The Geology of Big Cypress National Preserve: Limestone, Water and Karst

Big Cypress National Preserve, in the part of South Florida that includes the Everglades, is one of the most unusual landscapes on Earth. Its habitats range from estuaries, saline and freshwater marshes, slow-flowing sloughs and cypress swamps to prairies, mangroves, hardwood hammocks and pinelands — and geology underlies all of them. It controls how surface water flows, and so helps shape the region's climate, hydrology and topography; it also holds the great aquifers and the oil and gas beneath the preserve.

The preserve's Geologic Resources Inventory report. Credit: National Park Service.

A stable platform, a restless surface

Big Cypress is one of the lowest, youngest and most geologically stable platforms in North America. But that stability hides active processes. Low-lying South Florida has been repeatedly flooded and exposed as sea level rose and fell over the past 50,000 years. Beneath it lie thousands of feet of nearly horizontal layers rich in carbonate minerals, which dissolve in rainwater and groundwater made acidic by decaying plants. The result is karst: solution holes, sinkholes, disappearing streams and springs — a landscape that can store enormous amounts of water.

The deep record

Big Cypress sits in the northeastern part of the South Florida basin, about 80,000 square miles (200,000 km²), thickest beneath Florida Bay. Carbonate, evaporite and clastic sediments have accumulated there more or less continuously since the Late Jurassic, as the basin slowly sank. It is ringed by higher structures: the Florida Escarpment — a great barrier reef complex separating the Florida Shelf from the deep Gulf of Mexico — to the west, the Pine Key Arch to the southeast, the Tampa-Sarasota Arch to the northwest, and to the east the Peninsular Arch, a Paleozoic crystalline ridge running down the peninsula that controlled where Jurassic and Cretaceous sediments were laid down.

Oil-exploration cores show 15,000–17,000 feet (4,572–5,182 m) of sedimentary rock in the preserve's oil fields, resting on Jurassic volcanic rhyolite and basalt — some of Florida's oldest rocks.

Water in a flat land

South Florida's wetlands are defined by a gentle slope and a huge capacity to store water. Before people altered it, water moved very slowly through the system, buffering it against droughts and floods. At Big Cypress, water doesn't run in defined streams and rivers. It moves around and through:

• Sloughs — meandering marshy channels where the ground surface meets the shallow water table; a critical freshwater source for wildlife in dry seasons and a store of nutrients.
• Strands — long bands of cypress along depressions where water is high and flowing. Cypress covers about a third of the preserve, mostly dwarf pond cypress, with a few stands of giant bald cypress 600–700 years old.
• Cypress domes — small swamps with tall trees in the middle and shorter ones toward the edges, usually where near-surface limestone has dissolved and collapsed, collecting nutrients and letting roots reach deeper.
• Hardwood hammocks — dense, closed-canopy forests of evergreen and semi-deciduous trees on ground generally less than 3 feet (1 m) higher than the surrounding marsh, home to some of South Florida's rarest plants and animals.

Karst, caves and aquifers

The USGS classifies the entire preserve as karst. Florida's carbonate rocks have been weathered and dissolved since the last major interglacial flooded the platform some 130,000 years ago. Deep Lake, a water-filled sinkhole about 90 feet deep, probably formed then and is probably a flooded cave; more caves may exist in the preserve. All cave resources in national parks are protected under the Federal Cave Resources Protection Act of 1988.

Three aquifers lie beneath Big Cypress, which is a recharge area for them:

1. Biscayne Aquifer, at the surface, in sandstone and the Miami Limestone, which developed large solution cavities when sea level was lower — with very large long-term storage.
2. Gray Limestone Aquifer, in the Pliocene lower Tamiami Formation, a lightly cemented, fossil-rich sandy limestone.
3. Sandstone Aquifer, in the Lehigh Acres Sandstone.

Understanding how surface water and groundwater exchange is key to tracking how water, nutrients, wastes and metals move through the ecosystem.

A changed system

Since the late 1800s, a growing South Florida has built canals, ditches, dams and levees to drain swamps for farming, oil and gas, and cities — permanently altering the slow flow of fresh water from Lake Okeechobee south to Florida Bay along the Everglades' "River of Grass." The ecosystem is changing fast and is under threat, and a large multiagency effort is under way to restore water flow and save what remains. Understanding the geology is part of that work.

A canal at Big Cypress. Image from the National Park Service's Geodiversity Atlas page for Big Cypress National Preserve.

Fossils

Fossil-bearing units at the surface are the Upper Pliocene Tamiami Formation, Pliocene–Pleistocene shell-bearing sediments, the upper Pleistocene Miami Limestone, and Holocene deposits. The preserve's Geologic Resources Inventory report has more.

The preserve's geologic map. Credit: National Park Service.

Sources

Based on "NPS Geodiversity Atlas—Big Cypress National Preserve, Florida," National Park Service; a work of the United States government in the public domain.