NORTH AMERICAN STRATIGRAPHIC CODE
North American Commission on Stratigraphic Nomenclature
from The American Association of Petroleum Geologists Bulletin Volume 89, Number 11 (November, 2005), pp. 1547–1591, 11 Figures, 2 Tables with amendments published 2017, 2019, and 2020
Stratigraphy, Volume 13, Number 3 (April, 2017), pp. 220–222
Stratigraphy, Volume 16, Number 4 (December, 2019), pp. 279–281, 1 Table
Stratigraphy, Volume 17, Number 4 (December, 2020), pp. 315–316, 1 Table
FOREWORD TO THE 2021 EDITION
The 2021 version of the North American Stratigraphic Code is not a
major revision. It simply states the changes mandated by the three
approved amendments published in Easton et al. (2017), Brett et al.
(2019), and Aubry et al. (2020), which contain modifications to
Articles 13, 25, 26, 27, 37, 73, 81, 82, and Table 2. For
completeness, the composition of the North American Commission on
Stratigraphic Nomenclature has been updated in Appendix II, and
citations to Notes and Reports of the Commission since 2005 have been
added to Appendix III. These changes follow Code amendment procedures
as outlined in Article 21.
2021 North American Commission on Stratigraphic Nomenclature
FOREWORD TO THE REVISED 2005 EDITION
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,
text-figures 1, 4, 5, and 6, as well as Tables 1 and 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
FOREWORD TO THE 1983 CODE
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
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 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 Code Committee, chaired by Steven S. Oriel, who also served 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.
This Code is the product of a truly North American effort. Many former
and current commissioners representing not only the ten organizational
members of the North American Commission on Stratigraphic Nomenclature
(Appendix II) but other institutions as well, generated the product.
Endorsement by constituent organizations is anticipated, and
scientific communication will be fostered if Canadian, 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 Code are 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.
1982 North American Commission on Stratigraphic Nomenclature
PART I. PREAMBLE
BACKGROUND PERSPECTIVE SCOPE
RELATION OF CODES TO INTERNATIONAL GUIDE
Material Categories Based on Content or Physical Limits
Categories Expressing or Related to Geologic Age
FORMAL AND INFORMAL UNITS
PART II. ARTICLES
Article 1. Purpose
Article 2. Categories
DEFINITION OF FORMAL UNITS
Article 3. Requirements for Formally Named Geologic Units
Article 4. Publication
Remarks: (a) Inadequate publication
(c) Electronic publication
Article 5. Intent and Utility
Remark: (a) Demonstration of purpose served
Article 6. Category and Rank
Remark: (a) Need for specification
Article 7. Name
Remarks: (a) Appropriate geographic terms
(b) Duplication of names
(c) Priority and preservation of established names
(d) Differences of spelling and changes in name
(e) Names in different countries and different languages
Article 8. Stratotypes
Remarks: (a) Unit stratotypes
(b) Boundary stratotype
(c) Type locality
(d) Composite stratotype
(e) Reference sections
(f) Stratotype descriptions
Article 9. Unit Description
Article 10. Boundaries
Remarks: (a) Boundaries between intergradational units
(b) Overlaps and gaps
Article 11. Historical Background
Article 12. Dimensions and Regional Relations
Article 13. Age
Remarks: (a) Dating
(c) Convention and abbreviations
(d) Expression of “age” of lithodemic units
Article 14. Correlation
Article 15. Genesis
Article 16. Subsurface and Subsea Units
Remarks: (a) Naming subsurface units
(b) Additional recommendations
(c) Seismostratigraphic units
REVISION AND ABANDONMENT OF FORMAL UNITS
Article 17. Requirements for Major Changes
Remark: (a) Distinction between redefinition and revision
Article 18. Redefinition
Remarks: (a) Change in lithic designation
(b) Original lithic designation inappropriate
Article 19. Revision
Remarks: (a) Boundary change
(b) Change in rank
(c) Examples of changes from area to area
(d) Example of change in single area
(e) Retention of type section
(f) Different geographic name for a unit and its parts
(g) Undesirable restriction
Article 20. Abandonment
Remarks: (a) Reasons for abandonment
(c) Obsolete names
(d)Reference to abandoned names
Article 21. Procedure for Amendment
FORMAL UNITS DISTINGUISHED BY CONTENT PROPERTIES, OR PHYSICAL LIMITS
Nature and Boundaries
Article 22. Nature of Lithostratigraphic Units
Remarks: (a) Basic units
(b)Type section and locality
(c) Type section never changed
(d)Independence from inferred geologic history
(e) Independence from time concepts
(f) Surface form
(g) Economically exploited units
(h) Instrumentally defined units
(k) Soils and paleosols
(l) Depositional facies
Article 23. Boundaries
Remarks: (a) Boundary in a vertically gradational sequence
(b)Boundaries in lateral lithologic change
(c) Key beds used for boundaries
(d)Unconformities as boundaries
(e) Correspondence with genetic units
Ranks of Lithostratigraphic Units
Article 24. Formation 176
Remarks: (a) Fundamental unit
(c) Lithic characteristics
(d)Mappability and thickness
(e) Organic reefs and carbonate mounds
(f) Interbedded volcanic and sedimentary rock
(g) Volcanic rock
(h) Metamorphic rock
Article 25. Member
Remarks: (a) Mapping of members
(b)Lens and tongue
(c) Organic reefs and carbonate mounds
(d)Division of members
(e) Laterally equivalent members
Article 26. Submember
Remarks: (a) Mapping of submembers
(b) Division of submembers
(c)Laterally equivalent submembers
Article 27. Bed(s) and Flow(s)
Remarks: (a) Limitations
(b)Key or marker beds
Article 28. Group
Remarks: (a) Use and content
(b)Change in component formations
(c)Change in rank
Article 29. Supergroup
Remark: (a) Misuse of “series” for group or supergroup Lithostratigraphic Nomenclature
Article 30. Compound Character
Remarks: (a) Omission of part of a name
(b)Use of simple lithic terms
(c) Group names
(e) Member and submember names
(f) Names of reefs
(g) Bed and flow names
(h) Informal units
(i) Informal usage of identical geographic names
(j) Metamorphic rock
(k) Misuse of well-known name
Nature and Boundaries
Article 31. Nature of Lithodemic Units 179
Remarks: (a) Recognition and definition
(b)Type and reference localities
(c) Independence from inferred geologic history
(d)Use of “zone”
Article 32. Boundaries 180
Remark: (a) Boundaries within gradational zones Ranks of Lithodemic Units
Article 33. Lithodeme 180
Remarks: (a) Content
(b) Lithic characteristics
Article 34. Division of Lithodemes 180
Article 35. Suite 180
Remarks: (a) Purpose
(b) Change in component units
(c)Change in rank
Article 36. Supersuite 180
Article 37. Complex 180
Remarks: (a) Use of “complex”
(c) Structural complex
(e) Misuse of “complex”
Article 38. Misuse of “Series” for Suite, Complex, or Supersuite 181
Article 39. General Provisions
Article 40. Lithodeme Names
Remarks: (a) Lithic term
(b) Intrusive and plutonic rocks
Article 41. Suite names
Article 42. Supersuite Names
Nature and Boundaries
Article 43. Nature of Magnetostratigraphic Units
Remarks: (a) Definition
(b) Contemporaneity of rock and remanent magnetism
(c)Designations and scope
Article 44. Definition of Magnetopolarity Unit
Remarks: (a) Nature
(c) Independence from inferred history
(d)Relation to lithostratigraphic and biostratigraphic units
(e) Relation of magnetopolarity units to chronostratigraphic units
Article 45. Boundaries 182
Remark: (a) Polarity-reversal horizons and transition zones Ranks of Magnetopolarity Units
Article 46. Fundamental Unit 182
Remarks: (a) Content
(b) Thickness and duration
(c)Ranks Magnetopolarity Nomenclature
Article 47. Compound Name
Article 48. Fundamentals of Biostratigraphy
Remark: (a) Uniqueness Nature and Boundaries
Article 49. Nature of Biostratigraphic Units
Remarks: (a) Unfossiliferous rocks
(b)Contemporaneity of rocks and fossils
(c) Independence from lithostratigraphic units
(d)Independence from chronostratigraphic units
Article 50. Kinds of Biostratigraphic Units
Remarks: (a) Range biozone
(c) Lineage biozone
(e) Abundance biozone
(f) Hybrid or new kinds of biozones
Article 51. Boundaries
Remark: (a) Identification of biozones Article 52. [not used]
Ranks of Biostratigraphic Units
Article 53. Fundamental Unit
Remarks: (a) Scope
(c) Shortened forms of expression Biostratigraphic Nomenclature
Article 54. Establishing Formal Units
Remarks: (a) Name
(b) Shorter designations for biozone names
(e) Reference sections
Nature and Boundaries
Article 55. Nature of Pedostratigraphic Units
Remarks: (a) Definition
(c) Boundaries and stratigraphic position
(e) Distinction from pedologic soils
(f) Relation to saprolite and other weathered materials
(g) Distinction from other stratigraphic units
(h) Independence from time concepts Pedostratigraphic Nomenclature and Unit
Article 56. Fundamental Unit
Article 57. Nomenclature
Remarks: (a) Composite geosols
(c) Procedures for establishing formal pedostratigraphic units
Nature and boundaries
Article 58. Nature of Allostratigraphic Units
Remarks: (a) Purpose
(b) Internal characteristics
(e) Type locality and extent
(f) Relation to genesis
(g) Relation to geomorphic surfaces
(h) Relation to soils and paleosols
(i) Relation to inferred geologic history
(j) Relation to time concepts
(k) Extension of allostratigraphic units Ranks of Allostratigraphic Units
Article 59. Hierarchy
Remarks: (a) Alloformation
(d) Changes in rank Allostratigraphic Nomenclature
Article 60. Nomenclature
Remark: (a) Revision
FORMAL UNITS EXPRESSING OR RELATING TO GEOLOGIC AGE
KINDS OF GEOLOGIC-TIME UNITS
Nature and Kinds
Article 61. Kinds
Units Based on Material Referents
Article 62. Kinds Based on Referents
Article 63. Isochronous Categories
Remark: (a) Extent
Article 64. Diachronous Categories
Remarks: (a) Diachroneity
Units Independent of Material Referents
Article 65. Numerical Divisions of Time
Nature and Boundaries
Article 66. Definition
Remarks: (a) Purposes
Article 67. Boundaries
Remark: (a) Emphasis on lower boundaries of chronostratigraphic units
Article 68. Correlation
Ranks of Chronostratigraphic Units
Article 69. Hierarchy
Article 70. Eonothem
Article 71. Erathem
Remark: (a) Names
Article 72. System
Remark: (a) Subsystem and supersystem
Article 73. Series
Remark: (a) Subseries
Article 74. Stage
Remark: (a) Substage
Article 75. Chronozone
Remarks: (a) Boundaries of chronozones
(c) Practical utility Chronostratigraphic Nomenclature
Article 76. Requirements
Article 77. Nomenclature
Remarks: (a) Systems and units of higher rank
(b) Series and units of lower rank
Article 78. Stratotypes
Article 79. Revision of Units
Nature and Boundaries
Article 80. Definition and Basis
Ranks and Nomenclature of Geochronologic Units
Article 81. Hierarchy
Article 82. Nomenclature
Nature and Boundaries
Article 83. Definition
Remarks: (a) Nature
(b) Principal purposes
Article 84. Boundaries
Ranks and Nomenclature of Polarity-Chronostratigraphic Units
Article 85. Fundamental Unit
Remarks: (a) Meaning of term
Article 86. Establishing Formal Units
Article 87. Name
Remarks: (a) Preservation of established name
(b) Expression of doubt
Nature and Boundaries
Article 88. Definition
Ranks and Nomenclature of Polarity-Chronologic Units
Article 89. Fundamental Unit
Remark: (a) Hierarchy
Article 90. Nomenclature
Nature and Boundaries
Article 91. Definition
Remarks: (a) Purposes
Article 92. Boundaries
Remark: (a) Temporal relations
Ranks and Nomenclature of Diachronic Units
Article 93. Ranks
Remarks: (a) Diachron
(b) Hierarchical ordering permissible
Article 94. Name
Remarks: (a) Formal designation of units
(b) Interregional extension of geographic names
(c) Change from geochronologic to diachronic classification
Article 95. Establishing Formal Units
Remark: (a) Revision or abandonment
Nature and Boundaries
Article 96. Definition
Ranks and Nomenclature of Geochronometric Units
Article 97. Nomenclature
PART III. ADDENDA
I. PARTICIPANTS AND CONFEREES IN CODE REVISION
II. 1977-2020 COMPOSITION OF THE NORTH AMERICAN
COMMISSION ON STRATIGRAPHIC NOMENCLATURE
III. REPORTS AND NOTES OF THE AMERICAN COMMISSION ON STRATIGRAPHIC NOMENCLATURE
- Classes of units defined
- Categories and ranks of units defined in this code
- Relation of geologic time units to the kinds of rock-unit referents on which most are based
- Diagrammatic examples of lithostratigraphic boundaries and classification
- Lithodemic and lithostratigraphic units
- Examples of range, lineage, and interval biozones
- Examples of assemblage and abundance biozones
- Relation between pedostratigraphic units and pedologic profiles
- Example of allostratigraphic classification of alluvial and lacustrine deposits in a graben
- Example of allostratigraphic classification of contiguous deposits of similar lithology
- Example of allostratigraphic classification of lithologically similar, discontinuous terrace deposits
- Comparison of geochronologic, chronostratigraphic, and diachronic units
- Schematic relation of phases to an episode
PART I. PREAMBLE
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.
New concepts and techniques developed since 1961 have 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 of Geological Sciences (IUGS) and UNESCO, including working
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 was undertaken
by committees appointed by the North American 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, and
Mexico represent the geological profession throughout North America
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 electronic
publication of new and revised names and correcting inconsistencies to
improve clarity (Ferrusquía-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, although developed
initially to bring order to strata and the events recorded therein,
are applicable to all earth materials, not solely to strata. They
promote systematic and rigorous study of the composition, geometry,
sequence, history, and genesis of rocks and unconsolidated materials.
They provide the framework within 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 accepted and used; geologic organizations and
journals may 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, or physiographic terms. Terms
from these other 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 is not 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,
distinctive terms are needed to identify the property used in defining
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).
RELATION OF CODES TO INTERNATIONAL GUIDE
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 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:
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.
A biostratigraphic unit is a body of rock defined and
characterized by its fossil content. Example: Discoaster
multiradiatus Interval Biozone.
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.
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.
Classes of Units Defined.*
* Numbers in parentheses are the numbers of the Articles where units
** Italicized categories are those introduced or developed since
publication of the previous code (ACSN 1970).
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:
Material categories based on content, inherent attributes, or physical limits
Categories expressing or related to geologic age
- Material categories used to define temporal spans
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, or on their physical, chemical, or biologic 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 22) and lithodemic (defined
here in Article 31). A lithostratigraphic unit obeys the Law of
Superposition, whereas a lithodemic unit does not. A lithodemic unit
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), dipole-field-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 44) 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 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 pedostratigraphic 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
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-Climate units, defined in the 1970 Code (ACSN 1970, p. 31),
were abandoned in the 1983 Code because they 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).
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,”
text-fig. 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 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.
Relation of geologic time units to the kinds of referents on which
most are based.
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 which 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 (text-fig. 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 66, 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 Standard Global Chronostratigraphic Scale (ISSC 1976, p.
76-81; 1994, p. 85; Harland 1978), consisting of established systems
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 polarity-chronologic unit (Article 88).
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, biostratigraphic, pedostratigraphic,
allostratigraphic, or an assemblage 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
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).
