The Geological History of Florida


The Floridian peninsula is a region where geological action has been gentle, slow and very uniform.”


Jason B. Barr

ES767: Global Tectonics

Emporia State University

April 2009


Figure 1: General Tourist Map of the state of Florida


INTRODUCTION


Florida's geologic history begins deep beneath its surface where ancient rocks indicate that Florida was once a part of northwest Africa. As ancient supercontinents split apart, collided, and rifted again, a fragment of Africa remained attached to North America. This fragment formed the base for the carbonate buildup which includes the Florida and Bahamas Platforms. Florida in general, is a simple structure geologically speaking. The rocks are all sedimentary in origin, no igneous or greatly metamorphosed rocks occurring in the state. The formations of the coastal plain are sedimentary, containing much clay, limestone and sandstone. The sediment making up these deposits came from higher land to the north and west. The sea occupying the present position of Florida was in early times remote from the sources of sediment, so that the proportion of wash was much less here than nearer the original shore line. This sea was favorable to the existence of shell life, the remains accumulating to form shell rock; hence in the early history of Florida in particular a large amount of lime and shell rock accumulated.


Figure 2: Limestone Formations in Levy County/Gilchrist County, FL


GEOLOGICAL COMPOSITION OF FLORIDA


In Florida, sedimentary rocks are the most common type of rock. These rocks are made up of cemented mineral particles. One example is shell fragments (the mineral aragonite) that are cemented together by calcite to form the sedimentary rock coquina. Another type of sedimentary rock is limestone, which is mined for road building and other construction applications. This rock is made up of small calcite particles formed by marine organisms that build up large reefs in the oceans. During much of Florida's geologic history, it was covered by the ocean. Much of the surface of Florida is covered by sediment (loose mineral particles, such as quartz sand), or sedimentary rocks such as limestone and dolostones. Igneous and metamorphic rocks do not occur naturally at the surface, but are found in deep wells reaching from 3,500 feet to deeper than 18,670 feet below land surface.


Figure 3: Florida Bedrock Layers.


Since Florida is relatively a young terrain, its basement rock is composed of carbonate remains of marine algae and shells deposited in an ancient sea. Evidence indicates earliest geologic formations began during the Tertiary Eocene era. Those shallow sediments are called Suwannee Limestone (1). Florida's landscape varies widely. Many of Florida’s prominent features have resulted from karst, a landscape with a base layer of limestone. Because limestone is porous, freshwater gradually dissolves the rock and forms cracks and passages. The limestone layer of the state is honeycombed with underground rivers. Where the rivers break through to the surface, springs and sinkholes are found.


As coral, shellfish, and fish skeletons piled up, this created a layer of limestone hundreds (in some places thousands) of feet thick. As the Appalachian Mountains eroded, sand and clay were deposited over Florida’s limestone layer. Because limestone is porous, water gradually dissolves the rock and forms cracks and passages. The limestone layer of the state is honeycombed with underground rivers. Where the rivers break through to the surface, springs and sinkholes are found.



Lime, Limestone and Sandstone Formations, Central Florida


FLORIDA and PANGAEA


In earliest times, Florida was part of Gondwanaland, the super continent that later divided into Africa and South America. There is evidence that Florida separated from Gondwanaland about 300 million years ago. Florida eventually found itself wedged between Gondwanaland and North America when they combined to form the super continent Pangaea. When Pangaea began to break up, Florida remained behind with North America. Florida slipped slowly beneath the waves to become part of North America’s continental shelf. Florida finally emerged from the seas as part of the North American mainland.


Figure 4: Earth 255 million years ago, Pangaea




THE FORMATION OF FLORIDA


Geologists estimate the age of the Earth at about 4.6 billion years. An understanding of the geologic processes of Florida begins over 40 million years ago when the region was at the bottom of the ocean. The Florida peninsula is actually the emerging portion of a tectonic platform called the Florida Plateau; where the Florida peninsula and adjacent continental shelf occupies this plateau region. It has been suggested that the Florida plateau, since it is composed of a thick section of mostly undeformed carbonate rocks, that this carbonate platform that has been developing since the late Triassic opening of the North Atlantic which occurred about 180 million years ago. (1) The development of the platform was controlled by regional subsidence of the passive margin and eustatic sea level changes which allowed the deposition of a thick section of carbonate rock over many millions of years.


Beneath the sea, the plateau acted as a marine shelf. As sea levels rose and fell, the remains of large and small sea creatures deposited on the shallow ocean floor creating limestone bedrock. After sea levels dropped, a limestone peninsula, several thousand feet deep, emerged. The Florida plateau is one of the youngest areas geologically, in the continental United States. Though Florida was formed about 530 million years ago by a combination of volcanic activity and marine sedimentation during the early Ordovician Period; throughout most of its history, Florida has been under water. When the Florida plateau was part of the supercontinent Pangaea, Florida was sandwiched between what were to become North and South America and Africa.


Figure 5: Topography of Florida


Sea levels have had a profound effect on both Florida's geology throughout most of its history, which placed Florida under water.

The fossil record indicates that approximately 2 million years ago, when sea levels were much lower, a land bridge connected North and South

America. Portions of the Florida peninsula have been above or below sea level at least four times within that time. As glaciers of ice in the north expanded and melted, the Florida peninsula emerged and submerged. When the sea level was lowest, the land area of Florida was much larger than it is now. In addition to being much larger, Florida was as much as three times the current land area as it is today during the last ice age…making the region drier by savanna-like conditions.


Figure 6: Florida during the last ice age.


As the ice age ended, sea levels rose, Florida shrank in size, the climate became much wetter, and habitats changed. A notable example of these climatic changes is formation of the Everglades, which occurred sometime around 4,000-6,000 years ago. Many of Florida's modern topographic features and surficial sediments were created or deposited during periods when sea levels were high. Waves and currents in these ancient seas eroded the exposed formations of previous epochs, reshaping earlier landforms and redistributing eroded sediments over a wide area. Florida's geological history has been principally affected by changing sea levels, which influenced the formation of bedrock and its surface topography. In general, geological history and time, has played a major part in influencing Florida's environment, structure and topography, not only in the past, but during the present, and will continue in the future.

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