Facies in geology. Meaning, and Concepts

Facies (it means – facing) is a set of rock layers associated with a specific depositional environment. Facies in stratigraphy is defined as a body of strata (rock) characterized by a particular sequence of sedimentary structures, fossils, or lithology (sedimentary rock type). Facies was introduced as part of a classification system by British geologist William Smith, who first used them in the 18th century to describe the rocks exposed between coal mines. He noticed that the sequence of STRATIFICATION varied from place to place, but the rock layers still contained USED DOLOMITE, limestone, sandstone, and shales which always maintain their original order. A facies is a sequence of sedimentary rock layers with specific geographic distribution and a specific range of geological characteristics. In other words, facies are sedimentary rock deposits within a given region that share similar or identical characteristics. They often contain fossils from the era, and many can tell paleontologists valuable information about the ancient environments where they were formed. In this article, we'll talk about what facies are and why they're so important. Everybody of rock may be regarded as composed of facies, which are not necessarily distinct layers or strata but maybe beds interbedded with others. The principal distinction between facies and lithology relates to the origin of the rocks concerned. Facies is based on sedimentary process and environment, lithology on composition. Facies may be derived from any group of rocks; for example, igneous facies (based on cooling rate) and metamorphic facies (based on temperature).

Facies fluviales - Mioceno -Caracenilla-Cuenca


Types of Facies

Primary facies

Primary facies are formed by original sedimentary processes. This means they were deposited through underwater currents or the air. Examples include Terrigenous clastic – Gravel, sand, and mud deposited by gravity flow from land into water (alluvial fans, deltas, turbidites); Carbonates – Lime mud and limestone deposited from ocean water; Evaporites – Salt deposits laid down as the water evaporates from an enclosed basin; Chemical precipitates – Minerals laid down when ocean water evaporates or reacts with organic material; Volcaniclastic – Rock debris laid down by volcanic eruption and Biogenic – Material laid down by living organisms. For example skeletal fragments.

Main depositional environments.


Secondary facies

They are those that were formed after the original deposit has been subjected to alteration due to various physical and chemical processes including metamorphism and diagenesis. Secondary facies can be subdivided into autogenic, allogenic. Autogenic refers to changes in the sediment related to the physical environment or biota within the sediment, or processes that are caused by external forces and events, such as mass movements on submarine slopes (turbidite systems). while allogenic refers to changes in the sediment that are related to external processes and those that are produced by organisms, such as coral reefs (bioherms). Diagenesis is the process of changes in sediments after their deposition. These changes occur because of chemical or physical reactions that alter their composition. A good example of this is the formation of fossils from organic materials deposited millions of years ago in marine environments.

Abietane Diagenesis Scheme


Concept of Facies

The concept of facies is based on the principle that various types of sediment are laid down under distinct environments. The different types of environments create different types of sediments through distinct processes. Therefore, the nature and composition of sediments can be used to identify the environment in which they were laid down. Facies is one of the key concepts geologists use to study the relationships between sedimentary rocks, their depositional environments, and the time when they were formed. It is about recognizing changes in sedimentary rocks that are a result of physical, chemical, and hydrological processes. The concept of facies is an important one for understanding how sedimentary rocks can record past climate changes, sea-level changes, and tectonics. Facies can also help us to deduce what type of deposition occurred: whether it was deltaic or lacustrine, shallow or deepwater, continental or marginal marine sediments.

Stratigraphic column - Chacarilla Formation, Chile


Importances of Facies

Facies in geology play an important role in the study of geology. They are to be studied under the subject, Stratigraphy. Facies are types of rocks (they are also called facies series) that are deposited in a specific environment. By studying these rocks, one can determine what kind of environment existed when the rock was deposited. For example, if a rock is eroded, it leaves behind two or more layers of rocks on either side which are called strata. In this process, you will find a thin flat layer at the top followed by a thicker one at the bottom (it is not always like this). What happens later is that sediments carried by wind and water get deposited between them and form another thinner layer at the top, followed by a thicker layer at the bottom. This is how we often get three distinct layers of rocks with varying thicknesses. The uppermost layer is called 'covering' - it covers over the uppermost rock in a specific place which is called bedrock - lying underneath it and may include mudstone, siltstone or sandstone, etc. 

Uses of Facies in Geology

Facies are the main characteristics of a stratigraphic succession (strata). They can be used to determine the structural and sedimentary history of a rock body, to gain information about the environment in which it was deposited. Facies describe a set of rocks, not in terms of their age but by how they were formed and subsequently modified by environmental factors (such as deposition environment or erosion) over time. This allows them to be placed into the geologic history of an area. The information derived from facies is relevant to paleontology, biostratigraphy, paleogeography, and engineering geology. Facies are a reflection of the original environment of deposition. This environment is influenced by climate, energy system, water depth and currents, sediment supply, and other factors. A palaeogeographical map or a sequence stratigraphic model is built up with this type of information. Geological maps of an area will usually include a description of typical facies seen in that area.

Conclusion

  In the classic explanation of facies, the facies concept was presented as the theory of interpretation, which states that sedimentary rocks can be classified based on their typical appearance. We hope that the concept of facies discussed above has helped you to better understand it intimately. Facies play a vital role in geology and hence their significance should not be undermined. There are many different types of facies are present in the world. Each facies has its significance, type, and description.


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