A pedologic soil is composed of one or more soil
horizons3. 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 55),
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 (text-fig. 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.
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
FORMAL AND INFORMAL UNITS
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
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
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 craton were designated
“sequences” (example: Sauk sequence) 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
The terms magnafacies and parvafacies, coined by Caster (1934) to
emphasize the distinction between lithostratigraphic and
chronostratigraphic units 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 the
Bandelier Tuff), others bear the same name as the volcanic vent (e.g.,
Huckleberry Ridge ash bed of Izett and Wilcox 1981).
Informal geologic units are designated by ordinary nouns, 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
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
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
PART II. ARTICLES
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 terminology and, therefore,
promote unambiguous communication.
Article 2. − Categories. Categories of formal stratigraphic 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, and includes geochronologic,
polarity-chronologic, diachronic, and geochronometric units.
DEFINITION OF FORMAL UNITS
Article 3. − Requirements for Formally Named Geologic Units.
Naming, establishing, revising, redefining, and abandoning formal
geologic units require publication in a recognized scientific medium
of a comprehensive statement, which includes (i) intent to designate
or modify a formal unit; (ii) designation of category and rank of
unit; (iii) 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,
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)
distribution 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;
(11) 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).
(b) Guidebooks. − A guidebook with distribution limited to
participants of a field excursion does not meet the test of
availability. Some organizations publish and distribute widely large
editions of serial guidebooks that include refereed regional papers;
although these do meet the tests of scientific purpose and
availability, and therefore constitute valid publication, other
media are preferable.
(c) Electronic publication. − Publication in electronic medium,
which has become widespread since distribution of the Code in
1983, is confined to publication in a journal or other publication
series by a widely recognized (1) scientific society, (2) government agency, (3) academic institution, or (4) other respected scientific publisher. All versions distributed must be the
same, whether in paper or electronic form, without alteration. Other
requirements are as follows: (1) archival practices adequate for
future availability; (2) suitable typography; (3) coding and markup
practices that adhere to accepted standards; (4) database preparation
that includes satisfactory search and retrieval tools as well as the
capability for downloading to a researcher’s local printer; and (5)
adequate copyediting standards. New stratigraphic names can be
Article 5. − Intent and Utility. To be valid, a new unit must serve a clear purpose and be duly proposed and duly described, and the intent to establish it must be specified. Casual mention of a unit, such as “the granite exposed near the Middleville schoolhouse,” does not establish a new formal unit, nor does mere use in a table, columnar section, or map.
Remark. (a) Demonstration of purpose served. − The initial
definition or revision of a named geologic unit constitutes, in
essence, a proposal. As such, it lacks status until use by others
demonstrates that a clear purpose has been served. A unit becomes
established through repeated demonstration of its utility. The
decision not to use a newly proposed or a newly revised term requires
a full discussion of its unsuitability.
Article 6. − Category and Rank. The category and rank of a new or revised unit must be specified.
Categories and Ranks of Units Defined in This Code*
Remark. (a) Need for specification. − Many stratigraphic
controversies have arisen from confusion or misinterpretation of the
category of a unit (for example, lithostratigraphic vs.
chronostratigraphic). Specification and unambiguous description of the
category is of paramount importance. Selection and designation of an
appropriate rank from the distinctive terminology developed for each
category help serve this function (Table 2).
Article 7. − Name. The name of a formal geologic unit is compound. For most categories, the name of a unit should consist of a geographic name combined with an appropriate rank (Wasatch Formation) or descriptive term (Viola Limestone). Biostratigraphic units are designated by appropriate biologic forms (Exus albus Assemblage Biozone). Worldwide chronostratigraphic units bear long established and generally accepted names of diverse origins (Triassic System). The first letters of all words used in the names of formal geologic units are capitalized (except for the trivial species and subspecies terms in the name of a biostratigraphic unit).
Remarks. (a) Appropriate geographic terms. − Geographic names
derived from permanent natural or artificial features at or near which
the unit is present are preferable to those derived from impermanent
features such as farms, schools, stores, churches, crossroads, and
small communities. Appropriate names may be selected from those shown
on topographic, state, provincial, county, forest service,
hydrographic, or comparable maps, particularly those showing names
approved by a national board for geographic names. The generic part of
a geographic name, e.g., river, lake, village, should be omitted from
new terms, unless required to distinguish between two otherwise
identical names (e.g., Redstone Formation and Redstone River
Formation). Two names should not be derived from the same geographic
feature. A unit should not be named for the source of its components;
for example, a deposit inferred to have been derived from the Keewatin
glaciation center should not be designated the “Keewatin Till.”
(b) Duplication of names. − Responsibility for avoiding duplication,
either in use of the same name for different units (homonymy) or in
use of different names for the same unit (synonomy), rests with the
proposer. Although the same geographic term has been applied to
different categories of units (example: the lithostratigraphic Word
Formation and the chronostratigraphic Wordian Stage) now entrenched
in the literature, the practice is undesirable. The extensive
geologic nomenclature of North America, including not only names but
also nomenclatural history of formal units, is recorded in compendia
maintained by the Committee on Stratigraphic Nomenclature of the
Geological Survey of Canada, Ottawa, Ontario; by the Geologic Names
Committee of the United States Geological Survey, Reston, Virginia;
by the Instituto de Geología, Ciudad Universitaria, México, D.F.;
and by many state and provincial geological surveys. These
organizations respond to inquiries regarding the availability of
names, and some are prepared to reserve names for units that are
likely to be defined in the next year or two.
(c) Priority and preservation of established names. − Stability of
nomenclature is maintained by use of the rule of priority and by
preservation of well-established names. Names should not be
modified without explaining the need. Priority in publication is
to be respected, but priority alone does not justify displacing a
well-established name by one neither well-known nor commonly used;
nor should an inadequately established name be preserved merely on
the basis of priority. Redefinitions in precise terms are
preferable to abandonment of the names of well-established units
which may have been defined imprecisely but nonetheless in
conformance with older and less stringent standards.
(d) Differences of spelling and changes in name. − The geographic
component of a well-established stratigraphic name is not changed
due to differences in spelling or changes in the name of a
geographic feature. The name Bennett Shale, for example, used for
more than half a century, need not be altered because the town is
named Bennet. Nor should the Mauch Chunk Formation be changed
because the town has been renamed Jim Thorpe. Disappearance of an
impermanent geographic feature, such as a town, does not affect
the name of an established geologic unit.
(e) Names in different countries and different languages. − For
geologic units that cross local and international boundaries, a
single name for each is preferable to several. Spelling of a
geographic name commonly conforms to the usage of the country and
linguistic group involved. Although geographic names are not
translated (Cuchillo is not translated to Knife), lithologic or
rank terms are (Edwards Limestone, Caliza Edwards; Formación La
Casita, La Casita Formation).
Article 8. − Stratotypes. The designation of a unit or boundary stratotype (type section or type locality) is essential in the definition of most formal geologic units. Many kinds of units are best defined by reference to an accessible and specific sequence of rock that may be examined and studied by others. A stratotype is the standard (original or subsequently designated) for a named geologic unit or boundary and constitutes the basis for definition or recognition of that unit or boundary; therefore, it must be illustrative and representative of the concept of the unit or boundary being defined.
Remarks. (a) Unit stratotype. − A unit stratotype is the type
section for a stratiform deposit or the type area for a nonstratiform
body that serves as the standard for definition and recognition of a
geologic unit. The upper and lower limits of a unit stratotype are
designated points in a specific sequence or locality and serve as the
standards for definition and recognition of a stratigraphic unit’s
(b) Boundary stratotype. − A boundary stratotype is the type
locality for the boundary reference point for a stratigraphic unit.
Both boundary stratotypes for any unit need not be in the same
section or region. Each boundary stratotype serves as the standard
for definition and recognition of the base of a stratigraphic unit.
The top of a unit may be defined by the boundary stratotype of the
next higher stratigraphic unit.
(c) Type locality. − A type locality is the specified geographic
locality where the stratotype of a formal unit or unit boundary
was originally defined and named. A type area is the geographic
territory encompassing the type locality. Before the concept of a
stratotype was developed, only type localities and areas were
designated for many geologic units which are now longand
well-established. Stratotypes, though now mandatory in defining
most stratiform units, are impractical in definitions of many
large nonstratiform rock bodies whose diverse major components may
be best displayed at several reference localities.
(d) Composite-stratotype. − A composite-stratotype consists of
several reference sections (which may include a type section)
required to demonstrate the range or totality of a stratigraphic
(e) Reference sections. − Reference sections may serve as invaluable
standards in definitions or revisions of formal geologic units. For
those well-established stratigraphic units for which a type section
never was specified, a principal reference section (lectostratotype
of ISSC 1976, p. 26; 1994, p. 28) may be designated. A principal
reference section (neostratotype of ISSC 1976, p. 26; 1994, p. 28)
also may be designated for those units or boundaries whose
stratotypes have been destroyed, covered, or otherwise made
inaccessible. Supplementary reference sections often are designated
to illustrate the diversity or heterogeneity of a defined unit or
some critical feature not evident or exposed in the stratotype. Once
a unit or boundary stratotype section is designated, it is never
abandoned or changed; however, if a stratotype proves inadequate, it
may be supplemented by a principal reference section or by several
reference sections that may constitute a composite-stratotype.
(f) Stratotype descriptions. − Stratotypes should be described both
geographically and geologically. Sufficient geographic detail must
be included to enable others to find the stratotype in the field,
and may consist of maps and/or aerial photographs showing location
and access, as well as appropriate coordinates or bearings. Geologic
information should include thickness, descriptive criteria
appropriate to the recognition of the unit and its boundaries, and
discussion of the relation of the unit to other geologic units of
the area. A carefully measured and described section provides the
best foundation for definition of stratiform units. Graphic
profiles, columnar sections, structure-sections, and photographs are
useful supplements to a description; a geologic map of the area
including the type locality is essential.
Article 9. − Unit Description. A unit proposed for formal status
should be described and defined so clearly that any subsequent
investigator can recognize that unit unequivocally. Distinguishing
features that characterize a unit may include any or several of the
following: composition, texture, primary structures, structural
attitudes, biologic remains, readily apparent mineral composition
(e.g., calcite vs. dolomite), geochemistry, geophysical properties
(including magnetic signatures), geomorphic expression, unconformable
or cross-cutting relations, and age. Although all distinguishing
features pertinent to the unit category should be described
sufficiently to characterize the unit, those not pertinent to the
category (such as age and inferred genesis for lithostratigraphic
units, or lithology for biostratigraphic units) should not be made
part of the definition.
Article 10. − Boundaries. The criteria specified for the
recognition of boundaries between adjoining geologic units are of
paramount importance because they provide the basis for scientific
reproducibility of results. Care is required in describing the
criteria, which must be appropriate to the category of unit involved.
Remarks. (a) Boundaries between intergradational units. − Contacts
between rocks of markedly contrasting composition are appropriate
boundaries of lithic units, but some rocks grade into, or intertongue
with, others of different lithology. Consequently, some boundaries are
necessarily arbitrary as, for example, the top of the uppermost
limestone in a sequence of interbedded limestone and shale. Such
arbitrary boundaries commonly are diachronous.
(b) Overlaps and gaps. − The problem of overlaps and gaps between
long-established adjacent chronostratigraphic units is being addressed
by international IUGS and IGCP working groups appointed to deal with
various parts of the geologic column. The procedure recommended by the
Geological Society of London (George et al. 1969; Holland et al.
1978), of defining only the basal boundaries of chronostratigraphic
units, has been widely adopted (e.g., McLaren 1977) to resolve the
problem. Such boundaries are defined by a carefully selected and
agreed-upon boundary-stratotype (marker-point type section or “golden
spike”) that becomes the standard for the base of a
chronostratigraphic unit. The concept of the mutual-boundary
stratotype (ISSC 1976, p. 84-86), redesignated lower-boundary
stratotype (ISSC 1994, p. 90), based on the assumption of continuous
deposition in selected sequences, also has been used to define
Although international chronostratigraphic units of series and higher
rank are being redefined by IUGS and IGCP working groups, there may be
a continuing need for some provincial series. Adoption of the basal
boundary-stratotype concept is urged.
Article 11. − Historical Background. A proposal for a new name
must include a nomenclatorial history of constituent rocks assigned to
the proposed unit, describing how they were treated previously and by
whom (references), as well as such matters as priorities, possible
synonymy, and other pertinent considerations. Consideration of the
historical background of an older unit commonly provides the basis for
justifying definition of a new unit.
Article 12. − Dimensions and Regional Relations. A perspective on
the magnitude of a unit should be provided by such information as may
be available on the geographic extent of a unit; observed ranges in
thickness, composition, and geomorphic expression; relations to other
kinds and ranks of stratigraphic units; correlations with other nearby
sequences; and the bases for recognizing and extending the unit beyond
the type locality. If the unit is not known anywhere but in an area of
limited extent, informal designation is recommended.
Article 13. − Age. For most formal material geologic units, other
than chronostratigraphic and polarity-chronostratigraphic, inferences
regarding geologic age play no proper role in their definition.
Nevertheless, the age, as well as the basis for its assignment, are
important features of the unit and, where possible, should be stated.
For many lithodemic units, the age of the protolith should be
distinguished from that of the metamorphism or deformation. If the
basis for assigning an age is tenuous, a doubt should be expressed.
Remarks. (a) Dating. − The geochronologic ordering of the rock
record, whether in terms of radioactive-decay rates or other
processes, is generally called “dating.” However, the use of the noun
“date” to mean “isotopic age” is not recommended. Similarly, the
term “absolute age” should be suppressed in favor of “isotopic age”
for an age determined on the basis of isotopic ratios. The more
inclusive term “numerical age” is recommended for all ages determined
from isotopic ratios, fission tracks, and other quantifiable
(b) Calibration. − The dating of chronostratigraphic boundaries in
terms of numerical ages is a special form of dating for which the
word “calibration” should be used. The geochronologic time-scale
now in use has been developed mainly through such calibration of
(c) Convention and abbreviations. − The age of a stratigraphic
unit or the time of a geologic event, as commonly determined by
numerical dating or by reference to a calibrated time-scale, may
be expressed in years before the present. The unit of time is the
modern year as presently recognized worldwide. Recommended (but
not mandatory) abbreviations for such ages are SI (International
System of Units) multipliers coupled with “a” for annum: ka, Ma,
and Ga5 for kilo-annum (103 years),
Mega-annum (106 years), and Giga-annum (109
years), respectively. Use of these terms after the age value
follows the convention established in the field of C-14 dating.
The “present” refers to 1950 AD, and such qualifiers as “ago” or
“before the present” are omitted after the value because
measurement of the duration from the present to the past is
implicit in the designation. In contrast, the duration of a remote
interval of geologic time, as a number of years, should not be
expressed by the same symbols. Abbreviations for numbers of years,
without reference to the present, are informal (e.g., y or yr for
years; my, m.y., or m.yr. for millions of years; and so forth, as
preference dictates). For example, boundaries of the Late
Cretaceous Epoch currently  are calibrated at 66.0 Ma and 100.5 Ma, but the interval of time represented by this epoch is 34.5 m.y.
(d) Expression of “age” of lithodemic units. − The adjectives
“early,” “middle,” and “late” should be used with the appropriate
geochronologic term to designate the age of lithodemic units. For
example, a granite dated isotopically at 410 Ma should be referred
to using the geochronologic term “Early Devonian granite” rather
than either the chronostratigraphic term “Lower Devonian granite” or
the more cumbersome designation “granite of Early Devonian age.”
Article 14. − Correlation. Information regarding spatial and
temporal counterparts of a newly defined unit beyond the type area provides readers with an enlarged perspective. Discussions of criteria used in correlating a unit with those in other areas should
make clear the distinction between data and inferences.
