Trough, and the Apalachicola Embayment. A major, actively subsiding basin, the Gulf of Mexico Basin, lies west of the Florida Platform. To the east of the peninsula lie the Blake Plateau Basin and the Bahamas Basin.
 

The Quaternary Period encompasses the last 1.8-million years of geologic history. The Quaternary Period is made of two geologic epochs, the Pleistocene Epoch (1.8 million to 10,000 years ago) and the Holocene Epoch (10,000 years ago to the present) (Figure 2). It was a time of worldwide glaciations, widely fluctuating sea levels, unique animal populations, and the emergence of man. Seas alternately flooded and retreated from the land area of Florida. Most of the landforms characterizing Florida’s modern topography, as well as the springs, lakes and rivers dotting the state today formed during the Quaternary.

The Pleistocene Epoch, also known as "The Ice Age," was punctuated by at least four great glacial periods. During each glaciation, huge ice sheets formed and spread southward out of Canada, covering much of the northern United States. Sea water provided the primary source of water for the expanding glaciers. As the ice sheets enlarged, sea level dropped as much as 400 feet below present level, and the land area of Florida increased dramatically (Figure 16). During peak glacial periods when sea level was lowest, Florida’s Gulf of Mexico coastline was probably situated some 100 miles west of its current position.

The fresh-water table in Florida was probably much lower than today during the Pleistocene sea level low stands. The climate may have been significantly drier as a result. Surface water features such as springs and lakes were less abundant. Only the hardiest of trees, such as oaks, and varieties of ragweed and dry-tolerant grasses would have flourished, giving Pleistocene Florida the appearance of the modern African savannas.

The glaciations were interrupted by warmer interglacial intervals, with Earth’s climate warming considerably. As the climate warmed, the glaciers melted, raising sea level and flooding the Florida peninsula. At the peak interglacial stages, sea level stood at least 100 to 150 feet above the present level, and peninsular Florida probably consisted of islands. Figure 16 illustrates the probable Pleistocene shoreline positions in Florida during the glacial and interglacial periods.

Many of Florida’s modern topographic features and surficial sediments were created or deposited during the various Pleistocene sea level high stands. Waves and currents in these ancient seas eroded the exposed formations of previous epochs, reshaping the earlier landforms and redistributing the eroded sediments over a wide area. At the same time, rivers and longshore currents transported tremendous quantities of sediments into Florida from the coastal plain surrounding the Appalachian Mountains to the north. Much of the quartz sand covering the state today, as well as the heavy mineral deposits, trace their origin to rocks of this once-great mountain chain.

The Pleistocene seas spread a blanket of sand over the limestones underlying Florida’s Gulf coast, infilling the irregular rock surface, forming a relatively featureless sea bottom. During the sea-level high stands, and as the seas retreated, shore waves and near-shore currents eroded a series of relict, coast-parallel scarps and constructed sand ridges spanning the state. Many of these features are formed on or carved out of older geologic landforms and are today stranded many miles inland. Notable examples include the Cody Scarp, Trail Ridge, Brooksville Ridge, and Lake Wales Ridge (Figure 17). Some of the lowland valleys probably evolved largely from dissolution and lowering of the underlying limestones, and these areas may well have functioned as Pleistocene lagoons or waterways bordering the emergent ridges. The Eastern Valley probably contained such a waterway, situated between the relict Atlantic Coastal Ridge on the east and the higher ridges of the central peninsula.

The karst nature of the Eocene, Oligocene, and Miocene limestones comprising the foundation of Florida influenced the development of Pleistocene landforms. For millions of years, naturally acidic rain and ground water flowed through these limestones, dissolving a myriad of conduits and caverns out of the rock. In some cases, the caverns collapsed, forming new sinkholes and modifying the existing landforms through collapse and lowering of the limestone bedrock. In some areas large dissolution valleys formed, such as the Western and Central Valleys of the central peninsula, where dissolution processes lowered the valley floors relative to the surrounding highlands (Figure 17). Many of the larger Pleistocene sinkholes and collapse depressions remain today as lakes dotting the Florida landscape.

The unique geograpic position of southernmost Florida during the Pleistocene produced a terrain significantly different from the rest of the peninsula. Here, carbonate sediments predominate, and the sandy ridges of the central peninsula are absent. South of approximately Palm Beach, the marine continental slope approaches the edge of the Florida peninsula. Most of the continental quartz sands, moving southward with the coastal currents during the Pleistocene, were funneled offshore and lost down the continental slope. As the glaciers melted and sea level rose, nutrient-rich water flooded the southern tip of Florida. Calcium carbonate, in the form of broken shell fragments and chemically-precipitated particles, was the main source of sediments.

