Any observable attribute of rocks, such as overall appearance, composition, or conditions of formation, and changes that may occur in these attributes over a geographic area. The term facies is widely used in connection with sedimentary rock bodies, but is not restricted to them. In general, facies are not defined for sedimentary rocks by features produced during weathering, metamorphism, or structural disturbance. In metamorphic rocks specifically, however, facies may be identified by the presence of minerals that denote degrees of metamorphic change. Sedimentary facies
The term sedimentary facies is applied to bodies of sedimentary rock on the basis of descriptive or interpretive characteristics. Descriptive facies are based on lithologic features such as composition, grain size, bedding characteristics, and sedimentary structures (lithofacies) or on biological (fossil) components (biofacies), or on both. Individual lithofacies or biofacies may be single beds a few millimeters thick or a succession of beds tens to hundreds of meters thick. For example, a river deposit may consist of decimeters-thick beds of a conglomerate lithofacies interbedded with a cross-bedded sandstone lithofacies. The fill of certain major Paleozoic basins may be divided into units hundreds of meters thick comprising a shelly facies, containing such fossils as brachiopods and trilobites, and graptolitic facies.
The term facies can be used also in an interpretive sense for groups of rocks that are thought to have been formed under similar conditions. This usage may emphasize specific depositional processes, such as a turbidite facies, or a particular depositional environment such as a shelf carbonate facies, encompassing a range of depositional processes.
Groups of facies (usually lithofacies) that are commonly found together in the sedimentary record are known as facies assemblages or facies associations. These groupings provide the basis for defining broader, interpretive facies for the purpose of paleogeographic reconstruction.
A metamorphic facies is a collection of rocks containing characteristic mineral assemblages developed in response to burial and heating to similar depths and temperatures. It can represent either the diagnostic mineral assemblages that indicate the physical conditions of metamorphism or the pressure-temperature conditions that produce a particular assemblage in a rock of a specific composition.
The metamorphic facies to which a rock belongs can be identified from the mineral assemblage present in the rock; the pressure and temperature conditions represented by each facies are broadly known from experimental laboratory work on mineral stabilities. These facies names are based on the mineral assemblages that develop during metamorphism of a rock with the composition of a basalt, which is a volcanic rock rich in iron and magnesium and with relatively little silica. For example, the dominant mineral of the blueschist facies (in a rock of basaltic composition) is a sodium- and magnesium-bearing silicate called glaucophane, which is dark blue in outcrop and blue or violet when viewed under the microscope. Characteristic minerals of the greenschist facies include chlorite and actinolite, both of which are green in outcrop and under the microscope. Basaltic rocks metamorphosed in the amphibolite facies are largely composed of an amphibole called hornblende. The granulite facies takes its name from a texture rather than a specific mineral: the pyroxenes and plagioclase that are common minerals in granulite facies rocks typically form rounded crystals of similar size that give the rock a granular fabric.
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