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Abstract

Lithostratigraphic units are bodies of rocks, bedded or unbedded, that are defined and characterized on the basis of their observable lithologic properties. All stratigraphic units are composed of rock and thus have “rock character,” but only lithostratigraphic units are differentiated on the basis of the kind of rock: limestone, sandstone, sand, tuff, claystone, basalt, granite, schist, marble, and so on. The recognition of such units is useful in visualizing the physical picture of the Earth's rocks, in working out rock sequences, in determining local and regional structure, in investigating and developing natural resources, and in determining the origin of rocks.

Lithostratigraphic units are the basic units of geologic mapping and are an essential element of the stratigraphy of the area. Lithostratigraphic classification is usually the first approach in stratigraphic work in any new area and is always an important key to geologic history even if no ages are available from either fossils or isotopic age determinations (Figure 3).

Nature of Lithostratigraphic Units

Lithostratigraphic units are bodies of rocks, bedded or unbedded, that are defined and characterized on the basis of their observable lithologic properties. All stratigraphic units are composed of rock and thus have “rock character,” but only lithostratigraphic units are differentiated on the basis of the kind of rock: limestone, sandstone, sand, tuff, claystone, basalt, granite, schist, marble, and so on. The recognition of such units is useful in visualizing the physical picture of the Earth's rocks, in working out rock sequences, in determining local and regional structure, in investigating and developing natural resources, and in determining the origin of rocks.

Lithostratigraphic units are the basic units of geologic mapping and are an essential element of the stratigraphy of the area. Lithostratigraphic classification is usually the first approach in stratigraphic work in any new area and is always an important key to geologic history even if no ages are available from either fossils or isotopic age determinations (Figure 3).

Figure 3.

Cross section illustrating how the interrelations of lithostratigraphic units may be used in deciphering geologic history. The sequence in which the various stratigraphic units (rock bodies) shown in this cross section were formed can clearly be established on the basis of the stratigraphic relations of the units, even if their age is not known.

Figure 3.

Cross section illustrating how the interrelations of lithostratigraphic units may be used in deciphering geologic history. The sequence in which the various stratigraphic units (rock bodies) shown in this cross section were formed can clearly be established on the basis of the stratigraphic relations of the units, even if their age is not known.

The relation of lithostratigraphic units to other kinds of stratigraphic units is discussed in Chapter 10.

Definitions

Lithostratigraphy.

The element of stratigraphy that deals with the description and systematic organization of the rocks of the Earth's crust into distinctive named units based on the lithologic character of the rocks and their stratigraphic relations.

Lithostratigraphic Classification.

The organization of rock bodies into units on the basis of their lithologic properties.

Lithostratigraphic Unit.

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

A lithostratigraphic unit may consist of sedimentary, or igneous, or metamorphic rocks, or of an association of two or more of these. Lithostratigraphic units may be composed of consolidated or unconsolidated rocks. The critical requirement of the unit is a substantial degree of overall lithologic homogeneity. (Diversity in detail may in itself constitute a form of overall lithologic unity.) Lithostratigraphic units are recognized and defined by observable physical features and not by their inferred age, the time span they represent, inferred geologic history, or manner of formation. Definition and recognition of lithostratigraphic units must be based on description of the lithologic composition of actual rock material, not on the geophysical properties (electric, radioactive, density, sonic, and other inferred or measured physical properties) of the rocks.

Fossils may be important in the recognition of a lithostratigraphic unit either as minor but distinctive physical constituents or because of their rock-forming properties, as in coquinas, diatomites, coal beds, and so on. The geographic extent of lithostratigraphic units is controlled entirely by the continuity and extent of their diagnostic lithologic features. Only major lithologic features readily recognizable in the field should serve as the basis for litho-stratigraphic units.

A series of laterally discontinuous rock bodies having approximately the same lithologic properties and stratigraphic position may be considered as a single lithostratigraphic unit. An example is the Gila Conglomerate, which is a series of valley beds distributed in disconnected areas along the gorges of the upper Gila River. Similarly, a series of genetically related but discontinuous igneous bodies or a series of disconnected reef limestones or coal lenses lying at approximately the same stratigraphic position may all be included in the same named unit if their size and separation is not sufficient to warrant naming each one individually. Examples might be the Permian patch-reefs of West Texas or the Leduc reefs of Canada. When recognized as formal lithostratigraphic units, a reef system may constitute a member within a formation or may constitute a formation in itself.

