Guia de nomenclatura estratigráfica

Guia de nomenclatura estratigráfica

(Parte 6 de 15)

Article 3.—Lithodeme. The lithodeme is the fundamental unit in lithodemic classification. A lithodeme is a body of intrusive, pervasively deformed, or highly metamorphosed rock, generally non-tabular and lacking primary depositional structures, and characterized by lithic homogeneity. It is mappable at the Earth’s surface and traceable in the subsurface. For cartographic and hierarchical purposes, it is comparable to a formation (see Table 2).

Remarks. (a) Content.—A lithodeme should possess distinctive lithic features and some degree of internal lithic homogeneity. It may consist of (i) rock of one type, (i) a mixture of rocks of two or more types, or (i) extreme heterogeneity of composition, which may constitute in itself a form of unity when compared to adjoining rock-masses (see also ‘‘complex,’’ Article 37).

(b) Lithic characteristics.—Distinctive lithic characteristics may include mineralogy, textural features such as grain size, and structural

Figure 3. Lithodemic (upper case) and lithostratigraphic (lower case) units. A lithodeme of gneiss (A) contains an intrusion of diorite (B) that was deformed with the gneiss. A and B may be treated jointly as a complex. A younger granite (C) is cut by a dike of syenite (D) that is cut in turn by unconformity I. All the foregoing are in fault contact with a structural complex (E). A volcanic complex (G) is built upon unconformity I, and its feeder dikes cut the unconformity. Laterally equivalent volcanic strata in orderly, mappable succession (h) are treated as lithostratigraphic units. A gabbro feeder (G0), to the volcanic complex, where surrounded by gneiss is readily distinguished as a separate lithodeme and named as a gabbro or an intrusion. All the foregoing are overlain, at unconformity I, by sedimentary rocks (j) divided into formations and members.

North American Commission on Stratigraphic Nomenclature 1571 features such as schistose or gneissic structure. A unit distinguishable from its neighbors only by means of chemical analysis is informal.

(c) Mappability.—Practicability of surface or subsurface mapping is an essential characteristic of a lithodeme (see Article 24d).

Article 34.—Division of Lithodemes. Units below the rank of lithodeme are informal.

Article 35.—Suite. A suite (metamorphic suite, intrusive suite, plutonic suite) is the lithodemic unit next higher in rank to lithodeme. It comprises two or more associated lithodemes of the same class (e.g., plutonic, metamorphic). For cartographic and hierarchical purposes, suite is comparable to group (see Table 2).

Remarks. (a) Purpose.—Suites are recognized for the purpose of expressing the natural relations of associated lithodemes having significant lithic features in common, and of depicting geology at compilation scales too small to allow delineation of individual lithodemes. Ideally, a suite consists entirely of named lithodemes, but may contain both named and unnamed units.

(b) Change in component units.—The named and unnamed units constituting a suite may change from place to place, so long as the original sense of natural relations and of common lithic features is not violated.

(c) Change in rank.—Traced laterally, a suite may lose all of its formally named divisions but remain a recognizable, mappable entity. Under such circumstances, it may be treated as a lithodeme but retain the same name. Conversely, when a previously established lithodeme is divided into two or more mappable divisions, it may be desirable to raise its rank to suite, retaining the original geographic component of the name. To avoid confusion, the original name should not be retained for one of the divisions of the original unit (see Article 19g).

Article 36.—Supersuite. A supersuite is the unit next higher in rank to a suite. It comprises two or more suites or complexes having a degree of natural relationship to one another, either in the vertical or the lateral sense. For cartographic and hierarchical purposes, supersuite is similar in rank to supergroup.

Article 37.—Complex. An assemblage or mixture of rocks of two or more genetic classes, i.e., igneous, sedimentary, or metamorphic, with or without highly complicated structure, may be named a complex. The term ‘‘complex’’ takes the place of the lithic or rank term (for example, Boil MountainComplex, Franciscan Complex)and, althoughunranked, commonly is comparable to suite or supersuite and is named in the same manner (Articles 41, 42).

Remarks. (a) Use of ‘‘complex.’’—Identification of an assemblage of diverse rocks as a complex is useful where the mapping of each separate lithic component is impractical at ordinary mapping scales. ‘‘Complex’’ is unranked but commonly comparable to suite or supersuite; therefore, the term may be retained if subsequent, detailed mapping distinguishes some or all of the component lithodemes or lithostratigraphic units.

(b) Volcanic complex.—Sites of persistent volcanic activity commonly are characterized by a diverse assemblage of extrusive volcanic rocks, related intrusions, and their weathering products. Such an assemblage may be designated a volcanic complex.

