Skip to Main Content

Abstract

Unconformity-bounded units are bodies of rocks bounded above and below by significant unconformities.

Unconformity-bounded units are generally composed of diverse types of any kind or kinds of rocks (sedimentary, igneous, metamorphic, or any combination of two or more of these kinds). Unconformity-bounded units are mappable stratigraphic units differentiated and set apart from underlying and overlying units only by being separated from them by their bounding stratigraphic discontinuities. Lithologic properties, fossil content, and chronostratigraphic span of the rocks on either side of a bounding unconformity are significant for the recognition of an unconformity-bounded unit only to the extent that they serve to recognize the boundary unconformity.

Lithostratigraphic, biostratigraphic, and chronostratigraphic units will continue to be those most frequently used in stratigraphic work, but in certain areas and for certain purposes, unconformity-bounded units are invaluable stratigraphic units, almost “natural” units, which the stratigrapher may be able to use for a clear and pragmatic approach to stratigraphic analysis and for a descriptive, lucid interpretation of geologic history. Unconformity-bounded units lend themselves, for instance, to the expression of those aspects of the geologic development of the Earth dealing with its orogenic episodes, its epeirogenic cycles, and its phases of eustatic sea-level changes. These geological events are commonly recorded by unconformities in the stratigraphic succession. Unconformity-bounded units, for this reason, have sometimes been considered to be equivalent to “sedimentary cycles” or tectonically controlled stratigraphic units: stratotec-tonic, tectostratigraphic, tectonostratigraphic, or tectogenic units; tectonic cycles; tectosomes; structural or tectonic stages; and so on. All of these types

Nature of Unconformity-Bounded Units

Unconformity-bounded units are bodies of rocks bounded above and below by significant unconformities.

Unconformity-bounded units are generally composed of diverse types of any kind or kinds of rocks (sedimentary, igneous, metamorphic, or any combination of two or more of these kinds). Unconformity-bounded units are mappable stratigraphic units differentiated and set apart from underlying and overlying units only by being separated from them by their bounding stratigraphic discontinuities. Lithologic properties, fossil content, and chronostratigraphic span of the rocks on either side of a bounding unconformity are significant for the recognition of an unconformity-bounded unit only to the extent that they serve to recognize the boundary unconformity.

Lithostratigraphic, biostratigraphic, and chronostratigraphic units will continue to be those most frequently used in stratigraphic work, but in certain areas and for certain purposes, unconformity-bounded units are invaluable stratigraphic units, almost “natural” units, which the stratigrapher may be able to use for a clear and pragmatic approach to stratigraphic analysis and for a descriptive, lucid interpretation of geologic history. Unconformity-bounded units lend themselves, for instance, to the expression of those aspects of the geologic development of the Earth dealing with its orogenic episodes, its epeirogenic cycles, and its phases of eustatic sea-level changes. These geological events are commonly recorded by unconformities in the stratigraphic succession. Unconformity-bounded units, for this reason, have sometimes been considered to be equivalent to “sedimentary cycles” or tectonically controlled stratigraphic units: stratotec-tonic, tectostratigraphic, tectonostratigraphic, or tectogenic units; tectonic cycles; tectosomes; structural or tectonic stages; and so on. All of these types of units, however, have a definite genetic and causal meaning and require for their recognition an interpretation of the observed stratigraphic relationships. Unconformity-bounded units, in contrast, entail no such genetic or causal interpretation; they are objective, noninterpretative units. Calling a unit a “tectonic stage,” for instance, implies that the unconformities bounding the unit are the result of tectonic events; unconformity-bounded units, on the other hand, are established and recognized without any regard to the cause of their bounding unconformities, whether they are the result or orogenic events, epeirogenic episodes, eustatic sea-level changes, or any combination of them.

Unconformity-bounded units are useful and have been used extensively in stable cratonic areas where their boundary discontinuities are geographically extensive and where the stratigraphic “breaks” that these discontinuities represent have great significance in deciphering the geologic history of the area. They can be just as useful, however, in orogenic belts and other tectonically less stable areas, particularly if the units are properly established and not equated with lithostratigraphic units, tectonically controlled stratigraphic units, or chronostratigraphic units.

Unconformities, disconformities, and significant hiatuses in the stratigraphic section have long been used as boundaries of stratigraphic units. This is not surprising, as the abrupt lithologic changes, faunal “breaks,” and structural angularity that commonly occur at such stratigraphic discontinuities are some of the most noticeable and distinct features recognizable in any stratigraphic succession. Bounding unconformities, in fact, were the basis for establishing many of the earliest stratigraphic units. Many of the systems of the presently accepted Standard Global Chronostratigraphic Scale were originally unconformity-bounded units.

