Guia de nomenclatura estratigráfica

Guia de nomenclatura estratigráfica

(Parte 1 de 15)


North American Commission on Stratigraphic Nomenclature

By design, the North American Stratigraphic Code is meant to be an evolving document, one that requires change as the field of earth science evolves. The revisions to the Code that are included in this 2005 edition encompass a broad spectrum of changes, ranging from a complete revision of the section on Biostratigraphic Units (Articles 48 to 54), several wording changes to Article 58 and its remarks concerning Allostratigraphic Units, updating of Article 4 to incorporate changes in publishing methods over the last two decades, and a variety of minor wording changes to improve clarity and self-consistency between different sections of the Code. In addition, Figures 1, 4, 5, and 6, as well as Tables 1 and Tables 2 have been modified. Most of the changes adopted in this revision arose from Notes 60, 63, and 64 of the Commission, all of which were published in the AAPG Bulletin. These changes follow Code amendment procedures as outlined in Article 21.

We hope these changes make the Code a more usable document to professionals and students alike. Suggestions for future modifications or additions to the North American Stratigraphic Code are always welcome. Suggested and adopted modifications will be announced to the profession, as in the past, by serial Notes and Reports published in the AAPG Bulletin. Suggestions may be made to representatives of your association or agency who are current commissioners, or directly to the Commission itself. The Commission meets annually, during the national meetings of the Geological Society of America.

2004 North American Commission on Stratigraphic Nomenclature


The 1983 Code of recommended procedures for classifying and naming stratigraphic and related units was prepared during a four-year period, by and for North American earth scientists, under the auspices of the North American Commission on Stratigraphic Nomenclature. It represents the thought and work of scores of persons, and thousands of hours of writing and editing. Opportunities to participate in and review the work have been provided throughout its development, as cited in the Preamble, to a degree unprecedented during preparation of earlier codes.

Publication of the International Stratigraphic Guide in 1976 made evident some insufficiencies of the American Stratigraphic Codes of 1961 and 1970. The Commission considered whether to discard our codes, patch them over, or rewrite them fully, and chose the last. We believe it desirable to sponsor a code of stratigraphic practice for use in North America, for we can adapt to new methods and points of view more rapidly than a worldwide body. A timely example was the recognized need to develop modes of establishing formal nonstratiform (igneous and high-grade metamorphic) rock units, an objective that is met in this Code, but not yet in the Guide.

The ways in which the 1983 Code (revised 2005) differs from earlier American codes are evident from the Contents. Some categories have disappeared and others are new, but this Code has evolved from earlier codes and from the International Stratigraphic Guide. Some new units have not yet stood the test of long practice, and conceivably may not, but they are introduced toward meeting recognized and defined needs of the profession. Take this Code, use it, but do not condemn it because it contains something new or not of direct interest to you. Innovations that prove unacceptable to the profession will expire without damage to other concepts and procedures, just as did the geologic-climate units of the 1961 Code.

The 1983 Code was necessarily somewhat innovative because of (1) the decision to write a new code, rather than to revise the 1970 Code; (2) the open invitation to members of the geologic profession to offer suggestions and ideas, both in writing and orally; and (3) the progress in the earth sciences since completion of previous codes. This report

1Manuscript received November 12, 2004; provisional acceptance February 10, 2005; revised manuscript received May 19, 2005; final acceptance July 05, 2005. DOI:10.1306/07050504129 strives to incorporate the strength and acceptance of established practice, with suggestions for meeting future needs perceived by our colleagues; its authors have attempted to bring together the good from the past, the lessons of the Guide, and carefully reasoned provisions for the immediate future.

Participants in preparation of the 1983 Code are listed in Appendix I, but many others helped with their suggestions and comments. Major contributions were made by the members, and especially the chairmen, of the named subcommittees and advisory groups under the guidance of the CodeCommittee,chairedbyStevenS.Oriel,whoalsoserved as principal, but not sole, editor. Amidst the noteworthy contributions by many, those of James D. Aitken have been outstanding. The work was performed for and supported by the Commission, chaired by Malcolm P. Weiss from 1978 to 1982.