The area of modern-day Everglades was a shallow marine bank, similar to the present Bahama Banks. Carbonate sediment bars, some vegetated by mangrove trees, protected the eastern edge of the bank near Miami and to the south along the lower Florida Keys. Calcareous sediments and bryozoan reefs accumulated on the shallow bank under low wave energy conditions. These sediments compacted and eventually solidified to form the limestone that floors the Everglades today. Dissolution and cementation by rainwater and acidic organics has since produced the Everglade’s jagged, craggy rock surface. As sea level climbed to its present level in the Late Pleistocene and throughout the Holocene, modern surface-water drainage patterns formed, ultimately providing water for the immense, southward-flowing "river of grass" which would become the Everglades.

Florida Bay, stranded as dry land during glacial periods, was most likely a Pleistocene lagoon during high stands of sea level. It was protected from extensive wave activity on the south by a chain of the then-living coral reefs of the Florida Keys. Because of the protected, low-energy nature of the south Florida area during the high Pleistocene seas, relict wave-formed features such as bars, spits and beach ridges are rare.

Near the southern rim of the Florida Platform’s escarpment lies a fringeline of living and dead coral reefs (Figure 11). The dead coral reefs form the islands of the Florida Keys. The edge of the Florida Platform, marked by the 300-feet depth contour line, lies four-to-eight miles south of the Keys. Today, living coral reefs grow in the shallow waters seaward of the Keys. This environment is ideal for the growth of coral: a shallow-water shelf, subtropical latitude and the warm, nutrient-rich Gulf Stream nearby.

The geological history of the Florida Keys began about 1.8 million years ago, when a shallow sea covered what is now south Florida. From that time to about 10,000 years ago, often called the Pleistocene "Ice Ages," world sea levels underwent many fluctuations of several hundred feet, both above and below present sea level, in response to the repeated growth and melting of the great glaciers. Colonies of coral became established in the shallow sea along the rim of the broad, flat Florida Platform. The subtropical climate allowed the corals to grow rapidly and in great abundance, forming reefs. As sea levels fluctuated, the corals maintained footholds along the edge of the platform; their reefs grew upward when sea level rose, and their colonies retreated to lower depths along the platform’s rim when sea levels fell. During times of rising sea levels, dead reefs provided good foundations for new coral growth. In this manner, during successive phases of growth, the Key Largo Limestone accumulated from 75 to 200-feet thick in places. The Key Largo Limestone is a white-to-tan limestone that is primarily the skeletal remains of corals, with invertebrate shells, marine plant and algal debris and lime-sand. The last major drop in sea level exposed the ancient reefs, which are the present Keys. Exposures of the Key Largo Limestone can be seen in many places along the Keys: in canal cuts, at shorelines, and in construction spoil piles.

During reef growth, carbonate sand banks periodically accumulated behind the reef in environments similar to the Bahamas today. One such lime-sand bank covered the southwestern end of the coral reefs and, when sea level last dropped, the exposed lime-sand or oöid bank formed the Lower Keys. This white-to-light tan, granular rock, the Miami Limestone, is composed of tiny, spherical oöliths, lime-sand and shells. Oöliths may be up to 2 millimeters in diameter and are made of concentric layers of calcium carbonate deposited around a nucleus of sand, shell, or other foreign matter. Throughout the Lower Keys, the Miami Limestone lies on top of the coralline Key Largo Limestone, and varies from a few feet up to 35 feet in thickness. The northwest-southeast aligned channels between islands of the Lower Keys were cut in the broad, soft, oölite bank by tidal currents. Then, as today, the tidal currents flowed rapidly into and out of the shallow bay behind the reefs, keeping the channels scoured clean.
 

The Holocene Epoch began 10,000 years ago during a slow warming of the Earth’s climate. Sea level climbed intermittently toward its present level from a glacial low about 8,000 years ago. As the encroaching sea shrank the state to its present size, paleo-Indians spread throughout Florida, flourishing on the abundant resources. The first paleo-Indians probably migrated into the state from the continental mainland between 10,000 and 12,000 years ago. The earliest documentation of man’s presence in Florida comes from Little Salt Spring in Sarasota County. Paleo-Indian skeletal remains from this site have been dated at over 10,000 years old.

Sea level then was as much as 100 feet lower than at present, and the land area of Florida was much larger than it is n

During the paleo-Indian period (14,000 - 8,500 BP) and the Archaic period (8,500 - 3,000 BP) which followed, exploitation of the geologic resources of Florida was probably limited to the use of caves and sinks as water sources and possible shelter, and outcrops of chert for the production of projectile points, scrapers and other lithic tools. The next major advancement in the utilization of geologic resources was the manufacture of fired clay pottery. The earliest examples of pottery appear at various places in the state between 3,000 - 4,000 BP. The use of clay or mud to seal vertical post-walled structures and the use of sandstone scrapers has also been documented.