(c) Structural complex.—In some terranes, tectonic processes (e.g., shearing, faulting) have produced heterogeneous mixtures or disrupted bodies of rock in which some individual components are too small to be mapped. Where there is no doubt that the mixing or disruption is due to tectonic processes, such a mixture may be designated as a structural complex, whether it consists of two or more classes of rock, or a single class only. A simpler solution for some mapping purposes is to indicate intense deformation by an overprinted pattern.

(d) Misuse of ‘‘complex.’’—Where the rock assemblage to be united under a single, formal name consists of diverse types of a single class of rock, as in many terranes that expose a variety of either intrusive igneous or high-grade metamorphic rocks, the term ‘‘intrusive suite,’’ ‘‘plutonic suite,’’ or ‘‘metamorphic suite’’ should be used, rather than the unmodified term ‘‘complex.’’ Exceptions to this rule are the terms structural complex and volcanic complex (see remarks c and b, above).

Article 38.—Misuse of ‘‘Series’’ for Suite, Complex, or

Supersuite. The term ‘‘series’’ has been employed for an assemblage of lithodemes or an assemblage of lithodemes and suites, especially in studies of the Precambrian. This practice now is regarded as improper; these assemblages are suites, complexes, or supersuites. The term ‘‘series’’ also has been applied to a sequence of rocks resulting from a succession of eruptions or intrusions. In these cases a different term should be used; ‘‘group’’ should replace ‘‘series’’ for volcanic and low-grade metamorphic rocks, and ‘‘intrusive suite’’ or ‘‘plutonic suite’’ should replace ‘‘series’’ for intrusive rocks of group rank.

Lithodemic Nomenclature

Article 39.—General Provisions. The formal name of a lithodemic unit is compound. It consists of a geographic name combined with a descriptive or appropriate rank term. The principles for the selection of the geographic term, concerning suitability, availability, priority, etc., follow those established in Article 7, where the rules for capitalization are also specified.

Article 40.—Lithodeme Names. The name of a lithodeme combines a geographic term with a lithic or descriptive term, e.g., Killarney Granite, Adamant Pluton, Manhattan Schist, Skaergaard Intrusion, Duluth Gabbro. The term formation should not be used.

Remarks. (a) Lithic term.—The lithic term should be a common and familiar term, such as schist, gneiss, gabbro. Specialized terms and terms not widely used, such as websterite and jacupirangite, and compound terms, such as graphitic schist and augen gneiss, should be avoided.

(b) Intrusive and plutonic rocks.—Because many bodies of intrusive rock range in composition from place to place and are difficult to characterize with a single lithic term, and because many bodies of plutonic rock are considered not to be intrusions, latitude is allowed in the choice of a lithic or descriptive term. Thus, the descriptive term should preferably be compositional (e.g., gabbro, granodiorite), but may, if necessary, denote form (e.g., dike, sill), or be neutral (e.g., intrusion, pluton7). In any event, specialized

7Pluton—a mappable body of plutonic rock. 1572 North American Stratigraphic Code compositional terms not widely used are to be avoided, as are form terms that are not widely used, such as bysmalith and chonolith. Terms implying genesis should be avoided as much as possible, because interpretations of genesis may change.

Article41.—SuiteNames.Thenameofasuitecombines a geographic term, the term ‘‘ suite, ’’ and an adjective denoting the fundamental character of the suite; for example, Idaho SpringsMetamorphicSuite, TuolumneIntrusiveSuite, Cassiar Plutonic Suite. The geographic name of a suite may not be the same as that of a component lithodeme (see Article 19f). Intrusive assemblages, however, may share the same geographic name if an intrusive lithodeme is representative of the suite(e.g., the Methuen Plutonic Suite may include the Methuen, Deloro,Abinger and AddingtonGranites,[Easton,1992]. As the Methuen Granite, a lithodeme, is typical of the suite, the duplication of names is permissible).

Article 42.—Supersuite Names. The name of a supersuitecombinesageographictermwiththeterm‘‘supersuite.’’

Nature and Boundaries

Article 43.—Nature of Magnetostratigraphic Units.A magnetostratigraphic unit is a body of rock unified by specified remanent-magnetic properties and is distinct from underlying and overlying magnetostratigraphic units having different magnetic properties.

Remarks. (a) Definition.—Magnetostratigraphy is defined here as all aspects of stratigraphy based on remanent magnetism (paleomagnetic signatures). Four basic paleomagnetic phenomena can be determined or inferred from remanent magnetism: polarity, dipole-field-pole position (including apparent polar wander), the non-dipole component (secular variation), and field intensity.