Many former and current commissioners representing not only the ten organizational members of the North American Commission on Stratigraphic Nomenclature (Appendix I), but other institutions, as well, generated the product. En- dorsement by constituent organizations is anticipated, and scientificcommunicationwillbefosteredifCanadian,United States, and Mexican scientists, editors, and administrators consult Code recommendations for guidance in scientific reports. The Commission will appreciate reports of formal adoption or endorsement of the Code, and asks that they be transmitted to the Chairman of the Commission (c/o American Association of Petroleum Geologists, Box 979, Tulsa, Oklahoma 74101, U.S.A.).

Any code necessarily represents but a stage in the evolution of scientific communication. Suggestions for future changes of, or additions to, the North American Stratigraphic Codeare welcome. Suggestedand adopted modifications will be announced to the profession, as in the past, by serial Notes and Reports published in the AAPG Bulletin. Suggestions may be made to representatives of your association or agency who are current commissioners, or directly to the Commission itself. The Commission meets annually, during the national meetings of the Geological Society of America.

1982 North American Commission on Stratigraphic Nomenclature

Material Categories Based on Content or Physical Limits1557
Categories Expressing or Related to Geologic Age1558
Pedostratigraphic Terms1559
Article 1. Purpose1561
Article 2. Categories1561
Article 3. Requirements for Formally Named Geologic Units1561
Article 4. Publication1561
Remarks: a. Inadequate publication1561
b. Guidebooks1561
c. Electronic publication1561
Article 5. Intent and Utility1561
Remark: a. Demonstration of purpose served1561
Article 6. Category and Rank1561
Remark: a. Need for specification1561
Article 7. Name1561
Remarks: a. Appropriate geographic terms1562
b. Duplication of names1562
c. Priority and preservation of established names1562

