Guia de nomenclatura estratigráfica

Guia de nomenclatura estratigráfica

(Parte 7 de 15)

Article 47.—Compound Name. The formal name of a magnetopolarity zone should consist of a geographic name and the term Polarity Zone. The term may be modified by Normal, Reversed,o r Mixed (example: Deer Park Reversed Polarity Zone). In naming or revising magnetopolarity units, appropriate parts of Articles 7 and 19 apply. The use of informal designations, e.g., numbers or letters, is not precluded.

Preamble

Article 48.—Fundamentals of biostratigraphy. Biostratigraphy is the branch of stratigraphy that deals with the distribution of fossils in the stratigraphic record and the classification of bodies of rock or rock material into biostratigraphic units based on their contained fossils.

Remark. (a) Uniqueness.—Biostratigraphic units are distinct from all other kinds of stratigraphic units because their contained fossils record the unidirectional process of organic evolution. As such, the stratigraphic record as a whole contains an unrepeated sequence of fossil taxa that may be used to determine the relative age of their enclosing strata.

Nature and Boundaries

Article 49.—Nature of Biostratigraphic Units. A biostratigraphic unit is a body of rock that is defined or characterized by its fossil content.

Remarks. (a) Unfossiliferous rocks.—Those bodies of rock lacking named fossils have no biostratigraphic character and are, therefore, not amenable to biostratigraphic classification.

(b) Contemporaneity of rocks and fossils.—Most fossils are contemporaneous with the body of rock that contains them, including those derived from different, but coeval sedimentary environments. A body of rock, however, sometimes contains fossils derived from older or younger rocks. Fossils not contemporaneous with the enclosing body of rock should not be used to define, characterize, or identify a biostratigraphic unit.

(c) Independence from lithostratigraphic units.—Biostratigraphic units are based on criteria that differ fundamentally from those used for lithostratigraphic units. Their boundaries may or may not coincide with the boundaries of lithostratigraphic units, but they bear no inherent relation to them.

(d) Independence from chronostratigraphicunits.—The boundaries of most biostratigraphic units, unlike the boundaries of chronostratigraphic units, are both characteristically and conceptually diachronous. The vertical and lateral limits of the biostratigraphic unit represent the recorded limits in distribution of the defining or characterizing fossil elements. Regionally, the upper and lower boundaries of biostratigraphic units are rarely synchronous surfaces, whereas the lateral boundaries of biostratigraphic units are never synchronous surfaces. Nevertheless, biostratigraphic units are effective for interpreting chronostratigraphic relations.

Article 50.—Kinds of Biostratigraphic Units. The biozone is the fundamental biostratigraphic unit. Five specific kinds of biozones are recognized herein: range biozone, interval biozone, lineage biozone, assemblage biozone, and abundance biozone. These five kinds of biozones are not hierarchically interrelated. The words ‘‘range,’’ ‘‘interval,’’ ‘‘lineage,’’ ‘‘assemblage,’’ and ‘‘abundance’’ are merely descriptive terms. They represent different approaches in the process of setting up, and in the recognition of, a biozone. The kind of biozone chosen will depend on the nature of the biota, the approaches and preferences of the individual scientist, and the specific problem being investigated. The most common choice of biozone is one in which both the lower boundary and the upper boundary are based on the lowest occurrences of individual taxa; the two taxa may or may not have a direct phylogenetic link. The ranges of the taxa whose lowest or highest occurrences or maximum abundances define the boundaries of the biozone are not necessarily restricted to the biozone, nor is it necessary that they range through the entire biozone.

Remarks. (a) Range biozone.—A range biozone is a body of rock representing the known stratigraphic and geographic range of occurrence of any selected element or elements of the chosen fossil taxon, or taxa, present in the rock record. There are two kinds of range biozones: taxon-range biozone and concurrent-range biozone.

A taxon-range biozone (Figure 4A) is a body of rock representing the known stratigraphic and geographic range of a chosen taxon. A concurrent-range biozone (Figure 4B) is a body of rock including the concurrent, coincident, or overlapping part of the ranges of two specified taxa.

(b) Interval biozone.—An interval biozone is a body of rock between two specified biostratigraphic surfaces (biohorizons of the ISSC, 1994, p. 56). The features on which biohorizons are commonly based include lowest occurrences (Figure 4C), highest occurrences (Figure 4D), distinctive occurrences, or changes in the character of individual taxa (e.g., changes in the direction of coiling in foraminifera or in number of septa in corals).

(c) Lineage biozone.—A lineage biozone (Figure 4E) is a body of rock containing species representing a specific segment of an evolutionary lineage.

(d) Assemblage biozone.—An assemblage biozone (Figure 5A) is a body of rock characterized by a unique association of three or more taxa, the association of which distinguishes it in biostratigraphic character from adjacent strata. An assemblage biozone may be based on a single taxonomic group, for example, trilobites, or on more than one group, such as acritarchs and chitinozoans.