(b) Contemporaneity of rock and remanent magnetism.—

Many paleomagnetic signatures reflect earth magnetism at the time the rock formed. Nevertheless, some rocks have been subjected subsequently to physical and/or chemical processes that altered the magnetic properties. For example, a body of rock may be heated above the blocking temperature or Curie point for one or more minerals, or a ferromagnetic mineral may be produced by lowtemperature alteration long after the enclosing rock formed, thus acquiring a component of remanent magnetism reflecting the field at the time of alteration, rather than the time of original rock deposition or crystallization.

(c) Designations and scope.—The prefix magneto is used with an appropriate term to designate the aspect of remanent magnetism used to define a unit. The terms ‘‘magnetointensity’’ or ‘‘magnetosecular-variation’’ are possible examples. This Code considers only polarity reversals, which now are recognized widely as a stratigraphic tool. However, apparent-polar-wander paths offer increasing promise for correlations within Precambrian rocks.

Article 4.—Definition of Magnetopolarity Unit.A magnetopolarity unit is a body of rock unified by its remanent magnetic polarity and distinguished from adjacent bodies of rock that have different polarity.

Remarks. (a) Nature.—Magnetopolarity is the record in rocks of the polarity history of the Earth’s magnetic-dipole field. Frequent past reversals of the polarity of the Earth’s magnetic field provide a basis for magnetopolarity stratigraphy.

(b) Stratotype.—A stratotype for a magnetopolarity unit should be designated and the boundaries defined in terms of recognized lithostratigraphic and/or biostratigraphic units in the stratotype. The formal definition of a magnetopolarity unit should meet the applicable specific requirements of Articles 3 to 16.

(c) Independence from inferred history.—Definition of a magnetopolarity unit does not require knowledge of the time at which the unit acquired its remanent magnetism; its magnetism may be primary or secondary. Nevertheless, the unit’s present polarity is a property that may be ascertained and confirmed by others.

(d) Relation to lithostratigraphic and biostratigraphic units.—

Magnetopolarity units resemble lithostratigraphic and biostratigraphic units in that they are defined on the basis of an objective recognizable property, but differ fundamentally in that most magnetopolarity unit boundaries are thought not to be time transgressive. Their boundaries may coincide with those of lithostratigraphic or biostratigraphic units, or be parallel to but displaced from those of such units, or be crossed by them.

(e) Relation of magnetopolarity units to chronostratigraphic units.—Although transitions between polarity reversals are of global extent, a magnetopolarity unit does not contain within itself evidence that the polarity is primary, or criteria that permit its unequivocal recognition in chronocorrelative strata of other areas. Other criteria, such as paleontologic or numerical age, are required for both correlation and dating. Although polarity reversals are useful in recognizing chronostratigraphic units, magnetopolarity alone is insufficient for their definition.

Article 45.—Boundaries. The upper and lower limits of a magnetopolarity unit are defined by boundaries marking a change of polarity. Such boundaries may represent either a depositional discontinuity or a magnetic-field transition. The boundaries are either polarity-reversal horizons or polarity transition zones, respectively.

Remark. (a) Polarity-reversal horizons and transition zones.—

A polarity-reversal horizon is either a single, clearly definable surface or a thin body of strata constituting a transitional interval across which a change in magnetic polarity is recorded. Polarity-reversal horizons describe transitional intervals of 1 m or less; where the change in polarity takes place over a stratigraphic interval greater than 1 m, the term ‘‘polarity transition zone’’ should be used. Polarity-reversal horizons and polarity transition zones provide the boundaries for polarity zones, although they may also be contained within a polarity zone where they mark an internal change subsidiary in rank to those at its boundaries.

Ranks of Magnetopolarity Units

Article 46.—Fundamental Unit. A polarity zone is the fundamental unit of magnetopolarity classification. A polarity zone is a unit of rock characterized by the polarity of its magnetic signature. Magnetopolarity zone, rather than polarity zone, should be used where there is risk of confusion with other kinds of polarity.

Remarks. (a) Content.—A polarity zone should possess some degree of internal homogeneity. It may contain rocks of (1) entirely or predominantly one polarity, or (2) mixed polarity.

North American Commission on Stratigraphic Nomenclature 1573

(b) Thickness and duration.—The thickness of rock of a polarity zone or the amount of time represented should play no part in the definition of the zone. The polarity signature is the essential property for definition.

(c) Ranks.—When continued work at the stratotype for a polarity zone, or new work in correlative rock bodies elsewhere, reveals smaller polarity units, these may be recognized formally as polarity subzones. If it should prove necessary or desirable to group polarity zones, these should be termed polarity superzones. The rank of a polarity unit may be changed when deemed appropriate .

Magnetopolarity Nomenclature

(Parte 6 de 15)