No effort was made for many years, however, to recognize unconformity-bounded units as a distinct, independent category of stratigraphic units. This changed as geologists came to realize that rock bodies bounded by unconformities serve useful purposes in stratigraphy and that they require recognition and their own terminology to permit their differentiation from other kinds of stratigraphic units (International Subcommission on Stratigraphic Classification, 1987a).

The relation of unconformity-bounded units to other kinds of stratigraphic units is discussed in Chapter 10.

Definitions

Unconformity-bounded Unit.

A body of rocks bounded above and below by specifically designated, significant and demonstrable discontinuities in the stratigraphic succession (angular unconformities, disconformities, etc.), preferably of regional or interregional extent. The diagnostic criteria used to establish and recognize these stratigraphic units are its two designated bounding unconformities.

Unconformity-bounded units may include any number of other kinds of stratigraphic units (lithostratigraphic, biostratigraphic, chronostratigraphic, magnetostratigraphic, and so on), from a few to many, both in vertical and/or lateral succession (Figure 4).

Figure 4.

Relation of unconformity-bounded units to other kinds of stratigraphic units included within them. Unconformity-bounded units may include any number of other kinds of stratigraphic units: Lithostratigraphic (A), biostratigraphic (B), chronostratigraphic (C), and so on. The boundaries of unconformity-bounded units can be parallel or, most commonly, can intersect at an angle the boundaries of the stratigraphic units they include.

Figure 4.

Relation of unconformity-bounded units to other kinds of stratigraphic units included within them. Unconformity-bounded units may include any number of other kinds of stratigraphic units: Lithostratigraphic (A), biostratigraphic (B), chronostratigraphic (C), and so on. The boundaries of unconformity-bounded units can be parallel or, most commonly, can intersect at an angle the boundaries of the stratigraphic units they include.

Unconformity.

For the purpose of establishing and recognizing unconform-ity-bounded units, an unconformity is defined as a surface of erosion between rock bodies, representing a significant hiatus or gap in the stratigraphic succession. “Significant” is, of course, a subjective term difficult to quantify (see section 6.E). Unconformities result from subaerial or subaqueous exposure of rocks below the unconformity and erosion with loss of part of the older rocks. Several kinds of unconformities have been generally recognized:

Angular Unconformity.

An unconformity in which the bedding planes above and below the unconformity are at an angle to each other, denoting either tilting or folding before erosion and subsequent deposition, or strong onlap.

Disconformity.

An unconformity in which the bedding planes above and below the stratigraphic break are essentially parallel. This apparent parallelism, however, is for the most part restricted in areal extent. At a regional scale, some degree of angularity (truncation below or onlap above) is usually present at a disconformity. Even when angularity cannot be detected, the upper surface of the rock section directly underlying a disconformity often shows evidence of erosion and/or of a discontinuity in deposition, such as an irregular or uneven erosion surface or indications of weathering. A disconformity may be marked also by evidence of a significant depositional hiatus in the stratigraphic succession.

Diastem.

A short interruption in deposition with little or no erosion before resumption of sedimentation. These short hiatuses are generally very limited in lateral extent and are not an appropriate basis for establishing unconformitybounded units.

Kinds of Unconformity-Bounded Units

The basic unconformity-bounded unit is the synthem (from the Greek syn meaning “together” and them meaning “the deposit of”). When needed and useful, a synthem may be subdivided into two or more subsynthems and two or more synthems may be combined into a supersynthem.

Hierarchy of Unconformity-Bounded Units

To establish a useful hierarchy of unconformity-bounded units—supersyn-them-synthem-subsynthem—presents some problems: since the single diagnostic criterion for defining and recognizing this type of stratigraphic units is the presence or absence of their bounding stratigraphic discontinuities, any hierar-chy could only be based on the magnitude or importance of these discontinuities. Such magnitude or importance, however, can be judged from at least three points of view: the degree of structural angularity, the time span of the stratigraphic hiatus represented by the discontinuity, and the geographic extension of the discontinuity. All three criteria are often independent of one another. Each may be most diagnostic in any one case. Structural angularity, for example, may have more significance in mobile belts, whereas the magnitude of the time gap may be more important in stable cratonic areas. It is evident that the danger of a contradictory and inconsistent classification outweighs any benefits of a structured hierarchy. The recognition of supersynthems and subsynthems should, therefore, be restricted as much as possible.

Procedures for Establishing Unconformity-Bounded Units

The procedure for establishing unconformity-bounded units should follow the general procedure recommended for all stratigraphic units (see section 3.B). As in the case of other stratigraphic units, to establish a new unconformity-bounded unit requires the publication, in a recognized scientific medium, of a statement of intent to establish such a new unit, together with an appropriate description.