Article 15. − Genesis. Objective data are used to define and
classify geologic units and to express their spatial and temporal
relations. Although many of the categories defined in this Code (e.g.,
lithostratigraphic group, plutonic suite) have genetic connotations,
inferences regarding geologic history or specific environments of
formation may play no proper role in the definition of a unit.
However, observations, as well as inferences, that bear on genesis are
of great interest to readers and should be discussed.
Article 16. − Subsurface and Subsea Units. The foregoing
procedures for establishing formal geologic units apply also to
subsurface and offshore or subsea units. Complete lithologic and
paleontologic descriptions or logs of the samples or cores are
required in written or graphic form, or both. Boundaries and
divisions, if any, of the unit should be indicated clearly with their
depths from an established datum.
Remarks. (a) Naming subsurface units. − A subsurface unit may be
named for the borehole (Eagle Mills Formation), oil field (Smackover
Limestone), or mine which is intended to serve as the stratotype, or
for a nearby geographic feature. The hole or mine should be located
precisely, both with map and exact geographic coordinates, and
identified fully (operator or company, farm or lease block, dates
drilled or mined, surface elevation and total depth, etc).
(b) Additional recommendations. − Inclusion of appropriate borehole
geophysical logs is urged. Moreover, rock and fossil samples and
cores and all pertinent accompanying materials should be stored, and
available for examination, at appropriate federal, state,
provincial, university, or museum depositories. For offshore or
subsea units (Clipperton Formation of Tracey et al. 1971, p.22; Argo
Salt of McIver 1972, p. 57), the names of the project and vessel,
depth of sea floor, and pertinent regional sampling and geophysical
data should be added.
(c) Seismostratigraphic units. − High-resolution seismic methods now
can delineate stratal geometry and continuity at a level of
confidence not previously attainable. Accordingly, seismic surveys
have come to be the principal adjunct of the drill in subsurface
exploration. On the other hand, the method identifies rock types
only broadly and by inference. Thus, formalization of units known
only from seismic profiles is inappropriate. Once the stratigraphy
is calibrated by drilling, the seismic method may provide objective
REVISION AND ABANDONMENT OF FORMAL UNITS
Article 17. − Requirements for Major Changes. Formally defined and named geologic units may be redefined, revised, or abandoned, but revision and abandonment require as much justification as establishment of a new unit.
Remark. (a) Distinction between redefinition and revision. − Redefinition of a unit involves changing the view or emphasis on the content of the unit without changing the boundaries or rank, and differs only slightly from redescription. Neither redefinition nor redescription is considered revision. A redescription corrects an inadequate or inaccurate description, whereas a redefinition may change a descriptive (for example, lithologic) designation. Revision involves either minor changes in the definition of one or both boundaries or in the rank of a unit (normally, elevation to a higher rank). Correction of a misidentification of a unit outside its type area is neither redefinition nor revision.
Article 18. − Redefinition. A correction or change in the descriptive term applied to a stratigraphic or lithodemic unit is a redefinition which does not require a new geographic term.
Remarks. (a) Change in lithic designation. − Priority should not
prevent more exact lithic designation if the original designation is
not everywhere applicable; for example, the Niobrara Chalk changes
gradually westward to a unit in which shale is prominent, for which
the designation “Niobrara Shale” or “Formation” is more appropriate.
Many carbonate formations originally designated “limestone” or
“dolomite” are found to be geographically inconsistent as to
prevailing rock type. The appropriate lithic term or “formation” is
again preferable for such units.
(b) Original lithic designation inappropriate. − Restudy of some
long-established lithostratigraphic units has shown that the original
lithic designation was incorrect according to modern criteria; for
example, some “shales” have the chemical and mineralogical composition
of limestone, and some rocks described as felsic lavas now are
understood to be welded tuffs. Such new knowledge is recognized by
changing the lithic designation of the unit, while retaining the
original geographic term. Similarly, changes in the classification of
igneous rocks have resulted in recognition that rocks originally
described as quartz monzonite now are more appropriately termed
granite. Such lithic designations may be modernized when the new
classification is widely adopted. If heterogeneous bodies of plutonic
rock have been misleadingly identified with a single compositional
term, such as “gabbro,” the adoption of a neutral term, such as
“intrusion” or “pluton,” may be advisable.
Article 19. − Revision. Revision involves either minor changes in the definition of one or both boundaries of a unit, or in the unit’s rank.
Remarks. (a) Boundary change. − Revision is justifiable if a minor
change in boundary will make a unit more natural and useful. If
revision modifies only a minor part of the content of a previously
established unit, the original name may be retained.
(b) Change in rank. − Change in rank of a stratigraphic or
temporal unit requires neither redefinition of its boundaries nor
alteration of the geographic part of its name. A member may become
a formation or vice versa, a formation may become a group or vice
versa, and a lithodeme may become a suite or vice versa.
(c) Examples of changes from area to area. − The Conasauga Shale
is recognized as a formation in Georgia and as a group in eastern
Tennessee; the Osgood Formation, Laurel Limestone, and Waldron
Shale in Indiana are classed as members of the Wayne Formation in
a part of Tennessee; the Virgelle Sandstone is a formation in
western Montana and a member of the Eagle Sandstone in central
Montana; the Skull Creek Shale and the Newcastle Sandstone in
North Dakota are members of the Ashville Formation in Manitoba.
(d) Example of change in single area. − The rank of a unit may be
changed without changing its content. For example, the Madison
Limestone of early work in Montana later became the Madison Group,
containing several formations.
(e) Retention of type section. − When the rank of a geologic unit
is changed, the original type section or type locality is retained
for the newly ranked unit (see Article 22c).
(f) Different geographic name for a unit and its parts. − In
changing the rank of a unit, the same name may not be applied both
to the unit as a whole and to a part of it. For example, the
Astoria Group should not contain an Astoria Sandstone, nor the
Washington Formation, a Washington Sandstone Member.
(g) Undesirable restriction. − When a unit is divided into two or
more of the same rank as the original, the original name should
not be used for any of the divisions. Retention of the old name
for one of the units precludes use of the name in a term of higher
rank. Furthermore, in order to understand an author’s meaning, a
later reader would have to know about the modification and its
date, and whether the author is following the original or the
modified usage. For these reasons, the normal practice is to raise
the rank of an established unit when units of the same rank are
recognized and mapped within it.
Article 20. − Abandonment. An improperly defined or obsolete stratigraphic, lithodemic, or temporal unit may be formally abandoned, provided that (a) sufficient justification is presented to demonstrate a concern for nomenclatural stability, and (b) recommendations are made for the classification and nomenclature to be used in its place.
Remarks. (a) Reasons for abandonment. − A formally defined unit
may be abandoned by the demonstration of synonymy or homonymy, of
assignment to an improper category (for example, definition of a
lithostratigraphic unit in a chronostratigraphic sense), or of other
direct violations of a stratigraphic code or procedures prevailing at
the time of the original definition. Disuse, or the lack of need or
useful purpose for a unit, may be a basis for abandonment; so, too,
may widespread misuse in diverse ways that compound confusion. A unit
also may be abandoned if it proves impracticable, neither recognizable
nor mappable elsewhere.
(b) Abandoned names. − A name for a lithostratigraphic or lithodemic
unit, once applied and then abandoned, is available for some other
unit only if the name was introduced casually, or if it has been
published only once in the last several decades and is not in
current usage, and if its reintroduction will cause no confusion. An
explanation of the history of the name and of the new usage should
be a part of the designation.
(c) Obsolete names. − Authors may refer to national and provincial
records of stratigraphic names to determine whether a name is
obsolete (see Article 7b).
(d) Reference to abandoned names. − When it is useful to refer to an
obsolete or abandoned formal name, its status is made clear by some
such term as “abandoned” or “obsolete,” and by using a phrase such
as “La Plata Sandstone of Cross (1898).” (The same phrase also is
used to convey that a named unit has not yet been adopted for usage
by the organization involved.)
(e) Reinstatement. − A name abandoned for reasons that seem valid at
the time, but which subsequently are found to be erroneous, may be
reinstated. Example: the Washakie Formation, defined in 1869, was
abandoned in 1918 and reinstated in 1973.
Article 21. − Procedure for Amendment. Additions to, or changes of, this Code may be proposed in writing to the Commission by any geoscientist at any time. If accepted for consideration by a majority vote of the Commission, they may be adopted by a two-thirds vote of the Commission at an annual meeting not less than a year after publication of the proposal.
FORMAL UNITS DISTINGUISHED BY CONTENT, PROPERTIES, OR PHYSICAL LIMITS
Nature and Boundaries
Article 22. − Nature of Lithostratigraphic Units. A lithostratigraphic unit is a defined body of sedimentary, extrusive igneous, metasedimentary, or metavolcanic strata which is distinguished and delimited on the basis of lithic characteristics and stratigraphic position. A lithostratigraphic unit generally conforms to the Law of Superposition and commonly is stratified and tabular in form.
Remarks. (a) Basic units. − Lithostratigraphic units are the basic
units of general geologic work and serve as the foundation for
delineating strata, local and regional structure, economic resources,
and geologic history in regions of stratified rocks. They are
recognized and defined by observable rock characteristics; boundaries
may be placed at clearly distinguished contacts or drawn arbitrarily
within a zone of gradation. Lithification or cementation is not a
necessary property; clay, gravel, till, and other unconsolidated
deposits may constitute valid lithostratigraphic units.
(b) Type section and locality. − The definition of
lithostratigraphic unit should be based, if possible, on a
stratotype consisting of readily accessible rocks in place, e.g., in
outcrops, excavations, and mines, or of rocks accessible only to
remote sampling devices, such as those in drill holes and
underwater. Even where remote methods are used, definitions must be
based on lithic criteria and not on the geophysical characteristics
of the rocks, nor the implied age of their contained fossils.
Definitions must be based on descriptions of actual rock material.
Regional validity must be demonstrated for all such units. In
regions where the stratigraphy has been established through studies
of surface exposures, the naming of new units in the subsurface is
justified only where the subsurface section differs materially from
the surface section, or where there is doubt as to the equivalence
of a subsurface and a surface unit. The establishment of subsurface
reference sections for units originally defined in outcrop is
(c) Type section never changed. − The definition and name of a
lithostratigraphic unit are established at a type section (or
locality) that, once specified, must not be changed. If the type
section is poorly designated or delimited, it may be redefined
subsequently. If the originally specified stratotype is
incomplete, poorly exposed, structurally complicated, or
unrepresentative of the unit, a principal reference section or
several reference sections may be designated to supplement, but
not to supplant, the type section (Article 8e).
(d) Independence from inferred geologic history .− Inferred
geologic history, depositional environment, and biological
sequence have no place in the definition of a lithostratigraphic
unit, which must be based on composition and other lithic
characteristics; nevertheless, considerations of well-documented
geologic history properly may influence the choice of vertical and
lateral boundaries of a new unit. Fossils may be valuable during
mapping in distinguishing between two lithologically similar,
noncontiguous lithostratigraphic units. The fossil content of a
lithostratigraphic unit is a legitimate lithic characteristic; for
example, oyster-rich sandstone, coquina, coral reef, or
graptolitic shale. Moreover, otherwise similar units, such as the
Formación Mendez and Formación Velasco mudstones, may be
distinguished on the basis of coarseness of contained fossils
(e) Independence from time concepts. − The boundaries of most
lithostratigraphic units are time independent, but some may be
approximately synchronous. Inferred time spans, however measured,
play no part in differentiating or determining the boundaries of
any lithostratigraphic unit. Either relatively short or relatively
long intervals of time may be represented by a single unit. The
accumulation of material assigned to a particular unit may have
begun or ended earlier in some localities than in others; also,
removal of rock by erosion, either within the time span of
deposition of the unit or later, may reduce the time span
represented by the unit locally. The body in some places may be
entirely younger than in other places. On the other hand, the
establishment of formal units that straddle known, identifiable,
regional disconformities is to be avoided, if at all possible.
Although concepts of time or age play no part in defining
lithostratigraphic units nor in determining their boundaries,
evidence of age may aid recognition of similar lithostratigraphic
units at localities far removed from the type sections or areas.
(f) Surface form. − Erosional morphology or secondary surface form
may be a factor in the recognition of a lithostratigraphic unit,
but properly should play a minor part at most in the definition of
such units. Because the surface expression of lithostratigraphic
units is an important aid in mapping, it is commonly advisable,
where other factors do not countervail, to define
lithostratigraphic boundaries so as to coincide with lithic
changes that are expressed in topography.
(g) Economically exploited units. − Aquifers, oil sands, coal
beds, and quarry layers are, in general, informal units even
though named. Some such units, however, may be recognized formally
as beds, submembers, members, or formations because they are
important in the elucidation of regional stratigraphy.
(h) Instrumentally defined units. − In subsurface investigations,
certain bodies of rock and their boundaries are widely recognized
on borehole geophysical logs showing their electrical resistivity,
radioactivity, density, or other physical properties. Such bodies
and their boundaries may or may not correspond to formal
lithostratigraphic units and their boundaries. Where other
considerations do not countervail, the boundaries of subsurface
units should be defined so as to correspond to useful geophysical
markers; nevertheless, units defined exclusively on the basis of
remotely sensed physical properties, although commonly useful in
stratigraphic analysis, stand completely apart from the hierarchy
of formal lithostratigraphic units and are considered informal.
(i) Zone. − As applied to the designation of lithostratigraphic
units, the term “zone” is informal. Examples are “producing zone,”
“mineralized zone,” “metamorphic zone,” and “heavymineral zone.” A
zone may include all or parts of a bed, a member, a formation, or
even a group.
(j) Cyclothems. − Cyclic or rhythmic sequences of sedimentary
rocks, whose repetitive divisions have been named cyclothems, have
been recognized in sedimentary basins around the world. Some
cyclothems have been identified by geographic names, but such
names are considered informal. A clear distinction must be
maintained between the division of a stratigraphic column into
cyclothems and its division into groups, formations, and members.
Where a cyclothem is identified by a geographic name, the word
cyclothem should be part of the name, and the geographic term
should not be the same as that of any formal unit embraced by the
(k) Soils and paleosols. − Soils and paleosols are layers composed
of the in-situ products of weathering of older rocks that may be
of diverse composition and age. Soils and paleosols differ in
several respects from lithostratigraphic units, and should not be
treated as such (see “Pedostratigraphic Units,” Articles 55 et
(l) Depositional facies. − Depositional facies are informal units,
whether objective (conglomeratic, black shale, graptolitic) or genetic
and environmental (platform, turbiditic, fluvial), even when a
geographic term has been applied, e.g., Lantz Mills facies.
Descriptive designations convey more information than geographic terms
and are preferable.
Article 23. Boundaries. − Boundaries of lithostratigraphic units are placed at positions of lithic change. Boundaries are placed at distinct contacts or may be selected at some arbitrary level within zones of gradation (text-fig. 2A). Both vertical and lateral boundaries are based on the lithic criteria that provide the greatest unity and utility.
Remarks. (a) Boundary in a vertically gradational sequence. − A
named lithostratigraphic unit is preferably bounded by a single lower
and a single upper surface so that the name does not recur in a normal
stratigraphic succession (see Remark b). Where a rock unit passes
vertically into another by intergrading or interfingering of two or
more kinds of rock, unless the gradational strata are sufficiently
thick to warrant designation of a third, independent unit, the
boundary is necessarily arbitrary and should be selected on the basis
of practicality (text-fig. 2B). For example, where a shale unit
overlies a unit of interbedded limestone and shale, the boundary commonly is placed at the top of the
highest readily traceable limestone bed. Where a sandstone unit grades
upward into shale, the boundary may be so gradational as to be
difficult to place even arbitrarily; ideally it should be drawn at the
level where the rock is composed of one-half of each component.
