Wednesday, July 31, 2013

First Visit to the Florida Everglades: Part I - The Geology of Florida and the Everglades

“The miracle of light pours over the green and brown expanse of sawgrass and of water, shining and slowly moving, the grass and water that is the meaning and the central fact of the Everglades. It is a river of grass.”
 From The Everglades: River of Grass
by Marjory Stoneman Douglas, 1947

On an unseasonably chilly April morning, I visited the Everglades in South Florida for the first time. It was an unforgettable experience. With the sweet smell of sawgrass in the air, amid its pervasive silence punctuated by the sound of a thousand birds in flight, and with the sight of scores of alligators basking in the sun, the marshy wilderness seemed hauntingly beautiful and frozen in time. Changeless, yet changed. Wild, and yet tamed.

The Everglades is said to be highly imperiled and in decline. Others say it’s a "tiny, dying remnant of the once immense wetland system", and if it's to survive, it needs help probably now more than ever before (Marjory Stoneham Douglas). 

The Everglades has many narrow canals and trails that slice through the dense sawgrass.

Only recently have we begun to appreciate the importance of wetlands. Once thought to be useless, disease-ridden places, wetlands provide values that no other ecosystem can. Water quality improvement and sustainability, shoreline erosion control, flood protection, recreation, aesthetic refuge, wildlife and natural resources.

Florida has been infested with alligators almost since they first emerged from the sea.
The Florida Everglades ecosystem is the only place in the world
where alligators and crocodiles coexist side by side.

Seen from high above the Gold Coast, the Florida Everglades is one of the world’s great ecosystems, partially hidden in the haze immediately beyond the urban sprawl of Greater Miami. Only a small portion of the Everglades actual size is visible and only a fraction of its original extent of 4,000 square miles. Once a continuous, wide shallow river, it used to run unabated. Today, its flow is interrupted, confined, restricted, redirected, impounded, drained, leveed and channeled. But at what cost to the ecosystem?  

The population of urban Miami exceeds 500,000 people, while the population of alligators in the Everglades is over 1,300,000, far smaller than historical numbers. How do these closely-juxtaposed habitats co-exist? How CAN they co-exist? What is the future of this incongruous relationship?   

Looking west at North Miami, the Everglades abruptly begins about 20 miles from the coast,
when, in actuality, land development was reclaimed from the Everglades that was originally beneath it.

The first natives called it “Pa-hay-okee” for grassy water. On early Spanish maps, it appears as “El Laguno del Espiritu Santo”, and on a surveyor’s map as “River Glades.” On later English maps, the word “River” became “Ever”, and written as "The Ever Glades." The single word “Everglades” first appeared on a map of 1823, “one word and yet plural.”
Moving south from conflict in North Florida and Georgia, the Seminoles were the last native peoples
to make the Everglades their home. Finding refuge and adapting to the harsh conditions,

they created their own unique lifestyles. They constructed shallow canals for transportation,
many of which are maintained by “Gladesmen” that live in the Everglades
and used by airboat operators that serve the tourist trade.

It's been called a miasmic mire, a worthless swamp, an alligator-infested quagmire, a poisonous lagoon, a dismal marsh, a rotting inland sea, a stagnant wetland, the haunt of noxious vermin, the resort of pestilent reptiles and a “watery labyrinth of dark trees looped about with snakes and dripping mosses malignant with tropical fevers and malaria's evil to the white man.” These are things that the Everglades is NOT and are largely views taken by those that have desired to either drain, develop, diminish or don’t understand it.

A swooping flock of Black-necked Stilts

It IS a “treasured river”, an immense, freshwater flooded-grassland in a subtropical climate, and a stunningly rich and diverse ecosystem of plants and animals. It is literally a “river of grass” that both fascinates and repels, conjuring up a “vision of steaming swamps inhabited by dangerous alligators, swarms of mosquitoes and venomous snakes.” (Petuch et al).

It's a common misconception that the Everglades is a stagnant body of water. In reality¸ its waters are slowly on the move at 1/4 mile per day (that's 0.01 mph) on a southern slope that's visually imperceptible (averaging 2 inches per mile). For comparison, the Mississippi and Colorado Rivers average 2.1 and 4 mph, respectively.