Because the presence or absence of the bounding discontinuities is the single diagnostic criterion for establishing, defining, recognizing, and extending unconformity-bounded units, the definition and description of these units should emphasize the discussion of the nature, position, and characteristics of the discontinuities: geologic and geographic identification of their stratotypes and reference sections by means of verbal descriptions, maps, cross sections, photos, etc.; geographic extent, variation in the nature and character away from the stratotype or stratotypes and reference sections; regional stratigraphic relations to included and adjacent stratigraphic units.

The stratotypes of the lower and upper bounding unconformities need no be located in the same section.

The internal attributes are not diagnostic properties in the definition and identification of an unconformity-bounded unit, but descriptions of the lithology, thickness, fossil content, geomorphic expression, and age of the unit throughout the area where it is present are necessary in the recognition of an unconformity-bounded unit.

The description may also include a discussion of the origin of the unit and particularly of its bounding unconformities—their relationship to tectonism and/or eustatic sea-level changes—and the significance of these causative geologic processes in the geologic history of the area.

Unconformity-bounded units should be established only where and when they can fulfill a need that other kinds of stratigraphic units cannot meet, where they can contribute to the understanding of the stratigraphy and geologic history of an area, where they can provide the framework for the regional stratigraphic analysis of sedimentary basins, and where they can allow mapping procedures and the expression of stratigraphic concepts for which other stratigraphic units are inadequate.

Every pair of unconformities in a stratigraphic sequence does not require the establishment, recognition, and naming of an unconformity-bounded unit. Particularly toward the margins of sedimentary basins, unconformities develop at many levels in the section. If unconformity-bounded units were to be established for each pair of these many unconformities, the number of stratigraphic units would grow unmanageably. Most such unconformities are of limited geographic extent and do not lend themselves to the establishment of meaningful and useful unconformity-bounded units of regional or interregional scope. (The case is not different from that of lithostratigraphic and biostratigraphic units. In principle, the boundaries of these kinds of units can be established at every vertical and lateral change in lithologic properties or at every appearance or dis-appearance of particular fossil forms, but this is seldom done to avoid an unnecessarily complex subdivision and terminology.)

Stratigraphers, therefore, must decide when and where to establish uncon-formity-bounded units. Excesses may occur, but usage (or lack thereof) and time will rectify them. Stratigraphers will have to be as prudent in establishing and naming unconformity-bounded units as they generally are in establishing and naming lithostratigraphic and biostratigraphic units. They should establish and name only those unconformity-bounded units which they need and find useful, just as they recognize and name only needed and useful lithostratigraphic and biostratigraphic units among the infinite possibilities offered by the complexities of the stratigraphic record.

Procedures for Extending Unconformity-Bounded Units

An unconformity-bounded unit should be extended laterally only as far as both of its bounding unconformities are identifiable.

Naming of Unconformity-Bounded Units

The names of unconformity-bounded units should be formed from the name of an appropriate geographic feature at or near the location where the unit is well developed, combined with the term “synthem” (or “subsynthem” or “supersynthem”).

The choice of the geographic component of the name should follow the rules and procedures described in section 3.B.3.a.

Stratigraphic units similar to the synthems of this Guide have been named alio stratigraphic units (alio group, alloforrnation, allomember) in the North American Stratigraphic Code (1983, p. 865-867). The term “sequence,” used by Sloss et al. (1949), Krumbein and Sloss (1951), Sloss (1963), and many subsequent authors for a lithostratigraphic unit bounded by unconformities, has more recently been redefined (Mitchum, Vail, and Thompson, 1977, p. 53) to designate units composed of a relatively conformable succession of genetically related strata bounded at its top and base by unconformities or their correlative conformities. These redefined “sequences” are the basic units of what has been called “sequence stratigraphy” (see Glossary of Stratigraphic Terms).

Revision of Unconformity-Bounded Units

Revision or redefinition of an established formal unconformity-bounded unit without changing its name requires the same procedures as for proposing a new unit (see section 3.B.5).

Figures & Tables

Figure 4.

Relation of unconformity-bounded units to other kinds of stratigraphic units included within them. Unconformity-bounded units may include any number of other kinds of stratigraphic units: Lithostratigraphic (A), biostratigraphic (B), chronostratigraphic (C), and so on. The boundaries of unconformity-bounded units can be parallel or, most commonly, can intersect at an angle the boundaries of the stratigraphic units they include.

Figure 4.

Relation of unconformity-bounded units to other kinds of stratigraphic units included within them. Unconformity-bounded units may include any number of other kinds of stratigraphic units: Lithostratigraphic (A), biostratigraphic (B), chronostratigraphic (C), and so on. The boundaries of unconformity-bounded units can be parallel or, most commonly, can intersect at an angle the boundaries of the stratigraphic units they include.

Contents

GeoRef

References

Related

Citing Books via

Close Modal
This Feature Is Available To Subscribers Only

Sign In or Create an Account

Close Modal
Close Modal