8/8/2023 0 Comments Oolite compositionAlso known as "oolitic" sand, the sugar-sized round grains of this sand pass easily through the gills of gobies and other sand-sifting organisms. Oolites are often used in the home aquarium industry because their small grain size (0.2 to 1.22 mm) is ideal for shallow static beds and bottom covering of up to 1" in depth. The size of the oolites reflect the time that they were exposed to the water before they were covered with later sediment. The oolites are commonly found in large current bedding structures that resemble sand dunes. Strong intertidal currents wash the 'seeds' around on the seabed, where they accumulate layers of chemically precipitated calcite from the supersaturated water. The mechanism of formation starts with a small fragment of sediment acting as a 'seed', such as a piece of a shell. They are usually formed in warm, supersaturated, shallow, highly agitated marine water intertidal environments, though some are formed in inland lakes. Oolitic hematite occurs at Red Mountain near Birmingham, Alabama, along with oolitic limestone. Dolomitic and chert ooids are most likely the result of the replacement of the original texture in limestone. Ooids are most commonly composed of calcium carbonate ( calcite or aragonite), but can be composed of phosphate, clays, chert, dolomite or iron minerals, including hematite. The term oolith can refer to oolite or individual ooids. Strictly, oolites consist of ooids of diameter 0.25–2 millimetres rocks composed of ooids larger than 2 mm are called pisolites. Oolite or oölite (from Ancient Greek ᾠόν (ōión) 'egg stone') is a sedimentary rock formed from ooids, spherical grains composed of concentric layers. An attempt is made to reconstruct the paleohydrology of the study area in an attempt to explain the nature and distribution of the cements.Thin-section of calcitic ooids from an oolite within the Carmel Formation (Middle Jurassic) of southern Utah Very light oxygen values in veins are attributed to increased temperatures associated with deep burial. These changes occurred over an area at least the size of the study area. The pattern of changes seen in successive cement zones is explained in terms of changing pore water chemistry and isotopic composition with time. The values typical of a specific cement zone are thought to relate to the pore fluids from which it was precipitated. The successive cement zones do not show a progressive trend in caroon and oxygen values however, in different samples successive cement zones do show the same pattern of changes in carbon and oxygen values. The carbon isotopic composition of the allochems and cements pre-dating the exposure of the top of the Oolite Group is related to their distance below the exposure surface at the top of the Oolite Group and thus seems to reflect meteoric alteration. Carbon and oxygen stable isotope analysis of allochems and cement zones highlights the distinction between Areas 1 and 2 and are used to try and identify the nature of the pore waters affecting the Oolite Group during its diagenesis. The presence of solution surfaces is used to illustrate the constantly changing nature of the pore waters that have affected the Oolite Group. Solution affects that are previously unreported are described. four solution events are identified in Area 2. Using CL it is possible to identify solution surfaces, on a micron to millimetre scale, within the cements. Oolomitisation prevents correlation of cements in Area 1 over a distance of more than 1/2 km. It is possible to correlate the cement zones of Area 2 along the outcrop for a distance of 8 km and also with cements in the overlying Llanelly formation. Area 1 : the Pwll-y-cwm and Blaen Onneu Oolites in the Clydach area, in which Zones 1, 2b and 6 are present, Area 2 : the rest of the Oolite Group, in which Zones 2a, 3, 4, 5 and 6 are present. Two distinct diagenetic areas are recognised on the basis of the distribution of the cement zones. The age of these zones relative to the exposure of the top of the Oolite Group is established. Each zone has a characteristic luminescence. Seven cement zones are identified using stained specimens. CL makes it possible to trace the growth of crystals and monitor changes in crystallographic form during growth. The calcite cement crystals show chemical zonation through staining but cathodoluminescence (CL) has proved the most useful method for displaying internal features of calcite crystals. Cement types, solution events, dolomitisation, concretions and other authigenic minerals are described. The diagenetic history of the Oolite Group from Pwll du to Blaen Onneu has been unravelled.
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