Throughout much of the Neogene and Pleistocene, Florida was home to a diverse animal population (Figure 18, Figure 19, Figure 20, and Figure 21). Many unique and now-extinct species migrated into temperate Florida to escape the cold and ice of the huge glaciers to the north. Fossil remains found today in Neogene and Pleistocene deposits include mastodons, mammoths, black bears, giant sloths, capybaras, beavers, lemmings, dire wolves, horses, tapirs, camels, glyptodonts, llamas and saber-toothed cats. Florida may have been a final refuge for many species as extinction took its toll on the once-diverse animal populations. Animals such as the mastodon, mammoth, giant sloth, and saber-toothed cat disappeared forever.

th the rising sea level during the Holocene came a corresponding rise in the state’s ground-water table. Most of Florida’s springs, lakes, and spring-fed river systems developed during the Holocene Epoch. The rate of sea level rise slowed about 3,500 years ago when sea level was five feet below present level. By that time the beaches, barrier islands, and spits characterizing Florida’s modern coastline had evolved. The complex geologic processes which shaped Florida into its present form continue today. Florida continues to evolve as the sea shapes the coasts and redistributes the sands and other sediments which are to be the rocks of future epochs.
 

Florida is not generally thought of as a mining state, however, it ranked fifth nationally in total value of non-fuel minerals produced in 1990. Phosphate rock, crushed stone, and cement are the major commodities produced; but clay, heavy minerals, magnesium compounds, oil, natural gas, peat, and sand and gravel have also been produced in recent years. In addition, sulfur is produced as a byproduct of oil and gas production while fluorine and uranium are produced as byproducts of phosphate production. Figure 22 shows the main areas of mineral production.

All mining in Florida is by open pit methods. The mineral commodity being mined is removed by earthmoving equipment, dragline, or floating dredge (Figures 23 and 24). The equipment used depends on the pit depth, water table conditions, and hardness of the material. Where the water table is located below the base of the pit, or where conditions allow dewatering a pit by pumping, mining can be conducted under dry conditions. Where the pit cannot be economically dewatered, floating dredges or draglines are utilized to mine below the water table. In limestone and dolomite quarries, blasting is often required to break up the rock prior to mining.
 

Cement is produced by heating a finely ground mixture of lime, silica, alumina, and iron oxide in a rotary kiln, then pulverizing the clinker which is formed. All of the raw materials can be found in Florida. Cement production is closely tied to construction activity.
 

Clay is a general term for common materials which have a very fine particle size and which exhibit the property of plasticity when wet. Strictly speaking, clay is both a size term and the name of a group of minerals. Clay-sized particles are those which are less than 0.000154 inches (1/256 mm) in largest dimension. Clay minerals are composed of hydrous aluminum or magnesium silicates, which form, among others, the minerals kaolinite, smectite, illite, halloysite and palygorskite. These minerals combine with a large number of possible clay-sized impurities including silica, iron oxides, carbonates, mica, feldspar, potassium, sodium, and other ions. Clay deposits are found in many parts of Florida, but only in certain locations are they found with the proper mineralogy, purity, and volume necessary for commercial use. Clays that are presently mined in Florida include fuller’s earth that is used as a carrier to disperse insecticides and as cat litter, and kaolin and common clays for use as lightweight aggregrate, cement ingredients, and construction material.
 

Heavy minerals are associated with essentially all of the quartz sands and clayey sands in Florida, but economically valuable concentrations are known to occur only in limited areas. The areas which are of economic importance are the Trail Ridge and Green Cove Springs deposits located in northeast Florida. All of the commercially valuable heavy-mineral deposits in Florida are inland from the present shoreline and are genetically associated with older, higher shore lines, which were created during the Pleistocene. Heavy minerals (minerals having a specific gravity greater than 2.9) comprise approximately four percent of the economic deposits. The titanium minerals, rutile, ilmenite, and leucoxene, make up about 45 percent of the heavy-mineral fraction. Staurolite, zircon, kyanite, sillimanite, tourmaline, spinel, topaz, corundum, monazite and others make up the remainder of the heavy-mineral fraction.

Heavy minerals are mined by a floating suction dredge equipped with a cutter head, similar to the one shown in Figure 23. The heavy minerals are removed from the quartz sand by running the dredge’s output through a series of centrifugal, magnetic, and electrostatic separators.

The major use for the titanium-rich minerals (ilmenite, rutile, and leucoxene) is to make white titanium dioxide paint pigment and titanium alloys used in military and aerospace industries. Staurolite is utilized as a source of iron and alumina in cement production and as an abrasive. Zircon is utilized for foundry sands, refractories, ceramics, abrasives, zirconium metal and chemical manufacturing. Monazite is rich in the rare earth elements cerium, yttrium, lanthanum, and thorium. Major uses of rare earth minerals include catalysts in petroleum refining, high temperature metal alloys and optical glass.
 

Peat is formed when the rate of accumulation of dead plant material exceeds its decay. Water-logged sites such as estuaries, lagoons, and coastal marshes, largely poorly drained areas such as the Everglades, lake and river beds and surrounding marshes and swamps, and seasonally flooded depressions are common environments in which peat forms in Florida.