Kinds of Lithostratigraphic Units

Formal Lithostratigraphic Units.

Formal lithostratigraphic units are those that are defined and named in accordance with an explicitly established or conventionally accepted scheme of classification and nomenclature (see Table 1, and section 3.A.5). The conventional hierarchy of formal lithostratigraphic terms is as follows:

  • Group—two or more formations

  • Formation—primary unit of lithostratigraphy

  • Member—named lithologic entity within a formation

  • Bed—named distinctive layer in a member or formation

  • Flow—smallest distinctive layer in a volcanic sequence

Formation.

The formation is the primary formal unit of lithostratigraphic classification used to map, describe, and interpret the geology of a region; it is a body of rocks of intermediate rank in the hierarchy of lithostratigraphic units and is identified by its lithologic character and stratigraphic position. Formations are the only formal lithostratigraphic units into which the stratigraphic column everywhere should be divided completely on the basis of lithology.

Formations may be composed of any kind of rock—sedimentary rocks, ig-neous rocks, metamorphic rocks, or under some circumstances, associations of two or more kinds of rock.

The degree of change in lithology required to justify the establishment of distinct formations (or other lithostratigraphic units) is not amenable to strict and uniform rules. It may vary with the complexity of the geology of a region and the detail needed to portray satisfactorily its rock framework and to work out its geologic history.

The thickness of units of formation rank follows no standard and may range from less than a meter to several thousand meters, depending on the size of units required to interpret the lithologic development of a region.

Practicability of mapping and delineation on cross sections is an important consideration in the establishment of formations. No formation is considered justifiable and useful that cannot be delineated at the scale of geologic mapping practiced in the region where the formation is proposed. Although representation of a formation on maps and cross sections by a labeled line may be justified, proliferation of such thin units is undesirable.

Member.

A member is the formal lithostratigraphic unit next in rank below a formation and is always a part of a formation. It is recognized as a named entity within the formation because it possesses lithologic properties distinguishing it from adjacent parts of the formation. No fixed standard is required for the extent or 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; still others may have no members. A member may extend from one formation to another.

Lens (lentil, lenticle) and tongue have been used frequently as independent lithostratigraphic rank terms but more correctly they are only specially shaped forms of members or of formations. A lens is a lens-shaped body of rock of differ-ent lithology than the unit that encloses it. A tongue is a projecting part of a litho-stratigraphic unit extending out beyond its main body. The proper designation of these bodies rests in part on circumstances of known occurrence; a body seen as a lens might be part of a narrow tongue. Lenses and tongues may be named.

Bed.

A bed is the smallest formal unit in the hierarchy of sedimentary lithostratigraphic units. It is a unit layer in a stratified sequence of rocks which is lithologically distinguishable from other layers above and below from which it is separated by more or less well-defined bedding surfaces; for example, the Baker Coal Bed. Several contiguous beds of similar lithology may also together constitute a formal unit; for example, the Marcus Limestone Beds.

The term “bed” is applied customarily to layers from a centimeter to a few meters in thickness. Those of less thickness are called laminae. Stratification is not necessarily identical with splitting properties, and the two should not be confused.

A single named bed or named sequence of beds may constitute a member or a formation, and in such a case, the unit-term bed (or beds) is replaced by member or formation; for example, “Bracklesham Beds” = Bracklesham Formation, “Drusberg-Schichten” = Drusberg Formation. A specific bed conceivably may pass from one member or formation to another and still retain the same name.

Customarily only distinctive beds (commonly known as key beds or marker beds) particularly useful for stratigraphic purposes, such as correlation or reference, are given proper names and considered formal lithostratigraphic units. Intervening beds are left unnamed.

Flow.

