Guia de nomenclatura estratigráfica

Guia de nomenclatura estratigráfica

(Parte 2 de 15)

Table 1. Classes of Units Defined*

I. CATEGORIES EXPRESSING OR RELATED TO GEOLOGIC AGE A. Material Categories Used to Define Temporal Spans

B. Temporal (Non-Material) Categories

*Numbers in parentheses are the numbers of the Articles where units are defined.

**Italicized categories are those introduced or developed since publication of the previous code (ACSN, 1970).

North American Commission on Stratigraphic Nomenclature 1557 based on fossil content (biostratigraphic units, Article 48). Second, the irreversibility of organic evolution makes it possible to partition enclosing strata temporally. Third, biologic remains provide important data for the reconstruction of ancient environments of deposition.

Composition also is important in distinguishing pedostratigraphic units. A pedostratigraph ic unit is a body of rock that consists of one or more pedologic horizons developed in one or more lithic units now buried by a formally defined lithostratigraphic or allostratigraphic unit or units. A pedostratigraphic unit is the part of a buried soil characterized by one or more clearly defined soil horizons containing pedogenically formed minerals and organic compounds. Pedostratigraphic terminology is discussed below and in Article 5.

Many upper Cenozoic, especially Quaternary, deposits are distinguished and delineated on the basis of content, for which lithostratigraphic classification is appropriate. However, others are delineated on the basis of criteria other than content. To facilitate the reconstruction of geologic history, some compositionally similar deposits in vertical sequence merit distinction as separate stratigraphic units because they are the products of different processes; others merit distinction because they are of demonstrably different ages. Lithostratigraphic classification of these units is impractical and a new approach, allostratigraphic classification, is introduced here and may prove applicable to older deposits as well. An allostratigraphic unit is a mappable body of rock defined and identified on the basis of bounding discontinuities (Article 58 and related Remarks).

Geologic-Climateunits,definedinthe1970Code(ACSN, 1970, p. 31), were abandoned in the 1983 Code becausethey proved to be of dubious utility. Inferences regarding climate are subjective and too tenuous a basis for the definition of formal geologic units. Such inferences commonly are based on deposits assigned more appropriately to lithostratigraphic or allostratigraphic units and may be expressed in terms of diachronic units (defined below).

Categories Expressing or Related to Geologic Age

Time is a single, irreversible continuum. Nevertheless, various categories of units are used to define intervals of geologic time, just as terms having different bases, such as Paleolithic, Renaissance, and Elizabethan, are used to designate specific periods of human history. Different temporal categories are established to express intervals of time distinguished in different ways.

Major objectives of stratigraphic classification are to provide a basis for systematic ordering of the time and space relations of rock bodies and to establish a time framework for the discussion of geologic history. For such purposes, units of geologic time traditionally have been named to represent the span of time during which a well-described sequence of rock, or a chronostratigraphic unit, was deposited (‘‘time units based on material referents,’’ Figure 1). This procedure continues, to the exclusion of other possible approaches, to be standard practice in studies of Phanerozoic rocks. Despite admonitions in previous American codes and

Figure 1. Relation of geologic time units to the kinds of referents on which most are based.

1558 North American Stratigraphic Code the International Stratigraphic Guide (ISSC, 1976, p. 81; 1994, p. 87) that similar procedures should be applied to the Precambrian, no comparable chronostratigraphic units, or geochronologic units derived therefrom, proposed for the Precambrian have yet been accepted worldwide. Instead, the IUGS Subcommission on Precambrian Stratigraphy (Sims, 1979) and its Working Groups (Harrison and Peterman, 1980) recommend division of Precambrian time into geochronometric units having no material referents.

A distinction is made throughout this report between isochronous and synchronous, as urged by Cumming et al. (1959, p. 730), although the terms have been used synonymously by many. Isochronous means of equal duration; synchronous means simultaneous, or occurring at the same time. Although two rock bodies of very different ages may be formed during equal durations of time, the term isochronous is not applied to them in the earth sciences. Rather, isochronous bodies are those bounded by synchronous surfaces and formed during the same span of time. Isochron, in contrast, is used for a line connecting points of equal age on a graph representing physical or chemical phenomena; the line represents the same or equal time. The adjective diachronous is applied either to a rock unit with one or two bounding surfaces that are not synchronous, or to a boundary that is not synchronous (that ‘‘transgresses time’’).

