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Abstract

Stratigraphy makes use of numerous named divisions of the rock bodies making up the Earth's crust. It is essential that these named units have their attributes clearly stated and their boundaries clearly defined so that all who use them will start with the same basic understanding of their meaning and so that there will be a common standard for their identification away from the place where they were defined and named. An area of exposure (or a well, or mine) that may be examined and studied by those interested provides an essential part of the establishment of a stratigraphic unit and a useful aid to its identification—a stratotype (type section) for layered sedimentary and volcanic sequences, a type locality for units composed of nonlayered igneous or meta-morphic rocks.

Stratotypes in the Definition and Characterization of Stratigraphic Units

Standard Definitions.

Stratigraphy makes use of numerous named divisions of the rock bodies making up the Earth's crust. It is essential that these named units have their attributes clearly stated and their boundaries clearly defined so that all who use them will start with the same basic understanding of their meaning and so that there will be a common standard for their identification away from the place where they were defined and named. An area of exposure (or a well, or mine) that may be examined and studied by those interested provides an essential part of the establishment of a stratigraphic unit and a useful aid to its identification—a stratotype (type section) for layered sedimentary and volcanic sequences, a type locality for units composed of nonlayered igneous or meta-morphic rocks.

Reference to a Specific Rock Section.

The concept of a stratigraphic unit is based on properties or attributes of the rocks, such as lithology, fossil content, magnetic polarity, and age or time span. The type section or type locality of such a unit, therefore, constitutes the standard of reference on which the concept of the unit is based. Stratigraphic units are defined also by means of written descriptions; but, valuable as such descriptions are, they are always subject to misunderstanding due to differences in language, differences in interpretation of words, inadequacy or error in description, or other imperfections in our ability to convey concepts verbally. For many kinds of stratigraphic units and their bound-aries, therefore, a designated and identified interval or point in a specified body of rocks—a unit-stratotype, a boundary-stratotype, or a type locality—provides by far the most stable and unequivocal standard of definition.

For a few kinds of stratigraphic units, such as biostratigraphic range zones, the standard of the unit cannot be tied to a specified stratigraphic section or area because the stratigraphic scope of the unit may vary with increasing information.

However, the definition, characterization, and description of these and other biostratigraphic units can be enhanced by the designation of one or more specific reference sections (see section 7.F).

Definitions

Stratotype (Type Section).

The original or subsequently designated standard of reference of a named layered stratigraphic unit or of a stratigraphic boundary. A stratotype is a specific interval or point in a specific sequence of rock strata and constitutes the standard for the definition and characterization of the stratigraphic unit or boundary being defined.

Unit-stratotype.

The type section of a layered unit that serves as the standard of reference for the definition and characterization of the unit. In the case of a complete and well-exposed layered unit, the upper and lower limits of the unit-stratotype are its boundary-stratotypes (see Figure 2A).

Figure 2.

(A) Unit-stratotype and boundary-stratotypes for a lithostratigraphic unit (Formation B). (B) Boundary-stratotype for a chronostratigraphic unit. A point in the rock sequence defines the lower boundary of Stage B (see section 9.H.2).

Figure 2.

(A) Unit-stratotype and boundary-stratotypes for a lithostratigraphic unit (Formation B). (B) Boundary-stratotype for a chronostratigraphic unit. A point in the rock sequence defines the lower boundary of Stage B (see section 9.H.2).

Boundary-stratotype.

A specified sequence of rock strata in which a specific point is selected that serves as the standard for definition and recognition of a stratigraphic boundary (see Figure 2B and section 9.H.3 for boundary-stratotypes of chronostratigraphic units).

Composite-stratotype.

A unit-stratotype formed by the combination of several specified type intervals of strata, called component-stratotypes. Thus a certain lithostratigraphic unit may not be entirely exposed in any single section, and it may be necessary to designate one section as the type for part of the unit and another section as the type for the rest of the unit. In this case, one of the two component sections should be considered the holostratotype and the other a parastratotype (see section 4.B.7).

