Geologic unit

Geologic unit

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A geological unit is a volume of rock
Rock (geology)
In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...

 or ice
Glaciology Glaciology Glaciology (from Middle French dialect (Franco-Provençal): glace, "ice"; or Latin: glacies, "frost, ice"; and Greek: λόγος, logos, "speech" lit...

 of identifiable origin and age range that is defined by the distinctive and dominant, easily mapped and recognizable petrographic
Petrology is the branch of geology that studies rocks, and the conditions in which rocks form....

, lithologic
The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples or with low magnification microscopy, such as colour, texture, grain size, or composition. It may be either a detailed description of these characteristics or be a summary of...

 or paleontologic
Paleontology "old, ancient", ὄν, ὀντ- "being, creature", and λόγος "speech, thought") is the study of prehistoric life. It includes the study of fossils to determine organisms' evolution and interactions with each other and their environments...

 features (facies
In geology, facies are a body of rock with specified characteristics. Ideally, a facies is a distinctive rock unit that forms under certain conditions of sedimentation, reflecting a particular process or environment....

) that characterize it.

Units must be mappable and distinct from one another, but the contact need not be particularly distinct. For instance, a unit may be defined by terms such as "when the sandstone component exceeds 75% or more".

Lithostratigraphic units

  • Supergroup – two or more groups and lone formations
  • Group – two or more formations
  • Formation – primary unit of lithostratigraphy
  • Member – named lithologic subdivision of a formation
  • Bed – named distinctive layer in a member or formation
  • Flow – smallest distinctive layer in a volcanic sequence

The component units of any higher rank unit in the hierarchy need not be everywhere the same.

Supergroup:The term "supergroup" may be used for several associated groups or for associated groups and formations with significant lithologic properties in common.

Group:A succession of two or more contiguous or associated formations with significant and diagnostic lithologic properties in common. Formations need not be aggregated into groups unless doing so provides a useful means of simplifying stratigraphic classification in certain regions or certain intervals. Thickness of a stratigraphic succession is not a valid reason for defining a unit as a group rather than a formation. The component formations of a group need not be everywhere the same.

Subgroup:Exceptionally, a group may be divided into subgroups.

Geologic formation
A formation or geological formation is the fundamental unit of lithostratigraphy. A formation consists of a certain number of rock strata that have a comparable lithology, facies or other similar properties...

s are the primary formal unit of lithostratigraphic classification. Formations are the only formal lithostratigraphic units into which the stratigraphic column everywhere should be divided completely on the basis of lithology. The contrast in lithology between formations required to justify their establishment varies with the complexity of the geology of a region and the detail needed for geologic mapping and to work out its geologic history. No formation is considered justifiable and useful that cannot be delineated at the scale of geologic mapping practiced in the region. There is no formal limit to how thick or thin a formation may be.

Member:The formal lithostratigraphic unit next in rank below a formation.
  • It possesses lithologic properties distinguishing it from adjacent parts of the formation.
  • No fixed standard is required for the extent and thickness of a member.
  • A formation need not be divided into members unless a useful purpose is thus served.
  • Formations may have only certain parts designated as members. A member may extend from one formation to another.

Bed:The smallest formal unit in the hierarchy of sedimentary lithostratigraphic units, e.g. a single stratum lithologically distinguishable from other layers above and below. Customarily only distinctive beds (key beds, marker beds
Key bed
In geology, a key bed is a widespread sedimentary layer that formed at a single time, such that it is useful for geologic correlations and dating over a large area...

) particularly useful for stratigraphic purposes are given proper names and considered formal lithostratigraphic units.

Flow:A discrete extrusive volcanic body distinguishable by texture, composition, or other objective criteria. The designation and naming of flows as formal lithostratigraphic units should be limited to those that are distinctive and widespread.

Chemostratigraphic units

Whenever lithological grounds fail to provide significant ability to distinguish mappable rock units, it is possible to map lithology using geochemistry
The field of geochemistry involves study of the chemical composition of the Earth and other planets, chemical processes and reactions that govern the composition of rocks, water, and soils, and the cycles of matter and energy that transport the Earth's chemical components in time and space, and...

 to identify stratigraphy with the same or similar geochemical composition.

Chemostratigraphy can also be the basis for defining a member, bed or subdivision of a geologic unit. For instance, a shale unit may be more sulphidic in the base, and less so in the upper portions, allowing a subdivision into a sulphidic member.

The mapped chemostratigraphic units need not follow stratigraphic or lithostratigraphic units, as the chemical stratigraphy of an area may be independent of lithology. Any geochemical criteria could be used to define chemostratigraphic units; gold, nickel, carbonate, silica or aluminium content, or a ratio of one or more elements to another.

For instance, it would be possible to map a regolith
Regolith is a layer of loose, heterogeneous material covering solid rock. It includes dust, soil, broken rock, and other related materials and is present on Earth, the Moon, some asteroids, and other terrestrial planets and moons.-Etymology:...

 feature such as carbonate cement in a sandstone and siltstone area, which is independent of lithology. Similarly, it is possible to identify fertile nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...