Page 1548 North American Stratigraphic Code

e. Names in different countries and different languages1563
Article 8. Stratotypes1563
Remarks: a. Unit stratotype1563
b. Boundary stratotype1563
c. Type locality1563
d. Composite-stratotype1563
e. Reference sections1563
f. Stratotype descriptions1563
Article 9. Unit Description1563
Article 10. Boundaries1563
Remarks: a. Boundaries between intergradational units1563
b. Overlaps and gaps1563
Article 1. Historical Background1564
Article 12. Dimensions and Regional Relations1564
Article 13. Age1564
Remarks: a. Dating1564
b. Calibration1564
c. Convention and abbreviations1564
d. Expression of ‘‘age’’ of lithodemic units1564
Article 14. Correlation1564
Article 15. Genesis1564
Article 16. Surface and Subsea Units1564
Remarks: a. Naming subsurface units1564
b. Additional recommendations1564
c. Seismostratigraphic units1564
Article 17. Requirements for Major Changes1565
Remark: a. Distinction between redefinition and revision1565
Article 18. Redefinition1565
Remarks: a. Change in lithic designation1565
b. Original lithic designation inappropriate1565
Article 19. Revision1565
Remarks: a. Boundary change1565
b. Change in rank1565
c. Examples of changes from area to area1565
d. Example of change in single area1565
e. Retention of type section1565
f. Different geographic name for a unit and its parts1565
g. Undesirable restriction1565
Article 20. Abandonment1565
Remarks: a. Reasons for abandonment1565
b. Abandoned names1565
c. Obsolete names1565
d. Reference to abandoned names1566
e. Reinstatement1566
Article 21. Procedure for Amendment1566
Nature and Boundaries1566
Article 2. Nature of Lithostratigraphic Units1566
Remarks: a. Basic units1566
b. Type section and locality1566
c. Type section never changed1566
d. Independence from inferred geologic history1566
f. Surface form1566
g. Economically exploited units1566
h. Instrumentally defined units1566
i. Zone1567
j. Cyclothems1567
k. Soils and paleosols1567
l. Depositional facies1567
Article 23. Boundaries1567
Remarks: a. Boundary in a vertically gradational sequence1567
b. Boundaries in lateral lithologic change1567
c. Key beds used for boundaries1567
d. Unconformities as boundaries1567
e. Correspondence with genetic units1567
Ranks of Lithostratigraphic Units1567
Article 24. Formation1567
Remarks: a. Fundamental unit1567
b. Content1567
c. Lithic characteristics1567
d. Mappability and thickness1569
e. Organic reefs and carbonate mounds1569
f. Interbedded volcanic and sedimentary rock1569
g. Volcanic rock1569
h. Metamorphic rock1569
Article 25. Member1569
Remarks: a. Mapping of members1569
b. Lens and tongue1569
c. Organic reefs and carbonate mounds1569
d. Division of members1569
e. Laterally equivalent members1569
Article 26. Bed(s)1569
Remarks: a. Limitations1569
b. Key or marker beds1569
Article 27. Flow1569
Article 28. Group1569
Remarks: a. Use and content1569
b. Change in component formations1569
c. Change in rank1570
Article 29. Supergroup1570
Remark: a. Misuse of ‘‘series’’ for group or supergroup1570
Lithostratigraphic Nomenclature1570
Article 30. Compound Character1570
Remarks: a. Omission of part of a name1570
b. Use of simple lithic terms1570
c. Group names1570
d. Formation names1570
e. Member names1570
f. Names of reefs1570
g. Bed and flow names1570
h. Informal units1570
i. Informal usage of identical geographic names1570
j. Metamorphic rock1570
k. Misuse of well-known name1570
Nature and Boundaries1570
Article 31. Nature of Lithodemic Units1570
Remarks: a. Recognition and definition1570
c. Independence from inferred geologic history1571
d. Use of ‘‘zone’’1571
Article 32. Boundaries1571
Remark: a. Boundaries within gradational zones1571
Ranks of Lithodemic Units1571
Article 3. Lithodeme1571
Remarks: a. Content1571
b. Lithic characteristics1571
c. Mappability1572
Article 34. Division of Lithodemes1572
Article 35. Suite1572
Remarks: a. Purpose1572
b. Change in component units1572
c. Change in rank1572
Article 36. Supersuite1572
Article 37. Complex1572
Remarks: a. Use of ‘‘complex’’1572
b. Volcanic complex1572
c. Structural complex1572
d. Misuse of ‘‘complex’’1572
Article 38. Misuse of ‘‘Series’’ for Suite, Complex, or Supersuite1572
Lithodemic Nomenclature1572
Article 39. General Provisions1572
Article 40. Lithodeme Names1572
Remarks: a. Lithic term1572
b. Intrusive and plutonic rocks1572
Article 41. Suite Names1573
Article 42. Supersuite Names1573
Nature and Boundaries1573
Article 43. Nature of Magnetostratigraphic Units1573
Remarks: a. Definition1573
b. Contemporaneity of rock and remanent magnetism1573
c. Designations and scope1573
Article 4. Definition of Magnetopolarity Unit1573
Remarks: a. Nature1573
b. Stratotype1573
c. Independence from inferred history1573
d. Relation to lithostratigraphic and biostratigraphic units1573
e. Relation of magnetopolarity units to chronostratigraphic units1573
Article 45. Boundaries1573
Remark: a. Polarity-reversal horizons and transition zones1573
Ranks of Magnetopolarity Units1573
Article 46. Fundamental Unit1573
Remarks: a. Content1573
