View source for Facies

{{Short description|Body of rock with specified characteristic}}
{{Hatnote group|
{{Other uses|Facies (medical)|Metamorphic facies|Facies (disambiguation)}}
{{Distinguish|Feces|Faeces|Fasces|Face{{!}}Faces|l3=Faces|Fascism}}
}}

[[Image:EolianiteLongIsland.JPG|thumb|[[Eolianite]] carbonate facies (Holocene) on [[Long Island, Bahamas]]]]

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In [[geology]], a '''facies''' ({{IPAc-en|ˈ|f|eɪ|ʃ|ɪ|.|iː|z}} {{respell|FAY|shih|eez}}, {{IPAc-en|USalso|ˈ|f|eɪ|ʃ|iː|z}} {{respell|FAY|sheez}}; same pronunciation and spelling in the plural){{Efn|Other pronunciations include {{IPAc-en|ˈ|f|eɪ|s|iː|z}} {{respell|FAY|seez}} and {{IPAc-en|ˈ|f|æ|ʃ|iː|z}} {{respell|FASH|eez}}, but these are so rare that they are not recorded in any major dictionaries (incl. Collins, Merriam-Webster, Oxford (Lexico), and American Heritage).}} is a body of rock with distinctive characteristics.<ref name="Reading" /> The characteristics can be any observable attribute of rocks (such as their overall appearance, composition, or condition of formation) and the changes that may occur in those attributes over a geographic area. A facies encompasses all the characteristics of a rock including its chemical, physical, and biological features that distinguish it from adjacent rock.<ref>{{cite book|last1=Parker|first1=Sybil P.|title=McGraw-Hill Concise Encyclopedia of Science and Technology|date=1984|publisher=McGraw-Hill|isbn=0-07-045482-5|page=705}}</ref>

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]],<ref name="Cross" /> which replaced the earlier notions of ''[[Neptunism]]''.

== Walther's law ==
[[Image:IsfjordenSuperposition.jpg|thumb|Stratigraphic column on the north shore of [[Isfjord (Svalbard)|Isfjord]] in [[Svalbard]] [[Norway]]. The vertical succession of rock types (representing sedimentary facies) reflects lateral changes in paleoenvironment.]]
Walther's law of facies, or simply Walther's law, 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.<ref name="Stanley" /> In Russia the law is known as Golovkinsky-Walther's law, honoring also [[Nikolai A. Golovkinsky]].<ref>
{{cite journal |last1=Nurgalieva |first1=N. G. |last2=Vinokurov |first2=V. M. |last3=Nurgaliev |first3=D. K. |date=2007 |title=The Golovkinsky strata formation model, basic facies law and sequence stratigraphy concept: Historical sources and relations |journal=[[Russian Journal of Earth Sciences]] |volume=9 |pages=1–7 |doi=10.2205/2007ES000222 |s2cid=55280334 |quote=The fundamentals of the facies law, known in the West as Walther's Law and in Russia as Golovkinsky-Walther's Law, were also described in Golovkinsky's work long before Walther drew his conclusions on this subject. The present paper shows that the fundamentals of sequence stratigraphy were first set forth in the work of N. A. Golovkinsky.|doi-access=free }}
</ref> A classic example of this law is the vertical stratigraphic succession that typifies marine [[Marine transgression|transgressions]] and [[marine regression|regressions]].

== Types ==

===Sedimentary===
[[Image:Triassic Utah.JPG|thumb|Middle [[Triassic]] marginal marine siltstone and sandstone facies exposed in southern [[Utah]]]]

Ideally, a [[Sedimentary structures|sedimentary]] facies is a distinctive [[Stratigraphic unit|rock unit]] that forms under certain conditions of [[sedimentation]], reflecting a particular process or environment. Sedimentary facies are either descriptive or interpretative. Sedimentary facies are bodies of sediment that are recognizably distinct from adjacent sediments that resulted from different depositional environments. Generally, geologists distinguish facies by the [[aspect (geography)|aspect]] of the rock or sediment being studied.{{citation needed|date=June 2016}} Facies based on [[petrology|petrological]] characters (such as grain size and [[mineralogy]]) are called [[Lithology|lithofacies]], whereas facies based on [[Paleontology|fossil]] content are called ''biofacies''.

A facies is usually further subdivided. The characteristics of the [[Rock (geology)|rock]] unit come from the [[Sedimentary depositional environment|depositional environment]] and from the original composition. Sedimentary facies reflect their depositional environment, each facies being a distinct kind of sediment for that area or environment.

Since its inception in 1838, 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.

Sedimentary facies are described in a group of "facies descriptors" which must be distinct, reproducible and exhaustive. A reliable facies description of an outcrop in the field would include: composition, texture, sedimentary structure(s), bedding geometry, nature of bedding contact, fossil content and colour.

===Metamorphic===
{{Main|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''.
==Notes==
{{notelist}}

==References==
{{Wikibooks|Historical Geology|Walther's principle}}
<references>
<ref name="Reading">{{Cite book
  | last = Reading | first = H. G.
  | title = Sedimentary Environments and Facies
  | publisher = Blackwell Scientific Publications
  | year = 1996
  | isbn = 0-632-03627-3 }}</ref>
<ref name="Cross">{{Cite journal
  | last1 = Cross  | first1 = T. A.
  | last2 = Homewood  | first2 = P. W.
  | s2cid = 73609554
 | title = Amanz Gressly's role in founding modern stratigraphy
  | journal = Geological Society of America Bulletin
  | volume = 109 | issue = 12
  | pages = 1617–1630
  | publisher = Geological Society of America
  | year = 1997 | doi=10.1130/0016-7606(1997)109<1617:agsrif>2.3.co;2| bibcode = 1997GSAB..109.1617C
 }}</ref>
<ref name="Stanley">{{Cite book
  | last = Stanley | first = Steven M.
  | title = Earth System History
  | publisher = W.H. Freeman and Company
  | year = 1999
  | location = New York
  | pages = 134
  | isbn = 0-7167-2882-6 }}</ref>
</references>

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[[Category:Petrology]]
[[Category:Sedimentary rocks]]
[[Category:Stratigraphy]]