Two classes of time units based on material referents, or stratotypes, are recognized (Figure 1). The first is that of the traditional and conceptually isochronous units, and includes geochronologic units, which are based on chronostratigraphic units, and polarity-chronologic units. These isochronous units have worldwide applicability and may be used even in areas lacking a material record of the named span of time. The second class of time units, newly defined in this Code, consists of diachronic units (Article 91) that are based on rock bodies known to be diachronous. In contrast to isochronous units, a diachronic term is used only where a material referent is present; a diachronic unit is coextensive with the material body or bodies on which it is based.

A chronostratigraphic unit, as defined above and in

Article 6, is a body of rock established to serve as the material reference for all rocks formed during the same span of time; its boundaries are synchronous. It is the referent for a geochronologic unit, as defined above and in Article 80. Internationally accepted and traditional chronostratigraphic units were based initially on the time spans of lithostratigraphic units, biostratigraphic units, or other features of the rock record that have specific durations. In sum, they form the StandardGlobalChronostratigraphicScale(ISSC,1976,p.76– 81; 1994, p. 85; Harland, 1978), consisting of established systems and series.

A polarity-chronostratigraphic unit is a body of rock that contains a primary magnetopolarity record imposed when the rock was deposited or crystallized (Article 83). It serves as a material standard or referent for a part of geologic time during which the Earth’s magnetic field had a characteristic polarity or sequence of polarities; that is, for a polaritychronologic unit (Article 8).

A diachronic unit comprises the unequal spans of time represented by one or more specific diachronous rock bodies (Article 91). Such bodies may be lithostratigraphic, biostrati- graphic,pedostratigraphic,allostratigraphic, or anassemblage of such units. A diachronic unit is applicable only where its material referent is present.

A geochronometric (or chronometric) unit is an isochronous direct division of geologic time expressed in years (Article 96). It has no material referent.

Pedostratigraphic Terms

The definition and nomenclature for pedostratigraphic units2 in this Code differ from those for soil-stratigraphic units in the 1970 Code (ACSN, 1970, Article 18), by being more specific with regard to content, boundaries, and the basis for determining stratigraphic position.

The term ‘‘soil’’ has different meanings to the geologist, the soil scientist, the engineer, and the layman, and commonly has no stratigraphic significance. The term paleosol is currently used in North America for any soil that formed on a landscape of the past; it may be a buried soil, a relict soil, or an exhumed soil (Ruhe, 1965; Valentine and Dalrymple, 1976).

Apedologicsoiliscomposedofoneormoresoilhorizons3.

A soil horizon is a layer within a pedologic soil that (1) is approximately parallel to the soil surface, (2) has distinctive physical, chemical, biological, and morphological properties that differ from those of adjacent, genetically related, soil horizons, and (3) is distinguished from other soil horizons by objective compositional properties that can be observed or measured in the field. The physical boundaries of buried pedologic horizons are objective traceable boundaries with stratigraphic significance. A buried pedologic soil provides the material basis for definition of a stratigraphic unit in pedostratigraphic classification (Article 5), but a buried pedologic soil may be somewhat more inclusive than a pedostratigraphic unit. A pedologic soil may contain both an O horizon and the entire C horizon (Figure 6), whereas the former is excluded and the latter need not be included in a pedostratigraphic unit.

The definition and nomenclature for pedostratigraphic units in this Code differ from those of soil stratigraphic units proposed by the International Union for Quaternary Research and International Society of Soil Science (Parsons, 1981). The pedostratigraphic unit, geosol, also differs from the proposed INQUA-ISSS soil-stratigraphic unit, pedoderm, in several ways, the most important of which are the following: (1) a geosol may be in any part of the geologic column, whereas a pedoderm is a surficial soil; (2) a geosol is a buried soil, whereas a pedoderm may be a buried, relict, or exhumed soil; (3) the boundaries and stratigraphic position of a geosol are defined and delineated by criteria that differ from those for a pedoderm; and (4) a geosol may be either all or only a part of a buried soil, whereas a pedoderm is the entire soil.