All peat presently mined in Florida is marketed for horticultural purposes, such as soil conditioner for lawns, nurseries, and greenhouses. Extensive farming in the Everglades is the major consumptive, non-extractive use of peat in Florida. Even though the peat is not mechanically removed from the site by farming, the peat volume decreases due to biochemical oxidation, compaction, dessication, erosion, and fire.
 

Florida has led the nation in phosphate production for 95 years, providing about 80 percent of the U.S. production and approximately 25 percent of the world production in recent years. Phosphate is found in sediments throughout much of peninsular Florida, however, only two areas are currently economic to mine. The primary mining area is the Central Florida Phosphate District, located in Polk, Hillsborough, Manatee, and Hardee Counties. The other area in which mining is occurring at the present time is that portion of the Northern Phosphate District lying in Hamilton and Columbia Counties.

Approximately 90 percent of the phosphate mined in Florida is used for agricultural fertilizers. The remainder is used in food preservatives, dyes, hardeners for steel, gasoline and oil additives, toothpaste, plastics, optical glass, photographic film, insecticides, soft drinks, fire fighting compounds, and in numerous other ways.

Two potential byproducts of phosphate production are fluorine and uranium. Both are separated after the phosphate rock has been digested into phosphoric acid. Fluorine is used mostly to fluoridate public water supplies. Uranium oxide recovered from the phosphoric acid is used to produce uranium fuel for nuclear power plants.
 

Quartz sand is one of Florida’s most abundant natural resources. Almost all of Florida is blanketed by quartz sand. Very few areas within the state do not have deposits of sand located within reasonable hauling distances. Some construction sand is mined in the panhandle, but the majority of construction sand is mined from the ridges of the central peninsula region. Commercial quantities of gravel are present only in the western panhandle and are associated with river channel deposits.

Industrial sand accounts for less than 10 percent of the sand mined in Florida and is used primarily as glass or foundry sand or as abrasives. Construction sand is utilized for concrete aggregate, asphalt mixtures, roadbase material, and construction fill.
 

Limestones and dolostones ranging in age from late Middle Eocene to Pleistocene are presently mined in Florida. Limestone and dolostone are found at or near the surface in several general areas within the state: in the panhandle in Holmes, Jackson, and Washington Counties; along the west coast from Wakulla to Pasco County and extending eastward to Alachua, Marion, and Sumter Counties; along the southwest coast from Manatee to Collier County; and in a narrow band along the east coast from St. Johns County south into Dade County and the Keys.

Mining methods vary depending on the position of the water table and the hardness of the rock. The easiest mining occurs in dry pits with soft rock conditions, where bulldozers equipped with a claw can rip the rock loose. Where pits are flooded, draglines are utilized to remove the rock. Under certain conditions both methods may be used in mining the same pit. As rock hardness increases, blasting becomes necessary prior to mining. After the rock is mined it may be loaded directly for transport to a processing plant, or it may be crushed and stockpiled on site.

The major uses of crushed stone in Florida are for roadbase material, concrete and asphalt, cement manufacturing, fertilizer and soil conditioners, and rip-rap for erosion control.
 

Petroleum (rock-oil, from the Latin petra = rock or stone, and oleum = oil) is widespread throughout the world. It may be a gas, liquid, semi-solid, solid, or in more than one of these states at a single place. Any petroleum is a complex chemical mixture of hydrocarbons, which are compounds composed mainly of hydrogen and carbon, with smaller amounts of nitrogen, oxygen, and sulfur as impurities.

Scientists think that petroleum formation began many millions of years ago, when lower forms of plants and animals flourished in and near the oceans, as they do today. When these organisms died, their remains settled to the ocean bottoms where they gradually were deeply buried in mud and silt. Over eons of time, this abundant organic matter was transformed into oil and natural gas by high temperatures and pressures, decay, and bacterial processes, in a natural pressure cooker. At the same time, the enclosing sediments also were being transformed into consolidated rocks, such as sandstone, shale, or limestone. These rocks, in which the oil was formed, are called source rocks.

Contrary to popular belief, oil does not occur in underground, cistern-like "pools" that can be tapped and pumped dry. Pool is a term that has special meaning in the oil industry; it refers to an economically produceable quantity of oil dispersed in rock within the earth. Rock strata that contain economically recoverable concentrations of oil and gas are called reservoirs.

In order for oil and gas to be concentrated in porous reservoir rocks, natural traps, seals, or cap rocks must occur, in various forms. In south Florida the oil traps are due to denser, less permeable rocks that overlie the oil fields’ reservoir rocks. The traps in the north Florida panhandle fields are due to very impermeable beds of anhydrite (evaporitic salts), faulting, and stratigraphic traps.

