The Miami Limestone, originally called Miami Oolite, is a geologic formation of limestone in southeastern

Florida Florida ( ; ) is a U.S. state, state in the Southeastern United States, Southeastern region of the United States. It borders the Gulf of Mexico to the west, Alabama to the northwest, Georgia (U.S. state), Georgia to the north, the Atlantic ...

. Miami Limestone forms the Atlantic Coastal Ridge in southeastern Florida, near the coast in Palm Beach, Broward and Miami Dade counties. It also lies under the eastern (Miami-Dade County) part of the Everglades, Florida Bay, and the lower

Florida Keys The Florida Keys are a coral island, coral cay archipelago off the southern coast of Florida, forming the southernmost part of the continental United States. They begin at the southeastern coast of the Florida peninsula, about south of Miami a ...

from Big Pine Key to the Marquesas Keys. Mitchell-Tapping also states that a component of the Miami Limestone extends under the

Gulf of Mexico The Gulf of Mexico () is an oceanic basin and a marginal sea of the Atlantic Ocean, mostly surrounded by the North American continent. It is bounded on the northeast, north, and northwest by the Gulf Coast of the United States; on the southw ...

north to a point 112 kilometers west of

Tampa Tampa ( ) is a city on the Gulf Coast of the United States, Gulf Coast of the U.S. state of Florida. Tampa's borders include the north shore of Tampa Bay and the east shore of Old Tampa Bay. Tampa is the largest city in the Tampa Bay area and t ...

. The part of the Miami Limestone forming the Atlantic Coastal Ridge and the lower Florida Keys is an oolitic grainstone which includes fossils of corals, echinoids,

mollusk Mollusca is a phylum of protostomic invertebrate animals, whose members are known as molluscs or mollusks (). Around 76,000 extant species of molluscs are recognized, making it the second-largest animal phylum after Arthropoda. The ...

s, and algae. The oolitic formation in the lower Florida Keys has less quartz sand and fewer fossils than does the oolitic formation on the mainland. Based on those differences, Mitchel-Tapping divided the Miami Limestone into the Fort Dallas Oolite on the mainland and under most of Florida Bay, and the Key West Oolite, under the southernmost corner of Florida Bay within the Everglades park boundaries, along the west side of the middle Florida Keys, and in the lower Keys, including the Marquesas Keys to near the Dry Tortugas. The Fort Dallas Oolite, white to yellow in color, is soft but hardens on exposure to air or water.

Quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tet ...

sand grains occur throughout the Fort Dallas Oolite. The ooids have generally formed on a nucleus of

calcite Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...

crystals, and occasionally on shell fragments and quartz grains, and are covered with up to five layers of calcite. Fort Dallas ooids have a defined layer of calcite around the nucleus. The Key West Oolite is mostly white, includes very little quartz sand, does not harden on exposure to air or water, and its ooids do not have a calcite mosaic around the nucleus. Uranium-thorium dating indicates that the ooids in the Fort Dallas and Key West units formed at the same time. The fossils in the formation underlying the Everglades, which does not include any ooids, consists primarily of a single bryozoan species, '' Schizoporella floridana''. The Miami Limestone was deposited during the Sangamon interglacial, when southern Florida was under a shallow sea. Falling sea levels during the

Wisconsin glaciation The Wisconsin glaciation, also called the Wisconsin glacial episode, was the most recent glacial period of the North American ice sheet complex, peaking more than 20,000 years ago. This advance included the Cordilleran Ice Sheet, which nucleated ...

exposed the formation to air and rain, and rainwater percolating through the deposits replaced

aragonite Aragonite is a carbonate mineral and one of the three most common naturally occurring crystal forms of calcium carbonate (), the others being calcite and vaterite. It is formed by biological and physical processes, including precipitation fr ...

with

and formed an indurated rock. The Miami Limestone is underlain by the Anastasia Formation in eastern Broward and extreme southeastern Palm Beach counties, by the Fort Thompson Formation in southeastern Palm Beach County, central Broward County, all of Miami-Dade County, and Florida Bay, by the Tamiami Formation in Collier and mainland Monroe counties, and by the Key Largo Limestone in the lower Florida Keys.

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