A flow is the smallest formal lithostratigraphic unit of volcanic flow rocks. A flow is a discrete, extrusive, volcanic body distinguishable by texture, composition, order of superposition, or other objective criteria. It is equivalent in rank to a bed of the sedimentary-rock classification. Many flows are informal units. The designation and naming of flows as formal lithostratigraphic units should be limited to those that are distinctive and widespread.

Group.

A group is the formal lithostratigraphic unit next in rank above a formation. The term is applied most commonly to a sequence of two or more contiguous or associated formations with significant and diagnostic lithologic properties in common. The type or reference sections of a group are the type or reference sections of its component formations. The proposal for recognition of a group should outline clearly the diagnostic characteristics on which it is based and the formations of which it is composed. Formations need not be aggregated into groups. Establishing groups without constituent formations in anticipation of possible action of future workers should be avoided. It is considered prefer-able to establish the unit as a formation and allow future workers to subdivide it, establish constituent formations, and change the rank of the original formation to group while keeping the same geographic component of the name (see section 3.B.5). Thickness of a sequence is not a valid reason for defining a unit as a group rather than a formation.

The aggregation of formations into groups provides a useful means of simplifying and generalizing stratigraphic classification for those who may not need, or may wish to avoid, the often complex detail of formational subdivision in certain regions or certain intervals. Groups are useful in small-scale mapping and regional stratigraphic analysis.

The component formations of a group are not necessarily everywhere the same. For example, in parts of western Iran the Fars Group comprises simply the Gach Saran and Agha Jari Formations; in parts of the province of Fars in southern Iran the group consists of the Razak, Mishan, and Agha Jari Formations. A formation may extend laterally from one group to another.

The wedging-out of a component formation or formations may justify the reduction of a group to formation rank, retaining the same name.

Supergroup and Subgroup.

The term supergroup may be used for several associated groups or for associated formations and groups with significant lithologic properties in common. Supergroups should be established only where their recognition serves a clear purpose. Exceptionally, a group may be divided into subgroups.

Complex.

A complex is a lithostratigraphic unit composed of diverse types of any class or classes of rock (sedimentary, igneous, metamorphic) and characterized by irregularly mixed lithology or by highly complicated structural relations to the extent that the original sequence of the component rocks may be obscured, and the individual rocks or rock sequence cannot be readily mapped. The term may be used as a part of a formal name in the place of a lithologic or rank term; for example, Akkajaure Complex, Franciscan Complex.

Lithostratigraphic Horizon (Lithohorizon).

A lithohorizon is a surface of lithostratigraphic change, commonly the boundary of a lithostratigraphic unit, or a lithologically distinctive very thin marker bed within a lithostratigraphic unit. Lithohorizons are preeminently valuable for correlation (not necessarily time-correlation).

Informal Lithostratigraphic Units.

If a unit merits a name it merits proper formal definition and description. Such definition and description impose a discipline on a worker that reduces the risk of unsound units being created, leading to confusion in the literature. If informal names with uncapitalized lithologic or unit-terms are published, inevitably sooner or later they will be republished with a capital initial letter, and thus the intent of the original author will be lost. Furthermore, in conversation and oral presentations, it is not possible to determine if a unit-term is being capitalized or not. Finally, use of geographic names for informal units renders those names unavailable for formal lithostratigraphic units.

Whenever possible, therefore, it is preferable to avoid the recognition and use of informal lithostratigraphic units that could in the course of time be treated as formal units (see section 3.A.5.b).

The terms “lithozone” or “zone” have been used informally for lithostratigraphic units that include bodies of rock that can be recognized by their lithologic properties and to which casual reference is made but for which there is inappropriate basis to justify designation as a formal unit. The term “measures” has been used informally for a sequence of coal beds (e.g., The Mara coal measures), but “The Coal Measures” has also been used historically for a series of the Carboniferous System.

Lithostratigraphic bodies that are recognized more for utilitarian purposes than for their lithologic unity, such as aquifers, oil sands, quarry layers, coal seams, and orebearing “reefs,” are considered to be informal units even if named. Some such units, however, may be recognized formally as beds, members, or formations if they are important in the interpretation of regional stratigraphy.

Procedures for Establishing Lithostratigraphic Units

Stratotypes and Type Localities as Standards of Definition.

