facies

zeolitic facies

One of the major divisions of the mineral facies classification of metamorphic rocks, encompassing rocks that formed at the lowest temperatures and pressures associated with regional metamorphism. Typical minerals in these facies include the zeolites, albite, and quartz.

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Different, but contemporaneous and juxtaposed, sedimentary rocks. Terrigenous facies are accumulations of particles eroded from older rocks and transported to the depositional site. Biogenic facies are accumulations of whole or fragmented shells and other hard parts of animals. Chemical facies result from precipitation of inorganic material from solution. The shapes and characteristics of facies may change as conditions change over time.

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One of the major divisions of the mineral facies classification of metamorphic rocks, encompassing the rocks that formed under fairly low temperatures (480–660 °F, or 250–350 °C) and pressure conditions and usually produced by regional metamorphism. The minerals commonly found in such rocks include quartz, orthoclase, muscovite, chlorite, serpentine, talc, and epidote; carbonate minerals and amphibole (actinolite) may also be present.

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One of the major divisions of the mineral facies classification of metamorphic rocks, encompassing rocks that formed under intense temperature-pressure conditions (higher than 950°F, or 500°C). The minerals found in the rocks of granulite facies include hornblende, pyroxene, biotite, garnet, calcium plagioclase, and quartz or olivine. Seealso amphibolite facies.

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One of the major divisions of the mineral facies classification of metamorphic rocks, encompassing rocks whose peculiar mineralogy suggests that they formed under conditions of high pressure and relatively low temperature (generally less than about 662°F, or 350°C); such conditions are not typical of the normal geothermal gradient in the earth. The minerals that chiefly occur include soda amphibole (glaucophane), soda pyroxene (jadeite), garnet, lawsonite, and pumpellyite. Quartz, muscovite, chlorite, epidote, and plagioclase may also be present. Classic deposits occur in western California.

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One of the major divisions of the mineral facies classification of metamorphic rocks, encompassing rocks that formed under conditions of moderate to high temperatures (950°F, or 500°C, maximum) and pressures. Less-intense temperatures and pressures form rocks of the epidote-amphibolite facies, and more-intense temperatures and pressures form rocks of the granulite facies. Amphibole, diopside, epidote, plagioclase, certain types of garnet, and wollastonite are minerals typically found in amphibolite facies rocks. They are widely distributed in Precambrian gneisses and probably formed in the deeper parts of folded mountain belts.

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The term "Facies" can also refer to distinctive facial expressions associated with conditions such as Williams syndrome.

In geology, facies are a body of rock with specified characteristics. [Reading (1996)] 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, [Cross and Homewood (1997)] which replaced the earlier notions of Neptunism.

Facies types

Sedimentary facies

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 examples, 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

The sequence of minerals that develop during progressive metamorphism (that is, metamorphism at progressivley higher temperatures) define a facies series and depend on the pressure, or range of pressures, at which the progressive metamorphism occurred.

Walther's Law of Facies

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.(Stanley, 134) 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

References

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

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