A stratotype of a unit of higher rank formed by the combination of the stratotypes of its component units of lower rank is also a composite-stratotype. Thus, the stratotype of a group may be a composite of the stratotypes of its constituent formations. In such a case, the lower boundary-stratotype of the lowermost constituent formation is also the boundary-stratotype of the base of the group. If the components of a composite-stratotype are already established formal stratigraphic units, it is unnecessary to distinguish one as a holostratotype and others as parastratotypes.

Type Locality.

The type locality of a layered stratigraphic unit or of a boundary between layered units is the specific geographic locality in which the unit-stra-totype or the boundary-stratotype is situated; or lacking a designated stratotype, the locality where the unit or boundary was originally defined or named. A type locality differs from a stratotype in that it refers to a geographic locality rather than to the specific profile or stratigraphic section of a layered unit.

In the case of stratigraphic units composed of nonlayered igneous or metamorphic rock bodies, the type locality is the specific geographic locality where the unit was originally defined and named, the standard of its definition.

Type Area (or Type Region).

The geographic area or region that encompasses the stratotype or type locality of a stratigraphic unit or stratigraphic boundary.

Holo-, Para-, Neo-, Lecto-, and Hypostratotypes.

The following terminology is in common use in designating the stratotypes of layered stratigraphic units.

Holostratotype.

The original stratotype designated by the original author at the time of proposing a stratigraphic unit or boundary.

Parastratotype.

A supplementary stratotype used in the original definition by the original author to illustrate the diversity or heterogeneity of the defined stratigraphic unit or some critical feature not evident or exposed in the holostratotype.

Lectostratotype.

A stratotype for a previously described stratigraphic unit selected later in the absence of an adequately designated original stratotype (holostratotype).

Neostratotype.

A new stratotype selected to replace an older one which has been destroyed, covered, or otherwise made inaccessible.

Hypostratotype

(also called reference section, auxiliary reference section). A stratotype proposed after the original designation of the holostratotype (and parastratotype) in order to extend knowledge of the unit or boundary to other geographic areas. It is always subordinate to the holostratotype.

Thus a holostratotype and a parastratotype are originally designated primary types; a lectostratotype and a neostratotype are subsequently designated primary types; and a hypostratotype is a subsequently designated secondary (reference or auxiliary) type, always subsidiary to a primary type.

Holostratotypes and parastratotypes are generally situated within the type area. Neostratotypes and lectostratotypes are preferably chosen within the limits of the original type area. Hypostratotypes may be chosen beyond the limits of the original type area.

Similar conventions apply to the designation and description of type localities and reference localities for nonlayered igneous or metamorphic rock bodies.

In principle, stratotypes and type localities should not be altered or amended. There may be more than one typical section or locality but only one stratotype (type section) or type locality.

Requirements for Stratotypes (Type Sections)

Stratotypes for each different category of layered stratigraphic units— lithostratigraphic, magnetostratigraphic, chronostratigraphic, etc.—require individual consideration and are discussed in the chapters covering each type of unit. The following requirements apply to stratotypes in general.

Expression of Concept.

The most important requisite of a stratotype is that it adequately represents the concept for which it is the material type. A complete exposure of all rocks in a unit from bottom to top and throughout its entire lateral extent would be the ideal unit-stratotype. However, because it is impossible to find or establish such a comprehensive stratotype, reliance is placed on the single section or area with the best exposures. Lack of continuous exposure or the presence of structural complications may make it difficult to find even such a single complete outcrop through an entire stratigraphic unit. It is then necessary to resort to a composite-stratotype or to supplementary and reference sections or areas (para-stratotypes and hypostratotypes), or to express the unit-stratotype simply as the stratigraphic interval between a designated boundary-stratotype marking the base of the unit and another designated boundary-stratotype marking the top of the unit.