From The Everglades: River of Grass
by Marjorie Stoneham Douglas

The Everglades is a region of tropical wetlands that comprises the southern half of a massive watershed beginning in Central Florida. The system has headwaters south of Orlando within the Chain of Lakes and five main headwater sources, the largest of which is the Kissimmee River, which discharges into vast and shallow Lake Okeechobee. Water leaving the lake is channeled through the Everglades Agricultural Area (orange) and into the Everglades (green), which is a 60 mile-wide and 100 mile-long, slowly-moving shallow river.

The Everglades consists of a complex system of interdependent habitats and ecosystems that includes cypress swamps, freshwater marshes, wetland tree islands, mangrove forests, tropical hardwood hammocks and pinelands, and coastal flats, before ultimately discharging into Florida Bay and the Gulf of Mexico.

This describes the current pattern of flow, which has been altered greatly from the historic, natural pattern of flow. The change has occurred over the last 125 years in order to place man in charge of the watershed for purposes of flood control, agricultural irrigation (mostly sugar cane), habitable real estate, and fresh water for hot days, green golf courses and a cool dip in the pool.


Geologically, the Everglades is classified as a freshwater marsh, a shallow wetland (~1-2 feet deep in the wet season) with an open expanse of grasses, sedges, rushes and other herbaceous plants. Typically, freshwater marshes contain few, if any, trees and shrubs, and are dominated by herbaceous (non-woody) vegetation that actually look like prairies. Swamps, by comparison, possess dominant, wooded-areas where standing water occurs for at least part of the year, while during the dry season, their mucky soils dry out. Unlike bogs and fens, they have non-peat soils (partially decayed vegetation).

Staunch preservationist Marjory Stoneham Douglas' metaphor of the Everglades as a "river of grass" is most appropriate, since its waters flow through a shallow, sawgrass-dominated prairie in a vast sheet, both shallow and wide. Sawgrass is actually not a grass but a sedge, having derived its name from the tiny saw-like teeth that line its blades. “Sedges have edges; rushes are round; grasses are hollow right up from the ground.”

This map depicts the original Everglades before 1900.
In addition to the Everglades, South Florida has a combination of habitats unique in the world.

Although it’s the largest remaining tropical wilderness in the United States, it’s the last remnant of a huge series of marine environments going back in time some forty million years or more. Today, it retains less than 10% of its original habitat as the human population of southern Florida threatens to over-run its increasingly fragile ecosystem.
In Marjorie Stoneman Douglas’ classic 1947 book The Everglades: River of Grass, she states “Only one force can conquer it completely and that is fire.” In reality, the other is man and his seemingly unendless encroachment. No longer flowing freely, the Everglades' water flow is now controlled by man, not by nature.

On a map, Florida’s shape makes it appear to be dangling off the southeast coast of North America. As a child, I remember Florida being the easiest piece to recognize on a puzzle of the United States. 
It raises the obvious question, how did Florida form? Did it form pendulous or did it geologically evolve into that shape? Was it always part of Cenozoic North America or preceding Paleozoic Laurentia, the cratonic core of the previous tectonic continent? Is it an extension of the trend of the Appalachian Mountains below the surface or is it simply the cumulative result of millions of years of carbonate deposition?

How did the Everglades form? Why is South Florida so flat? The low relief of the landscape implies it was covered by a shallow sea. Is the Everglades merely a persistence of this condition as an exposed ocean floor? The attraction of Florida, other than its weather, is the ocean. It comes as no surprise that Florida was born from the sea. The hordes of people that live there (19 million by last count) and the throngs that come to visit, see the geology of the present, not for want of science, but for recreation. Yet the geology of the present is dictated by the geology of the past. 

Indigenous landform or exotic terrane?
Appalachian extension or accreted platform?

Carbonate deposition or persistent flat seabed?

The highest natural elevation in the state is in the North Highlands of the Panhandle on Britton Hill at 345 feet. It’s also the lowest high point of any state in the United States. Literally the highest point in the state is the top of the 70-story Four Season's Hotel in Miami at 789 feet. Indeed, the majority of Florida’s geologic past is buried out of sight. The landscape of South Florida is so flat that driving around for a week, I didn’t see a single road cut, a unique experience for this New Englander. The bedrock is exposed, but it’s generally found in man-made canals, landfill-quarries and rail cuts that require a field guide to locate and permission to enter.