b. Thickness and duration1574
c. Ranks1574
Magnetopolarity Nomenclature1574
Article 47. Compound Name1574
Article 48. Fundamentals of Biostratigraphy1574
Remark: a. Uniqueness1574
Nature and Boundaries1574
Article 49. Nature of Biostratigraphic Units1574
Remarks: a. Unfossiliferous rocks1574
b. Contemporaneity of rocks and fossils1574
d. Independence from chronostratigraphic units1574
Article 50. Kinds of Biostratigraphic Units1574
Remarks: a. Range biozone1574
b. Interval biozone1574
c. Lineage biozone1574
d. Assemblage biozone1574
e. Abundance biozone1574
f. Hybrid or new types of biozones1575
Article 51. Boundaries1575
Remark: a. Identification of biozones1575
Article 52. [not used]1576
Ranks of Biostratigraphic Units1576
Article 53. Fundamental Unit1576
Remarks: a. Scope1576
b. Divisions1576
c. Shortened forms of expression1576
Biostratigraphic Nomenclature1576
Article 54. Establishing Formal units1576
Remarks: a. Name1576
b. Shorter designations for biozone names1576
c. Revision1576
d. Defining taxa1576
e. Reference sections1576
Nature and Boundaries1576
Article 5. Nature of Pedostratigraphic Units1576
Remarks: a. Definition1577
b. Recognition1577
c. Boundaries and stratigraphic position1577
d. Traceability1577
e. Distinction from pedologic soils1577
f. Relation to saprolite and other weathered materials1577
g. Distinction from other stratigraphic units1577
h. Independence from time concepts1578
Pedostratigraphic Nomenclature and Unit1578
Article 56. Fundamental Unit1578
Article 57. Nomenclature1578
Remarks: a. Composite geosols1578
b. Characterization1578
c. Procedures for establishing formal pedostratigraphic units1578
Nature and Boundaries1578
Article 58. Nature of Allostratigraphic Units1578
Remarks: a. Purpose1578
b. Internal characteristics1578
c. Boundaries1578
d. Mappability1578
e. Type locality and extent1578
f. Relation to genesis1578
g. Relation to geomorphic surfaces1578
h. Relation to soils and paleosols1578
i. Relation to inferred geologic history1578
j. Relation to time concepts1578
k. Extension of allostratigraphic units1578
Ranks of Allostratigraphic Units1578
Article 59. Hierarchy1578
Remarks: a. Alloformation1578
c. Allogroup1578
d. Changes in rank1579
Allostratigraphic Nomenclature1579
Article 60. Nomenclature1579
Remark: a. Revision1579
Nature and Kinds1579
Article 61. Kinds1579
Units Based on Material Referents1580
Article 62. Kinds Based on Referents1580
Article 63. Isochronous Categories1580
Remark: a. Extent1580
Article 64. Diachronous Categories1580
Remarks: a. Diachroneity1580
b. Extent1581
Units Independent of Material Referents1581
Article 65. Numerical Divisions of Time1581
Nature and Boundaries1581
Article 6. Definition1581
Remarks: a. Purposes1581
b. Nature1581
c. Content1581
Article 67. Boundaries1581
Remark: a. Emphasis on lower boundaries of chronostratigraphic units1581
Article 68. Correlation1581
Ranks of Chronostratigraphic Units1581
Article 69. Hierarchy1581
Article 70. Eonothem1581
Article 71. Erathem1581
Remark: a. Names1581
Article 72. System1582
Remark: a. Subsystem and supersystem1582
Article 73. Series1582
Article 74. Stage1582
Remark: a. Substage1582
Article 75. Chronozone1582
Remarks: a. Boundaries of chronozones1582
b. Scope1582
c. Practical utility1582
Chronostratigraphic Nomenclature1582
Article 76. Requirements1582
Article 7. Nomenclature1582
Remarks: a. Systems and units of higher rank1582
b. Series and units of lower rank1582
Article 78. Stratotypes1582
Article 79. Revision of Units1583
Nature and Boundaries1583
Article 80. Definition and Basis1583
Ranks and Nomenclature of Geochronologic Units1583
Article 81. Hierarchy1583
Article 82. Nomenclature1583
Nature and Boundaries1583
Article 83. Definition1583
Remarks: a. Nature1583
b. Principal purposes1583
c. Recognition1583
Article 84. Boundaries1583
Ranks and Nomenclature of Polarity-Chronostratigraphic Units1583
Article 85. Fundamental Unit1583
Remarks: a. Meaning of term1583
b. Scope1583
c. Ranks1583
Article 86. Establishing Formal Units1583
Article 87. Name1583
Remarks: a. Preservation of established name1583
b. Expression of doubt1584
Nature and Boundaries1584
Article 8. Definition1584
Ranks and Nomenclature of Polarity-Chronologic Units1584
Article 89. Fundamental Unit1584
Remark: a. Hierarchy1584
Article 90. Nomenclature1584
Nature and Boundaries1584
Article 91. Definition1584
Remarks: a. Purposes1584
b. Scope1584
c. Basis1584
d. Duration1584
Article 92. Boundaries1584
Remark: a. Temporal relations1584
Ranks and Nomenclature of Diachronic Units1584
Article 93. Ranks1584
Remarks: a. Diachron1584
b. Hierarchical ordering permissible1584
c. Episode1584
Article 94. Name1585
Remarks: a. Formal designation of units1585
b. Interregional extension of geographic names1585
c. Change from geochronologic to diachronic classification1585
Article 95. Establishing Formal Units1585
Remark: a. Revision or abandonment1585
Nature and Boundaries1585
Article 96. Definition1585
Ranks and Nomenclature of Geochronometric Units1586
Article 97. Nomenclature1586


