facies

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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. See also Paleogeography.

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. See also Amphibolite; Basalt; Blueschist; Glaucophane; Granulite; Metamorphic rocks; Metamorphism; Pyroxene.

The characteristics of a rock, such as fossil content, or chemical composition, which distinguish it from other formations and give some indication of the way it was formed.

Facial expression.

The features, general appearance, and expression of a face.

• Materials, Formations, and Substances - facies: sedimentological record of depositional environment

For other uses, see facies (disambiguation).

Eolianite carbonate facies (Holocene) on Long Island, Bahamas.

In geology, a facies (pronounced variously as 'faysheez', 'fayseez' or 'fash-eeze'; plural also 'facies') is a body of rock with specified characteristics.^[1] Ideally, a facies is a distinctive rock unit that forms under certain conditions of sedimentation, reflecting a particular process or environment.

The term facies was introduced by the Swiss geologist Amanz Gressly in 1838 and was part of his significant contribution to the foundations of modern stratigraphy,^[2] which replaced the earlier notions of Neptunism.

Contents 1 Facies types 1.1 Sedimentary facies 1.2 Metamorphic facies 2 Walther's Law of Facies 3 See also 4 References

Facies types

Sedimentary facies

Middle Triassic marginal marine siltstone and sandstone facies exposed in southern Utah.

Sedimentary facies are bodies of sediment recognizably different from adjacent sediment deposited in a different depositional environment. Generally, facies are distinguished by what aspect of the rock or sediment is being studied. Thus, facies based on petrological characters such as grain size and mineralogy are called lithofacies, whereas facies based on fossil content are called biofacies.

These facies types are usually further subdivided, for example, you might refer to a "tan, cross-bedded oolitic limestone facies" or a "shale facies". The characteristics of the rock unit come from the depositional environment and original composition. Sedimentary facies reflect depositional environment, each facies being a distinct kind of sediment for that area or environment.

Since its inception, the facies concept has been extended to related geological concepts. For example, characteristic associations of organic microfossils, and particulate organic material, in rocks or sediments, are called palynofacies. Discrete seismic units are similarly referred to as seismic facies.

Metamorphic facies

Main article: metamorphic facies

The sequence of minerals that develop during progressive metamorphism (that is, metamorphism at progressively higher temperatures and/or pressures) define a facies series.

Walther's Law of Facies

Stratigraphic column on north shore of Isfjord, Svalbard, Norway. The vertical succession of rock types (representing sedimentary facies) reflects lateral changes in paleoenvironment.

Walther's Law of Facies, named after the geologist Johannes Walther, states that the vertical succession of facies reflects lateral changes in environment. Conversely, it states that when a depositional environment "migrates" laterally, sediments of one depositional environment come to lie on top of another.^[3] A classic example of this law is the vertical stratigraphic succession that typifies marine trangressions and regressions. However, the law is not applicable where the contact between different lithologies is non-conformable (i.e. sedimentation was not continuous), or in instances of rapid environmental change where non-adjacent environments may replace one another.

See also

• Lithofacies
• Metamorphic Rock
• Palynofacies
• Sequence stratigraphy
• Stratum

References

1. ^ ^{a b} Reading, H. G. (1996). Sedimentary Environments and Facies. Blackwell Scientific Publications. ISBN 0-632-03627-3. 
2. ^ ^{a b} Cross, T. A.; Homewood, P. W. (1997). "Amanz Gressly's role in founding modern stratigraphy". Geological Society of America Bulletin (Geological Society of America) 109 (12): 1617–1630. 
3. ^ ^{a b} Stanley, Steven M. (1999). Earth System History. New York: W.H. Freeman and Company. pp. 134. ISBN 0-7167-2882-6. 

v t e Geologic Principles & Processses Stratigraphic Principles Principle of original horizontality Law of superposition Principle of lateral continuity Principle of cross-cutting relationships Principle of faunal succession Principle of inclusions and components Walther's law Petrologic Principles Intrusive Extrusive Exfoliation Weathering Pedogenesis Diagenesis Compaction Metamorphism Geomorphologic Processes Plate tectonics Salt tectonics Tectonic uplift Subsidence Marine transgression Marine regression Sediment transport Fluvial processes Aeolian processes Glacial processes Mass wasting processes

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