Facies
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Facies (medical)
:''The term "[[Facies (medical)|Facies]]" can also refer to distinctive [[facial expressions]] associated with conditions such as [[Williams syndrome]].''
[[Image:Diagonallylayeringkornberg.PNG||thumb|250px|right|Cornberg sandstone facies]]
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 [[petrology|petrological]] characters such as grain size and mineralogy are called [[lithofacies]], whereas facies based on [[Paleontology|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 (geology)|rock]] unit come from the [[Sedimentary depositional environment|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 [[seismology|seismic]] units are similarly referred to as seismic facies.
===Metamorphic facies===
{{Main|metamorphic facies}}
<!-- Unsourced image removed: [[Image:Metamorphic Facies.JPG|thumb|right|220px|Pressure-Temperature diagram of 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 [[Transgression (geology)|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 ==
* 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
[[Category:Metamorphic rocks]]
[[Category:Sedimentary rocks]]
[[Category:Stratigraphy]]
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