1. Classes of units defined1557
2. Categories and ranks of units defined in this Code1562

1554 North American Stratigraphic Code

1. Relation of geologic time units to the kinds of rock-unit referents on which most are based1558
2. Diagrammatic examples of lithostratigraphic boundaries and classification1568
3. Lithodemic and lithostratigraphic units1571
4. Examples of range, lineage, and interval biozones1575
5. Examples of assemblage and abundance biozones1576
6. Relation between pedostratigraphic units and pedologic profiles1577
7. Example of allostratigraphic classification of alluvial and lacustrine deposits in a graben1579
8. Example of allostratigraphic classification of contiguous deposits of similar lithology1579
9. Example of allostratigraphic classification of lithologically similar, discontinuous terrace deposits1580
10. Comparison of geochronologic, chronostratigraphic, and diachronic units1584
1. Schematic relation of phases to an episode1585

Codes of Stratigraphic Nomenclature prepared by the

North American Commission on Stratigraphic Nomenclature in 1983, the American Commission on Stratigraphic Nomenclature (ACSN, 1961), and its predecessor (Committee on Stratigraphic Nomenclature, 1933) have been used widely as a basis for stratigraphic terminology. Their formulation was a response to needs recognized during the past century by government surveys (both national and local) and by editors of scientific journals for uniform standards and common procedures in defining and classifying formal rock bodies, their fossils, and the time spans represented by them. The 1970 Code (ACSN, 1970) is a slightly revised version of that published in 1961, incorporating some minor amendments adopted by the Commission between 1962 and 1969. The 2005 edition of the 1983 Code incorporates amendments adopted by the Commission between 1983 and 2003. The Codes have served the profession admirably and have been drawn upon heavily for codes and guides prepared in other parts of the world (ISSC, 1976, p. 104–106; 1994, p. 143–147). The principles embodied by any code, however, reflect the state of knowledge at the time of its preparation.

Newconceptsandtechniquesdevelopedsince1961have revolutionized the earth sciences. Moreover, increasingly evident have been the limitations of previous codes in meeting some needs of Precambrian and Quaternary geology and in classification of plutonic, high-grade metamorphic, volcanic, and intensely deformed rock assemblages. In addition, the important contributions of numerous international stratigraphic organizations associated with both the International Union ofGeologicalSciences(IUGS)andUNESCO,includingworking groups of the International Geological Correlation Programme (IGCP), merit recognition and incorporation into a North American code.

For these and other reasons, revision of the 1970 Code wasundertakenbycommitteesappointedbytheNorthAmerican Commission on Stratigraphic Nomenclature (NACSN). The Commission, founded as the American Commission on Stratigraphic Nomenclature in 1946 (ACSN, 1947), was renamed the NACSN in 1978 (Weiss, 1979b) to emphasize that delegates from ten organizations in Canada, the United States,andMexicorepresentthegeologicalprofessionthroughout North America (Appendix I).

Although many past and current members of the Commission helped prepare the 1983 Code, the participation of all interested geologists was sought (for example, Weiss, 1979a). Open forums were held at the national meetings of both the Geological Society of America at San Diego in November, 1979, and the American Association of Petroleum Geologists at Denver in June, 1980, at which comments and suggestions were offered by more than 150 geologists. The resulting draft of this report was printed, through the courtesy of the Canadian Society of Petroleum Geologists, on October 1, 1981, and additional comments were invited from the profession for a period of one year before submittal of this report to the Commission for adoption. More than 50 responses were received with sufficient suggestions for improvement to prompt moderate revision of the printed draft (NACSN, 1981). We are particularly indebted to Hollis D. Hedberg and Amos Salvador for their exhaustive and perceptive reviews of early drafts of this Code, as well as to those who responded to the request for comments. Participants in the preparation and revisions of this report, and conferees, are listed in Appendix I.

Recent amendments to the 1983 Code include allowing electronicpublication of new andrevisednamesand correcting inconsistenciesto improveclarity(Ferrusquıa-Villafranca et al., 2001). Also, the Biostratigraphic Units section (Articles 48 to 54) was revised (Lenz et al., 2001).

Some of the expenses incurred in the course of this work were defrayed by National Science Foundation Grant EAR 7919845, for which we express appreciation. Institutions represented by the participants have been especially generous in their support.

The North American Stratigraphic Code seeks to describe explicit practices for classifying and naming all formally defined geologic units. Stratigraphic procedures and principles,althoughdeveloped initiallytobringordertostrata and the events recorded therein, are applicable to all earth materials, not solely to strata. They promote systematic and

North American Commission on Stratigraphic Nomenclature 1555 rigorous study of the composition, geometry, sequence, history, and genesis of rocks and unconsolidated materials. They provide the frameworkwithin which time and space relations among rock bodies that constitute the Earth are ordered systematically. Stratigraphic procedures are used not only to reconstruct the history of the Earth and of extra-terrestrial bodies, but also to define the distribution and geometry of some commodities needed by society. Stratigraphic classification systematically arranges and partitions bodies of rock or unconsolidated materials of the Earth’s crust into units on the basis of their inherent properties or attributes.