During the course of oil and gas formation and accumulation in reservoirs, some of the original sea water was displaced and gravity separated the gas, oil, and water into layers. Figure 25 illustrates this in principle, but in reality the situation within a reservoir is much more complex. Oil is only a small fraction of the fluids in the pores of a reservoir, but the discovery and recovery of this small fraction is the basis of the oil industry -- and most of the world’s energy. Most of the contained fluid is salt water, or brine, since its dissolved salt content may be higher than in sea water. Almost all crude oil has some gas dissolved in it under pressure. In some cases, excess gas forms a "gas-cap" above the oil zone. Figure 25 shows a small part of the rock that has in its pores quantities of oil, gas, and brine, all under pressure. Some pores may contain only oil, or only gas, or only brine, or mixtures of all. Some of the oil is coated on the rock, while some is suspended in the brine. If a well were to penetrate this zone, the pressure would try to drive the oil, gas, and brine out of the rock and into the well. Not all of the gas and liquids would be driven out, however, no matter how great the driving pressure. Much of the oil would still remain in the rock due to capillary and molecular attraction between the rock and oil. Several techniques have been devised to increase the yield of oil from reservoirs, such as water, steam, or gas injection, and even igniting some of the oil, but recovery usually is relatively low; a recovery of 30 to 40 percent of the in-place oil is considered good.

There are two oil-producing areas in Florida. One is in south Florida, with 14 fields, and the other is in the western panhandle, with seven fields. The south Florida fields are located in Lee, Hendry, Collier, and Dade Counties (Figure 26a). Florida’s first oil field, the Sunniland field, in Collier County, was discovered in 1943 (Table 1). It has since produced over 18 million barrels of oil. Subsequently, 13 more field discoveries were found to lie along the northwest-southeast trend through Lee, Hendry, Collier, and Dade Counties. Although these fields are relatively small, production is significant. Together, the three Felda fields (West Felda, Mid-Felda, and Sunoco Felda) in Hendry County have produced over 54 million barrels of oil (Table 1).

South Florida fields produce oil from small "patch reefs" within the Lower Cretaceous Sunniland Formation (Figure 26b), from between 11,500 and 12,000 feet below land surface (Table 1). The strata of rock from which oil and gas can be produced to a well is called a pay zone. Sunniland pay zones vary from about 5 to 30-feet thick.

The depositional environment during the Lower Cretaceous in south Florida was one of a shallow sea with a very slowly subsiding sea bottom. The time interval was characterized by numerous transgressions and regressions of the sea over the land, which created the carbonate-evaporite sequence of geologic formations shown on Figure 26b. The Sunniland "reefs" are not true patch reefs but were localized mounds of marine animals and debris on the sea floor. The primary mound-builders found in the Sunniland limestone were rudistids, oyster-like mollusks that existed only during the Cretaceous. They lived in great profusion and were widely distributed in clear, shallow Cretaceous seas. Other marine life found in the Sunniland patch reefs, or mounds, included calcareous algae, seaweed, foraminifera, and gastropods, such as snails.

Foraminifera, usually quite small, are single-celled animals with external skeletons or tests. Because of their incalculable numbers in the seas, their tests and remains can represent significant amounts of organic debris on the ocean bottom. Pellets and other organic debris also accumulated in these mounds. The remains of the rudistids, other marine life and debris were deposited on the sea floor, forming porous limestones. Porosity within the limestones was enhanced over succeeding eons by the gradual transformation of limestone to dolostone, which resulted in good reservoir rocks to hold the oil.

The porous limestones and dolostones grade laterally into non-porous, chalky lime mudstones. These dense limestones form a barrier to oil migration, thus trapping the oil in the more porous rocks. Research indicates that the dense mudstones are probably the source rocks for the Sunniland oil. The Sunniland Formation, therefore, appears to include its own oil source rocks and some of its own seals. Additional seals are provided by the evaporites of the overlying Lake Trafford Formation.

Production in the western panhandle began with the discovery of Jay field in June 1970 (Figure 27a, and Table 1). Jay is the largest oil field discovered in North America since the discovery on the Alaskan North Slope of the giant Prudhoe Bay field in 1968. Since then, an additional six oil fields have been discovered in the western panhandle of Florida (Figure 27a). These fields’ pay zones are from about 14,500 to 16,800 feet below land surface and vary in thickness from about 5 to 259 feet.

North Florida has dominated Florida oil production since the discovery of Jay field. North Florida oil fields account for 83 percent of the state’s cumulative production through January 1988. Jay field alone is responsible for 71 percent of the state’s cumulative production.

Jay field is located within the "Jay trend" of Escambia and Santa Rosa Counties in Florida, and Escambia County, Alabama. The Jay trend fields produce oil from Jurassic-age Smackover Formation carbonates and Norphlet Sandstone sands. In Florida, the Jay fields include Jay, Mt. Carmel, Coldwater Creek, and Blackjack Creek. The Jay trend fields in Florida and Alabama are associated with a normal fault complex which rims the Gulf Coast and is believed to extend to the south-southwest into the Gulf of Mexico.