2From Greek, pedon, ground or soil. 3As used in a geological sense, a horizon is a surface or line. In pedology, however, it is a body of material, and such usage is continued here.

North American Commission on Stratigraphic Nomenclature 1559

The term geosol, as defined by Morrison (1967, p. 3), is a laterally traceable, mappable, geologic weathering profile that has a consistent stratigraphic position. The term is adopted and redefined here as the fundamental and only unit in formal pedostratigraphic classification (Article 56).

Although the Code emphasizes formal categories of geologic units, informal nomenclature is highly useful in stratigraphic work.

Formally named units are those that are named in accordance with an established scheme of classification; the fact of formality is conveyed by capitalization of the initial letter of the rank or unit term (for example, Morrison Formation). Informal units, whose unit terms are ordinary nouns, are not protected by the stability provided by proper formalization and recommended classification procedures. Informal terms are devised for both economic and scientific reasons. Formalization is appropriate for those units requiring stability of nomenclature, particularly those likely to be extended far beyond the locality in which they were first recognized. Informal terms are appropriate for casually mentioned and innovative units. Also, most economic units, those defined by unconventional criteria, and those that may be too thin to map at usual scales may be informal.

Casually mentioned geologic units not defined in accordance with this Code are informal. For many of these, there may be insufficient need or information, or perhaps an inappropriate basis, for formal designations. Informal designations as beds or lithozones (the pebbly beds, the shaly zone, third coal) are appropriate for many such units.

Most economic units, such as aquifers, oil sands, coal beds, quarry layers, and ore-bearing ‘‘reefs,’’ are informal, even though they may be named. Some such units, however, are so significant scientifically and economically that they merit formal recognition as beds, members, or formations.

Innovative approaches in regional stratigraphic studies have resulted in the recognition and definition of units best left as informal, at least for the time being. Units bounded by major regional unconformities on the North American cratonweredesignated‘‘sequences’’(example:Sauksequence) by Sloss (1963). Major unconformity-bounded units also were designated ‘‘synthems’’ by Chang (1975), who recommended that they be treated formally. Marker-defined units that are continuous from one lithofacies to another were designated ‘‘formats’’ by Forgotson (1957). The term ‘‘chronosome’’ was proposed by Schultz (1982) for rocks of diverse facies corresponding to geographic variations in sedimentation during an interval of deposition identified on the basis of bounding stratigraphic markers. Successions of faunal zones containing evolutionally related forms, but bounded by non-evolutionary biotic discontinuities, were termed ‘‘biomeres’’ (Palmer, 1965). The foregoing are only a few selected examples to demonstrate how informality provides a continuing avenue for innovation.

The terms magnafacies and parvafacies, coined by Caster (1934)to emphasize the distinctionbetweenlithostratigraphic and chronostratigraphicunits in sequences displaying marked facies variation, have remained informal despite their impact on clarifying the concepts involved.

Tephrochronologic studies provide examples of informal units that are too thin to map at conventional scales but yet invaluable for dating important geologic events. Although some such units are named for physiographic features and places where first recognized (e.g., Guaje pumice bed, where it is not mapped as the Guaje Member of theB andelier Tuff), others bear thes amen amea st he volcanic vent (e.g., Huckleberry Ridge ash bed of Izett and Wilcox, 1981).

Informalgeologicunitsaredesignatedbyordinarynouns, adjectives, or geographic terms and lithic or unit terms that are not capitalized (chalky formation or beds, St. Francis coal).

No geologic unit should be established and defined, whether formally or informally, unless its recognition serves a clear purpose.

Correlation is a procedure for demonstrating correspondence between geographically separated parts of a geologic unit. The term is a general one having diverse meanings in different disciplines. Demonstration of temporal correspondence is one of the most important objectives of stratigraphy. The term correlation frequently is misused to express the idea that a unit has been identified or recognized.

