Defining Lithostratigraphic Formations
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.
Formations must be able to be delineated at the scale of geologic mapping practiced in the region. The thickness of formations may range from less than a meter to several thousand meters.
Geologic formations are typically named for the geographic area in which they were first described.
Strictly, formations cannot be defined on any other criteria except primary lithology. However, it is often useful to define biostratigraphic units based on paleontological criteria, chronostratigraphic units based on the age of the rocks, and chemostratigraphic units based on geochemical criteria.
Sequence stratigraphy is a concept which challenges the idea of strict lithostratigraphic units by defining units based on events in sedimentary basins such as oceanic regressions and transgressions. These sequences are a combination of chronostratigraphic units, linked by time, and depositional environment linked by the geologic events which occurred at that time, regardless of the grain size of the sediments.
The term "formation" is often used informally to refer to a specific grouping of rocks, such as those encountered within a certain depth range in an oil well.
Read more about this topic: Geologic Formations
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