Because of creep in outcrops and caving in boreholes, it is generally
best to define such arbitrary boundaries by the highest occurrence of
a particular rock type, rather than the lowest.
TEXT FIGURE 2
Digrammatic examples of lithostratigraphic boundaries and
(b) Boundaries in lateral lithologic change. − Where a unit changes
laterally through gradation into, or intertongues with, a markedly
different kind of rock, a new unit should be proposed for the
different rock type. An arbitrary lateral boundary may be placed
between the two equivalent units. Where the area of lateral
intergradation or intertonguing is sufficiently extensive, a
transitional interval of interbedded rocks may constitute a third
independent unit (text-fig. 2C). Where tongues (Article 25b) of
formations are mapped separately or otherwise set apart without
being formally named, the unmodified formation name should not be
repeated in a normal stratigraphic sequence, although the modified
name may be repeated in such phrases as “lower tongue of Mancos
Shale” and “upper tongue of Mancos Shale.” To show the order of
superposition on maps and cross sections, the unnamed tongues may be
distinguished informally (text-fig. 2D) by number, letter, or other
means. Such relations may also be dealt with informally through the
recognition of depositional facies (Article 22l).
(c) Key beds used for boundaries. − Key beds (Article 27b) may be
used as boundaries for a formal lithostratigraphic unit where the
internal lithic characteristics of the unit remain relatively
constant. Even though bounding key beds may be traceable beyond the
area of the diagnostic overall rock type, geographic extension of
the lithostratigraphic unit bounded thereby is not necessarily
justified. Where the rock between key beds becomes drastically
different from that of the type locality, a new name should be
applied (text-fig. 2E), even though the key beds are continuous
(Article 27b). Stratigraphic and sedimentologic studies of
stratigraphic units (usually informal) bounded by key beds may be
very informative and useful, especially in subsurface work where the
key beds may be recognized by their geophysical signatures. Such
units, however, may be a kind of chronostratigraphic, rather than
lithostratigraphic, unit (Article 75, 75c), although others are
diachronous because one, or both, of the key beds are also
(d) Unconformities as boundaries. − Unconformities, where
recognizable objectively on lithic criteria, are ideal boundaries
for lithostratigraphic units. However, a sequence of similar rocks
may include an obscure unconformity so that separation into two
units may be desirable but impracticable. If no lithic distinction
adequate to define a widely recognizable boundary can be made, only
one unit should be recognized, even though it may include rock that
accumulated in different epochs, periods, or eras.
(e) Correspondence with genetic units. − The boundaries of
lithostratigraphic units should be chosen on the basis of lithic
changes and, where feasible, to correspond with the boundaries of
genetic units, so that subsequent studies of genesis will not have
to deal with units that straddle formal boundaries.
Ranks of Lithostratigraphic Units
Article 24. − Formation. The formation is the fundamental unit in lithostratigraphic classification. A formation is a body of rock identified by lithic characteristics and stratigraphic position; it is prevailingly but not necessarily tabular and is mappable at the Earth’s surface or traceable in the subsurface.
Remarks. (a) Fundamental unit. − Formations are the basic
lithostratigraphic units used in describing and interpreting the
geology of a region. The limits of a formation normally are those
surfaces of lithic change that give it the greatest practicable unity
of constitution. A formation may represent a long or short time
interval, may be composed of materials from one or several sources,
and may include breaks in deposition (see Article 23d).
(b) Content. − A formation should possess some degree of internal
lithic homogeneity or distinctive lithic features. It may contain
between its upper and lower limits (i) rock of one lithic type, (ii)
repetitions of two or more lithic types, or (iii) extreme lithic
heterogeneity which in itself may constitute a form of unity when
compared to the adjacent rock units.
(c) Lithic characteristics. − Distinctive lithic characteristics
include chemical and mineralogical composition, texture, and such
supplementary features as color, primary sedimentary or volcanic
structures, fossils (viewed as rock-forming particles), or other
organic content (coal, oil-shale). A unit distinguishable only by
the taxonomy of its fossils is not a lithostratigraphic but a
biostratigraphic unit (Article 48). Rock type may be distinctively
represented by electrical, radioactive, seismic, or other properties
(Article 22h), but these properties by themselves do not describe
adequately the lithic character of the unit.
(d) Mappability and thickness. − The proposal of a new formation
must be based on tested mappability. Well-established formations
commonly are divisible into several widely recognizable
lithostratigraphic units; where formal recognition of these smaller
units serves a useful purpose, they may be established as members
and beds, for which the requirement of mappability is not mandatory.
A unit formally recognized as a formation in one area may be treated
elsewhere as a group, or as a member of another formation, without
change of name. Example: the Niobrara is mapped at different places
as a member of the Mancos Shale, of the Cody Shale, or of the
Colorado Shale, and also as the Niobrara Formation, as the Niobrara
Limestone, and as the Niobrara Shale.
Thickness is not a determining parameter in dividing a rock succession
into formations; the thickness of a formation may range from a feather
edge at its depositional or erosional limit to thousands of meters
elsewhere. No formation is considered valid that cannot be delineated
at the scale of geologic mapping practiced in the region when the
formation is proposed. Although representation of a formation on maps
and cross sections by a labeled line may be justified, proliferation
of such exceptionally thin units is undesirable. The methods of
subsurface mapping permit delineation of units much thinner than those
usually practicable for surface studies; before such thin units are
formalized, consideration should be given to the effect on subsequent
surface and subsurface studies.
(e) Organic reefs and carbonate mounds. − Organic reefs and
carbonate mounds (“buildups”) may be distinguished formally, if
desirable, as formations distinct from their surrounding, thinner,
temporal equivalents. For the requirements of formalization, see
(f) Interbedded volcanic and sedimentary rock. − Sedimentary rock
and volcanic rock that are interbedded may be assembled into a
formation under one name that should indicate the predominant or
distinguishing lithology, such as Mindego Basalt.
(g) Volcanic rock. − Mappable distinguishable sequences of
stratified volcanic rock should be treated as formations or
lithostratigraphic units of higher or lower rank. A small
intrusive component of a dominantly stratiform volcanic assemblage
may be treated informally.
(h) Metamorphic rock. − Formations composed of low-grade metamorphic
rock (defined for this purpose as rock in which primary structures
are clearly recognizable) are, like sedimentary formations,
distinguished mainly by lithic characteristics. The mineral facies
may differ from place to place, but these variations do not require
definition of a new formation. High-grade metamorphic rocks whose
relation to established formations is uncertain are treated as
lithodemic units (see Articles 31 et seq.).
Article 25. − Member. A member is the formal lithostratigraphic unit next in rank below a formation and is always a part of some formation. It is recognized as a named entity within a formation because it possesses characteristics distinguishing it from adjacent parts of the formation. A formation need not be divided into members unless a useful purpose is served by doing so. Some formations may be divided completely into members; others may have only certain parts designated as members; still others may have no members. A member may extend laterally from one formation to another.
Remarks. (a) Mapping of members. − A member is established when it
is advantageous to recognize a particular part of a heterogeneous
formation. A member, whether formally or informally designated, need
not be mappable at the scale required for formations. Even if all
members of a formation are locally mappable, it does not follow that
they should be raised to formational rank, because proliferation of
formation names may obscure rather than clarify relations with other
(b) Lens and tongue. − A geographically restricted member that
terminates on all sides within a formation may be called a lens
(lentil). A wedging member that extends outward beyond a formation
or wedges (“pinches”) out within another formation may be called a
(c) Organic reefs and carbonate mounds. − Organic reefs and
carbonate mounds may be distinguished formally, if desirable, as
members within a formation. For the requirements of formalization,
see Article 30f.
(d) Division of members. − A formally or informally recognized
division of a member composed of multiple beds is called a
submember. Members and submembers may be formally or informally
divided into a bed or beds, except for volcanic flow rocks, for
which the smallest formal unit is a flow. Members may contain beds
or flows, but may never contain other members. Distinctive marker
beds may be recognized within members without the need to subdivide
(e) Laterally equivalent members. − Although members normally are in
vertical sequence, laterally equivalent parts of a formation that
differ recognizably may also be considered members.
Article 26. — Submember. A submember is the lithostratigraphic
unit next in rank below a member and is always a part of some member.
It is recognized as a named entity within a member because it
possesses characteristics distinguishing it from adjacent parts of the
member. A member need not be divided into submembers unless a useful
purpose is served by doing so. Some members may be divided completely
into submembers; others may have only certain parts designated as
submembers; still others may have no submembers. A submember may
extend laterally from one member to another.
Remarks. (a) Mapping of submembers. — A submember is established
when it is advantageous to recognize a particular part of a
heterogeneous member. A submember, whether formally or informally
designated, need not be mappable at the scale required for formations.
Even if all submembers of a member are locally mappable, it does not
follow that they should be raised to member rank, because
proliferation of names may obscure rather than clarify relations with
(b) Division of submembers. — A formally or informally recognized
division of a submember is called a bed or beds, except for volcanic
flow rocks, for which the smallest formal unit is a flow. Submembers
may contain beds or flows, but may not contain other submembers. A
member need not be divided into submembers in order to be divided
into beds or flows.
(c) Laterally equivalent submembers. — Although submembers normally
are in vertical sequence, laterally equivalent parts of a member
that differ recognizably may also be considered submembers.
Article 27.— Bed(s) and Flow(s). A bed, or beds, is the smallest
formal lithostratigraphic unit of sedimentary rocks. A flow is the
smallest formal lithostratigraphic unit of volcanic flow rocks. A flow
is a discrete, extrusive, volcanic rock body distinguishable by
texture, composition, order of superposition, paleomagnetism, or other
objective criteria. It is part of a member and thus is equivalent in
rank to a bed or beds of sedimentary-rock classification.
Remarks. (a) Limitations. — The designation of a bed or a unit of
beds as a formally named lithostratigraphic unit generally should be
limited to certain distinctive beds whose recognition is particularly
useful. Coal beds, oil sands, and other beds of economic importance
commonly are named, but such units and their names usually are not a
part of formal stratigraphic nomenclature (Articles 22g and 30g). The
designation and naming of flows as formal rock-stratigraphic units
should be limited to those that are distinctive and widespread. Many
flows are informal units.
(b) Key or marker beds. — A key or marker bed is a thin bed of
distinctive rock that is widely distributed. Such beds may be named,
but usually are considered informal units. Individual key beds and
individual flows may be traced beyond the lateral limits of a
particular formal unit (Article 23c).
Article 28. − Group. A group is the lithostratigraphic unit next higher in rank to formation; a group may consist entirely of named formations, or alternatively, need not be composed entirely of named formations.
Remarks. (a) Use and content. − Groups are defined to express the
natural relations of associated formations. They are useful in
small-scale mapping and regional stratigraphic analysis. In some
reconnaissance work, the term “group” has been applied to
lithostratigraphic units that appear to be divisible into formations,
but have not yet been so divided. In such cases, formations may be
erected subsequently for one or all of the practical divisions of the
(b) Change in component formations. − The formations making up a
group need not necessarily be everywhere the same. The Rundle Group,
for example, is widespread in western Canada and undergoes several
changes in formational content. In southwestern Alberta, it
comprises the Livingstone, Mount Head, and Etherington Formations in
the Front Ranges, whereas in the foothills and subsurface of the
adjacent plains, it comprises the Pekisko, Shunda, Turner Valley,
and Mount Head Formations. However, a formation or its parts may not
be assigned to two vertically adjacent groups.
(c) Change in rank. − The wedge-out of a component formation or
formations may justify the reduction of a group to formation rank,
retaining the same name. When a group is extended laterally beyond
where it is divided into formations, it becomes in effect a
formation, even if it is still called a group. When a previously
established formation is divided into two or more component units
that are given formal formation rank, the old formation, with its
old geographic name, should be raised to group status. Raising the
rank of the unit is preferable to restricting the old name to a part
of its former content, because a change in rank leaves the sense of
a well-established unit unchanged (Articles 19b, 19g).
Article 29. − Supergroup. A supergroup is a formal assemblage of related or superposed groups, or of groups and formations. Such units have proved useful in regional and provincial syntheses. Supergroups should be named only where their recognition serves a clear purpose.
Remark. (a) Misuse of “series” for group or supergroup. − Although
“series” is a useful general term, it is applied formally only to a
chronostratigraphic unit and should not be used for a
lithostratigraphic unit. The term “series” should no longer be
employed for an assemblage of formations or an assemblage of
formations and groups, as it has been, especially in studies of the
Precambrian. These assemblages are groups or supergroups.
Article 30. − Compound Character. The formal name of a lithostratigraphic unit is compound. It consists of a geographic name combined with a descriptive lithic term or with the appropriate rank term, or both. Initial letters of all words used in forming the names of formal rock-stratigraphic units are capitalized.Remarks. (a) Omission of part of a name. − Where frequent repetition would be cumbersome, the geographic name, the lithic term, or the rank term may be used alone, once the full name has been introduced; as “the Burlington,” “the limestone,” or “the formation,” for the Burlington Limestone.
(b) Use of simple lithic terms.− The lithic part of the name should
indicate the predominant or diagnostic lithology, even if
subordinate lithologies are included. Where a lithic term is used in
the name of a lithostratigraphic unit, the simplest generally
acceptable term is recommended (for example, limestone, sandstone,
shale, tuff, quartzite). Compound terms (for example, clay shale)
and terms that are not in common usage (for example, calcirudite,
orthoquartzite) should be avoided. Combined terms, such as “sand and
clay,” should not be used for the lithic part of the names of
lithostratigraphic units, nor should an adjective be used between
the geographic and the lithic terms, as “Chattanooga Black Shale”
and “Biwabik Iron-Bearing Formation.”
(c) Group names. − A group name combines a geographic name with the
term “group,” and no lithic designation is included; for example,
San Rafael Group.
(d) Formation names. − A formation name consists of a geographic
name followed by a lithic designation or by the word “formation.”
Examples: Dakota Sandstone, Mitchell Mesa Rhyolite, Monmouth
Formation, Halton Till.
(e) Member and submember names. − All member and submember names
include a geographic term and the word “member” or “submember;”
some have an intervening lithic designation, if useful; for
example, Wedington Sandstone Member of the Fayetteville Shale.
Members and submembers designated solely by lithic character (for
example, siliceous shale member), by position (upper, lower), or
by letter or number, are informal.
(f) Names of reefs. − Organic reefs identified as formations or
members are formal units only where the name combines a geographic
name with the appropriate rank term, e.g., Leduc Formation (a name
applied to the several reefs enveloped by the Ireton Formation),
Rainbow Reef Member.
(g) Bed and flow names. − The names of beds or flows combine a
geographic term, a lithic term, and the term “bed” or “flow;” for
example, Knee Hills Tuff Bed, Ardmore Bentonite Beds, Negus
(h) Informal units. − When geographic names are applied to such
informal units as oil sands, coal beds, mineralized zones, and
informal members (see Articles 22g and 27a), the unit term should
not be capitalized. A name is not necessarily formal because it is
capitalized, nor does failure to capitalize a name render it
informal. Geographic names should be combined with the terms
“formation” or “group” only in formal nomenclature.
(i) Informal usage of identical geographic names. − The application
of identical geographic names to several minor units in one vertical
sequence is considered informal nomenclature (lower Mount Savage
coal, Mount Savage fireclay, upper Mount Savage coal, Mount Savage
rider coal, and Mount Savage sandstone). The application of
identical geographic names to the several lithologic units
constituting a cyclothem likewise is considered informal.