Another observation. Much of the surface rock is covered with a veneer of quartz sand. Knowing something of Florida's carbonate-past, I expected to find its beaches consisting of pristine-white, carbonate sand. So, where did the siliciclastic, brownish sand come from? The only weathered, silicate source-rocks (igneous granites, metamorphic schists, quartzites and gneisses, and sedimentary quartz-rich sandstones) are hundreds of miles to the north.  How did it travel to South Florida?

Why isn't Miami Beach's sand a pristine, carbonate-white?

The congested Gold Coast on the southeast where I was staying is actually a topographic high called the Atlantic Coastal Ridge, some 20 feet above sea level. There's one on the west coast as well. Had it not been for the "highs", the Everglades would have flowed to the coasts (although it does at various outlets, some man-made and some natural). Its modest relief was acquired relatively recently during past elevated stands of high sea level. Yet, its elevation is imperceptible, having been bulldozed and concealed under the concrete and landscaping vestiges of human occupation. To appreciate the geology of the Everglades, we must look back in time. For that, we need to look to the rocks buried deep underfoot up to three miles.

Florida's modern coastline is shaped by its shoreline with 50% of the real Florida lying underwater. The political boundaries of the state are smaller than the geological entity called the Florida Platform, which extends well offshore in every direction. Think of it as a huge, escarpment-bounded plateau with a dip that rarely exceeds 10 feet per mile at 0.1 degrees. Punctuating its flatness, structural features of modest north and central highlands and coastal lowlands have dictated sediment deposition on the platform beginning around 40 million years ago, channeling the Everglades' flow to the south.

The platform is a distinct geological entity. Geologists call it a terrane, a physiographic province with a geomorphic structure that contributes to its uniqueness. On its periphery, it has long, sloping flanks that drop off into a deep water abyss, whose sea level has fluctuated markedly in cycles lasting from thousands to millions of years. Rising seas that we're now experiencing are nothing new to this planet. During the last 542 million years of the Phanerozoic eon, the Earth has experienced five major global floodings and countless minor ones. They generally occur in association with major continental glaciation events (glacio-eustasy) and major tectonic events (tectono-eustasy).

Look at Florida the state and compare it to the dimensions of the submerged Florida Platform.
If you measured Florida's relief, at ~3,400 meters it has more relief than many of the fifty states.
Geological looks are deceiving!
While you're here, notice the Bahamas off to the east and curvy Cuba to the south.
They bear important evolutionary relationships to one another.
Google Earth

We're in one such cycle now, actually  a "warm" interglacial with the last advance some 18,000 years ago. The Laurentide continental ice sheet never reached Florida with its southern extent to about 38 degrees latitude, but its effects surely did. The last Glacial Maximum lowered sea level globally as much as 120 meters. If we define Florida by its coastline, that would have placed its western paleo-shoreline 150 km further west on the continental shelf. The other glacial effect was depositional, and it relates to the Everglades evolution! More on that later.

Florida the State rests on Florida the Platform, which was once part of the larger Florida-Bahamas Platform. During the middle to Late Cretaceous the two became separate carbonate depositional areas with different geological stories. For simplicity, we'll focus solely on Florida's evolution knowing the development of both are inseparable.

The Florida Platform resides on the North American tectonic plate, but it wasn't always the case! When mobile tectonic plates collide, they transfer crustal components. It's how continents evolve, and it's how Florida started out on one plate and ended up on another. To comprehend the geology, let's begin at the site of Florida's earliest known origin. Although Florida is among the younger additions to the North American continent, we must look back in time to a Late Precambrian supercontinent that was rifting apart. 

The Florida Platform, measured above the 300 foot isobath, spans more than 350 miles
at its greatest width and extends southward more than 450 miles.
Notice that the Keys define the southern perimeter of the platform and how relatively small
the modern Florida peninsula is lying on the platform. The Panhandle is a part
of the Gulf Coastal Plain. 

Although poorly understood and controversial, the world's landmasses were united into a supercontinent called Rodinia late in the Proterozoic (~1.1 Ga). Its formation culminated with the formation of a transglobal, mountain-building event called the Grenville orogeny. Meaning "motherland" in Russian, by ~750 Ma, Rodinia's fragmentation led to the development of two large megacontinents: equatorial-based Laurentia (the geological core of the North American continent) and South Hemisphere-situated Gondwana (Antarctica, South America, Africa, Madagascar, Australia, Arabia and India).

