Edward J. Petuch - Ancient Seas of Southern Florida: The Geology and Paleontology of the Everglades Region

The principal physiographic feature of southern Florida, the Everglades, extends from north of Lake Okeechobee southward to the Gulf of Mexico. Within its central region, in Palm Beach, Hendry, Broward, Dade, and Collier Counties, the Everglades contains a subtropical wetland with the largest sawgrass prairies found anywhere in the United States. In reference to this seemingly-endless vista, the native Seminole people called this region the Pa-Hay-Okee, or the River of Grass. Myriads of small uninhabited tree islands are also scattered across the Everglades wet prairies, with each harboring a distinctive ecosystem that is composed of various combinations of cypresses, tropical hardwoods, and Sabal Palmettos. Everglades-type environments, including extensive cypress forests, also range into the surrounding counties of southern Florida, from Sarasota and Okeechobee Counties in the north to Lee and Monroe Counties in the south, making them the dominant ecological features across the southern part of the state. As described by Marjory Stoneman Douglas in her book (1947), this special North American natural treasure has always fascinated, and most often repelled, non-native visitors and settlers. Even today, the word Everglades conjures up visions of steaming swamps filled with voracious alligators, swarms of disease-bearing mosquitoes, and venomous snakes. For thousands of years, only the Native Americans found refuge in these marshlands, adapting to the harsh conditions and creating their own unique lifestyles.

The Everglades and southern Florida that we see today are actually the latest iteration of a large sequence of marine environments that stretches back for tens of millions of years. At first glance, no one would consider the modern Everglades to be part of the marine realm; but shallow excavations across the region have exposed ancient marine worlds that are hidden just below the surface. These prehistoric sea floors, one stacked upon the other for thousands of meters, are geographically important, as they have created the topography that governs the water flow and sedimentary deposition within the modern Everglades. The spectacular subtropical world that was so poetically described by Marjory Stoneman Douglas is now known to be only a thin film of terrestrial sediments that covers the record of Florida's ancient seas. Any geologic or stratigraphic study of the Everglades region, consequently, must be approached from the oceanographic perspective. This oceanic connection becomes strikingly apparent when studying exposures of the amazingly rich Neogene marine fossil beds that are buried just beneath the Everglades region. In this book, we illustrate almost 800 of the most important and diagnostic stratigraphic index fossils found in these beds, including over 50 species of corals and almost 700 species of mollusks, along with echinoderms, crustaceans, echinoids, petrified wood, and aquatic vertebrates. The following book is, in essence, a second edition of The Geology of the Everglades and Adjacent Areas(Petuch & Roberts, 2007) and includes new, updated geological data and concepts, and an expanded iconography of stratigraphic index fossils. Based on the data gleaned from these fossils, we also offer a series of geomorphological visualizations, showing the possible appearances of the Florida Peninsula during the times when it was covered by tropical seas, from the Oligocene to the late Pleistocene.

Looking across the Everglades and the southern Florida of today, it is intriguing to imagine what the area may have looked like when it was completely covered by shallow marine environments. This ancient marine world, referred to here as the Okeechobean Sea(named for present-day Lake Okeechobee; see Petuch, 2004), was essentially land-locked for much of its later history, being separated from the Gulf of Mexico and Atlantic Ocean by a narrow semicircular chain of shallow oyster banks, coral reef tracts, and small coral islands. Imagine a cross between present-day Florida Bay, with its mangrove jungles and sea grass beds, and the Bahamas, with their coral cays and coral reef complexes; this would have typified the appearance of the Okeechobean Sea over the past 12 million years. As sea levels rose and fell over the millenia, the Okeechobean Sea area was completely under the influence of this large-scale rhythmic pattern, which is recorded in the geology as alternating episodes of terrestrial and marine sedimentation. For cyclostratigraphers and climatologists, these well-preserved marine sediments and fossil beds contain one of the most detailed records of Neogene global climate change and sea level fluctuations found anywhere on Earth.

Considering the serene beauty of the present-day Everglades area, it is hard to imagine that the predecessor Okeechobean Sea had been formed in the unimaginable chaos and destruction that resulted from an asteroid impact (astrobleme). At the end of the Eocene Epoch, around 35 million years ago, the southern end of what is now Chesapeake Bay in Virginia was struck by an asteroid with a diameter of approximately 13 km (8 miles), producing immense incandescent debris clouds and multiple mega-tsunami waves of over 500 m (1640 feet). These monster waves, which were generated over and over again for weeks on end due to the collapsing crater, roared down the coast and essentially obliterated every coastal geologic feature from Maryland south to the Florida Peninsula; the evidence for this massive destruction lies buried deep below the modern Everglades (discussed later in this book). The catastrophism that resulted from this impact laid the groundwork for the structural features that have shaped the various basins of the Okeechobean Sea over its history and, also in a muted form, the surface of the modern Everglades.