oolite

Ooids on the surface of a limestone; Carmel Formation (Middle Jurassic) of southern Utah. The scale bar is 2.0 mm.

Thin-section of calcitic ooids from an oolite within the Carmel Formation (Middle Jurassic) of southern Utah. The largest ooid is 1.2 mm in diameter.

Oolite (egg stone) is a sedimentary rock formed from ooids, spherical grains composed of concentric layers. The name derives from the Hellenic word òoion for egg. Strictly, oolites consist of ooids of diameter 0.25–2 mm: rocks composed of ooids larger than 2 mm are called pisolites. The term oolith can refer to oolite or individual ooids.

Composition

Ooids are most commonly composed of calcium carbonate (calcite or aragonite), but can be composed of phosphate, chert, dolomite or iron minerals, including hematite. Dolomitic and chert ooids are most likely the result of the replacement of the original texture in limestone. Oolitic hematite occurs at Red Mountain near Birmingham, Alabama along with oolitic limestone.

Occurrence

Some exemplar oolitic limestone, a common term for an oolite, was formed in England during the Jurassic period, and forms the Cotswold Hills, the Isle of Portland with its famous Portland Stone^[1], and part of the North Yorkshire Moors. A particular type, Bath Stone, gives the buildings of the World Heritage City of Bath their distinctive appearance.

The islands of the Lower Keys in the Florida Keys, as well as some barrier islands east of Miami bordering Biscayne Bay, are mainly oolitic limestone, which was formed by deposition when shallow seas covered the area between periods of glaciation. The material consolidated and eroded during later exposure above the ocean surface.

This type of limestone is also found in Indiana in the United States. The town of Oolitic, Indiana was founded for the trade of limestone and bears its name. Quarries in Bedford, Oolitic, and Bloomington contributed the materials for such iconic US landmarks as the Empire State Building and the Pentagon. The Soldiers' and Sailors' Monument in downtown Indianapolis is built almost solely of grey oolitic limestone.

The movie Breaking Away centers around the sons of quarry workers in Bloomington, the home of Indiana University. Almost all of the buildings on the Indiana University campus are built with native oolitic limestone material.

Roggenstein is a term describing a specific type of oolite one in which the cementing matter is argillaceous.

See also

• Geologic timescale
• Geology of Great Britain
• Pearls: also formed from concentric layers of calcium carbonate
• Oolitic aragonite sand

References

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