A stratigraphic code or guide is a formulation of current views on stratigraphic principles and procedures designed to promote standardized classification and formal nomenclature of rock materials. It provides the basis for formalization of the language used to denote rock units and their spatial and temporal relations. To be effective, a code must be widely acceptedand used;geologic organizations and journalsmay adopt its recommendations for nomenclatural procedure. Because any code embodies only current concepts and principles, it should have the flexibility to provide for both changes and additions to improve its relevance to new scientific problems.

Any system of nomenclature must be sufficiently explicit to enable users to distinguish objects that are embraced in a class from those that are not. This stratigraphic code makes no attempt to systematize structural, petrographic, paleontologic,orphysiographicterms.Termsfrom theseother fields that are used as part of formal stratigraphic names should be sufficiently general as to be unaffected by revisions of precise petrographic or other classifications.

The objective of a system of classification is to promote unambiguous communication in a manner not so restrictive as to inhibit scientific progress. To minimize ambiguity, a code must promote recognition of the distinction between observable features (reproducible data) and inferences or interpretations. Moreover, it should be sufficiently adaptable and flexible to promote the further development of science.

Stratigraphic classification promotes understanding of the geometry and sequence of rock bodies. The development of stratigraphy as a science required formulation of the Law of Superposition to explain sequential stratal relations. Although superposition isnot applicable to many igneous, metamorphic, and tectonic rock assemblages, other criteria (such as cross-cutting relations and isotopic dating) can be used to determine sequential arrangements among rock bodies.

The term stratigraphic unit may be defined in several ways. Etymological emphasis requires that it be a stratum or assemblage of adjacent strata distinguished by any or several of the many properties that rocks may possess (ISSC, 1976, p. 13; 1994, p. 13–14). The scope of stratigraphic classification and procedures, however, suggests a broader definition: a naturally occurring body of rock or rock material distinguished from adjoining bodies of rock on the basis of some stated property or properties. Commonly used properties include composition, texture, included fossils, magnetic signature, radioactivity, seismic velocity, and age. Sufficient care is required in defining the boundaries of a unit to enable others to distinguish the material body from those adjoining it. Units based on one property commonly do not coincide with those based on another and, therefore, dis- tinctive terms are needed to identify the property used in defining each unit.

The adjective stratigraphic is used in two ways in the remainder of this report. In discussions of lithic (used here as synonymous with ‘‘lithologic’’) units, a conscious attempt is made to restrict the term to lithostratigraphic or layered rocks and sequences that obey the Law of Superposition. For nonstratiform rocks (of plutonic or tectonic origin, for example), the term lithodemic (see Article 27) is used. The adjective stratigraphic is also used in a broader sense to refer to those procedures derived from stratigraphy that are now applied to all classes of earth materials.

An assumption made in the material that follows is that the reader has some degree of familiarity with basic principles of stratigraphy as outlined, for example, by Dunbar and Rodgers (1957), Weller (1960), Shaw (1964), Matthews (1974), Blatt et al. (1990), Boggs (2001), or the International Stratigraphic Guide (ISSC, 1976, 1994).

Publication of the International Stratigraphic Guide by the International Subcommission on Stratigraphic Classification (ISSC, 1976), which is being endorsed and adopted throughout the world, played a part in prompting examination of the American Stratigraphic Code and the decision to revise it.

The International Guide embodies principles and procedures that had been adopted by several national and regional stratigraphic committees and commissions. More than two decades of effort by H. D. Hedberg and other members of the Subcommission (ISSC, 1976, p. VI, 1, 3) developed the consensus required for preparation of the Guide. Although the Guide attempts to cover all kinds of rocks and the diverse ways of investigating them, it is necessarily incomplete. Mechanisms are needed to stimulate individual innovations toward promulgating new concepts, principles, and practices that subsequently may be found worthy of inclusion in later editions of the Guide. The flexibility of national and regional committees or commissions enables them to perform this function more readily than an international subcommission, even while they adopt the Guide as the international standard of stratigraphic classification.

