Sedimentary marginal marine siltstone and sandstone facies exposed in southern
Utah Ideally, a
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. 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.
Metamorphic The sequence of minerals that develop during progressive
metamorphism (that is, metamorphism at progressively higher temperatures and/or pressures) define a
facies series. ==Notes==