Lithostratigraphic units are bodies of rocks, bedded or unbedded, that are defined and characterized on the basis of their lithologic properties and their stratigraphic relations. Lithostratigraphic units are the basic units of geologic mapping.
The relationship of lithostratigraphic units to other kinds of stratigraphic units is discussed in Chapter 10.
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.
The conventional hierarchy of formal lithostratigraphic terms is as follows:
The component units of any higher rank unit in the hierarchy need not be everywhere the same.
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.
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.
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.
Exceptionally, a group may be divided into subgroups.
If a unit merits a formal name it merits proper formal definition and description.
Each formal lithostratigraphic unit should have a clear and precise definition or characterization.
The designation of a stratotype for a layered unit or a type locality for a nonlayered unit is essential.
Designation of auxiliary reference sections or additional type localities may be used to supplement the definition of a lithostratigraphic unit. Where a complete section of a unit does not crop out in an area, the lower and upper boundary-stratotypes at specific sections are designated.
Boundaries of lithostratigraphic units commonly cut across time surfaces, across the limits of fossil ranges, and across the boundaries of any other kind of stratigraphic units.
Local or minor hiatuses, disconformities or unconformities within a sequence of similar lithologic composition should not be considered reason for recognition of more than one lithostratigraphic unit.
A lithostratigraphic unit and its boundaries are extended away from the type section or type locality only as far as the diagnostic lithologic properties on which the unit is based may be identified.
In the case of lithostratigraphic units, a simple lithologic term indicating its dominant rock type may be used instead of the unit-term indicating its rank (group, formation, member, bed).
However, the use of the unit-term is preferable; and the use of both the lithologic term and the unit-term should be discouraged.
The terms "lower", "middle", and "upper" should not be used for formal subdivisions of lithostratigraphic units.
In the case of lateral changes in lithologic composition, change in the geographic term is desirable for important regional changes, but the indiscriminate proposal of new names for minor lithologic variations is undesirable.
Compound, combined or lithogenetic terms should not be used.
Nonlayered intrusive rocks and bodies of metamorphic rocks that are deformed and/or recrystallized so that their original layering and stratigraphic succession can no longer be ascertained require a somewhat different treatment.
As lithostratigraphic units, their name should be composed of an appropriate local geographic term combined with either a unit-term or a simple field lithologic term. However, since most geologists may agree that unit-terms such as "group", "formation", or "member" imply stratification and position within a stratified sequence, it is more appropriate to use simple field lithologic terms such as "granite", "gneiss", or "schist" for these nonlayered units.
Also appropriate is the use of the terms "complex", "melange", and "ophiolite".
On the other hand, the use of the term "suite" seems inadvisable. The term has been commonly used for associations of comagmatic intrusive igneous rock bodies of similar or related lithologies and close association in time, space, and origin.
The use of adjectival qualifiers such as "plutonic", "igneous", or "volcanic", though preferably minimized in the formal nomenclature of lithostratigraphic units, may be used when they help to clarify the nature of a unit, as for instance a complex, e.g., "igneous complex", "volcanic complex".
Adjectives used as nouns, such as "volcanics" or "metamorphics", preferably should be avoided even though they have been used widely.
The lithostratigraphic names of igneous and metamorphic rock bodies should not include terms that express form or structure such as "dike", "sill", "pluton", and "neck", or the more general term "intrusion". These terms do not indicate lithology, are not unit-terms in the lithostratigraphic hierarchy, and are not, therefore, lithostratigraphic terms.
See sections 3.B.5, 5.F.2, and 5.F.3.