In
geology
Geology () is a branch of natural science concerned with Earth and other Astronomical object, astronomical objects, the features or rock (geology), rocks of which it is composed, and the processes by which they change over time. Modern geology ...
, a facies ( , ; same pronunciation and spelling in the plural) is a body of rock with specified characteristics,
which 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 of the characteristics of a rock including its chemical, physical, and biological features that distinguish it from adjacent rock.
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
Stratigraphy is a branch of geology concerned with the study of rock layers ( strata) and layering (stratification). It is primarily used in the study of sedimentary and layered volcanic rocks.
Stratigraphy has three related subfields: lithost ...
,
which replaced the earlier notions of
Neptunism
Neptunism is a superseded scientific theory of geology proposed by Abraham Gottlob Werner (1749–1817) in the late 18th century, proposing that rocks formed from the crystallisation of minerals in the early Earth's oceans.
The theory took i ...
.
Types of facies
Sedimentary facies
Ideally, a
sedimentary
Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...
facies is a distinctive
rock unit
A stratigraphic unit is a volume of rock of identifiable origin and relative age range that is defined by the distinctive and dominant, easily mapped and recognizable petrographic, lithologic or paleontologic features (facies) that characterize ...
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 of the rock or sediment being studied. Facies based on
petrological characters (such as grain size and
mineralogy
Mineralogy is a subject of geology specializing in the scientific study of the chemistry, crystal structure, and physical (including optical) properties of minerals and mineralized artifacts. Specific studies within mineralogy include the proce ...
) are called
lithofacies, whereas facies based on
fossil
A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...
content are called ''biofacies''.
A facies is usually further subdivided, for example, one might refer to a "tan,
cross-bedded oolitic limestone
Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms w ...
facies" or a "
shale facies". The characteristics of the
rock unit come from the
depositional environment and from the original composition.
Sedimentary
Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...
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
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.
Walther's law of facies
Walther's law of facies, or simply Walther's law, named after the geologist
Johannes Walther (1860-1937), 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.
In Russia the law is known as Golovkinsky-Walther's law, honoring also
Nikolai A. Golovkinsky[
]
(1834-1897). A classic example of this law is the vertical stratigraphic succession that typifies marine
transgressions and
regressions.
Metamorphic facies
The sequence of minerals that develop during progressive
metamorphism
Metamorphism is the transformation of existing rock (the protolith) to rock with a different mineral composition or texture. Metamorphism takes place at temperatures in excess of , and often also at elevated pressure or in the presence of ch ...
(that is, metamorphism at progressively higher temperatures and/or pressures) define a ''facies series''.
Seismic facies
Seismic facies are mappable three-dimensional seismic units composed of reflection units whose parameters differ from adjacent facies units.
See also
*
Lithofacies
*
Metamorphic rock
Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, caus ...
*
Sequence stratigraphy
*
Stratum
In geology and related fields, a stratum ( : strata) is a layer of rock or sediment characterized by certain lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by visible surfaces known as e ...
Notes
References
{{Authority control
Metamorphic petrology
Petrology concepts
Sedimentary rocks
Stratigraphy