For the record, the Late Proterozoic was a pivotal interval in the Earth's history. Irreversible global change occurred with worldwide orogeny, rapid continental growth, profound changes in ocean chemistry and an explosion of biological activity.

Florida's deepest sub-basement possesses rocks possibly with a billion year old, Late Proterozoic, Grenville-affinity from Rodinia. It is thought that it acquired this earliest crustal foundation while on the West African craton of Rodinia before rifting tore apart the supercontinent.

Traditional model of Rodinia prior to fragmentation showing posited rifting events on Laurentia's
east and west margins. A speculative location of origin is shown for the future Florida terrane.
Modified from Dalziel (1997), Torsvik (1996) and Meert (2003)

In the early Paleozoic, Rodinia's fragmented continental siblings tectonically-drifted throughout the globe. Seen here in the Silurian, the future platform of peninsular Florida (red arrow) as the Florida-Bahamas Block and the Suwannee Basin Block have been traced to the northwest coast of an amalgamated Africa-South America (red arrow) within the megacontinent of Gondwana located in the high southern latitudes.

The pendulous shape of peninsular Florida did not exist before 200 Ma, but neither did the Atlantic Ocean, the Caribbean Sea or the Gulf of Mexico. But its basement rocks did, lying above its Rodinian sub-foundation, embedded within the continent of Gondwana. They include igneous and metamorphosed-sedimentary rocks of Precambrian-Cambrian and Triassic-Jurassic ages, and sedimentary rocks of Ordovician-Devonian age.  

Global Palemap during the Silurian

CONVERGENCE (~490 to ~250 Ma)
Throughout the Paleozoic, Gondwana migrated towards Laurentia, but its collision was incremental. It occurred on Laurentia's east coast (present-day co-ordinates): first with Taconic island arcs in the early Paleozoic; later with peri-Gondwanan arcs and a Baltican micro-continent in the middle Paleozoic; and finally with arrival of the unwieldy mass of Gondwana in the late Paleozoic. It was during the final convergence that the Florida-Bahamas Platform was transported to Laurentia along with its basement that it had acquired from its Gondwanan affiliation, a distance of over 8,000 miles! The platform acquired its Ordovician-Devonian sedimentary basement while in transit. 

The entire series of successive Paleozoic collisions between Gondwana and Laurentia's east coast are collectively referred to as the Appalachian orogenic episode. Its tri-phasic components are the Taconic (Ordovician-Silurian), Acadian (Devonian-Mississippian) and Alleghanian (Mississippian-Permian) orogenies. The penultimate collision resulted in the formation of another supercontinent called Pangaea ("all lands") and delivered Florida to its new home on the North American plate (~250 Ma). The formation of a succession of supercontinents in a predictable cycle in association with the opening and closing of ocean basins is the hallmark of the plate tectonic theory.

Depicted in the Devonian (below), a massive continent-continent collision is imminent as Gondwana converges upon Laurentia's east coast. Contimitant with the welding of Gondwana to Laurentia, the Florida Platform (red arrow) is on a collision course as well. Peri-Gondwanan island arcs and micro-terranes (Taconic and Acadian orogenies) previously welded to Laurenta's east coast, adding crust and building mountains with each collision. 

Remniscent of the Late Proterozoic collision that built Rodinia and formed the Grenville orogen, Gondwana's oblique convergence with Laurentia built Pangaea and culminated in the formation of a near-continuous, 6,000 mile-long, transglobal, Himalayan-style orogen during the Pennsylvanian and Permian. This final collision is referred to as the unwieldy Ouachita-Alleghenian-Caledonian orogen (Central Pangaean on the map).

On North America’s present-day east coast, the Alleghenian orogeny is represented by the eroded Appalachian Mountain chain. The chain's southern extension includes mountains from Arkansas and Alabama through east Texas and into Mexico, formed during the Ouachita orogeny. It was during the Ouachita that the Florida-Bahamas Platform became sutured to Laurentia's southeast coast.