Each named lithostratigraphic unit of whatever rank or lithologic composition should have a clear and precise standard definition based on the fullest possible knowledge of its lateral and vertical variations. The designation of a stratotype (type section) or type locality is essential in the definition of a lithostratigraphic unit.

For a layered lithostratigraphic unit the ideal standard is a specifically designated sequence of rock strata (a stratotype) on which the definition of the unit is based.

In the case of a nonlayered lithostratigraphic unit (an intrusive igneous rock body or a body of massive metamorphic rocks of unknown origin), the standard of definition should be a specified locality where the unit is defined and where it can be studied. Such a locality would be the type locality of the unit.

Stratotypes of layered lithostratigraphic units and type localities of nonlayered lithostratigraphic units may be designated in specific outcrops or in specific excavations, mines, or drill holes, preferably those from which the name of the unit is derived (see Chapter 4). Once a stratotype or type locality is designated, it may be changed only if it was poorly designated or delimited. If the originally specified stratotype (holostratotype) or type locality is, for example, poorly exposed, structurally complicated, of difficult access, or unrepresentative of the unit, a principal reference section (hypostratotype) or reference locality or several reference sections or localities may be designated to supplement, but not to supplant, the holostratotype or type locality. Reference sections and localities, however, must always be considered as subsidiary to the original stratotype or type locality.

Designation of one or more auxiliary reference sections (hypostratotypes) or of additional type localities may be useful to supplement the definition of a lithostratigraphic unit even when the originally designated stratotype (holostratotype) or type locality is well exposed, accessible, and representative of the unit.

The stratotypes of formations or lithostratigraphic units of lesser rank are commonly unit-stratotypes. The stratotypes of lithostratigraphic units larger than a formation (e.g., groups) are commonly composite-stratotypes, that is, a composite of the stratotypes of the component formations of the group.

Where strata are nearly horizontal or poorly exposed and a complete section of a unit does not crop out in a reasonably limited area, it may not always be practicable to designate any one specific complete and continuous section as the unit-stratotype. Only a type area or a type locality, rather than a type section, can be indicated. In such cases it is essential to identify explicitly the lower and upper boundary-stratotypes at specified sections where the change to underlying and overlying units can be seen. The unit-stratotype in this case comprises the exposures in the type locality or type area lying stratigraphically between the lower and upper boundary-stratotypes.

Although the type section of a layered lithostratigraphic unit is a specific interval of strata in a specific section or area, the unit, as recognized elsewhere, may contain a greater or lesser thickness of strata than in the type section and may span a greater or lesser time interval than the type section. The only critical requirement of the unit as identified elsewhere is that it have essentially the same lithologic composition and relative stratigraphic position as in the type section.

Boundaries.

Boundaries of lithostratigraphic units are placed at positions of lithologic change. They are usually designated at sharp lithologic contacts, but also may be placed arbitrarily within zones of lithologic gradation. Both vertical and horizontal boundaries should be drawn to express most usefully lithostratigraphic development.

Where one rock unit passes vertically or laterally into another by intergrading or complex intertonguing of two or more kinds of rock, the boundary is necessarily arbitrary and should be selected to provide the most practicable assignment of units. For example, in a gradation upward from a limestone unit to a shale unit through interbedded shale and limestone, the boundary may be placed arbitrarily at the top of the highest readily traceable limestone bed in the sequence or at the base of the lowermost distinct shale interval. In subsurface work, because of caving in drill holes, it is generally best to define such arbitrary boundaries at the highest occurrence of a particular rock type rather than at the lowest. In a lateral gradation from a sandstone unit to a shale unit through increasingly shaly sandstone, the boundary may be placed arbitrarily at the limits of where the rock is still considered to be dominantly a sandstone. Where a unit changes laterally through abrupt gradation into, or intertongues with, a markedly different kind of rock, a new unit should be proposed for the different rock type. An arbitrary lateral boundary may be placed between the two laterally equivalent units (the “arbitrary cutoff of Wheeler and Mallory, 1953).

If the zone of vertical or lateral intergradation or intertonguing between two units is sufficiently extensive, the rocks of intermediate or mixed lithology may form the basis for establishing and naming a third independent unit. In general, the number of units should be kept to a minimum compatible with the greatest practical utility.