A guiding principle in preparing this Code has been to make it as consistent as possible with the International Guide, and at the same time to foster further innovations to meet the expanding and changing needs of earth scientists on the North American continent.

An attempt is made to strike a balance between serving the needs of those in evolving specialties and resisting the proliferation of categories of units. Consequently, additional

1556 North American Stratigraphic Code formal categories are recognized here relative to previous codes or in the International Guide (ISSC, 1994). On the other hand, no special provision is made for formalizing certain kinds of units (deep oceanic, for example) that may be accommodated by available categories.

Four principal categories of units have previously been used widely in traditional stratigraphic work; these have been termed lithostratigraphic, biostratigraphic, chronostratigraphic, and geochronologic and are distinguished as follows: 1. A lithostratigraphic unit is a stratum or body of strata, generally but not invariably layered, generally but not invariably tabular, that conforms to the Law of Superposition and is distinguished and delimited on the basis of lithic characteristics and stratigraphic position. Example: Navajo Sandstone. 2. A biostratigraphic unit is a body of rock defined and characterized by its fossil content. Example: Discoaster multiradiatus Interval Biozone. 3. A chronostratigraphic unit is a body of rock established to serve as the material reference for all rocks formed during the same span of time. Example: Devonian System. Each boundary of a chronostratigraphic unit is synchronous. Chronostratigraphy provides a means of organizing strata into units based on their age relations. A chronostratigraphic body also serves as the basis for defining the specific interval of geologic time, or geochronologic unit, represented by the referent. 4. A geochronologic unit is a division of time distinguished on the basis of the rock record preserved in a chronostratigraphic unit. Example: Devonian Period.

The first two categories are comparable in that they consist of material units defined on the basis of content. The third category differs from the first two in that it serves primarily as the standard for recognizing and isolating materials of a specific age. The fourth, in contrast, is not a material, but rather a conceptual, unit; it is a division of time. Although a geochronologic unit is not a stratigraphic body, it is so intimately tied to chronostratigraphy that the two are discussed properly together.

Properties and procedures that may be used in distinguishing geologic units are both diverse and numerous (ISSC, 1976,p.1,96;1994,p.102–103;Harland,1977,p.230),but all may be assigned to the following principal classes of categories used in stratigraphic classification (Table 1), which are discussed below:

I. Material categories based on content, inherent attributes, or physical limits

I. Categories expressing or related to geologic age A. Material categories used to define temporal spans B. Temporal (non-material) categories

Material Categories Based on Content or Physical Limits

The basic building blocks for most geologic work are rock bodies, defined on the basis of composition and related lithic characteristics,orontheirphysical,chemical,orbiologic content or properties. Emphasis is placed on the relative objectivity and reproducibility of data used in defining units within each category.

Foremost properties of rocks are composition, texture, fabric, structure, and color, which together are designated lithic characteristics. These serve as the basis for distinguishing and defining the most fundamental of all formal units. Such units based primarily on composition are divided into two categories (Henderson et al., 1980): lithostratigraphic (Article 2) and lithodemic (defined here in Article 31). A lithostratigraphicunitobeystheLawofSuperposition,whereas a lithodemicunit does not. A lithodemicunit is a defined body of predominantly intrusive, highly metamorphosed, or intensely deformed rock that, because it is intrusive or has lost primary structure through metamorphism or tectonism, generally does not conform to the Law of Superposition.

Recognition during the past several decades that remanent magnetism in rocks records the Earth’s past magnetic characteristics (Cox, et al., 1963) provides a powerful new tool encompassed by magnetostratigraphy (McDougall, 1977; McElhinny, 1978). Magnetostratigraphy (Article 43) is the study of remanent magnetism in rocks; it is the record of the Earth’s magnetic polarity (or field reversals), dipolefield-pole position (including apparent polar wander), the non-dipole component (secular variation), and field intensity. Polarity is of particular utility and is used to define a magnetopolarity unit (Article 4) as a body of rock identified by its remanent magnetic polarity (ACSN, 1976; ISSC, 1979). Empirical demonstration of uniform polarity does not necessarily have direct temporal connotations because the remanent magnetism need not be related to rock deposition or crystallization. Nevertheless, polarity is a physical attribute that may characterize a body of rock.

Biologic remains contained in, or forming, strata are uniquely important in stratigraphic practice. First, they provide the means of defining and recognizing material units

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