In the case of global chronostratigraphic units (e.g., systems, series, stages) it has been recommended (Geological Society of London, 1967; McLaren, 1977) that the definition should place emphasis in the selection of the boundary-stratotype of its lower boundary; its upper boundary is defined as the lower boundary of the overlying unit. The term “Global boundary stratotype section and point (GSSP)” has been proposed for these standard boundary-stratotypes of units of the Global Chronostratigraphic Scale (Cowie et ah, 1986; Cowie, 1986) (see sections 9.H.2 and 9.H.3 and Figure 2B).

Description.

The description of a stratotype is both geographic and geologic. The geographic description enables anyone to find the stratotype readily in the field. It includes a detailed location map and indication of the means of access to the type locality. It is desirable to include air photographs and other photographs at an appropriate scale to show the geographic extent of the unit in the type area and the geographic position of its boundaries.

The geologic description covers lithology, thickness, paleontology, mineralogy, structure, geomorphic expression, and other geologic features of the type section. The boundaries and relations with adjacent units particularly are described in detail, and reasons for choice of boundaries given. The description is accompanied by graphic profiles, columnar sections, structure sections, and photographs. A geologic map of the area including the stratotype is essential (see section 3.B.1).

Identification and Marking.

An important requirement of a stratotype is that it be clearly marked. A boundary-stratotype should be based on a single point in a designated sequence of rock strata, serving to indicate the position of the boundary horizon at one place. (Lateral extension of the boundary horizon in any direction from this point is accomplished by stratigraphic correlation.) A unit-stratotype should be clearly delimited by boundary-stratotypes for its base and for its top. Preferably, a boundary-stratotype or the limits of a unit-stratotype should be indicated by a permanent artificial marker, but in any case boundary points should be described geographically and geologically in such detail that there can be no doubt as to their exact location.

Accessibility and Assurance of Preservation.

If the stratotype is to fulfill the role of a standard, it should be situated in an area geographically accessible to all who are interested, regardless of political or other circumstances, and it should offer reasonable assurance of long-range preservation.

Subsurface Stratotypes.

Subsurface stratotypes are acceptable if adequate surface sections are lacking and if adequate subsurface samples and logs are available. Definition of a subsurface stratotype is also justified where the subsurface section differs materially from the laterally equivalent surface section or where there is doubt about the equivalence of a subsurface and a surface section (see section 3.B.2).

Acceptability.

The usefulness of the stratotypes for stratigraphic units of international extent is directly related to the extent to which they are generally accepted or acknowledged as the standard of reference for the units. It is, therefore, always desirable, and in due course to be expected, that the designation of a stratotype be submitted for approval to the geologic body having the highest standing in any particular case.

Stratotypes for the boundaries of chronostratigraphic units of international or worldwide application must be approved by appropriate bodies of the highest international or worldwide geologic standing, at present the IUGS International Commission on Stratigraphy (Cowie, 1986; Cowie et al, 1986). On the other hand, stratotypes of units of only local extent and interest may only require approval from local or national surveys or stratigraphic commissions.

Requirements for Type Localities of Nonlayered Igneous or Metamorphic Rock Bodies

Requirements for the proper selection of type localities and reference localities for nonlayered igneous or metamorphic rock bodies are similar to those that apply to the selection of stratotypes (type sections) of layered stratigraphic units. For example, the type localities and reference localities should represent the concept of the unit, should be carefully described, both geographically and geologically, and should be easily accessible (see section 4.C).

Figures & Tables

Figure 2.

(A) Unit-stratotype and boundary-stratotypes for a lithostratigraphic unit (Formation B). (B) Boundary-stratotype for a chronostratigraphic unit. A point in the rock sequence defines the lower boundary of Stage B (see section 9.H.2).

Figure 2.

(A) Unit-stratotype and boundary-stratotypes for a lithostratigraphic unit (Formation B). (B) Boundary-stratotype for a chronostratigraphic unit. A point in the rock sequence defines the lower boundary of Stage B (see section 9.H.2).

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