Lithodemic (upper case) and lithostratigraphic (lower case) units. A
lithodeme of gneiss (A) contains an intrusion of diorite (B)
that was deformed with gneiss. Aand B may be treated jointly as a
complex. Ayounger 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 lithotratigraphic units. A gabbro feeder (G’). 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 II, by
sedimentary rocks (j) divided into formations and members.
(j) Metamorphic rock. − Metamorphic rock recognized as a normal
stratified sequence, commonly low-grade metavolcanic or
metasedimentary rocks, should be assigned to named groups,
formations, and members, such as the Deception Rhyolite, a
formation of the Ash Creek Group, or the Bonner Quartzite, a
formation of the Missoula Group. High-grade metamorphic and
metasomatic rocks are treated as lithodemes and suites (see
Articles 31, 33, 35).
(k) Misuse of well-known name. − A name that suggests some
well-known locality, region, or political division should not be
applied to a unit typically developed in another less well-known
locality of the same name. For example, it would be inadvisable to
use the name “Chicago Formation” for a unit in California.
Nature and Boundaries
Article 31. − Nature of Lithodemic Units. A lithodemic6 unit is a defined body of predominantly intrusive, highly deformed, and/or highly metamorphosed rock, distinguished and delimited on the basis of rock characteristics. In contrast to lithostratigraphic units, a lithodemic unit generally does not conform to the Law of Superposition. Its contacts with other rock units may be sedimentary, extrusive, intrusive, tectonic, or metamorphic (text-fig. 3).
Remarks. (a) Recognition and definition. − Lithodemic units are
defined and recognized by observable rock characteristics. They are
the practical units of general geological work in terranes in which
rock bodies generally lack primary stratification; in such terranes
they serve as the foundation for studying, describing, and delineating
lithology, local and regional structure, economic resources, and
(b) Type and reference localities. − The definition of a lithodemic
unit should be based on as full knowledge as possible of its lateral
and vertical variations and its contact relations. For purposes of
nomenclatural stability, a type locality and, wherever appropriate,
reference localities should be designated.
(c) Independence from inferred geologic history. − Concepts based on
inferred geologic history properly play no part in the definition of
a lithodemic unit. Nevertheless, where two rock masses are
lithically similar but display objective structural relations that
preclude the possibility of their being even broadly of the same
age, they should be assigned to different lithodemic units.
(d) Use of “zone.” − As applied to the designation of lithodemic
units, the term “zone” is informal. Examples are: “mineralized
zone,” “contact zone,” and “pegmatitic zone.”
Article 32. − Boundaries. Boundaries of lithodemic units are placed at positions of lithic change. They may be placed at clearly distinguished contacts or within zones of gradation. Boundaries, both vertical and lateral, are based on the lithic criteria that provide the greatest unity and practical utility. Contacts with other lithodemic and lithostratigraphic units may be depositional, intrusive, metamorphic, or tectonic.
Remark. (a) Boundaries within gradational zones. − Where a
lithodemic unit changes through gradation into, or intertongues with,
a rock mass with markedly different characteristics, it is usually
desirable to propose a new unit. It may be necessary to draw an
arbitrary boundary within the zone of gradation. Where the area of
intergradation or intertonguing is sufficiently extensive, the rocks
of mixed character may constitute a third unit.
Ranks of Lithodemic Units
Article 33. − 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, (ii) a mixture of rocks of two or
more types, or (iii) 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 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
(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 typically
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 Mountain Complex, Franciscan Complex)
and, although unranked, 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
(d) Intrusive complex. − Some areas of igneous rock consist of
mixed intrusive and/or extrusive rocks composed of a variety of
igneous rock types and/or intrusive forms (e.g., pluton, stock,
dike) that are the result of the multiple, coeval, emplacement
events. Where there is no doubt that the complexity is due to the
presence of multiple intrusive bodies and/or related extrusive
rocks, such a mixture may be designated as an “intrusive complex.”
An “intrusive complex” differs from a “volcanic complex” in that
it consists largely or entirely of intrusive rocks. Intrusive
complex is unranked but, if useful, it may form part of ranked
lithodemic units (e.g., an intrusive complex) and at least one
lithodeme could be grouped together into an intrusive suite.
(e) 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, volcanic complex, and
intrusive complex (see Remarks b, c, and d, 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.
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
(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 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.
Article 41. − Suite Names. The name of a suite combines a geographic term, the term " suite, " and an adjective denoting the fundamental character of the suite; for example, Idaho Springs Metamorphic Suite, Tuolumne Intrusive Suite, 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 Addington Granites, 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 supersuite combines
a geographic term with the term “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 low-temperature
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
(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 44. − 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.
(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.
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, or 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.
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
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
Remarks. (a) Unfossiliferous rocks .− Those bodies of rock lacking
named fossils have no biostratigraphic character and are, therefore,
not amenable to biostratigraphic classification.
Contemporaneity of rocks and fossils. − Most fossils are
contemporaneous with the body of rock that contains them, in cluding
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
Examples of range, lineage and interval biozones.
Examples of assemblage and abundance biozones.
(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 chronostratigraphic units. − 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 (text-fig. 4A) is a body of rock representing
the known stratigraphic and geographic range of a chosen taxon. A
concurrent-range biozone (text-fig. 4B) is a body of rock including
the concurrent, coincident, or overlapping part of the ranges of two
(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 (text-fig. 4C), highest occurrences
(text-fig. 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 (text-fig. 4E) is a body of
rock containing species representing a specific segment of an
(d) Assemblage biozone. − An assemblage biozone (text-fig. 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 (text-fig. 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 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
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.
Article 54. − Establishing Formal Units. Formal establishment of a biozone must meet the requirements of Article 3 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 lundgreniTestograptus 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
re-designate, 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 55. − 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 (text-fig. 6) and is overlain by one or more formally defined lithostratigraphic or allostratigraphic units.
Relation between pedostratigraphic units and pedologic profiles. The
base of a geosol is the lowest clearly defined physical boundary of a
pedologic horizon in a buried soil profile. In this example, it is the
lower boundary of the B horizon because the base of the C horizon is
not a clearly defined physical boundary. In other profiles, the base
may be the lower boundary of a C horizon. Pedologic profile modified
from Ruhe (1965) and Pawluk (1978).
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, organic-matter 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
(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 unique identifying physical
and chemical characteristics. 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
(text-fig. 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
Example of allostratigraphic classification of alluvial and lacustrine
deposits in a graben. The alluvial and lacustrine deposits may be
included in a single formation, or may be separated laterally into
formations distinguished on the basis of contrasting texture (gravel,
clay). Textural changes are abrupt and sharp, both vertically and
laterally. The gravel deposits and clay deposits, respectively, are
lithologically similar and thus cannot be distinguished as members of
a formation. Four allostratigraphic units, each including two or three
textural facies, may be defined on the basis of laterally traceable
discontinuities (buried soils or disconformities).
(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 it is a product of surface alteration of one or more older
material units by specific processes (pedogenesis), (2) its lithology
and other properties differ markedly from those of the parent
material(s), and (3) a single pedostratigraphic unit may be formed in
situ in parent material units of diverse compositions and ages.
(h) Independence from time concepts.− The boundaries of a
pedostratigraphic unit are time-transgressive. Concepts of time
spans, however measured, play no part in defining the boundaries
of a pedostratigraphic unit. Nonetheless, evidence of age, whether
based on fossils, numerical ages, or geometrical or other
relations, may play an important role in distinguishing and
identifying non-contiguous pedostratigraphic units at localities
away from the type areas. The name of a pedostratigraphic unit
should be chosen from a geographic feature in the type area, and
not from a time span.
Pedostratigraphic Nomencslature and Unit
Article 56. − Fundamental Unit. The fundamental and only unit in pedostratigraphic classification is a geosol.
Article 57. − Nomenclature. The formal name of a pedostratigraphic
unit consists of a geographic name combined with the term “geosol.”
Capitalization of the initial letter in each word serves to identify
formal usage. The geographic name should be selected in accordance
with recommendations in Article 7 and should not duplicate the name of
another formal geologic unit. Names based on subjacent and superjacent
rock units, for example the super-Wilcoxsub-Claiborne soil, are
informal, as are those with time connotations
Example of allostratigraphic classification of contiguous deposits of
similar lithology. Allostratigraphic units 1, 2, and 3 are physical
records of three glaciations. They are lithologically similar,
reflecting derivation from the same bedrock, and constitute a single
Remarks. (a) Composite geosols. − Where the horizons of two or
more merged or “welded” buried soils can be distinguished, formal
names of pedostratigraphic units based on the horizon boundaries can
be retained. Where the horizon boundaries of the respective merged or
“welded” soils cannot be distinguished, formal pedostratigraphic
classification is abandoned and a combined name such as
Hallettville-Jamesville geosol may be used informally.
(b) Characterization. − The physical and chemical properties of a
pedostratigraphic unit commonly vary vertically and laterally
throughout the geographic extent of the unit. A pedostratigraphic
unit is characterized by the range of physical and chemical
properties of the unit in the type area, rather than by “typical”
properties exhibited in a type section. Consequently, a
pedostratigraphic unit is characterized on the basis of a composite
stratotype (Article 8d).
(c) Procedures for establishing formal pedostratigraphic units. − A
formal pedostratigraphic unit may be established in accordance with
the applicable requirements of Article 3. The definition should
include a description of major soil horizons and their lateral
Nature and Boundaries
Article 58. − Nature of Allostratigraphic Units. An allostratigraphic unit is a mappable body of rock that is defined and identified on the basis of its bounding discontinuities.
Remarks. (a) Purpose. − Formal allostratigraphic9 units
may be defined to distinguish between different (1) superposed
discontinuity-bounded deposits of similar lithology (text-figs. 7, 9),
(2) contiguous discontinuity-bounded deposits of similar lithology
(text-fig. 8), or (3) geographically separated discontinuity-bounded
units of similar lithology (text-fig. 9). Formal allostratigraphic
units may also be defined to distinguish as single units
discontinuity-bounded deposits characterized by lithic heterogeneity
(units 1–4 in text-fig. 7). Allostratigraphic units are distinguished
by bounding discontinuities. The lithology of an allostratigraphic
unit plays no part in its definition.
(b) Internal characteristics. − Internal characteristics (physical,
chemical, and paleontological) may vary laterally and vertically
throughout the unit.
(c) Boundaries. − Boundaries of allostratigraphic units are
laterally traceable discontinuities (text-figs. 7–9).
(d) Mappability. − A formal allostratigraphic unit must be mappable
at the scale practiced in the region where the unit is defined.
(e) Type locality and extent. − A type locality and type area must
be designated; a composite stratotype or a type section and several
reference sections are desirable. An allostratigraphic unit may be
laterally contiguous with all or part of a formally defined
lithostratigraphic unit, but as the two kinds of units are defined
by entirely different criteria, both kinds of units may be formally
recognized in the same area.10
(f) Relation to genesis. − Genetic interpretation is an
inappropriate basis for defining an allostratigraphic unit. However,
genetic interpretation may influence the choice of its boundaries.
(g) Relation to geomorphic surfaces. − A geomorphic surface may be
used as a boundary of an allostratigraphic unit, but the unit should
not be given the geographic name of the surface.
(h) Relation to soils and paleosols. − Soils and paleosols are
composed of products of weathering and pedogenesis and differ in
many respects from allostratigraphic units, which are depositional
units (see “Pedostratigraphic Units,” Article 55). The upper
boundary of a surface or buried soil may be used as a boundary of an
(i) Relation to inferred geologic history. − Inferred geologic
history is not used to define an allostratigraphic unit. However,
well-documented geologic history may influence the choice of the
(j) Relation to time concepts. − Inferred time spans, however
measured, are not used to define an allostratigraphic unit. However,
age relations may influence the choice of the unit’s boundaries.
(k) Extension of allostratigraphic units. − An allostratigraphic
unit is extended from its type area by tracing the boundary
discontinuities or by tracing or matching the deposits between the
Ranks of Allostratigraphic Units
Article 59. − Hierarchy. The hierarchy of allostratigraphic units,
in order of decreasing rank, is allogroup, alloformation, and
Example of allostratigraphic classification of lithologically similar,
discontinuous terrace deposits. A, B, C, and D are terrace gravel
units of similar lithology at different topographic position on a
valley wall. The deposits may be defined as separate formal
allostratigraphic units if such units are useful and if bounding
discontinuities can be traced laterally. Terrace gravels of the same
age commonly are separated geographically by exposure of older rocks.
Where the bounding discontinuities cannot be traced continuously, they
may be extended geographically on the basis of objective correlation
of internal properties of the deposits other than lithology (e.g.,
fossil content, included tephras), topographic position, numerical
ages, or relative-age criteria (e.g., soils or other weathering
phenomena). The criteria for such extension should be documented.
Slope deposits and eolian deposits(s) that mantle terrace surfaces may
be of diverse ages and are not included in a terrace-gravel
allostratigraphic unit. A single terrace surface may be underlain by
more than one allostratigraphic unit (units B and C in text-fig. 9B,
Remarks. (a) Alloformation. − The alloformation is the fundamental
unit in allostratigraphic classification. An alloformation may be
completely or only partly divided into allomembers, if some useful
purpose is served, or it may have no allomembers.
(b) Allomember. − An allomember is the formal allostratigraphic unit
next in rank below an alloformation.
(c) Allogroup. − An allogroup is the allostratigraphic unit next in
rank above an alloformation. An allogroup is established only if a
unit of that rank is essential to elucidation of geologic history.
An allogroup may consist entirely of named alloformations or,
alternatively, may contain one or more named alloformations that
jointly do not comprise the entire allogroup.
(d) Changes in rank. − The principles and procedures for elevation
and reduction in rank of formal allostratigraphic units are the same
as those in Articles 19b, 19g, and 28).
Article 60.−Nomenclature. The principles and procedures for naming allostratigraphic units are the same as those for naming of lithostratigraphic units (see Articles 7, 30).
Remark. (a) Revision. − Allostratigraphic units may be revised or
otherwise modified in accordance with the recommendations in Articles
17 to 20.
FORMAL UNITS EXPRESSING OR RELATING TO GEOLOGIC AGE
KINDS OF GEOLOGIC-TIME UNITS
Nature and Kinds
Article 61. − Kinds. Geologic-time units are conceptual, rather than material, in nature. Two kinds are recognized: those based on material standards or referents (specific rock sequences or bodies), and those independent of material referents (text-fig. 1).
Units Based on Material Referents
Article 62. − Kinds Based on Referents. Two kinds of formal geologic-time units that are based on material referents are recognized: they are isochronous and diachronous units.
Article 63. − Isochronous Categories. Isochronous time units and
the material bodies from which they are derived are twofold:
geochronologic units (Article 80), which are based on corresponding
material chronostratigraphic units (Article 66), and
polarity-chronologic units (Article 88), based on corresponding
material polarity-chronostratigraphic units (Article 83).
Remark. (a) Extent. − Isochronous units are applicable worldwide;
they may be referred to even in areas lacking a material record of the
named span of time. The duration of the time may be represented by a
unit-stratotype referent. The beginning and end of the time are
represented by point-boundary-stratotypes either in a single
stratigraphic sequence or in separate stratotype sections (Articles
Article 64. − Diachronous Categories. Diachronic units (Article 91) are time units corresponding to diachronous material
allostratigraphic units (Article 58), pedostratigraphic units (Article
55), and most lithostratigraphic (Article 22) and biostratigraphic
(Article 48) units.
Remarks. (a) Diachroneity. − Some lithostratigraphic and
biostratigraphic units are clearly diachronous, whereas others have
boundaries that are not demonstrably diachronous within the resolving
power of available dating methods. The latter commonly are treated as
isochronous and are used for purposes of chronocorrelation (see
biochronozone, Article 75). However, the assumption of isochroneity
must be tested continually.
