through
Jurassic strata of the
Colorado Plateau area of southeastern
Utah demonstrate the principles of
stratigraphy. These strata make up much of the famous prominent rock formations in widely spaced protected areas such as
Capitol Reef National Park and
Canyonlands National Park. From top to bottom: Rounded tan domes of the
Navajo Sandstone, layered red
Kayenta Formation, cliff-forming, vertically jointed, red
Wingate Sandstone, slope-forming, purplish
Chinle Formation, layered, lighter-red
Moenkopi Formation, and white, layered
Cutler Formation sandstone. Picture from
Glen Canyon National Recreation Area, Utah. Sequences of
sedimentary and
volcanic rocks are subdivided on the basis of their shared or associated
lithology. Formally identified lithostratigraphic units are structured in a hierarchy of
lithostratigraphic rank, higher rank units generally comprising two or more units of lower rank. Going from smaller to larger in rank, the main lithostratigraphic ranks are bed, member, formation, group and supergroup. Formal names of lithostratigraphic units are assigned by
geological surveys. Units of formation or higher rank are usually named for the unit's
type location, and the formal name usually also states the unit's rank or lithology. A lithostratigraphic unit may have a change in rank over a some distance; a group may thin to a formation in another region and a formation may reduce in rank for member or bed as it "pinches out".
Bed A
bed is a lithologically distinct layer within a member or formation and is the smallest recognisable stratigraphic unit. These are not normally named, but may be in the case of a
marker horizon.
Member A member is a named lithologically distinct part of a formation. Not all formations are subdivided in this way and even where they are recognized, they may only form part of the formation.
Formation Formations are the primary units used in the subdivision of a sequence and may vary in scale from tens of centimetres to kilometres. They should be distinct lithologically from other formations, although the boundaries do not need to be sharp. To be formally recognised, a formation must have sufficient extent to be useful in mapping an area.
Group A group is a set of two or more formations that share certain lithological characteristics. A group may be made up of different formations in different geographical areas and individual formations may appear in more than one group. Groups are occasionally divided into subgroups, but subgroups are not mentioned in the North American Stratigraphic Code, and are permitted under International Commission on Stratigraphy guidelines only in exceptional circumstances.
Supergroup A supergroup is a set of two or more associated groups and/or formations that share certain lithological characteristics. A supergroup may be made up of different groups in different geographical areas. ==Biostratigraphic units==