The other panhandle oil fields are Bluff Springs, McLellan, Sweetwater Creek, and McDavid. Bluff Springs field probably formed as the result of a structure created by movement of the underlying Louann Salt (Figure 27b). McLellan and Sweetwater Creek are probably associated with small salt structures or with the stratigraphic pinchout of the Smackover Formation.

Production for all of the panhandle oil fields, except Mt. Carmel, is from Jurassic-age Smackover dolostones and limestones. Mt. Carmel field produces from both the Smackover and the underlying Jurassic-age Norphlet Sandstone (Figure 27b). Although a mixture of carbonates and clastics can be found within the Smackover, in the western panhandle producing area, it is almost purely a sequence of dolostones and limestones. The underlying Norphlet Sandstone is primarily an arkosic sandstone. The Norphlet is underlain by the Louann Salt. The Smackover Formation is overlain by the Buckner Member of the Haynesville Formation. The Buckner is composed of anhydrite, and other evaporites, and forms the seal to some of the Smackover producing zones.

Figure 28 shows the historical trend of oil and gas production from Florida fields. The bell-shapes of the curves indicate that production of both commodities peaked in 1979, and it has been declining sharply since. This trend will continue unless significant discoveries are made in the future.
 

DISCOVERY DATE	FIELD	COUNTY	APPROX. DEPTH BELOW SURFACE	NAME OF PRODUCING FORMATION	OIL PRODUCTION THROUGH 1991, x 1,000 BARRELS
9-26-43	Sunniland	Collier	11,626	Sunniland	18,445
2-1-54	40-Mile Bend	Dade	11,557	Sunniland	33
7-22-64	Sunoco Felda	Hendry	11,485	Sunniland	11,584
8-2-66	West Felda	Hendry	11,675	Sunniland	41,959
3-30-69	Lake Trafford	Collier	11,987	Sunniland	278
6-15-70	Jay	Santa Rosa	15,984	Sunniland	372,072
12-19-71	Mt. Carmel	Santa Rosa	15,399	Smackover & Norphlet	4,706
2-14-72	Blackjack Creek	Santa Rosa	16,235	Smackover	55,395
12-5-72	Bear Island	Collier	11,817	Sunniland	10,905
11-14-73	Seminole	Hendry	11,651	Sunniland	85
7-30-74	Lehigh Park	Lee	11,630	Sunniland	5,272
4-22-77	Sweetwater Creek	Santa Rosa	14,611	Smackover	14
8-11-77	Baxter Island	Collier	11,823	Sunniland	2
10-13-77	Mid-Felda	Hendry	11,686	Sunniland	1,365
6-20-78	Raccoon Point	Collier	11,658	Sunniland	5,567
9-28-78	Pepper Hammock	Collier	11,897	Sunniland	0
6-27-82	Townsend Canal	Hendry	11,462	Sunniland	472
3-25-82	Bluff Springs	Escambia	16,800	Smackover	242
11-10-85	Corkscrew	Collier	11,565	Sunniland	703
2-19-86	McLellan	Santa Rosa	14,475	Smackover	241
6-4-88	Coldwater Creek	Santa Rosa	15,400	Smackover	27
6-14-88	McDavid	Escambia	16,800	Smackover	150
				TOTAL	529,517

The continuous movement of water in all its phases on the Earth’s surface is called the hydrologic cycle (Figure 29). The hydrologic cycle begins with the evaporation of sea water by the sun. Evaporated water is transported through the atmosphere by convective currents. Condensation of water vapor forms clouds, which produce precipitation as rain, snow, or hail. Once precipitation reaches the land surface it takes one of two paths depending on terrain slope, soil permeability (or lack of permeability), soil moisture content, and vegetation cover. Steep slopes, low permeability and soil saturation increases the quantity of water which runs off into lakes, streams and rivers. Conversely, shallow slopes, permeable surficial and near-surface materials and vegetative cover increase the quantity of water which infiltrates into the surficial material. Some of the precipitation returns to the atmosphere because of evaporation from land and open bodies of water, such as lakes and streams, and by transpiration of plants. Some of the water which infiltrates into the ground flows to lower levels into streams and lakes. Some of the ground water recharges the regional aquifer system. Depending on local geologic conditions and the relative levels of the water, water in lakes and streams may either recharge the aquifer or the aquifer may discharge into the lakes and streams as springs. Eventually the water is returned to the ocean.

The majority of the potable water used in Florida is obtained from subsurface rock units called aquifers (Figure 30). An aquifer must be both porous and permeable (i.e., contain interconnected pores), so that water may move freely within and through it.