(b) Extent. − Diachronic units are coextensive with the
diachronous material stratigraphic units on which they are based and
are not used beyond the extent of their material referents.
Units Independent of Material Referents
Article 65. − Numerical Divisions of Time. Isochronous geologic-time units based on numerical divisions of time in years are geochronometric units (Article 96) and have no material referents.
Nature and Boundaries
Article 66. − Definition. A chronostratigraphic unit is a body of rock established to serve as the material reference for all constituent rocks formed during the same span of time. Each boundary is synchronous. The body also serves as the basis for defining the specific interval of time, or geochronologic unit (Article 80), represented by the referent.
Remarks. (a) Purposes. − Chronostratigraphic classification
provides a means of establishing the temporally sequential order of
rock bodies. Principal purposes are to provide a framework for (1)
temporal correlation of the rocks in one area with those in another,
(2) placing the rocks of the Earth’s crust in a systematic sequence
and indicating their relative position and age with respect to earth
history as a whole, and (3) constructing an internationally recognized
Standard Global Chronostratigraphic Scale.
(b) Nature. − A chronostratigraphic unit is a material unit and
consists of a body of strata formed during a specific time span. Such
a unit represents all rocks, and only those rocks, formed during that
(c) Content. − A chronostratigraphic unit may be based upon the
time span of a biostratigraphic unit, a lithic unit, a magnetopolarity
unit, or any other feature of the rock record that has a time range.
Or it may be any arbitrary but specified sequence of rocks, provided
it has properties allowing chronocorrelation with rock sequences
Article 67. − Boundaries. Boundaries of chronostratigraphic units should be defined in a designated stratotype on the basis of observable paleontological or physical features of the rocks.
Remark. (a) Emphasis on lower boundaries of chronostratigraphic
units. − Designation of point boundaries for both base and top of
chronostratigraphic units is not recommended, because subsequent
information on relations between successive units may identify
overlaps or gaps. One means of minimizing or eliminating problems of
duplication or gaps in chronostratigraphic successions is to define
formally as a point-boundary stratotype only the base of the unit.
Thus, a chronostratigraphic unit with its base defined at one
locality, will have its top defined by the base of an overlying unit
at the same, but more commonly another, locality (Article 8b).
Article 68. − Correlation. Demonstration of time equivalence is required for geographic extension of a chronostratigraphic unit from its type section or area. Boundaries of chronostratigraphic units can be extended only within the limits of resolution of available means of chronocorrelation, which currently include paleontology, numerical dating, remanent magnetism, thermoluminescence, relative-age criteria (examples are superposition and cross-cutting relations), and such indirect and inferential physical criteria as climatic changes, degree or weathering, and relations to unconformities. Ideally, the boundaries of chronostratigraphic units are independent of lithology, fossil content, or other material bases of stratigraphic division, but, in practice, the correlation or geographic extension of these boundaries relies at least in part on such features. Boundaries of chronostratigraphic units commonly are intersected by boundaries of most other kinds of material units.
Ranks of Chronostratigraphic Units
Article 69. − Hierarchy. The hierarchy of chronostratigraphic
units, in order of decreasing rank, is eonothem, erathem,
system, series, and stage. Of these, system is the primary unit
of worldwide major rank; its primacy derives from the history of
development of stratigraphic classification. All systems and units of
higher rank are divided completely into units of the next lower rank.
Chronozones are non-hierarchical and commonly lower-rank
chronostratigraphic units. Stages and chronozones in sum do not
necessarily equal the units of next higher rank and need not be
contiguous. The rank and magnitude of chronostratigraphic units are
related to the time interval represented by the units, rather than to
the thickness or areal extent of the rocks on which the units are
Article 70. − Eonothem. The unit highest in rank is eonothem. The
Phanerozoic Eonothem encompasses the Paleozoic, Mesozoic, and Cenozoic
Erathems. Although older rocks have been assigned heretofore to the
Precambrian Eonothem, they also have been assigned recently to other
(Archean and Proterozoic) eonothems by the IUGS Precambrian
Subcommission. The span of time corresponding to an eonothem is an
Article 71. − Erathem. An erathem is the formal
chronostratigraphic unit of rank next lower to eonothem and consists
of several adjacent systems. The span of time corresponding to an
erathem is an era.
Remark. (a) Names. − Names given to traditional Phanerozoic
erathems were based upon major stages in the development of life on
Earth: Paleozoic (old), Mesozoic (intermediate), and Cenozoic (recent)
life. Although somewhat comparable terms have been applied to
Precambrian units, the names and ranks of Precambrian divisions are
not yet universally agreed upon and are under consideration by the
IUGS Subcommission on Precambrian Stratigraphy.
Article 72. − System. The unit of rank next lower to erathem is the system. Rocks encompassed by a system represent a time span and an episode of Earth history sufficiently great to serve as a worldwide chronostratigraphic reference unit. The temporal equivalent of a system is a period.
Remark. (a) Subsystem and supersystem. − Some systems initially
established in Europe later were divided or grouped elsewhere into
units ranked as systems. Subsystems (Mississippian Subsystem of the
Carboniferous System) and supersystems (Karoo Supersystem) are more
Article 73. − Series. Series is a conventional chronostratigraphic
unit that ranks below a system and always is a division of a system. A
series commonly constitutes a major unit of chronostratigraphic
correlation within a province, between provinces, or between
continents. Although many European series are being adopted
increasingly for dividing systems on other continents, provincial
series of regional scope continue to be useful. The temporal
equivalent of a series is an epoch.
Remark. (a) Subseries. − Series may be, but need not be, divided
completely into subseries. A series may comprise two subseries (using
the capitalized adjectives Lower and Upper) or three subseries (using
the capitalized adjectives Lower, Middle and Upper); a subseries may
comprise one or several stages. The temporal equivalent of a subseries
is a subepoch.
Article 74.−Stage. A stage is a chronostratigraphic unit of smaller scope and rank than a series. It is most commonly of greatest use in intra-continental classification and correlation, although it has the potential for worldwide recognition. The geochronologic equivalent of a stage is an age.
Remark. (a) Substage.− Stages may be, but need not be, divided
completely into substages.
Article 75.−Chronozone. A chronozone is a non-hierarchical, but commonly small, formal Chronostratigraphic unit, and its boundaries may be independent of those of ranked chronostratigraphic units such as stage or series. Although a chronozone is an isochronous unit, it may be based on a biostratigraphic unit (example: Cardioceras cordatum Biochronozone), a lithostratigraphic unit (Woodbend Lithochronozone), or a magnetopolarity unit (Gilbert Reversed-Polarity Chronozone). Modifiers (litho-, bio-, polarity) used in formal names of the units need not be repeated in general discussions where the meaning is evident from the context, e.g., Exus albus Chronozone.
Remarks. (a) Boundaries of chronozones. − The base and top of a
chronozone correspond in the unit’s stratotype to the observed,
defining, physical and paleontological features, but they are extended
to other areas by any means available for recognition of synchroneity.
The temporal equivalent of a chronozone is a chron.
(b) Scope. − The scope of the non-hierarchical chronozone may
range markedly, depending upon the purpose for which it is defined
either formally or informally. The informal “biochronozone of the
ammonites,” for example, represents a duration of time which is
enormous and exceeds that of a system. In contrast, a biochronozone
defined by a species of limited range, such as the Exus albus
Chronozone, may represent a duration equal to or briefer than that of
(c) Practical utility. − Chronozones, especially thin and informal
biochronozones and lithochronozones bounded by key beds or other
“markers,” are the units used most commonly in industry
investigations of selected parts of the stratigraphy of economically
favorable basins. Such units are useful to define geographic
distributions of lithofacies or biofacies, which provide a basis for
genetic interpretations and the selection of targets to drill.
Article 76. − Requirements. Requirements for establishing a formal chronostratigraphic unit include: (i) statement of intention to designate such a unit; (ii) selection of name; (iii) statement of kind and rank of unit; (iv) statement of general concept of unit including historical background, synonymy, previous treatment, and reasons for proposed establishment; (v) description of characterizing physical and/or biological features; (vi) designation and description of boundary type sections, stratotypes, or other kinds of units on which it is based; (vii) correlation and age relations; and (viii) publication in a recognized scientific medium as specified in Article 4.
Article 77. − Nomenclature. A formal chronostratigraphic unit is
given a compound name, and the initial letters of all words, except
for trivial taxonomic terms, are capitalized. Except for chronozones
(Article 75), names proposed for new chronostratigraphic units should
not duplicate those for other stratigraphic units. For example, naming
a new chronostratigraphic unit simply by adding “-an” or “-ian” to the
name of a lithostratigraphic unit is improper.
Remarks. (a) Systems and units of higher rank. − Names that are
generally accepted for systems and units of higher rank have diverse
origins, and they also have different kinds of endings (Paleozoic,
Cambrian, Cretaceous, Jurassic, Quaternary).
(b) Series and units of lower rank. − Series and units of lower
rank are commonly known either by geographic names (Virgilian Series,
Ochoan Series) or by names of their encompassing units modified by the
capitalized adjectives Upper, Middle, and Lower (Lower Ordovician).
Names of chronozones are derived from the unit on which they are based
(Article 75). For series and stage, a geographic name is preferable
because it may be related to a type area. For geographic names, the
adjectival endings -an or -ian are recommended (Cincinnatian Series),
but it is permissible to use the geographic name without any special
ending, if more euphonious. Many series and stage names already in use
have been based on lithic units (groups, formations, and members) and
bear the names of these units (Wolfcampian Series, Claibornian Stage).
Nevertheless, a stage preferably should have a geographic name not
previously used in stratigraphic nomenclature. Use of internationally
accepted (mainly European) stage names is preferable to the
proliferation of others.
Article 78. − Stratotypes. An ideal stratotype for a chronostratigraphic unit is a completely exposed unbroken and continuous sequence of fossiliferous stratified rocks extending from a well-defined lower boundary to the base of the next higher unit. Unfortunately, few available sequences are sufficiently complete to define stages and units of higher rank, which therefore are best defined by boundary-stratotypes (Article 8b).
Boundary-stratotypes for major chronostratigraphic units ideally
should be based on complete sequences of either fossiliferous
monofacial marine strata or rocks with other criteria for
chronocorrelation to permit widespread tracing of synchronous horizons. Extension of synchronous surfaces should be based on as many indicators of age as possible.
Article 79. − Revision of Units. Revision of a chronostratigraphic unit without changing its name is allowable but requires as much justification as the establishment of a new unit (Articles 17, 19, 76). Revision or redefinition of a unit of system or higher rank requires international agreement. If the definition of a chronostratigraphic unit is inadequate, it may be clarified by establishment of boundary stratotypes in a principal reference section.
Nature and Boundaries
Article 80. − Definition and Basis. Geochronologic units are divisions of time traditionally distinguished on the basis of the rock record as expressed by chronostratigraphic units. A geochronologic unit is not a stratigraphic unit (i.e., it is not a material unit), but it corresponds to the time span of an established chronostratigraphic unit (Articles 65 and 66), and its beginning and ending corresponds to the base and top of the referent.
Ranks and Nomenclature of Geochronologic Units
Article 81. − Hierarchy. The hierarchy of geochronologic units in order of decreasing rank is eon, era, period, epoch, and age. Intermediate ranks superperiod, subperiod, subepoch, and subage may be recognized and formalized. Chron is a non-hierarchical, but commonly brief, geochronologic unit. Ages in sum do not necessarily equal epochs and subepochs and need not form a continuum. An eon is the time represented by the rocks constituting an eonothem; era by an erathem; period by a system; epoch by a series; age by a stage; and chron by a chronozone.
Article 82. − Nomenclature. Names for periods and units of lower
rank are identical with those of the corresponding chronostratigraphic
units; the names of some eras and eons are independently formed. Rules
of capitalization for chronostratigraphic units (Article 77) apply to
geochronologic units. The adjectives Early, Middle, and Late are used
for the geochronologic epochs and subepochs where appropriate,
equivalent to the corresponding chronostratigraphic Lower, Middle, and
Upper series, where these are formally established.
Nature and Boundaries
Article 83. − Definition. A polarity-chronostratigraphic unit is a body of rock that contains the primary magnetic-polarity record imposed when the constituent rock was deposited, or crystallized, during a specific interval of geologic time.
Remarks. (a) Nature. − Polarity-chronostratigraphic units depend
fundamentally for definition on actual sections or sequences, or
measurements on individual rock units, and without these standards
they are meaningless. They are based on material units, the polarity
zones of magnetopolarity classification. Each
polarity-chronostratigraphic unit is the record of the time during
which the rock formed and the Earth’s magnetic field had a designated
polarity. Care should be taken to define polaritychronologic units in
terms of polarity-chronostratigraphic units, and not vice versa.
(b) Principal purposes. − Two principal purposes are served by
polarity-chronostratigraphic classification: (1) correlation of rocks
at one place with those of the same age and polarity at other places;
and (2) delineation of the polarity history of the Earth’s magnetic
(c) Recognition. − A polarity-chronostratigraphic unit may be
extended geographically from its type locality only with the support
of physical and/or paleontologic criteria used to confirm its age.
Article 84. − Boundaries. The boundaries of a polarity chronozone are placed at polarity-reversal horizons or polarity transition zones (see Article 45).
Ranks and Nomenclature of Polarity-Chronostratigraphic Units
Article 85. − Fundamental Unit. The polarity chronozone consists of rocks of a specified primary polarity and is the fundamental unit of worldwide polarity-chronostratigraphic classification.
Remarks. (a) Meaning of term. − A polarity chronozone is the
worldwide body of rock that is collectively defined as a
(b) Scope. − Individual polarity zones are the basic building
blocks of polarity chronozones. Recognition and definition of polarity
chronozones may thus involve step-by-step assembly of carefully dated
or correlated individual polarity zones, especially in work with rocks
older than the oldest ocean-floor magnetic anomalies. This procedure
is the method by which the Brunhes, Matuyama, Gauss, and Gilbert
Chronozones were recognized (Cox et al. 1963) and defined originally
(Cox et al. 1964).
(c) Ranks. − Divisions of polarity chronozones are designated
polarity subchronozones. Assemblages of polarity chronozones may be
termed polarity superchronozones.
Article 86. − Establishing Formal Units. Requirements for
establishing a polarity-chronostratigraphic unit include those
specified in Articles 3 and 4, and also (1) definition of boundaries
of the unit, with specific references to designated sections and data;
(2) distinguishing polarity characteristics, lithologic descriptions,
and included fossils; and (3) correlation and age relations.
Article 87. − Name. A formal polarity-chronostratigraphic unit is
given a compound name beginning with that for a named geographic
feature; the second component indicates the normal, reversed, or mixed
polarity of the unit, and the third component is chronozone. The
initial letter of each term is capitalized. If the same geographic
name is used for both a magnetopolarity zone and a
polarity-chronostratigraphic unit, the latter should be distinguished
by an -an or -ian ending. Example: Tetonian Reversed-Polarity
Remarks. (a) Preservation of established name. − A particularly
well-established name should not be displaced, either on the basis of
priority, as described in Article 7c, or because it was not taken from
a geographic feature. Continued use of Brunhes, Matuyama, Gauss, and
Gilbert, for example, is endorsed so long as they remain valid units.
(b) Expression of doubt. − Doubt in the assignment of polarity
zones to polarity-chronostratigraphic units should be made explicit if
criteria of time equivalence are inconclusive.
Nature and Boundaries
Article 88. − Definition. Polarity-chronologic units are divisions of geologic time distinguished on the basis of the record of magnetopolarity as embodied in polarity-chronostratigraphic units. No special kind of magnetic time is implied; the designations used are meant to convey the parts of geologic time during which the Earth’s magnetic field had a characteristic polarity or sequence of polarities. These units correspond to the time spans represented by polarity chronozones, e.g., Gauss Normal-Polarity Chronozone. They are not material units.