The Florida Platform's tectonic journey from high South Polar latitudes trans-equatorially
to southeast Laurentia exceeded 5,000 miles of drift and the closure of two oceans.
Global Paleomap during the Permian
The weld occurred at the Suwannee suture, uniting Gondwanan and Laurentian bedrock. Florida didn't arrive as an isolated, peninsular platform. Instead, it welded to Laurentia with Gondwana on the leading edge of the collision as part of Africa (Senegal and Guinea most often cited) and likely some of South America. The Florida Platform (and previously-accreted peri-Gondwanan terranes) are "exotic" in that their basement rocks have little resemblance to those constituting Laurentia, having originated elsewhere with a distinctive stratigraphy and geologic history. Today, Florida's exotic foundation is buried up to a depth of three miles. It presence is known from wells and seismic sounding. 

Pangaea’s assembly was complete by the Permian with Florida submerged at the juncture of the North American, South American and African plates. Thus, the Florida Platform did not originate on the North American plate, but through the magic of plate tectonics (rifting-drifting-accretion-rifting) it became a terrane of the New World. 

The modern east coast of North America is veneered with accreted Gondwanan-derived terranes.
Notice the Gondwanan terranes in Great Britain across the Atlantic that were torn from Pangaea
when it rifted apart. The red line is the Iapetus suture of the Alleghenian-Caledonide orogeny.
Modern Map of North America and part of Eurasia
Modified from Cocks and Torsvik, 2011

Massive and unstable, Pangaea began to break up (as Rodinia had done in the Late Proterozoic) between the Late Triassic and Early Jurassic after being assembled for ~85 Myr. Its cleavage sent Africa and Europe adrift across the newly-forming Atlantic Ocean, while our Gondwanan-derived Florida-Bahamas Platform remained welded to Laurentia, now called North America.

Pendulous-Florida (along with Georgia and southeastern South Carolina) acquired its oceanic real estate on the east coast of North America, now a tectonically-stable, passive continental margin (no major earthquakes, no volcanoes, lots of subsidence and sedimentation) as a result of the break up of Pangaea. During this time (~180 Ma to ~140 Ma), the North Atlantic Ocean, the Gulf of Mexico and the Caribbean Sea had their origins as well. The South Atlantic opened later (beginning ~125 Ma) separating South America from Africa.

Accompanying the rifting of Pangaea in the early Mesozoic, widespread volcanic rocks extruded from the spreading, mid-Atlantic sea floor. Known as the Central Atlantic Magmatic Province (CAMP for short), it blanketed North America's new coast (actually both opposing, Atlantic-coasts since rifting is a bi-coastal event). That is how the Florida Platform acquired its basement's Mesozoic component. The platform's tectonic journey was complete, but its geological evolution was far from finished. The Everglades had not yet formed, but its basement had!

Pangaea has broken apart. The new continents are separated by the new Atlantic Ocean.
The Florida Platform has become a peninsular fixture on the North America plate,
totally submerged by global Cretaceous high seas.
The Everglades have not yet begun to form, but its basement now in place, lies in wait.
Global Paleomap during the Cretaceous

In summary, convergent plate tectonics delivered the Florida Platform to its new home on the North American plate. Rifting apart of Pangaea not only created the Atlantic Ocean and ultimately the Gulf of Mexico and the Caribbean Sea, but it established Florida as a pendulous landmass attached to the newly formed continent of North America. It was all fortuitous for the evolution of South Florida. Had the Gulf of Mexico never formed and the region's wet climate never evolved, Florida would still have been tropical but likely more arid, and the wetlands of Florida would never have evolved.

The stage was set for Florida to build 1-6 km of carbonate cover rock on its basement. Carbonate formation (think limestone) is derived biologically (from microbes, plants and animals) and non-biologically (via precipitation from sea water). Its formation requires clear, shallow, well-illuminated, warm, normal saline water for carbonate-secreting biota to produce carbonate-containing sediments. As the Atlantic Ocean continued to open in the Late Jurassic, conditions were perfect for building an extensive carbonate platform.

Cross-section through the Florida Platform showing range of thickness of carbonate rocks
covering basement rocks. The Peninsular Arch forms the backbone of peninsular Florida.
The west Florida shelf is a low-gradient carbonate ramp.
Modified from Randazzo, 1997

The Florida Platform was elevated and stable (being a tectonically-inactive, passive continental margin), shallow (allowing solar photosynthetic processes to occur), exhibited rapid subsidence (lowering) that provided accommodation space (room to accumulate) for the formation of extensive carbonate stratigraphy, and its waters were well-oxygenated (circulated) and warm (in the Cretaceous Greenhouse’s equatorial latitudes). Lastly, the platform was “protected” from the influx of turbid, nutrient-rich, siliciclastic sediments that would otherwise foul the “carbonate factory.”