-bearing volcanic flows in heavily sheared greenstone
Greenstone belt
Greenstone belts are zones of variably metamorphosed mafic to ultramafic volcanic sequences with associated sedimentary rocks that occur within Archaean and Proterozoic cratons between granite and gneiss bodies....

 terranes by utilising a chemostratigraphic approach.

In mapping of igneous rocks, particularly volcanic rocks and intrusive rocks
An intrusion is liquid rock that forms under Earth's surface. Magma from under the surface is slowly pushed up from deep within the earth into any cracks or spaces it can find, sometimes pushing existing country rock out of the way, a process that can take millions of years. As the rock slowly...

, particularly layered intrusions and granite
Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...

s, chemical stratigraphy and chemical differentiation of phases of these intrusives is warranted and in many cases necessary.

Chemical stratigraphy is useful in areas of sparse outcrop for making correlattions between separate, distant sections of stratigraphy, especially in layered intrusions and granite terrains which have poor outcrop. Here, chemical trends in the stratigraphy and between intrusive phases can be used to correlate individual sections within the larger intrusive stratigraphy, or group outcrops into their respective intrusive phases and make rough correlative maps.

Chemical stratigraphy is often used with drilling
Drilling is a cutting process that uses a drill bit to cut or enlarge a hole in solid materials. The drill bit is a multipoint, end cutting tool...

 information to assist in correlating between drill holes on a section, to resolve dips and pick formation boundaries. Downhole geophysical logging can produce a form of chemical stratigraphy via logging of radioactive properties of a rock.

Often when compared to lithostratigraphic units, chemostratigraphic units will not always clearly match. Thus, it is wise to map both lithology and geochemistry and provide separate interpretations and map units.

Biostratigraphic units

Biostratigraphic units are defined by the presence of biological markers, usually fossils, which place the rock into a chronostratigraphic
Chronostratigraphy is the branch of stratigraphy that studies the age of rock strata in relation to time.The ultimate aim of chronostratigraphy is to arrange the sequence of deposition and the time of deposition of all rocks within a geological region, and eventually, the entire geologic record of...


Biostratigraphic units are defined by assemblages of fossils. Few biostratigraphic intervals are entirely distinctive as to an age of a rock, and often the best chronostratigraphic resolution that can be provided by biostratigraphy can be a range in ages from a maximum to a minimum when that fossil assemblage is known to have coexisted.

Biostratigraphic units can be defined by as little as one fossil, and can be marker beds or members within a formally identified Formation, for instance an ammonoid bearing bed. This can be a valuable tool for orienting oneself within a stratigraphic section or within a thick lithostratigraphic unit.

Biostratigraphic units can overlap lithostratigraphic units, as the habitat of a fossil may extend from areas where sediment was being deposited as sandstone into areas where it was a being deposited as a siltstone. An example would be a trilobite
Trilobites are a well-known fossil group of extinct marine arthropods that form the class Trilobita. The first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period , and they flourished throughout the lower Paleozoic era before...


Other biostratigraphic markers are restricted to certain environments, for instance, graptolites are generally only found in shale
Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering...


Biostratigraphic units can also be used in archaeology
Archaeology, or archeology , is the study of human society, primarily through the recovery and analysis of the material culture and environmental data that they have left behind, which includes artifacts, architecture, biofacts and cultural landscapes...

, for instance where introduction of a plant species can be identified by different pollen
Pollen is a fine to coarse powder containing the microgametophytes of seed plants, which produce the male gametes . Pollen grains have a hard coat that protects the sperm cells during the process of their movement from the stamens to the pistil of flowering plants or from the male cone to the...

 assemblages through time or the presence of the bones of different vertebrate animals in midden
A midden, is an old dump for domestic waste which may consist of animal bone, human excrement, botanical material, vermin, shells, sherds, lithics , and other artifacts and ecofacts associated with past human occupation...


See also

  • Sequence stratigraphy
    Sequence stratigraphy
    Sequence stratigraphy is a branch of geology that attempts to subdivide and link sedimentary deposits into unconformity bound units on a variety of scales and explain these stratigraphic units in terms of variations in sediment supply and variations in the rate of change in accommodation space...

  • Stratigraphy
    Stratigraphy, a branch of geology, studies rock layers and layering . It is primarily used in the study of sedimentary and layered volcanic rocks....

  • Sedimentology
    Sedimentology encompasses the study of modern sediments such as sand, mud , and clay, and the processes that result in their deposition. Sedimentologists apply their understanding of modern processes to interpret geologic history through observations of sedimentary rocks and sedimentary...

  • Glaciology
    Glaciology Glaciology Glaciology (from Middle French dialect (Franco-Provençal): glace, "ice"; or Latin: glacies, "frost, ice"; and Greek: λόγος, logos, "speech" lit...