Correlation is used in this Code as the demonstration of correspondence between two geologic units in both some defined property and relative stratigraphic position. Because correspondence may be based on various properties, three kinds of correlation are best distinguished by more specific terms. Lithocorrelation links units of similar lithology and stratigraphic position (or sequential or geometric relation for lithodemic units). Biocorrelation expresses similarity of fossil content and biostratigraphic position. Chronocorrelation expresses correspondence in age and in chronostratigraphic position.

Other terms that have been used for the similarity of content and stratal succession are homotaxy and chronotaxy. Homotaxy is the similarity in separate regions of the serial arrangement or succession of strata of comparable compositions or of included fossils. The term is derived from homotaxis, proposed by Huxley (1862, p. xlvi) to emphasize that similarity in succession does not prove age equivalence of comparable units. The term chronotaxy has been applied to similar stratigraphic sequences composed of units that are of equivalent age (Henbest, 1952, p. 310).

Criteria used for ascertaining temporal and other types of correspondence are diverse (ISSC, 1976, p. 86–93; 1994, p. 92–97) and new criteria will emerge in the future. Evolving statistical tests, as well as isotopic and paleomagnetic techniques, complement the traditional paleontologic and lithologic procedures. Boundaries defined by one set of criteria need not correspond to those defined by others.

1560 North American Stratigraphic Code

Article 1.—Purpose. This Code describes explicit stratigraphic procedures for classifying and naming geologic units accorded formal status. Such procedures, if widely adopted, assure consistent and uniform usage in classification and terminologyand,therefore,promoteunambiguouscommunication.

Article2.—Categories. Categoriesof formalstratigraphic units, though diverse, are of three classes. The first class (I on Table 1) is of rock-material categories based on content, inherent attributes, or physical limits, and includes lithostratigraphic, lithodemic, magnetopolarity, biostratigraphic, pedostratigraphic, and allostratigraphic units. The second class (IIA on Table 1) is of material categories used as standards for defining spans of geologic time, and includes chronostratigraphic and polarity-chronostratigraphic units. The third class (IIB on Table 1) is of non-material temporal categories, andincludesgeochronologic,polarity-chronologic,diachronic, and geochronometric units.

Article 3.—Requirements for FormallyNamedGeologic

Units. Naming, establishing, revising, redefining, and abandoning formal geologic units require publication in a recognizedscientificmediumofacomprehensivestatement,which includes (i) intent to designate or modify a formal unit; (i) designation of category and rank of unit; (ii) selection and derivation of name; (iv) specification of stratotype (where applicable); (v) description of unit; (vi) definition of boundaries;(vii)historical background;(viii)dimensions,shape,and other regional aspects; (ix) geologic age; (x) correlations; and possibly (xi) genesis (where applicable). These requirements apply to subsurface and offshore, as well as exposed, units.

Article 4.—Publication.4 ‘‘Publication in a recognized scientific medium’’ in conformance with this Code means that a work, when first issued, must (1) be reproduced in ink on paper; be reproduced electronically on CD-ROM, on the Internet, or by another electronic method widely accepted by the scientific community; or be reproduced by some method that assures numerous identical copies and wide distribution; (2) be issued for the purpose of scientific, public, permanent record; (3) be readily obtainable by purchase or free distribution; and (4) have undergone adequate peer review.

Remarks. (a) Inadequate publication.—The following do not constitute publication within the meaning of the Code: (1) dis- tribution of microfilms, microcards, or matter reproduced by similar methods; (2) distribution to colleagues or students of a note, even if printed, in explanation of an accompanying illustration; (3) distribution of proof sheets; (4) open-file release; (5) theses, dissertations, and dissertation abstracts; (6) mention at a scientific or other meeting; (7) mention in an abstract, map explanation, or figure caption; (8) labeling of a rock specimen in a collection; (9) mere deposit of a document in a library; (10) anonymous publication; (1) mention in the popular press or in a legal document; (12) distribution by an author by posting on the Internet, or by another electronic medium, a document that has not undergone the procedures stated below (Remark c).

(Parte 2 de 15)

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