Initially as a small, shallow carbonate bank 150 km off the coast of Georgia, a thick succession of carbonate rocks in the form of marine limestones and reefs began to develop. It eventually extended along the east coast of North America from the Yucatan, Bahamas and Florida up to Nova Scotia as the Bahamas-Grand Banks Gigaplatform. Later in the Mesozoic, production on the gigaplatform ceased from the introduction of siliciclastics off the eroding Appalachians, while on the Florida-Bahamas Platform, carbonate production thrived, promoted by global high seas. The Gigaplatform is presently buried on the continental margin of North America under a thick layer of siliciclastic sediment.
This shallow, subtidal shelf environment on the tropical South Florida-Bahamas platform
is likely how the region appeared in the Paleogene.

In the course of Florida’s birth from the sea, other geologic processes began to alter it almost immediately beginning in the Cretaceous. They are integral to the understanding of Florida's geological evolution, but for purposes of simplicity, I have included only those most relevant to our discussion.

Florida's west margin began to tectonically subside at a rate greater than the ability of shallow water carbonate production to keep pace in the Early Cretaceous. With rising levels of the sea due to greenhouse Earth global warming, the platform subsided beyond the photic zone for photosynthesis to occur, "drowning" carbonate production of the carbonate factory. Formerly in shallow water, it was now ~1.8 km below the surface.

This created a gently west-sloping ramp to the end of the platform in the Gulf of Mexico, while Florida’s east Bahamas Escarpment curiously remained topographically higher, narrower and drier, and today “supports” the state. The west margin widened and isolated the Florida Platform, and would dictate drainage patterns in the as yet-unformed Everglades in South Florida.﻿

False color image of the canyon-features along the
West Florida Escarpment overlaid on a satellite-derived seafloor map.
Deep blue is 3,400 m (2.1 miles) deep; yellow and orange are 700 m (0.4 miles) deep.
From Schmidt Ocean Institute and Google Earth

Not without controversy and still debated as to its tectonic origin, the Cretaceous Antilles volcanic arc system in the eastern Pacific Ocean is thought to have migrated 2,000 km northeast into the as yet unformed Caribbean Sea through a 3,000 km land gap between North America and South America in the region of present-day Central America.

By the Late Paleocene, a collision between the arc system and the “passive” Florida carbonate platform resulted in the Antillean orogeny. Where the Caribbean and North American plate converged, the orogeny had many effects upon the developing Caribbean Sea, Cuba and the Greater Antilles islands chain. Although the Florida Platform was deformationally-unscathed, the orogeny created the Straits of Florida (separating the Florida and Bahamas Platforms), drowned a portion of the Florida-Bahamas Platform (actually a foreland basin), and contributed to Florida’s further isolation.

One of several proposed tectonic scenarios involving the generation of the Caribbean Sea,
here on a northeast track toward the Florida-Bahamas Platform
Modified from Stanek, 2000

Beginning in the Late Cretaceous, the Georgia Channel Seaway Complex (also called the Gulf Trough, Suwannee Strait and Georgia Rift Valley) separated peninsular Florida from the North American mainland. The trough was one of many failed, Atlantic Ocean rift-basins from the Triassic (the Mid Atlantic Ridge being the “successful” one that formed the ocean), although most of it lies buried. Its northeast-directed current prevented southern Appalachian (mainly from the Piedmont and Blue Ridge) turbid siliciclastic sediments from reaching the platform across the strait, which would otherwise suppress carbonate production.

