carbonate facies (Holocene) on Long Island, Bahamas
, a facies ( FAY-sheez; plural also 'facies') 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.
It is the sum total characteristics of a rock including its chemical, physical, and biological features that distinguishes 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
which replaced the earlier notions of Neptunism
Types of facies
marginal marine siltstone and sandstone facies exposed in southern Utah
Ideally, a Sedimentary structures|sedimentary
facies is a distinctive 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
of the rock or sediment being studied. Facies based on petrological
characters (such as grain size and mineralogy
) are called lithofacies
, whereas facies based on fossil
content are called ''biofacies''.
A facies is usually further subdivided, for example, one 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 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 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
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
(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
The sequence of minerals that develop during progressive metamorphism
(that is, metamorphism at progressively higher temperatures and/or pressures) define a ''facies series''.
Seismic facies are mappable three-dimensional seismic units composed of reflection units whose parameters differ from adjacent facies units.
* Metamorphic rock
* Sequence stratigraphy