(e) Abundance biozone.—An abundance biozone (Figure 5B) is a body of rock in which the abundance of a particular taxon or specified group of taxa is significantly greater than in adjacent parts of the section. Abundance zones may be of limited, local utility

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because abundances of taxa in the geologic record are largely controlled by paleoecology, taphonomy, and diagenesis. The only unequivocal way to identify a particular abundance zone is to trace it laterally.

(f) Hybrid or new kinds of biozones.—As specific problems are faced, biostratigraphic analysis progresses, and new technologies appear, other forms of biozones may prove useful and are not prohibited under this Code.

Article 51.—Boundaries. The boundaries of a biozone are drawn at surfaces that mark the lowest occurrence, highest occurrence, limit, increase in abundance, or decrease in abundance of one or more components of the fauna or flora. Furthermore, the base or top of one kind of biozone may not, or need not, coincide with the base or top of another kind of biozone.

Remark. (a) Identification of biozones.—Boundaries of range biozones are the horizons of lowest and highest stratigraphic occurrence of the specified taxon or taxa. When two taxa are involved, the concurrent-range biozone is present only where both taxa are present. Boundaries of interval biozones are defined by two specified biostratigraphic surfaces, in which case the base of one biozone usually defines the top of the underlying biozone. Boundaries of

Figure 4. Examples of range, lineage, and interval biozones.

North American Commission on Stratigraphic Nomenclature 1575

lineage biozones are determined by the biohorizons representing the lowest occurrence(s) of successive elements in the evolutionary lineage under consideration. Boundaries of assemblage biozones may be difficult to define precisely, but such biozones are readily characterized and identified by the fully or partially overlapping ranges of enclosed taxa or groups of taxa. In any one section, however, not all characterizing taxa need be present in order to recognize the biozone, and the biozone may be characterized or identified by other taxa. Boundaries of abundance biozones are defined by marked changes in relative abundance of preserved taxa.

Article 52.—not used.

Ranks of Biostratigraphic Units

Article 53.—Fundamental Unit. The biozone is the fundamental unit of biostratigraphic classification.

Remarks. (a) Scope.—A single body of rock may be divided into more than one kind of biozone. A biozone may be based on a single taxonomic group or on several different taxonomic groups. Biozone boundaries derived from one taxonomic group need not, and commonly do not, coincide with those of another taxonomic group. Biozones vary greatly in their stratigraphic thickness and geographic extent, and taxonomic refinement or revision may increase or decrease the extent of a biozone.

(b) Divisions.—A biozone may be completely or partly divided into subbiozones. All rules for defining and characterizing biozones are also applicable to subbiozones.

(c) Shortened forms of expression.—‘‘Biozone’’ is a condensed expression for ‘‘biostratigraphic zone.’’ ‘‘Bio’’ should be used in front of ‘‘zone’’ to differentiate it from other types of zones, but the unadorned term ‘‘zone’’ may be used once it is clear that the term is a substitute for ‘‘biozone.’’ Furthermore, once it has been made clear what kind of biozone has been employed, the descriptive term is not required to become part of the formal name; for example, the Eurekaspirifer pinyonensis Assemblage Biozone can be designated simply as the Eurekaspirifer pinyonensis Biozone. When a biozone is described for the first time, however, the descriptive term should be capitalized; e.g., Exus albus Assemblage Biozone. Similarly, ‘‘subbiozone’’ may be shortened to ‘‘subzone’’ when the meaning is clear.

Biostratigraphic Nomenclature

Article 54.—Establishing Formal Units. Formal establishmentofabiozonemustmeettherequirementsofArticle3 and requires a unique name, a description of its fossil content and stratigraphic boundaries, and a discussion of its spatial extent.

Remarks. (a) Name.—The name of a biozone consists of the name of one or more distinctive taxa or parataxa (for trace fossils) found in the biozone, followed by the word ‘‘Biozone.’’ (e.g., Turborotalia cerrozaulensis Biozone or Cyrtograptus lundgreni-Testograptus testis Biozone). The name of the species whose lowest occurrence defines the base of the zone is the most common choice for the biozone name. Names of the nominate taxa, and hence the names of the biozones, conform to the rules of the international codes of zoological or botanical nomenclature or, in the case of trace fossils, internationally accepted standard practice.

(b) Shorter designations for biozone names.—Once a formal biozone has been established, an abbreviation or alpha-numeric designation that represents the name of the biozone may be a convenient substitute. For example, the Icriodus woschmidti Biozone was termed the woschmidti Zone by Klapper and Johnson (1980), and the Rhombodinium porosum Assemblage Zone in the Barton Beds was termed BAR-3 by Bujak et al. (1980).

