Facies, as a geologic term, are a way to distinguish bodies of rock into mappable units in terms of physical characteristics, composition, formation, or various other attributes. Facies are used mostly to establish different units of rock from adjacent units within a contiguous body of rock by physical, chemical, or biological means. Facies were an important development into the concept of stratigraphy because when compiled together, facies can generate a succession that can give insight into an assortment of different process and systems that acted on or within the region and rock record. When modeled, facies can give insight into regional biologic and ecologic activity, water chemistry and properties, igneous events, sedimentary processes, climate records, and tectonics movement.
Facies were first described by paleontologist and geologist Amanz Gressly in 1838 as a component of his then-revolutionary idea of modern stratigraphy, wherein he used the concept of facies to determine the origin of rock bodies through fossil assemblages and petrographic attributes.Throughout his life, Gressly made contributions and developed three key areas of geology; facies concepts and applications, stratigraphic correlations, and paleogeographic construction. Gressly is mostly remembered for his contribution of facies, but his devotion to fundamental stratigraphy would lay the groundwork for many geologists for years to come. 
Walther’s Law of Facies
Famous geologist Johannes Walther in 1894 developed the hypothesis that states when depositional environments shift, freshly deposited sediment will overlay previous depositional environments, and that vertical succession of facies reflect lateral changes in the environment of deposition. Simplified, the law states that if two facies are found to be adjacent within the rock record, with no unconformities, then they must have been deposited laterally in formation. The most common example of Walther’s law is shown in transgressive and regressive cycles seen in marine and coastal deposits.
Application of Facies
Facies may be used by themselves in order to create an idea of the geology of a region, but when combined, can be used to build an extensive network of models for a better understanding. Facies descriptors like sedimentary and seismic facies can give a comprehensive guide to mapping the subsurface and depositional systems. Biofacies and lithofacies can lead to dating bodies of rock. Metamorphic and lithofacies can be used to describe the tectonic regime of the region. Facies are vital when attempting to create a geologic, economic overview of a region. They are commonly used in all types of geologic industry, such as the oil and gas industry to create petroleum systems, or to map economically rich veins for a mining company, or just locate groundwater for municipal utilities.
Common Types of Facies
Facies described by a combination of physical rock characteristics such as mineral composition, grain size, color, and texture. One of the more common forms of facies modeling as most of the facies are discernible on an outcrop scale.
Facies based on a rocks fossil content and paleoecologic characteristics in order to create a facies model of the paleoenvironment at the time of deposition. Can give insight on climate shifts and environmental evolution using trace fossils and biomarkers within the rock record.
- Sedimentary Facies
Since facies are used to make distinctions within bodies of rock, sedimentary facies focus on the sediments that make up the rock. Sedimentary rocks are valuable because of their ability to supply information from their deposition, such as the lithofacies of the different units and the sedimentary structures found within. These descriptors, in turn, relate sedimentary facies, which can be characterized and compiled into a model to give an overview of the regional depositional environment.
- Metamorphic Facies
As rocks are exposed to increasing degrees of metamorphism, a body of rock will be altered on a mineralogic, textural, chemical, and physical scale. Using sequences of relatable metamorphic facies can help define the different stressors and intensity of pressures and temperatures on a given region.
- Seismic Facies
When looking at collected seismic data, facies can be described from units with similar seismic characteristics, such as high acoustic impedance changes or reflectors with similar properties. This can be used to discern different horizons within the subsurface that could relate to things such as hydrocarbon exploration or sequence stratigraphic boundaries.
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