Geological formation - Knowledge (XXG)

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The contrast in lithology between formations required to justify their establishment varies with the complexity of the geology of a region. Formations must be able to be delineated at the scale of geologic mapping normally practiced in the region; the thickness of formations may range from less than

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of their rock strata, which can vary widely. They are usually, but not universally, tabular in form. They may consist of a single lithology (rock type), or of alternating beds of two or more lithologies, or even a heterogeneous mixture of lithologies, so long as this distinguishes them from adjacent

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of Arizona. The names must not duplicate previous formation names, so, for example, a newly designated formation could not be named the Kaibab Formation, since the Kaibab Limestone is already established as a formation name. The first use of a name has precedence over all others, as does the first

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The boundaries of a formation are chosen to give it the greatest practical lithological consistency. Formations should not be defined by any criteria other than lithology. The lithology of a formation includes characteristics such as chemical and mineralogical composition, texture, color, primary

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may be defined. Long-established formations dating to before the modern codification of stratigraphy, or which lack tabular form (such as volcanic formations), may substitute a type locality for a type section as their stratotype. The geologist defining the formation is expected to describe the

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in his theory of the origin of the Earth, which was developed over the period from 1774 to his death in 1817. The concept became increasingly formalized over time and is now codified in such works as the North American Stratigraphic Code and its counterparts in other regions.

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Geologic formations are typically named after a permanent natural or artificial feature of the geographic area in which they were first described. The name consists of the geographic name plus either "Formation" or a descriptive name. Examples include the

363:"Formation" is also used informally to describe the odd shapes (forms) that rocks acquire through erosional or depositional processes. Such a formation is abandoned when it is no longer affected by the geologic agent that produced it. Some well-known 265:

A type section is ideally a good exposure of the formation that shows its entire thickness. If the formation is nowhere entirely exposed, or if it shows considerably lateral variation, additional

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A formation must be large enough that it can be mapped at the surface or traced in the subsurface. Formations are otherwise not defined by the

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is composed of alternating thin beds of two lithologies, mudstone and sandstone, penetrated by veins of a third lithology, gypsum.

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Although formations should not be defined by any criteria other than primary lithology, it is often useful to define

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is underlain by the Mutitjulu Arkose, a formation composed almost entirely of a single lithology (arkosic sandstone).

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were described and put in chronological order by the geologists and stratigraphers of the 18th and 19th centuries.

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The concept of a geologic formation goes back to the beginnings of modern scientific geology. The term was used by

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stratotype in sufficient detail that other geologists can unequivocally recognize the formation.

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of formations and white numbers correspond to formations (click on picture for more information)

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showing where various formations are exposed at the surface are fundamental to such fields as

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The concept of formally defined layers or strata is central to the geologic discipline of

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As with other stratigraphic units, the formal designation of a formation includes a

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North American Commission on Stratigraphic Nomenclature (November 2005).

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on geochemical criteria, and these are included in stratigraphic codes.

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regarded as rock-forming particles, or other organic materials such as

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Structural geology : fundamentals and modern developments

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