Observe the relationships of the Florida-Bahamas Platform, the location of the Georgia Channel Seaway,
the Antillean collision margin and Cuba, the Straits of Florida (connecting the Gulf with the Atlantic) and the Bahamas Fracture Zone (a major transform fault along which the Florida-Bahamas Platform moved southeast).
From Albert C. Hine, Cengage Learning, 2009

Carbonate sedimentation continued through the Early Oligocene (~30 Ma) on most of the platform. But following the Oligocene, siliciclastic sediments (whether under the influence of renewed southern Appalachian uplift, a warmer Eocene climate and/or a global sea level lowstand related to South Polar glaciation in response to the closing of the Tethys Ocean) allowed prograding river deltas from the north to obliterate the Georgia Seaway. In the absence of this sediment-barrier, carbonate production on the platform was shut down. The subsequent introduction of siliciclastic sedimentation on the Florida Platform marked a fundamental and permanent change in deposition across the platform.

Florida’s Siliciclastic Transport System
With the obliteration of the Georgia Seaway, sediments were delivered to the south 
by longshore transport, and rivers and streams.
From Albert C. Hine et al, 2007

The platform’s 2-6 km of thick carbonates became buried under a veneer of quartz sand at a depth of a few meters and extended some 40 km onto the continental shelf. It’s what we see when we walk the state’s world-renown beaches. But not all beaches are sandy, quartz-sandy that is. How did it arrive over 1,000 km from its weathering Appalachian source? It appears that the sand's north to south primary transport occurred via longshore currents during sea level highstands in conjunction with secondary sediment movement via fluvial-deltaic transport on land. Marine currents and downslope gravity processes carried the sediments to deeper waters. 

The result is that sand in the northeast of Florida is composed mainly of quartz, while sand to the south possesses more calcium carbonate. In the Keys, the beaches consist almost entirely of skeletal debris (biogenic) from plants and animals. By the way, the obliteration of the Georgia Seaway also provided a land bridge for the introduction of the first terrestrial animals onto the peninsula.

Miami Beach sand exhibits a color indicative of its mixed siliciclastic-carbonate composition.

The Georgia Seaway’s presence likely relates to deeper structural features, since it roughly coincides with the Suwannee suture between Laurentia and Gondwana. What's more, the Suwannee Basin block developed its Ordovician through Devonian sedimentary bedrock when it was still part of the pre-rift margin of Africa.

Modified from William A. Thomas, GSA Presidential Address, 2005

Today, the Suwannee River (spelled “Swanee” in the 1851 American Folk song by Stephen Foster, which is also the official song of the state of Florida) courses through this ancient landscape in a southerly direction from the Okeefenokee Swamp to the Gulf of Mexico. The river topographically slices the panhandle from the rest of the state and whose flow is partially dictated by inherited structural features. It’s an excellent example of how sedimentary cover and major morphological features are controlled by antecedent (pre-existing) topography.

1950's postcard "Way Down Upon the Swanee River"

The Florida Platform possesses an elaborate internal plumbing system within its buried, highly-permeable carbonate rocks called the Floridan Aquifer, an important freshwater resource that sustains the state. Like all carbonate rocks, it is susceptible to dissolution in the presence of acidic water (carbonic acid) absorbed from carbon dioxide in the atmosphere and in the soil.

Acid rain has severe environmental consequences, but its action on carbonate bedrock causes it to dissolve into karstic topography on the surface, and in the subsurface, it forms caves, caverns, and depressions called sinkholes. Enhanced by warmer and wetter climates during interglacial periods, the dissolution of carbonate on and within the Florida Platform likely has resulted in its slow isostatic uplift during the last 1.5 million years, in addition to the liberation of carbon sequestered within its rocks (i.e. the carbon cycle).

From the Southwest Florida Water Management District

The influx of quartz-sand from the southern Appalachians continued into the Pliocene and built a south-sloping, gently-seaward ramp through South Florida. This siliciclastic sediment package provided a shallow-water seafloor for the resumption of carbonate sedimentation in the Pleistocene. The limestone formations that followed created the present topography of South Florida and provided the multi-tiered oceanic floor of the Everglades.

The period of warmth was ending as the Earth approached the ice ages of the last 2-3 million years. Carbonate sedimentation had returned to South Florida, but its deposition fluctuated with the levels of the sea, which in turn was tied to cycles of glaciation and deglaciation. A subject of conflicting opinions, the closure of the Central American Seaway may have affected circulation between the Atlantic and Pacific Oceans by triggering or facilitating climate change with recurring continental ice sheets in the North Hemisphere and alpine glaciers at high altitudes. An alternate theory states that diminished Atlantic seafloor spreading and associated volcanic activity resulted in a reduction in atmospheric carbon dioxide with a cooler icehouse Earth.

