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Folk Classification
The Folk classification, in geology, is a technical descriptive classification of sedimentary rocks devised by Robert L. Folk, an influential sedimentary petrologist and Professor Emeritus at the University of Texas. Folk's sandstone (clastic) classification Folk's philosophy is that the name of a rock must convey as much information as possible without being a complete description. For this, he proposed five important properties of sandstones to use as defining characteristics. These five properties are: grain size, chemically precipitated cements, textural maturity, miscellaneous transported constituents, and clan designation. Folk's fivefold name must be in the following format: :(Grain size): (chemically precipitated cements) (textural maturity) (miscellaneous transported constituents) (clan designation) However, Folk stated that cements and miscellaneous transported constituents are optional categories as they are not always observed. The other three properties should always ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Geology
Geology () is a branch of natural science concerned with Earth and other astronomical objects, the features or rocks of which it is composed, and the processes by which they change over time. Modern geology significantly overlaps all other Earth sciences, including hydrology, and so is treated as one major aspect of integrated Earth system science and planetary science. Geology describes the structure of the Earth on and beneath its surface, and the processes that have shaped that structure. It also provides tools to determine the relative and absolute ages of rocks found in a given location, and also to describe the histories of those rocks. By combining these tools, geologists are able to chronicle the geological history of the Earth as a whole, and also to demonstrate the age of the Earth. Geology provides the primary evidence for plate tectonics, the evolutionary history of life, and the Earth's past climates. Geologists broadly study the properties and processes of E ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Allochems
Allochem is a term introduced by FolkFolk, R. L. (1959) ''Practical petrographic classification of limestones.'' American Association of Petroleum Geologists Bulletin. 43, pp. 1–38. to describe the recognisable "grains" in carbonate rocks. Any fragment from around 0.5 mm upwards in size may be considered an allochem. Examples would include ooids, peloids, oncolites, pellets, fossil or pre-existing carbonate fragments. Fragments are still termed allochems if they have undergone chemical transformations – for example if an aragonite shell were to dissolve and be later replaced by calcite, the replacement would still be deemed an allochem.Scholle, P. A., and D. S. Ulmer-Scholle (2003) ''A Color Guide to the Petrography of Carbonate Rocks: Grains, textures, porosity, diagenesis''. American Association of Petroleum Geologists Memoir no. 77. Tulsa, Oklahoma, American Association of Petroleum Geologists. 474 pp. The allochems are typically embedded in a matrix of micrite Micrite ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Geopetal
A way up structure'', ''way up criterion, or geopetal indicator is a characteristic relationship observed in a sedimentary or volcanic rock, or sequence of rocks, that makes it possible to determine whether they are the right way up (i.e. in the attitude in which they were originally deposited, also known as "stratigraphic up" or "younging upwards") or have been overturned by subsequent deformation. This technique is particularly important in areas affected by thrusting and where there is a lack of other indications of the relative ages of beds within the sequence, such as in the Precambrian where fossils are rare. The original definition comes from Bruno Sander in 1936, translated from German to English in 1951, which states: ''Geopetal Fabrics'' - All the widely distributed spatial characters of a fabric that enable us to determine what was the relation of "top" to "bottom" at the time when the rock was formed are termed ''geopetal'' fabrics. Such fabrics are mechanical and che ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Thin Section
In optical mineralogy and petrography, a thin section (or petrographic thin section) is a thin slice of a rock or mineral sample, prepared in a laboratory, for use with a polarizing petrographic microscope, electron microscope and electron microprobe. A thin sliver of rock is cut from the sample with a diamond saw and ground optically flat. It is then mounted on a glass slide and then ground smooth using progressively finer abrasive grit until the sample is only 30 μm thick. The method uses the Michel-Lévy interference colour chart to determine thickness, typically using quartz as the thickness gauge because it is one of the most abundant minerals. When placed between two polarizing filters set at right angles to each other, the optical properties of the minerals in the thin section alter the colour and intensity of the light as seen by the viewer. As different minerals have different optical properties, most rock forming minerals can be easily identified. Plagioclase ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Jurassic
The Jurassic ( ) is a geologic period and stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The Jurassic constitutes the middle period of the Mesozoic Era and is named after the Jura Mountains, where limestone strata from the period were first identified. The start of the Jurassic was marked by the major Triassic–Jurassic extinction event, associated with the eruption of the Central Atlantic Magmatic Province. The beginning of the Toarcian Stage started around 183 million years ago and is marked by an extinction event associated with widespread oceanic anoxia, ocean acidification, and elevated temperatures likely caused by the eruption of the Karoo-Ferrar large igneous provinces. The end of the Jurassic, however, has no clear boundary with the Cretaceous and is the only boundary between geological periods to remain formally undefined. By the beginning of the Jurassi ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Carmel Formation