Because the many lithologic variations of rocks offer a wide choice in drawing boundaries of lithostratigraphic units, the selection of these boundaries may be properly influenced by such other factors as lateral traceability, physiographic expression, fossil content, and well log character, as long as the requirement of a substantial degree of lithologic homogeneity is maintained.

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.

Unconformities and Hiatuses.

A sequence of rocks of closely similar lithology but including a local or minor hiatus, disconformity, or unconformity should not be separated into more than one lithostratigraphic unit merely because these types of sedimentary breaks occur, unless there is a lithologic distinction adequate to define a boundary. However, the union of adjacent strata separated by regional unconformities or major hiatuses into a single lithostratigraphic unit should preferably be avoided even if no more than minor lithologic differences can be found to justify the separation.

Procedures for Extending Lithostratigraphic Units—Lithostratigraphic Correlation

A lithostratigraphic unit and its boundaries should be extended away from the type section or type locality only as far as the diagnostic lithologic properties on which the unit is based in its stratotype or type locality are known to exist or, through indirect evidence, may be presumed with assurance to exist.

Use of Indirect Evidence for Identification of Units and Their Bound aries.

Where lithologic identity is probable but difficult to demonstrate with certainty because exposures are poor or lacking, the unit and its boundaries may be identified and correlated on the basis of criteria which only indirectly indicate lithologic composition. Geomorphic expression, lithogenetic evidence, electric log character, seismic reflections, and distinctive vegetation have been used in this manner. The occurrence of distinctive fossils has also been useful in establishing the probable presence of a lithostratigraphic unit. However, it should be recognized that tracing or identification of a lithostratigraphic unit by such means is using only inferred evidence of lithologic properties and is not based directly on lithostratigraphic criteria.

Because the surface expression of lithostratigraphic units is an important aid in mapping, it is advisable, where other factors do not countervail, to define lithostratigraphic boundaries so as to coincide with lithologic changes that are expressed in the topography.

Marker Beds Used as Boundaries.

The top or the base of a marker bed may be used as a boundary for a formal lithostratigraphic unit where the marker bed occurs at or near a recognizable vertical change in lithology. Where the rock between two boundary marker beds becomes substantially different from that of the stratotype, a new unit should be recognized; even though the bounding marker beds may be traceable beyond the stratotype, geographic extension of the lithostratigraphic unit may not be justified.

Naming of Lithostratigraphic Units

General.

The name of lithostratigraphic units should follow the general rules for naming stratigraphic units (see section 3.B.3). Some additional rules apply specifically to lithostratigraphic units.

The name of a lithostratigraphic unit should be formed from the name of an appropriate local geographic feature, combined either with the appropriate unit-term indicating its rank (group, formation, member, bed) or with a simple lithologic term indicating the dominant rock type of which the unit is composed (e.g., Gafsa Formation, Spiti Shale, Manhattan Schist, Concord Granite). As will be discussed later, it is preferable to use the unit-term rather than the lithologic term in combination with the geographic term for the name of a layered sedimentary lithostratigraphic unit (see section 5.F.3). The use of both the lithologic term and the unit-term (Victoria Limestone Formation) should be discouraged. In any case, the initial letters of all words used in forming the names of formal lithostratigraphic units should be capitalized.

Descriptive adjectives, such as soft, hard, folded, and brecciated, should not be included in the lithologic term of the name of a lithostratigraphic unit.

The adjectival endings “ian” or “an” should not be used for a lithostratigraphic unit since they are customarily reserved for chronostratigraphic units.

The terms “lower,” “middle,” and “upper” should not be used for formal subdivisions of lithostratigraphic units.

Geographic Component of Name.

Recommendations for the use of geographic terms in naming stratigraphic units are included in section 3.B.3.a. Some additional recommendations apply more specifically to lithostratigraphic units.

Where a lithostratigraphic unit changes laterally in lithologic properties due either to sedimentological reasons or to metamorphism or diagenesis, the need for a change in the geographic name depends on the degree of change, the persistence of the change, and the assurance of correlation and continuity. For regional changes, a change of name is desirable, but it should be balanced against the confusing multiplicity of terms arising from the indiscriminate proposal of new names for minor lithologic variations. Especially in cases of subtle lateral transition, where boundaries have to be arbitrary, the use of one name in a broad sense may be preferable to the use of two or more names.