Ranks and Nomenclature of Polarity-Chronologic Units
Article 89. − Fundamental Unit. The polarity chron is the
fundamental unit of geologic time designating the time span of a
Remark. (a) Hierarchy. − Polarity-chronologic units of decreasing
hierarchical ranks are polarity superchron, polarity chron, and
Article 90. − Nomenclature. Names for polarity chronologic units are identical with those of corresponding polarity-chronostratigraphic units, except that the term chron (or superchron, etc.) is substituted for chronozone (or superchronozone, etc.).
Nature and Boundaries
Article 91. − Definition. A diachronic unit comprises the unequal spans of time represented either by a specific lithostratigraphic, allostratigraphic, biostratigraphic, or pedostratigraphic unit, or by an assemblage of such units.
Remarks. (a) Purposes. − Diachronic classification provides (1) a
means of comparing the spans of time represented by stratigraphic
units with diachronous boundaries at different localities, (2) a basis for broadly establishing in time the beginning and ending of deposition of diachronous stratigraphic units at different sites, (3) a basis for inferring the rate of change in areal extent of depositional processes, (4) a means of determining and comparing rates and durations of deposition at different localities, and (5) a means of comparing temporal and spatial relations of diachronous stratigraphic units (Watson and Wright 1980).
Comparison of geochronologic, chronostratigraphic, and diachronic
(b) Scope. − The scope of a diachronic unit is related to (1) the
relative magnitude of the transgressive division of time represented
by the stratigraphic unit or units on which it is based and (2) the areal extent of those units. A diachronic unit is not extended beyond the geographic limits of the stratigraphic unit or units on
which it is based.
(c) Basis. − The basis for a diachronic unit is the diachronous
(d) Duration. − A diachronic unit may be of equal duration at
different places despite differences in the times at which it began
and ended at those places.
Article 92.− Boundaries. The boundaries of a diachronic unit are the times recorded by the beginning and end of deposition of the material referent at the point under consideration (text-figs. 10, 11).
Remark. (a) Temporal relations. − One or both of the boundaries of
a diachronic unit are demonstrably time-transgressive. The varying
time significance of the boundaries is defined by a series of boundary
reference sections (Article 8b, 8e). The duration and age of a
diachronic unit differ from place to place (text-fig. 10, 11).
Ranks and Nomenclature of Diachronic Units
Article 93. − Ranks. A diachron is the fundamental and non-hierarchical diachronic unit. If a hierarchy of diachronic units is needed, the terms episode, phase, span, and cline, in order of decreasing rank, are recommended. The rank of a hierarchical unit is determined by the scope of the unit (Article 91b), and not by the time span represented by the unit at a particular place.
Remarks. (a) Diachron. − Diachrons may differ greatly in magnitude
because they are the spans of time represented by individual or
grouped lithostratigraphic, allostratigraphic, biostratigraphic,
and/or pedostratigraphic units.
(b) Hierarchical ordering permissible. − A hierarchy of diachronic
units may be defined if the resolution of spatial and temporal
relations of diachronous stratigraphic units is sufficiently precise
to make the hierarchy useful (Watson and Wright 1980). Although all
hierarchical units of rank lower than episode are part of a unit
next higher in rank, not all parts of an episode, phase, or span
need be represented by a unit of lower rank.
Episode. − An episode is the unit of highest rank and greatest
scope in hierarchical classification. If the “Wisconsinan Age” were to
be redefined as a diachronic unit, it would have the rank of episode.
Schematic relation of phases to an episode. Parts of a phase may be
divided into spans, and spans into clines. Formal definition of spans
and clines is unnecessary in most diachronic unit hierarchies.
Article 94. − Name. The name for a diachronic unit should be compound, consisting of a geographic name followed by the term diachron or a hierarchical rank term. Both parts of the compound name are capitalized to indicate formal status. If the diachronic unit is defined by a single stratigraphic unit, the geographic name of the unit may be applied to the diachronic unit. Otherwise, the geographic name of the diachronic unit should not duplicate that of another formal stratigraphic unit. Genetic terms (e.g., alluvial, marine) or climatic terms (e.g., glacial, interglacial) are not included in the names of diachronic units.
Remarks. (a) Formal designation of units. − Diachronic units
should be formally defined and named only if such definition is
(b) Interregional extension of geographic names. − The geographic
name of a diachronic unit may be extended from one region to another
if the stratigraphic units on which the diachronic unit is based
extend across the regions. If different diachronic units in
contiguous regions eventually prove to be based on laterally
continuous stratigraphic units, one name should be applied to the
unit in both regions. If two names have been applied, one name
should be abandoned and the other formally extended. Rules of
priority (Article 7c) apply. Priority in publication is to be
respected, but priority alone does not justify displacing a
well-established name by one not well-known or commonly used.
(c) Change from geochronologic to diachronic classification. - Lithostratigraphic units have served as the material basis for
widely accepted chronostratigraphic and geochronologic
classifications of Quaternary nonmarine deposits, such as the
classifications of Frye et al (1968), Willman and Frye (1970),
and Dreimanis and Karrow (1972). In practice, time-parallel
horizons have been extended from the stratotypes on the basis of
markedly time-transgressive lithostratigraphic and
pedostratigraphic unit boundaries. The time (“geochronologic”)
units, defined on the basis of the stratotype sections but
extended on the basis of diachronous stratigraphic boundaries,
are diachronic units. Geographic names established for such
“geochronologic” units may be used in diachronic
classification if (1) the chronostratigraphic and geochronologic
classifications are formally abandoned and diachronic
classifications are proposed to replace the former
“geochronologic” classifications, and (2) the units are
redefined as formal diachronic units. Preservation of
well-established names in these specific circumstances retains
the intent and purpose of the names and the units, retains the
practical significance of the units, enhances communication, and
avoids proliferation of nomenclature.
Article 95. − Establishing Formal Units. Requirements for establishing a formal diachronic unit, in addition to those in Article 3, include (1) specification of the nature, stratigraphic relations, and geographic or areal relations of the stratigraphic unit or units that serve as a basis for definition of the unit, and (2) specific designation and description of multiple reference sections that illustrate the temporal and spatial relations of the defining stratigraphic unit or units and the boundaries of the unit or units.
Remark. (a) Revision or abandonment. − Revision or abandonment of
the stratigraphic unit or units that serve as the material basis for
definition of a diachronic unit may require revision or abandonment of
the diachronic unit. Procedure for revision must follow the
requirements for establishing a new diachronic unit.
Nature and Boundaries
Article 96. − Definition. Geochronometric units are units established through the direct division of geologic time, (Article 80), geochronometric units are abstractions, i.e., they are not material units. Unlike geochronologic units, geochronometric units are not based on the time span of designated chronostratigraphic units (stratotypes), but are simply time divisions of convenient magnitude for the purpose for which they are established (e.g., Hofmann 1990), such as the development of a time scale for the Precambrian. Their boundaries are arbitrarily chosen or agreed-upon ages in years.
Ranks and Nomenclature of Geochronometric Units
Article 97. − Nomenclature. Geochronologic rank terms (eon,
era, period, epoch, age, and chron) may be used for
geochronometric units when such terms are formalized. For example,
Archean Eon and Proterozoic Eon, as recognized by the IUGS
Subcommission on Precambrian Stratigraphy, are formal geochronometric
units in the sense of Article 96, distinguished on the basis of an
arbitrarily chosen boundary at 2.5 Ga. Geochronometric units are not
defined by, but may have, corresponding chronostratigraphic units
(eonothem, erathem, system, series, stage, and
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NORTH AMERICAN COMMISSION ON STRATIGRAPHIC NOMENCLATURE (NACSN), 1981. Draft North American Stratigraphic Code. Calgary: Canadian Society of Petroleum Geologists, 63 pp.
———, 1983. North American Stratigraphic Code. American Association of Petroleum Geologists Bulletin, 67: 841–875.
PALMER, A. R., 1965. Biomere-a new kind of biostratigraphic unit. Journal of Paleontology, 39 (l): 149–153.
PARSONS, R. B., 1981. Proposed soil-stratigraphic guide. In: International Union for Quaternary Research and International Society of Soil Science. INQUA Commission 6 and ISSS Commission 5 Working Group, Pedology, Report: 6–12.
PAWLUK, S., 1978. The pedogenic profile in the stratigraphic section. In: W. C. Mahaney, Ed., Quaternary soils. Norwich, England: GeoAbstracts, Ltd., 61–75.
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SALVADOR, A., Ed., 1994. International Stratigraphic Guide: A Guide to Stratigraphic Classification, Terminology, and Procedure, 2nd Edition. Boulder, Colorado: Geological Society of America, 214pp.
SCHULTZ, E. H., 1982. The chronosome and supersome-terms proposed for low-rank chronostratigraphic units. Canadian Petroleum Geology, 30 (1): 29–33.
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WEISS, M. P., 1979a. Comments and suggestions invited for revision of American Stratigraphic Code. Geological Society of America, News and Information, 1 (7): 97–99.
———, 1979b. Note 50 Proposal to change name of commission (American Commission on Stratigraphic Nomenclature). American Association of Petroleum Geologists Bulletin, 63 (10): 1986.
WELLER, J. M., 1960. Stratigraphic principles and practice. New York: Harper and Brothers, 725 pp.
WILLMAN, H. B. and FRYE, J. C., 1970. Pleistocene stratigraphy of Illinois. Illinois State Geological Survey Bulletin 94, 204 pp.
Participants and Conferees in 1983 Code Revision
Steven S. Oriel (U.S. Geological Survey), chairman, Hubert Gabrielse
(Geological Survey of Canada), William W. Hay (Joint Oceanographic
Institutions), Frank E. Kottlowski (New Mexico Bureau of Mines), John
B. Patton (Indiana Geological Survey).
James D. Aitken (Geological Survey of Canada), chairman, Monti Lerand
(Gulf Canada Resources, Ltd.), Mitchell W. Reynolds (U.S. Geological
Survey), Robert J. Weimer (Colorado School of Mines), Malcolm P. Weiss
(Northern Illinois University).
Allison R. (Pete) Palmer (Geological Society of America), chairman,
Ismael Ferrusquia (University of Mexico), Joseph E. Hazel (U.S.
Geological Survey), Erle G. Kauffman (University of Colorado), Colin
McGregor (Geological Survey of Canada), Michael A. Murphy (University of California, Riverside), Walter C. Sweet (Ohio State University).
Zell E. Peterman (U.S. Geological Survey), chairman, Zoltan de Cserna
(Sociedad Geológica Mexicana), Edward H. Schultz (Suncor, Inc.,
Calgary), Norman F. Sohl (U.S. Geological Survey), John A. Van
Couvering (American Museum of Natural History).
Jack E. Harrison (U.S. Geological Survey), chairman, John B. Henderson
(Geological Survey of Canada), Harold L. James (retired), Leon T.
Silver (California Institute of Technology), Paul C. Bateman (U.S. Geological Survey).
Magnetostratigraphic Advisory Group
Roger W. Macqueen (University of Waterloo), chairman, G. Brent
Dalrymple (U.S. Geological Survey), Walter F. Fahrig (Geological
Survey of Canada), J. M. Hall (Dalhousie University).
Volcanic Advisory Group
Richard V. Fisher (University of California, Santa Barbara), chairman,
Thomas A. Steven (U.S. Geological Survey), Donald A. Swanson (U.S. Geological Survey).
Tectonostratigraphic Advisory Group
Darrel S. Cowan (University of Washington), chairman, Thomas W.
Donnelly (State University of New York at Binghamton), Michael W.
Higgins and David L. Jones (U.S. Geological Survey), Harold Williams
(Memorial University, Newfoundland).
Quaternary Advisory Group
Norman P. Lasca (University of Wisconsin-Milwaukee), chairman, Mark M.
Fenton (Alberta Research Council), David S. Fullerton (U.S. Geological
Survey), Robert J. Fulton (Geological Survey of Canada), W. Hilton
Johnson (University of Illinois), Paul F. Karrow (University of
Waterloo), Gerald M. Richmond (U.S. Geological Survey).
W. G. E. Caldwell (University of Saskatchewan), Lucy E. Edwards (U.S.
Geological Survey), Henry H. Gray (Indiana Geological Survey), Hollis
D. Hedberg (Princeton University), Lewis H. King (Geological Survey of
Canada), Rudolph W. Kopf (U.S. Geological Survey), Jerry A. Lineback
(Robertson Research U.S.), Marjorie E. MacLachlan (U.S. Geological
Survey), Amos Salvador (University of Texas, Austin), Brian R. Shaw
(Samson Resources, Inc.), Ogden Tweto (U.S. Geological Survey).
1977–2021 COMPOSITION OF THE NORTH AMERICAN COMMISSION ON
Each Commissioner is appointed, with few exceptions, to serve a 3-year
term (shown by such numerals as 80–82 for 1980–1982) and a few are
reappointed. At the 2006 Meeting of the Commission, SEPM Society for
Sedimentary Geology and the Servicio Géologico Mexicano were added to membership.
American Association of Petroleum Geologists
Timothy A. Anderson 1977–1983, Orlo E. Childs 1976–1979, Richard Denne
2018–2021, Art D. Donovan 2005–2014, Kenneth J. Englund 1974–1977,
Katherine A. Giles 2019–2022, Robert R. Jordan 1985–2009, Susan
Longacre 1978–2005, Donald E. Owen 1979–1985, 1987–2015, Nahum
Schneidermann 1983–1986, Robert W. Scott 2012–2018, Doug Sprinkel
2020–2023, Grant Steele 1975–1978.
Association of American State Geologists
M. Lee Allison 1988–1991, Thomas M. Berg 1991–1994, John P. Bluemle 1992–2003, David Dockery 2018–2021, Robert H. Fakundiny 1981–1992, 1995–2007, Larry D. Fellows 1981–1982, 1991–1994, Conrad Gazzier 1988–1991, Lee C. Gerhard 1979–1981, Gary B. Glass 1984–1987, Donald C. Haney 1980–1983,
Norman C. Hester 1986–1987, 1997–2002, William T. Hill 1986–1989, Wallace B. Howe 1974–1977, Matt Joeckel
2017–2020, Robert R. Jordan 1978–1984, Frank E. Kottlowski 1976–1979,
Ernest A. Mancini 1983–1986, 1998–2011, Robert C. Milici 1987–1990, Meredith E. Ostrom 1977–1980,
John B. Patton 1975–1978, James Robertson 1992–1996, Thomas M. Scott
2007–2012, John Steinmetz 2012–2015, Berry H. (Nick) Tew 2004–2022,
David Wunsch 2009–2014.
Geological Society of America
Marie-Pierre J. Aubry 2007–2010, Donald L. Baars 1988–1995, Gordon C.
Baird 2020–2023, William A. Berggren 2016–2019, E. Arthur Bettis 2008–2011, Carlton E. Brett 2014–2017, Charles W. Copeland, Jr. 1983–1986, John M. Dennison 1985–1988, Marieke Deschesne
2017–2020, Frank R. Ettenson 2012–2015, Stanley C. Finney 2009–2012,
Richard H. Fleugeman 2010–2013, Lee C. Gerhard 1993–1996, Clarence A. Hall, Jr. 1978–1981, Joe Hannibal 2018–2021, Ardith K. Hansel 1998–2001, W. Burleigh Harris 1995–1998, Jack E. Harrison 1974–1977,
Donald E. Hattin, 1987–1990, William W. Hay 1975–1978, Robert S.