The Carmel Formation is a geologic formation in the San Rafael Group that is spread across the U.S. states of Wyoming, Utah, Colorado, north east Arizona and New Mexico. Part of the Colorado Plateau, this formation was laid down in the Middle Jurassic during the late Bajocian, through the Bathonian and into the early Callovian stages. Description The Carmel Formation consists of up to of mudrock and sandstone interbedded with limestone and gypsum. It was laid down in a shallow marine to sabkha environment, into which terrigenous sediment was periodically carried. This gives the formation considerable lithological complexity. The formation is underlain by the Navajo Sandstone, with the regional J-2 unconformity separating the two formations, or by the Temple Cap Formation. Portions of the Carmel Formation grade laterally eastward into the Page Sandstone. The Carmel Formation in turn is overlain by the Entrada Formation. In the type area of southern Utah, the Carmel Formati ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Calcite
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on the Mohs scale of mineral hardness, based on scratch hardness comparison. Large calcite crystals are used in optical equipment, and limestone composed mostly of calcite has numerous uses. Other polymorphs of calcium carbonate are the minerals aragonite and vaterite. Aragonite will change to calcite over timescales of days or less at temperatures exceeding 300 °C, and vaterite is even less stable. Etymology Calcite is derived from the German ''Calcit'', a term from the 19th century that came from the Latin word for lime, ''calx'' (genitive calcis) with the suffix "-ite" used to name minerals. It is thus etymologically related to chalk. When applied by archaeologists and stone trade professionals, the term alabaster is used not just as in geology and mineralogy, where it is res ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Intraclasts
Intraclasts are irregularly shaped grains that form by syndepositional erosion of partially lithified sediment. Gravel grade material is generally composed of whole disarticulated or broken skeletal fragments together with sand grade material of whole, disaggregated and broken skeletal debris. Such sediments can contain fragments of early cemented limestones of local origin which are known as intraclasts. The sediments that contain fragments of early cemented limestones of extra-basinal origin are called extraclasts. Examples of intraclasts include mudlumps that are torn up from the bottoms of lagoons during storms, hardened desiccated mudflakes produced in intertidal and supratidal environments and fragments broken from cemented deep-sea crusts. Other intraclasts are aggregate Aggregate or aggregates may refer to: Computing and mathematics * collection of objects that are bound together by a root entity, otherwise known as an aggregate root. The aggregate root guarantees the ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Diagenesis
Diagenesis () is the process that describes physical and chemical changes in sediments first caused by water-rock interactions, microbial activity, and compaction after their deposition. Increased pressure and temperature only start to play a role as sediments become buried much deeper in the Earth's crust. In the early stages, the transformation of poorly consolidated sediments into sedimentary rock (lithification) is simply accompanied by a reduction in porosity and water expulsion (clay sediments), while their main mineralogical assemblages remain unaltered. As the rock is carried deeper by further deposition above, its organic content is progressively transformed into kerogens and bitumens. The process of diagenesis excludes surface alteration (weathering) and deep metamorphism. There is no sharp boundary between diagenesis and metamorphism, but the latter occurs at higher temperatures and pressures. Hydrothermal solutions, meteoric groundwater, rock porosity, permeabili ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Peloids
Peloids are allochems that are composed of micrite, irrespective of size, shape, or origin. The two primary types of peloids are pellets and intraclasts Intraclasts are irregularly shaped grains that form by syndepositional erosion of partially lithified sediment. Gravel grade material is generally composed of whole disarticulated or broken skeletal fragments together with sand grade material of wh .... Another type of peloid is pseudo-oolith.Folk, R.L. (1959) ''Practical petrographic classification of limestones.'' American Association of Petroleum Geologists Bulletin. 43, pp. 1-38.Neuendorf, K.K.E., J.P. Mehl, Jr., and J.A. Jackson, J.A., eds. (2005) ''Glossary of Geology'' (5th ed.). Alexandria, Virginia, American Geological Institute. 779 pp. Scholle, P.A., and D.S. Ulmer-Scholle (2003) ''A Color Guide to the Petrography of Carbonate Rocks: Grains, textures, porosity, diagenesis. '' American Association of Petroleum Geologists Memoir no. 77. Tulsa, Oklahoma, American Associati ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Ossicle (echinoderm)
Ossicles are small calcareous elements embedded in the dermis of the body wall of echinoderms. They form part of the endoskeleton and provide rigidity and protection. They are found in different forms and arrangements in sea urchins, starfish, brittle stars, sea cucumbers, and crinoids. The ossicles and spines (which are specialised sharp ossicles) are the only parts of the animal likely to be fossilized after an echinoderm dies. Formation Ossicles are created intracellularly by specialised secretory cells known as sclerocytes in the dermis of the body wall of echinoderms. Each ossicle is composed of microcrystals of calcite arranged in a three-dimensional lattice known as a stereom. Under polarized light the ossicle behaves as if it were a single crystal because the axes of all the crystals are parallel. The space between the crystals is known as the stroma and allows entry to sclerocytes for enlargement and repair. The honeycomb structure is light but tough and collagenous li ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Ooids
Ooids are small (commonly ≤2 mm in diameter), spheroidal, "coated" (layered) sedimentary grains, usually composed of calcium carbonate, but sometimes made up of iron- or phosphate-based minerals. Ooids usually form on the sea floor, most commonly in shallow tropical seas (around the Bahamas, for example, or in the Persian Gulf). After being buried under additional sediment, these ooid grains can be cemented together to form a sedimentary rock called an '' oolite''. Oolites usually consist of calcium carbonate; these belong to the limestone rock family. Pisoids are similar to ooids, but are larger than 2 mm in diameter, often considerably larger, as with the pisoids in the hot springs at Carlsbad ( Karlovy Vary) in the Czech Republic. Formation An ooid forms as a series of concentric layers around a nucleus. The layers contain crystals arranged radially, tangentially or randomly. The nucleus can be a shell fragment, quartz grain or any other small fragment. Most m ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
