These waters are approximately two meters in depth, although ooids can form in waters of up to 10 to 15 meters in depth (Figure 2) (6). Marine ooids are formed in shallow marine environments, such as the tropics, in waters that are supersaturated with calcium carbonate. Marine ooids, as opposed to lacustrine ooids, tend to have higher surface polish and tangentially arranged crystals Aragonite rods in marine ooids have an average length of 1 micron and a maximum length of 3 microns (5). These ooids usually consist of aragonite rods without crystallographic terminations, oriented tangentially (parallel to the ooid lamination). Marine ooids are generally found in shallow, warm marine environments, often in locations at low latitudes. Lacustrine ooids are also primarily composed of aragonite, and are often dull, have a radial fabric, and may have a bumpy surface, known as a “cerebroid” surface (1). A prime example of lacustrine ooid formation is the Great Salt Lake in Utah. In addition to variations in structure, ooids can also be classified as lacustrine or marine in origin. Some geologists recognize a third type of ooid structure – the “recrystallized structure.” In this type of ooid, large irregular crystals either converge toward the center or have no special orientation (5). The arrangement of the calcium carbonate within the ooid is generally dependent upon the process by which it was formed: physical processes produce ooids with concentric lamellae, and chemical processes produce ooids with radiating crystals (4). Figure 1 illustrates the difference between radial and tangential classifications. The crystals can be arranged radially, tangentially, or randomly. Ooids are spherical or ellipsoid concretions, usually less than 2 mm in diameter, of calcium carbonate and aragonite crystals arranged around a nucleus – typically a small particle such as a quartz grain or shell fragment (2, 3). This allows for a progressive sorting of ooid size along Long Bay Beach, with larger, coarser particles observed near the northeastern end of the beach, and smaller, finer ooid particles observed at the southwestern end. The ooids studied in this project are produced along the southeastern coast of Providenciales and transported westward by longshore currents along the coast. Observations were also made regarding ooid transport direction, visible as highly reflective sub-aqueous sand bodies in multispectral scanner satellite and high resolution aerial photos. The samples taken from the most recent beach ridges close to the modern shore show a general trend of decreased porosity, increased weathering, and higher coral content, with minor variations in ooid size, when compared to samples from older beach ridges further inland, about 1.2 km away.
#Oolite depositional environment series#
In a field research trip to Providenciales, Turks and Caicos, samples of oolitic limestone were collected at a series of successive Pleistocene and Holocene beach ridges, and brought back for analysis. By understanding modern ooid-producing environments, we can begin to interpret and understand ancient carbonate facies and sediments that include such particles. Ooids are defined as spherical, concentric accretions of calcium carbonate, usually less than 2 mm in diameter, developed around a nucleus of some previously existing particle. Of particular interest are ooids – non-skeletal grains that precipitate predominantly in the warm, shallow waters of the tropics (1).
The minerologic and fabric character of these limestones generally reflect the complex biological, physical, and climactic character of the depositional systems under which they were created. Limestone is common in rocks from all geologic periods of the Phanerozoic era as well as in many Proterozoic assemblages (1). Share on Twitter Share on Facebook Share on Email
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