Offshore drilling in many areas presents problems in applying formal geographic names to the lithostratigraphic units penetrated in the wells. In some cases, the lithostratigraphic units penetrated in the wells cannot be correlated with the surface units in the bordering region, and it may be very difficult to find locality names for new formations. If the offshore well in which a new lithostratigraphic unit has been penetrated has been given a name taken from coastal, oceanographic, or other features, this name can be used for a subsurface unit providing that the requirements of section 3.B.2 are followed. However, it may often be necessary to use purely arbitrary nomenclatural designations for offshore subsurface units. The same rules apply to units recognized in underwater mapping.

Lithologic Component of Name.

If a lithologic term is used in the name of a lithostratigraphic unit, it should be the simplest generally accepted term that indicates the predominant or diagnostic lithology of the unit (e.g., limestone, sandstone, shale, tuff, granite, quartzite, gneiss, serpentinite). Compound terms such as “clay shale” or “sandy limestone” or combined terms such as “shale and sandstone” should not be used for the lithologic component of the name of a lithostratigraphic unit, nor should an adjective be used between the geographic and the lithologic terms, as “Chattanooga Black Shale” or the “Biwabik Iron-Bearing Formation.”

Lithogenetic terms such as “turbidite” or “flysch” should be avoided for formally named lithostratigraphic units.

The use of a lithologic term in combination with a geographic term for the name of a layered sedimentary lithostratigraphic unit, while permissible and often employed, has numerous disadvantages. First, it creates problems when the unit changes laterally in lithologic character due either to sedimentological reasons or to metamorphism or diagenesis, so that the lithologic term no longer indicates the diagnostic lithology of the unit. More flexibility is possible if unit-terms like “formation” or “member” are used. In addition, lithologic terms do not indicate the position of the unit in the hierarchy of lithostratigraphic units. Finally, the use of a lithologic term may enshrine debatable or erroneous lithologic identifications in the formal name of a lithostratigraphic unit.

In the case of groups, it is seldom possible to characterize their lithology in a name, except in unusual circumstances where the group consists of formations of very similar lithology.

As in the case of so many other stratigraphic problems, no hard and fast rule can be given concerning the use of lithologic terms for the name of lithostratigraphic units, and decisions should be based on common sense and concern for clarity and accuracy.

Some Special Aspects of Igneous and Metamorphic Rocks.

Stratified volcanic rocks and bodies of metamorphic rocks that can be recognized as of sedimentary and/or extrusive volcanic origin can be treated in every respect as unmetamorphosed sedimentary lithostratigraphic units because their characteristic lithologic features, original layering, and stratigraphic relationships are readily distinguishable.

The same is not the case of nonlayered intrusive rocks and of the bodies of metamorphic rocks that are deformed and/or recrystallized so that their original layering and stratigraphic succession may no longer be ascertained. These intrusive igneous bodies and nonlayered metamorphic rocks of undetermined origin, however, are defined, classified, and mapped on the basis of their distinguishing lithologic properties and stratigraphic relationships and are considered lithostratigraphic units. Even though in many cases their precise stratigraphic position is not known, they can often effectively contribute to a clear determination of stratigraphic succession, as they are a source of geochronometric (numerical) ages determined by isotopic or other methods and provide stratigraphic information through the establishment of their cross-cutting and boundary relationships with their associated layered and/or nonlayered rocks.

As lithostratigraphic units, the names of bodies of intrusive igneous rocks and nonlayered metamorphic rocks of undetermined origin should be composed of an appropriate local geographic term combined with either a unit-term indicating rank or a simple field lithologic term indicating dominant rock type. All terms of the name (geographic term and unit-term or simple field lithologic term) should be capitalized. However, since most geologists may agree that the terms “group,” “formation,” and “member” imply stratification and position within a stratified sequence showing original layering, it may be more appropriate to use simple field lithologic terms such as “granite,” “gneiss,” and “schist” for intrusive igneous and nonlayered metamorphic rock bodies.