The Cenozoic sediments in Florida form the several ground-water aquifer systems that provide the vast majority of the state’s water supplies. The Paleogene carbonate rocks, for the most part, make up the Floridan aquifer system, which is one of the world’s most productive aquifers. A variable series of highly permeable rocks separated by low permeability rocks comprise the Floridan aquifer system. The base of the aquifer occurs where the evaporite minerals fill the pores in the Paleocene to Early Eocene rocks. The early Neogene siliciclastic sediments form the top of the aquifer system by providing a relatively impermeable cap. Where these sediments are present, the Floridan aquifer system is under confined conditions and acts as an artesian aquifer. In areas where the overlying confining beds are absent, the system is unconfined.

In southern Florida, an extremely permeable and porous zone occurs in the lower part of the Floridan aquifer system. This zone, called the "Boulder Zone," is thought to be the result of dissolution of the carbonate rocks by ground water. Cavities formed by the dissolution are interconnected allowing vast amounts of water to flow easily through this zone. The term "Boulder Zone" arises from the drilling characteristics of this unit. When drilling operations encounter this zone, pieces of rock (boulders) break from the ceilings of the cavities, fall to the cavities’ floors and, when the drill bit encounters them on the bottom, cause the bit to bounce around, impeding drilling. This zone contains highly saline water and is often used for the subsurface disposal of waste water.

The intermediate aquifer system or intermediate confining unit, where they occur, lie above and are separated from the Floridan aquifer system by beds having lower permeability, such as clay, which retard the exchange of water between the two units. Often the intermediate aquifer system consists of interbedded carbonate and clastic rocks, some of which are permeable enough to provide water to wells. Water within this system is under confined conditions. The base of the intermediate aquifer system (or intermediate confining unit) is the same as the top of the Floridan aquifer system (Figure 30).

The surficial aquifer system is at or near land surface and is generally composed of loose sediments, such as sand or gravel. The surficial aquifer system contains the water table, and water is generally unconfined.

Potable water sources are a vitally important natural resource and are extremely vulnerable to pollution due to the shallow and unconfined nature of many of the aquifers in the state. Even the confined aquifers at deeper depths are vulnerable due to recharge from point source situations, such as poorly constructed wells and from sinkholes which breach confining layers.
 

Much of Florida is karst terrain. Karst terrain is the generic term for landforms that have been shaped by dissolution of the underlying carbonate rocks. Karst terrains have drainage systems distinctly different from the usual surface drainage systems that have connected streams, rivers, and lakes. Karst drainage is characterized by sinkholes, springs, caves, disappearing streams, and underground drainage channels. The genesis of karst involves the development of underground drainage systems. Karst processes tend to be secretive and imperceptible because most development occurs underground over long periods of time. The results of these persistent processes will be manifested, sooner or later, in the subsidence of surficial sediments to form swales, the formation of a new sinkhole, a sudden influx of muddy water in a water-well after a heavy rain or some other karst phenomenon that may disturb or disrupt man’s activities. Figures 31ato to 31d illustrate the evolution of karst terrain, as described below.

Chemical weathering is the predominant erosive process that forms karst terrain. Chemical weathering of limestone removes rock-mass through solution activity. As rain falls through the atmosphere, some carbon dioxide and nitrogen gases dissolve in it, forming a weak acidic solution. When the water comes in contact with decaying organic matter in the soil, it becomes more acidic. Upon contact with limestone, a chemical reaction takes place that dissolves some of the rock. All rocks and minerals are soluble in water to some extent, but limestone is especially susceptible to dissolution by acidic water. Limestones, by nature, tend to be fractured, jointed, laminated, and have units of differing texture, all characteristics which, from the standpoint of percolating ground water, are potential zones of weakness. These zones of weakness in the limestone are avenues of attack that, given time, the acidic waters will enlarge and extend. Given geologic time, conduits will permeate the rock that allow water to flow relatively unimpeded for long distances.

During the chemical process of dissolving the limestone, the water takes into solution some of the minerals. The water containing the dissolved minerals moves to some point of discharge, which may be a spring, a stream bed, the ocean, or a well.

Removal of the rock, with the continuing formation or enlargement of cavities, can ultimately lead to the collapse of overlying rocks or sediments, sometimes revealing the cavity in the rock. More often, though, debris or water covers the entrance to subterranean drainage. Partial subsidence of the overburden into cavities will form swales at the surface, producing undulating topography. By this slow, persistent process of dissolution of limestone and subsequent collapse of overburden, the land is worn down to form a karst terrain.

At some point in this process of dissolution of underground rocks, a normal surface drainage system will begin to be transformed into a dry or disappearing stream system. Continuing dissolution of the limestone will create more swales and sinkholes, which will divert more of the surface water into the underground drainage. Eventually, all of the surface drainage may be diverted underground, leaving dry stream channels that flow only during floods, or disappearing streams that flow down swallow holes (sinkholes in stream beds) and reappear at distant points to flow as springs or resurgent streams.

Inherent in the formation of karst terrain is the lowering of land surface on a regional scale, in contrast to the very localized lowering at a sinkhole. Regional lowering of the land surface takes place through the cumulative effects of thousands of individual, localized events and through the continual removal of carbonate rock by dissolution.
 

