Lithostratigraphy

 

Lithostratigraphy. The element of stratigraphy that deals with the description and nomenclature of the rocks of the Earth based on their lithology and their stratigraphic relations.

Lithostratigraphic classification. The organization of rock bodies into units on the basis of their lithologic properties and their stratigraphic relations.

Lithostratigraphic unit. A body of rocks that is defined and recognized on the basis of its lithologic properties or combination of lithologic properties and stratigraphic relations.

A lithostratigraphic unit may consist of sedimentary, or igneous, or metamorphic rocks. Lithostratigraphic units are defined and recognized by observable physical features and not by their inferred age, the time span they represent, inferred geologic history, or manner of formation.

The geographic extent of a lithostratigraphic unit is controlled entirely by the continuity and extent of its diagnostic lithologic features.

Kinds of lithostratigraphic Units

·       Formal lithostratigraphic units. 

The conventional hierarchy of formal lithostratigraphic terms is as follows:

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.

 Formation. 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. The thickness of formations may range from less than a meter to several thousand meters.

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.

Some formations may be completely divided into members; others may have only certain parts designated as members.

A member may extend from one formation to another.

Specially shaped forms of members (or of formations) are lenses and tongues.

A lens is a lens-shaped body of rock of different lithology than the unit that encloses it.

A tongue is a projecting part of a lithostratigraphic unit extending out beyond its main body.

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

Group. A succession of two or more contiguous or associated formations with significant and diagnostic lithologic properties in common

Complex. A lithostratigraphic unit composed of diverse types of any class or classes or rocks (sedimentary, igneous, metamorphic) and characterized by irregularly mixed lithology or by highly complicated structural relations.

Lithostratigraphic horizon (Lithohorizon). A surface of lithostratigraphic change, commonly the boundary of a lithostratigraphic unit, or a lithologically distinctive very thin marker bed within a lithostratigraphic unit.

·       Informal lithostratigraphic units. Lithostratigraphic units recognized in preliminary studies and not fully described and characterized are sometimes given names. Such names should be considered informal and should not be included in published documents.