The term “complex” may be used for igneous and/or metamorphic rock bodies of diverse and irregularly mixed lithology, whether or not they are strongly deformed and/or metamorphosed. The term indicates that the stratigraphic relations of the individual lithologies forming the body of rock are poorly known or unidentifiable and that the body, therefore, cannot be subdivided on stratigraphic grounds. “Complex” also may still be used for sedimentary rock bodies or for combinations of sedimentary and igneous rocks (see section 5.C.8).

Terms such as “melange” or “ophiolite” may also be advantageously applied in naming igneous and metamorphic rock bodies, particularly if the sense in which they are being used is clearly defined. On the other hand, the use of the term “suite” for nonlayered igneous and metamorphic rock bodies seems inadvisable. The term has been commonly used for associations of apparently co-magmatic intrusive igneous rock bodies of similar or related lithologies and close association in time, space, and origin. It has also been used widely in the former Soviet Union and some eastern European countries for local stratigraphic units corresponding approximately or precisely to a formation.

The use of adjectival qualifiers such as “plutonic,” “igneous,” “intrusive,” “extrusive,” and so on should be minimized in the formal nomenclature of lithostratigraphic units. They make the names long and cumbersome and will inevitably be dropped with time, losing in this way the original purpose of their use. They may be used, however, when they help to clarify the nature of a unit or provide valuable descriptive information. For example, the terms “igneous,” “metamorphic,” and “volcanic” may be employed to advantage to clarify the nature of a complex: “igneous complex,” “metamorphic complex,” “volcanic complex.”

Adjectives used as nouns, such as “volcanics,” “metamorphics,” “intrusives,” and “extrusives,” preferably should not be used in naming igneous and metamorphic rock bodies. These terms, however, have been used widely in many parts of the world and may reluctantly be accepted in formal lithostratigraphic nomenclature when judged useful.

Lithostratigraphic names of igneous and metamorphic rock bodies should not include terms that express form or structure, as, for example, “dike,” “sill,” “batholith,” “pluton,” “diapir,” “stock,” “pipe,” and “neck” or the more general term “intrusion.” These terms do not indicate the lithology of the rock body, are not unit-terms in the lithostratigraphic hierarchy, and are not, therefore, lithostratigraphic terms. They are useful geologic terms that can be employed to characterize, in a nonstratigraphic sense, the geometry of igneous and metamorphic rock bodies. The term “dike” may be more informative, perhaps, than “dolerite,” but it expresses form rather than lithology and is, therefore, not a lithostrati-graphic term. “Deltaville Dolerite Dike” is the name of an igneous feature, not a stratigraphic unit. Form terms should be used to express form, and structural terms should be used to express structure. Stratigraphic characters and attributes, stratigraphic relations, and stratigraphic position should be expressed with stratigraphic terms.

The same rules for the naming of sedimentary and layered extrusive igneous rock bodies are applied to nonlayered igneous and metamorphic rocks. The proposal of a new unit or the redefinition of an already established one should include a comprehensive and clear description of the unit, express the intent to introduce a new unit or to revise an existing one, designate a type locality (and reference localities, if appropriate), cite the geographic feature from which the name of the unit is taken, and so on.

Revision of Lithostratigraphic Units

Revision or redefinition of an established formal lithostratigraphic unit without changing its name requires as much justification and the same procedures as for proposing a new unit (see section 3.B.5). For change of the geographic term of the name see section 5.F.2; for change of the lithologic term of the name, see section 5.F.3; for change of rank, see section 3.B.5.

Figures & Tables

Figure 3.

Cross section illustrating how the interrelations of lithostratigraphic units may be used in deciphering geologic history. The sequence in which the various stratigraphic units (rock bodies) shown in this cross section were formed can clearly be established on the basis of the stratigraphic relations of the units, even if their age is not known.

Figure 3.

Cross section illustrating how the interrelations of lithostratigraphic units may be used in deciphering geologic history. The sequence in which the various stratigraphic units (rock bodies) shown in this cross section were formed can clearly be established on the basis of the stratigraphic relations of the units, even if their age is not known.

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