Flood prone areas in Florida are associated primarily with either low-lying coastal areas or with inland rivers and lakes. Coastal areas, including the barrier islands and estuarine areas which are so highly developed in Florida are subject to flooding. Severe flooding problems can result from the storm surge developed as hurricanes or "northeasters" approach the shoreline. Storm surge is created as water is pushed ahead of the storm and piled up against the shoreline. Normal tidal action is added to the storm surge, which can produce waves several feet above normal.

Away from the coastline, flooding is associated with river and stream floodplains, lake margins, low-lying, and poorly drained areas such as the Everglades. The majority of stream or river-related flooding problems can be avoided by simply not building on floodplains. Heavy rainfall events in flat lying, poorly-drained areas can result in flooding because the water drains off at such a slow rate. Extensive areas of Florida have been subjected to major drainage enhancing projects. Typically these projects include river channelization, drainage ditching and storm water impoundments. These projects often provide flood relief but the environment may suffer.
 

The presence of unstable or plastic geologic materials in the near-surface can create foundation problems in construction projects. Organic materials, such as peat or muck deposits, do not have the strength to support structures. Some clays shrink and swell upon drying or wetting, which can stress buildings’ foundations to the point of failure. These materials must generally be removed or addressed in the design of roadbeds or foundations.

Figures 32and 33 show a landslide that flowed into a nearby stream channel. Heavy local rains weakened and lubricated the unconsolidated sediments of the hillside causing sudden failure.
 

Historically, Florida has had very little earthquake activity since the earliest recorded tremor, reported to have been felt at St. Augustine on October 29, 1727. Since then, about 30 tremors have been reported throughout Florida (Table 2). The strongest tremors that have been felt over the widest areas of the state were associated with the great earthquake of August 31, 1886, in Charleston, South Carolina, about 180 miles northeast of Jacksonville. Tremors from this earthquake were felt all over north Florida; church bells rang in St. Augustine, severe shocks were felt along the east coast, and tremors were felt in Tampa. Jacksonville felt more aftershocks from that quake during September and November 1886.

Two geological events cause earthquakes: active faults and volcanic eruptions. Florida has no volcanoes and no documented acti

There has been recent evidence that some very localized rumblings or "earth tremors" are, in fact, caused by cold air masses associated with frontal weather systems that have high altitude winds from the southwest. Some of these weather systems are known to have layers of air which have been stratified due to temperature differences, creating, in effect "tunnels" of air of differing densities. In addition, there is circumstantial evidence that sonic booms from military aircraft flying over the Gulf of Mexico could be "focused onshore" through such temperature stratified layers of air.

Florida is classified as a stable geological area. This means that, with respect to probable damage from the largest expected distant earthquake, some areas may experience tremors, with only minor damage, such as broken windows or glassware. Severe weather events, such as hurricanes and tornadoes, pose tremendously greater threats to Florida than do earthquakes.

The severity of an earthquake is expressed by two different methods: the Richter Scale and the Modified Mercalli Intensity Scale. The magnitude of an earthquake -- expressed by the Richter Scale -- is related to the amount of seismic energy released by the earthquake. The Richter Scale is based on the amplitude of earthquake waves recorded by seismometers. Richter values are given as whole numbers and decimals, such as 5.1 or 7.2, with larger earthquakes assigned larger numbers. The Richter Scale is not used to assess damage; the Modified Mercalli Intensity Scale was devised to do this.

The intensity of an earthquake -- expressed by the Modified Mercalli Intensity Scale (usually written as MM) -- is based on observed effects of ground shaking on people, buildings, and natural features. Values of MM range from MM I to MM XII. MM I is defined as: not felt except by a very few people under especially favorable conditions. A mid-range value of MM VI is: damage slight; felt by all; many frightened; some heavy furniture moved; a few instances of fallen plaster. A value of MM XII represents catastrophic destruction, and is defined as: damage total; objects thrown into the air; lines of sight and level are distorted. While the MM scale is an arbitrary ranking based on personal observations, it does provide a meaningful measure of severity to a non-scientist, because it refers to effects actually experienced.

The energy released by an earthquake travels as seismic waves through the earth and along the surface. Seismic waves of energy passing through rock strata cause alternate expansion and compression of the rocks. One result of this is that the seismic waves can cause the water level to fluctuate in a cased well, and a water level recorder installed on the well can, under special conditions, act as a crude seismograph. As the water level changes, a record is preserved of the earthquake, as on a real seismogram. The Florida Geological Survey has an instrumented observation water well which is sensitive to some of the larger earthquakes. It has recorded several of the world’s major earthquakes (Figures 34and 35). Figure 36 shows the effect the great 1964 Alaskan earthquake had on another instrumented observation water well that was located north of Lake Butler, Union County, Florida.

<