(c) Revision.—Biozones and subbiozones are established empirically and may be modified on the basis of new evidence. Positions of established biozone or subbiozone boundaries may be refined stratigraphically, new characterizing taxa may be recognized, or original characterizing taxa may be superseded. If the concept of a particular biozone or subbiozone is substantially modified, a new unique designation is desirable.

(d) Defining taxa.—When a biozone or subbiozone is formally described, or later emended, it is necessary to designate, or redesignate, the defining or characterizing taxa, and/or to document the lowest and highest occurrences of the taxa that mark the biozone or subbiozone boundaries.

(e) Reference sections.—Biostratigraphic units do not have stratotypes in the sense of Article 3, item (iv), and Article 8. Nevertheless, it is desirable to designate a reference section in which the biostratigraphic unit is characteristically developed.

Nature and Boundaries

Article 5.—Nature of Pedostratigraphic Units. A pedostratigraphic unit is a body of rock that consists of one or more pedologic horizons developed in one or more lithostratigraphic, allostratigraphic,or lithodemic units (Figure 6)

Figure 5. Examples of assemblage and abundance biozones.

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and is overlain by one or more formally defined lithostratigraphicor allostratigraphicunits.

Remarks. (a) Definition.—A pedostratigraphic8 unit is a buried, traceable, three-dimensional body of rock that consists of one or more differentiated pedologic horizons.

(b) Recognition.—The distinguishing property of a pedostratigraphic unit is the presence of one or more distinct, differentiated, pedologic horizons. Pedologic horizons are products of soil development (pedogenesis) that occurred subsequent to formation of the lithostratigraphic, allostratigraphic, or lithodemic unit or units on which the buried soil was formed; these units are the parent materials in which pedogenesis occurred. Pedologic horizons are recognized in the field by diagnostic features such as color, soil structure, organicmatter accumulation, texture, clay coatings, stains, or concretions. Micromorphology, particle size, clay mineralogy, and other properties determined in the laboratory also may be used to identify and distinguish pedostratigraphic units.

(c) Boundaries and stratigraphic position.—The upper boundary of a pedostratigraphic unit is the top of the uppermost pedologic horizon formed by pedogenesis in a buried soil profile. The lower boundary of a pedostratigraphic unit is the lowest definite physical boundary of a pedologic horizon within a buried soil profile. The stratigraphic position of a pedostratigraphic unit is determined by its relation to overlying and underlying stratigraphic units (see Remark d).

(d) Traceability.—Practicability of subsurface tracing of the upper boundary of a buried soil is essential in establishing a pedostratigraphic unit because (1) few buried soils are exposed continuously for great distances, (2) the physical and chemical properties of a specific pedostratigraphic unit may vary greatly, both vertically and laterally, from place to place, and (3) pedostratigraphic units of different stratigraphic significance in the same region generally do not have uniqueidentifyingphysicalandchemicalcharacteristics.Consequently, extension of a pedostratigraphic unit is accomplished by lateral tracing of the contact between a buried soil and an overlying, formally defined lithostratigraphic or allostratigraphic unit, or between a soil and two or more demonstrably correlative stratigraphic units.

(e) Distinction from pedologic soils.—Pedologic soils may include organic deposits (e.g., litter zones, peat deposits, or swamp deposits) that overlie or grade laterally into differentiated buried soils. The organic deposits are not products of pedogenesis, and therefore, O horizons are not included in a pedostratigraphic unit (Figure 6); they may be classified as biostratigraphic or lithostratigraphic units. Pedologic soils also include the entire C horizon of a soil. The C horizon in pedology is not rigidly defined; it is merely the part of a soil profile that underlies the B horizon. The base of the C horizon in many soil profiles is gradational or unidentifiable; commonly it is placed arbitrarily. The need for clearly defined and easily recognized physical boundaries for a stratigraphic unit requires that the lower boundary of a pedostratigraphic unit be defined as the lowest definite physical boundary of a pedologic horizon in a buried soil profile, and part or all of the C horizon may be excluded from a pedostratigraphic unit.

(f) Relation to saprolite and other weathered materials.— A material derived by in situ weathering of lithostratigraphic, allostratigraphic, and/or lithodemic units (e.g., saprolite, bauxite, residuum) may be the parent material in which pedologic horizons form, but is not a pedologic soil. A pedostratigraphic unit may be based on the pedologic horizons of a buried soil developed in the product of in-situ weathering, such as saprolite. The parents of such a pedostratigraphic unit are both the saprolite and, indirectly, the rock from which it formed.

(g) Distinction from other stratigraphic units.—A pedostratigraphic unit differs from other stratigraphic units in that (1) it is a 8Terminology related to pedostratigraphic classification is summarized on p. 1559.

(Parte 7 de 15)

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