Closure of the Central American Seaway and its affect on ocean circulation
From the Woods Hole Oceanographic Institute

Regardless, tectonic conditions may have allowed ever-present Milankovitch climate cycles to be expressed during the Pleistocene in the form of cyclical glacial and interglacial cycles, driven by the Earth's orbit around the sun and variations of the Earth's rotation on its axis. These astronomical events altered the amount of solar radiation that reached the Earth's surface (insolation), changing its climate, generating ice and affecting the level of the sea.  Low-lying regions worldwide (and certainly South Florida) were susceptible to these changes, which affected the evolution of the Everglades! 

Tourists visiting Florida during the Pleistocene would not only have been disappointed with the cooler and drier Floridian climate but the lack of plush beach sands along the coast without siliciclastic invasion from the north. Glacial ice never reached Florida, but global-scale climate change and high seas played a major role in converting siliciclastic to widespread carbonate deposition. Interglacial sea level highstands resulted in the accumulation of shallow water carbonate deposition; whereas, glacial lowstands exposed subaerial surfaces to erosion. At Glacial Maximum during the latest Wisconsinin Stage of the Pleistocene, the lowstand exposed the Florida Platform and its carbonate bedrock with up to three times the current land area.

Glacial Maximum during the Wisconsin Stage of the Pleistocene Period (~20 ka).
North America is covered by the Laurentide continental ice sheet,
while northern Europe and Asia is covered by the Fenno-Sandinavian ice sheet.
Sequestered-ice removed water from the planet's hydrologic budget,
thereby lowering sea level globally. Glacio-eustasy fundamentally affected
cyclical carbonate deposition in south Florida and the Everglades.
Modified from Ron Blakey and Colorado Plateau Geosystems, Inc.

In addition to being much larger during the last ice age, Florida was also much drier. It was dominated by savanna-like conditions that supported a diverse megafauna including mastodons, saber-toothed cats and giant armadillos.

As Milankovitch cycles were expressed physically, the fluctuating sea level history was recorded in multiple stratigraphic units of South Florida as the platform was alternately submerged and exposed. Somewhere between 5 and 10 highstands, the last of which occurred about 125,000 years ago, blanketed South Florida with limestone. Beneath the present-day Everglades resides the latest seafloor stacked upon the previous one in a rhythmic and cyclical sequence. The most recent units include the oolitic Miami Limestone, the reef rock of the Key Largo Limestone, and the coquina shell-rich rock of the Anastasia Formation, all of which formed coevally and represent lateral facies changes in the depositional environment.

Cenozoic Stratigraphy of the State of Florida
Ascending the column: Carbonate platform deposits of the Eocene Avon Park and Ocala Limestones with
North African fossil-affinities; Oligocene Suwannee Strait Limestones; the phosphorus and fossil-rich deposits of the Miocene Hawthorn Group formed from buried marine plant matter; and thick, fossiliferous carbonate strata from Pliocene through the Pleistocene ending with the members of the Okeechobee Group: Miami, Key Largo and Anastasia Formation Limestones formed during the latest sea level highstand. Notably, surficial Holocene deposits within the Everglades include peat, quartz sand and lime muds.
Modified from

In the early Holocene about 7,000 years before the present, the southward-flowing sheet of freshwater wetlands of the Everglades developed on these gently-tilted, limestone platforms confined by elevated lateral margins. As the climate warmed in the interglacial period we are currently experiencing, sheet flow in the developing Everglades increased, and sawgrass that had begun to proliferate began to accumulate thick layers of peat. Along with the Everglades, the environments that we associate with South Florida formed such as sawgrass prairies, cypress communities, pinelands and mangrove jungles.

They are a product of the state’s low elevation near sea level, flat topography, poor surface drainage, high water table, sloping platform, the region’s humid subtropical climate and abundant rainfall (~50-60 inches per year).

The Everglades in the southern tip of the peninsula below Lake Okeechobee
can be clearly seen surrounded by coastal population centers on the east and west coasts
that occur at topographic highlands, all of which funnel waters to the south.
NASA Space Shuttle Photo

Please join me on my next post Part II – Intended Change. Unintended Environmental Consequences.


Geologic History of Florida by Albert C. Hine, 2013.