Houston 1977–1980, James O. Jones 1994–1997, David T. King 1997–2000,
Ed Landing 2005–2008, H. Richard Lane 2001–2004, Norman P. Lasca 1982–1985, Robert F. Lundin
1987–1989, Ernest A. Mancini 1996–1998, Walter L. Manger 2000–2003,
Christopher G. Maples 2000–2003, Brett McLauren 2006–2009, Glenn B.
Morey 1991–1994, Jared R. Morrow 2003–2006, Michael A. Murphy
1981–1984, Allison R. Palmer 1980–1983, Shanan Peters 2015–2018,
Matthew R. Saltzman 2004–2007, Paul R. Seaber 1989–1992, Jon J. Smith
2019–2022, Patrick K. Sutherland 1984–1987, Peter R. Vail 1990–1993,
Bridget Wade 2011–2014, Malcolm P. Weiss 1976–1982.
United States Geological Survey
Earl E. Brabb 1978–1982, Lucy E. Edwards 1986–2022, David S. Fullerton 1978–1984, 2005–2014, Julie A. Herrick 2017–2023, E. Dale Jackson 1976–1978, Randall C. Orndorff 1997–2018, Kenneth L. Pierce
1975–1978, C. Wylie Poag 1983–1986, John Pojeta, Jr. 1993–1996,
Forrest G. Poole 1988–1994, Mitchell W. Reynolds 1992–1995, Norman F.
Sohl 1971–1983, Nancy Stamm 2018–2021, John H. Stewart 1984–1993,
Joshua I. Tracey, Jr. 1982–1988, Bruce R. Wardlaw 1995–2005.
Geological Survey of Canada
James D. Aitken 1975–1978, R.G. Anderson 2004–2010, 2013–2016, Benoit
Beauchamp 1992–1997, Kenneth D. Card 1980–1983, Michael P. Cecile
1988–1991, Fred W. Chandler 1988–1991, Donald G. Cook 1978–1981,
1992–1994, Lynn Dafoe 2019–2022, Anthony Davidson 1984–1987, 1997–2002, Lynda Dredge
1990–1994, Ashton F. Embry, III 1982–1988, Robert J. Fulton 1981–1984,
1992–1996, A.P. (Tony) Hamblin 1997–2015, Jim Haggart 2015–2021, John
B. Henderson 1974–1977, Lewis H. King 1979–1982, Maurice B. Lambert
1977–1980, Denis Lavoie 1992–1995, Robert MacNaughton 2016–2019, 2020–2023, John A. Percival 1990–1994, Terry Poulton 1998–2001, Robert Rainbird 2002–2014, Rod Smith 2015–2018,
R.I. Thompson 1983–1986, Graham L. Williams 1987–1989, Christopher J.
Canadian Society of Petroleum Geologists
William R. Arnott 1998–2001, Wayne Brideaux 1987–1988, Octavian
Catuneanu 2005–2015, Donald G. Cook 1989–1991, Roland F. deCaen
1979–1985, J. Ross McWhae 1977–1980, Timothy R. Marchant 1983–1986, Brian Pratt 1992–2021, Edward H.
Schultz 1974–1977, 1980–1983, Ulrich Wissner 1976–1979, C.E. Wright
1985–1989, Raymond W. Yole 1991–2002, John-Paul Zonneveld 2020–2023.
Geological Association of Canada
William R. Arnott 1998–2000, Frank Brunton 2005–2013, 2019–2022,
W.G.E. Caldwell 1976–1979, R. Michael Easton 1991–2017, John Johnston
2017–2021, R. K. Jull 1978–1979, Paul S. Karrow 1981–1984, Alfred C. Lenz 1979–1981, 1985–1988,
1990–1998, Grant D. Mossop 1982–1985, David E. Pearson 1979–1982, Paul E. Schenk 1975–1978, James T. Teller 1984–1987, John A. Westgate 1987–1990.
SEPM (Society for Sedimentary Geology)
Vitor Abreu 2006–2009, Marie-Pierre J. Aubry 2012–2021, Howard Harper 2009–2022, Peter Sadler 2008–2011.
Asociación Mexicana de Geólogos Petróleros
Jose Carillo-Bravo 1978–1981, Baldomerro CarrascoVelazquez 1975–1978,
1985–1988, Carlos Manuel Cantu-Chapa 1998–2001, 2008–2011, Hilarión Sánchez-Hernández 2017–2020.
Sociedad Geólogica Mexicana
Jose Carrillo-Bravo 1982–1985, 1996–2001, Zoltan de Cserna 1976–1982,
Emiliano Campos-Madrigal 1998–2001, 2008–2011, 2013–2016, René Alejandro Téllez-Flores 2016–2023.
Instituto de Geología de la Universidad Nacional Autónoma de México
Jorge J. Aranda-Gomez 1990–1993, Ricardo Barragán-Manzo 2007–2019,
Diego A. Cordoba-Mendez 1990–1993, Ismael Ferrusquia-Villafranca 1976–1981, 1992–2006, Fernando Núñez-Useche 2019–2022, Fernando Ortega-Gutiérrez 1981–1990, Elizabeth
Servico Geólogico Mexicano
Natalia Amecua-Torres 2013–2016, Edgar Juarez-Arroaga 2008–2011, Rosario Isabel López-Palomino 2008–2015, 2017–2020, Arturo Palma-Ramirez 2019–2022.
Jorge J. Aranda-Gomez 1994, Carlton E. Brett 2017–2022, Diego A.
Cordoba-Mendez 1992–1994, Thomas W. Dignes 2016–2022, R. Michael
Easton 2018–2021, Ashton F. Embry 2005–2011, Ismael
Ferrusquia-Villafranca 2006–2022, Stanley C. Finney 2012–2021, Richard H. Fluegeman 2013–2022, Donald E. Hattin 1991–1994, Ed Landing 2018–2021, Norman P. Lasca 1992–2021, Walter M.
Manger 2005–2009, Jared R. Morrow 2006–2009, Randall C. Orndorff
2018–2021, Robert Scott 2019–2022, Paul R. Seaber 1992–2000, Janet
Slate 2020–2023, John Van Couvering 2005–2016.
Reports And Notes Of The American Commission On Stratigraphic Nomenclature
Reports (formal declarations, opinions, and recommendations)
MOORE, R. C., 1949. Declaration on naming of subsurface stratigraphic units. American Association of Petroleum Geologists Bulletin, 33: 1280–1282.
HEDBERG, H. D., 1952. Nature, usage, and nomenclature of time-stratigraphic and geologic-time units. American Association of Petroleum Geologists Bulletin, 36: 1627–1638.
HARRISON, J. M., 1955. Nature, usage, and nomenclature of time-stratigraphic and geologic-time units as applied to the Precambrian. American Association of Petroleum Geologists Bulletin 39: 1859–1861.
COHEE, G. V., et al, 1956. Nature, usage, and nomenclature of rock-stratigraphic units: American Association of Petroleum Geologists Bulletin, 40: 2003–2014.
McKEE, E. D., 1957. Nature, usage and nomenclature of biostratigraphic units: American Association of Petroleum Geologists Bulletin, 41: 1877–1889.
RICHMOND, G. M., 1959. Application of stratigraphic classification and nomenclature to the Quaternary: American Association of Petroleum Geologists Bulletin, 43: 663–675.
LOHMAN, K. E., 1963. Function and jurisdictional scope of the American Commission on Stratigraphic Nomenclature: American Association of Petroleum Geologists Bulletin, 47: 853–855.
HENDERSON, J. B., CADWELL, W. G. E., and HARRISON, J. E., 1980. Amendment of code concerning terminology for igneous and high-grade metamorphic rocks: Geological Society of America Bulletin, pt. I, 91: 374–376.
HARRISON, J. E., and PETERMAN, Z. E. 1982. Adoption of geochronometric units for divisions of Precambrian time: American Association of Petroleum Geologists Bulletin, 66: 801–802.
OWEN, D. E., LASCA, N. P., and SCHULTZ, E. H., 1985. New articles of organization and procedure of North American Commissionon Stratigraphic Nomenclature: American Association of Petroleum Geologists Bulletin, 69: 872–873.
OWEN, D. E., LASCA, N. P., and EDWARDS, L. E., 2010. Revised Articles of Organization and Procedure of the North American Commission on Stratigraphic Nomenclature: Stratigraphy, 6 (2): 183–184.
EASTON, R. M., EDWARDS, L. E., ORNDORFF, R. C., DUGUET, M., and FERRUSQUIA-VILLAFRANCA, I., 2017. Revision of Article 37, Lithodemic Units, of the North American Stratigraphic Code: Stratigraphy, 13: 220–222.
BRETT, C. E., PRATT, B. R. and LANDING, E. 2019. Revision of Articles 25–27 of the North American Stratigraphic Code to allow formal submembers: Stratigraphy, 16: 279–281.
AUBRY, M.-P., FLUEGEMAN, R. H., EDWARDS, L. E., PRATT, B. R., and BRETT, C. E., 2020. Revision of Articles 73, 81, 82, and Table 2 of the North American Stratigraphic Code to formalize
subseries and subepochs, Stratigraphy, 17: 315–316.
Notes (informal statements, discussions, and outlines of problems)
MOORE, R. C., 1947. Organization and objectives of the Stratigraphic Commission. American Association of Petroleum Geologists Bulletin, 31: 513–518.
MOORE, R. C., 1947. Nature and classes of stratigraphic units. American Association of Petroleum Geologists Bulletin, 31: 519–528.
MOORE, R. C., 1948. Rules of geologic nomenclature of the Geological Survey of Canada. American Association of Petroleum Geologists Bulletin, 32: 366–367.
JONES, W. V. and MOORE, R. C., 1948. Naming of subsurface stratigraphic units. American Association of Petroleum Geologists Bulletin, 32: 367–371.
FLINT, R. F., and MOORE, R. C., 1948. Definition and adoption of the terms stage and age. American Association of Petroleum Geologists Bulletin, 32: 372–376.
MOORE, R. C., 1948. Discussion of nature and classes of stratigraphic units. American Association of Petroleum Geologists Bulletin, 32: 376–381.
MOORE, R. C., 1949. Records of the Stratigraphic Commission for 1947–1948. American Association of Petroleum Geologists Bulletin, 33: 1271–1273.
MOORE, R. C., 1949. Australian Code of Stratigraphical Nomenclature. American Association of Petroleum Geologists Bulletin, 33: 1273–1276.
MOORE, R. C., 1949. The Pliocene-Pleistocene boundary. American Association of Petroleum Geologists Bulletin, 33: 1276–1280.
MOORE, R. C., 1950. Should additional categories of stratigraphic units be recognized? American Association of Petroleum Geologists Bulletin, 34: 2360–2361.
MOORE, R. C., 1951. Records of the Stratigraphic Commission for 1949–1950. American Association of Petroleum Geologists Bulletin, 35: 1074–1076.
MOORE, R. C., 1951. Divisions of rocks and time. American Association of Petroleum Geologists Bulletin, 35: 1076.
WILLIAMS, J. S., and CROSS, A. T., 1952. Third Congress of Carboniferous Stratigraphy and Geology. American Association of Petroleum Geologists Bulletin, 36: 169–172.
Official report of round table conference on stratigraphic nomenclature at Third Congress of Carboniferous Stratigraphy and Geology, Heerlen, Netherlands, June 26–28, 1951, 1952.
American Association of Petroleum Geologists Bulletin, 36: 2044–2048.
Records of the Stratigraphic Commission for 1951–1952, 1953. American Association of Petroleum Geologists Bulletin, 37: 1078–1080.
Records of the Stratigraphic Commission for 1953–1954, 1955. American Association of Petroleum Geologists Bulletin, 39: 1861–1863.
Suppression of homonymous and obsolete stratigraphic names, 1956. American Association of Petroleum Geologists Bulletin, 40: 2953–2954.
GILLULY, J., 1957. Records of the Stratigraphic Commission for 1955–1956. American Association of Petroleum Geologists Bulletin, 41: 130–133.
RICHMOND, G. M., and FRYE, J. C., 1957. Status of soils in stratigraphic nomenclature. American Association of Petroleum Geologists Bulletin, 31: 758–763.
FRYE, J. C., and RICHMOND, G. M., 1958. Problems in applying standard stratigraphic practice in nonmarine Quaternary deposits. American Association of Petroleum Geologists Bulletin, 42: 1979–1983.
FRYE, J. C., 1958. Preparation of new stratigraphic code by American Commission on Stratigraphic Nomenclature. American Association of Petroleum Geologists Bulletin, 42: 1984–1986.
Records of the Stratigraphic Commission for 1957–1958, 1959. American Association of Petroleum Geologists Bulletin, 43: 1967–1971.
RODGERS, J. and McCONNELL, R. B., 1959. Need for rock-stratigraphic units larger than group. American Association of Petroleum Geologists Bulletin, 43: 1971–1975.
WHEELER, H. E., 1959. Unconformity-bounded units in stratigraphy. American Association of Petroleum Geologists Bulletin, 43: 1975–1977.
BELL, W. C., et al, 1961. Geochronologic and chronostratigraphic units. American Association of Petroleum Geologists Bulletin, 45: 666–670.
Records of the Stratigraphic Commission for 1959–1960, 1961. American Association of Petroleum Geologists Bulletin, 45: 670–673.
FRYE, J. C., and WILLMAN, H. B., 1962. Morphostratigraphic units in Pleistocene stratigraphy. American Association of Petroleum Geologists Bulletin, 46: 112–113.
SHAVER, R. H., 1962. Application to American Commission on
Stratigraphic Nomenclature for an amendment of Article 4f of the
Code of Stratigraphic Nomenclature on informal status of named
aquifers, oil sands, coal beds, and quarry layers. American
Association of Petroleum Geologists Bulletin, 46: 1935.
PATTON, J. B., 1963. Records of the Stratigraphic Commission for 1961–1962. American Association of Petroleum Geologists Bulletin, 47: 1987–1991.
RICHMOND, G. M., and FYLES, J. G., 1964. Application to American Commission on Stratigraphic Nomenclature for an amendment of Article 31, Remark (b) of the Code of Stratigraphic Nomenclature on misuse of the term “stage”. American Association of Petroleum Geologists Bulletin, 48: 710–711.
COHEE, G. V., 1965. Records of the Stratigraphic Commission for 1963–1964. American Association of Petroleum Geologists Bulletin, pt. I of II, 49: 296–300.
HEDBERG, H. D., Editor, 1965. International Subcommission on Stratigraphic Terminology, Definition of geologic systems. American Association of Petroleum Geologists Bulletin, 49: 1694–1703.
HEDBERG, H. D., 1966. Application to American Commission on Stratigraphic Nomenclature for amendments to Articles 29, 31, and 37 to provide for recognition of erathem, substage, and chronozone as time-stratigraphic terms in the Code of Stratigraphic Nomenclature. American Association of Petroleum Geologists Bulletin 50: 560–561.
HARKER, P., 1967. Records of the Stratigraphic Commission for
1964–1966. American Association of Petroleum Geologists
Bulletin, 51: 1862–1868.
DeFORD, R. K., WILSON, J. A. and SWAIN, F. M., 1967. Application to American Commission on Stratigraphic Nomenclature for an amendment of Article 3 and Article 13, Remarks (c) and (e), of the
Code of Stratigraphic Nomenclature to disallow recognition of new
stratigraphic names that appear only in abstracts, guidebooks,
microfilms, newspapers, or in commercial or trade journals. American
Association of Petroleum Geologists Bulletin, 51: 1868–1869.
COHEE, G. V., DeFORD, R. K. and WILLMAN, H. B., 1969. Amendment of Article 5, Remarks (a) and (e) of the Code of Stratigraphic Nomenclature for treatment of geologic names in a
gradational or interfingering relationship of rock-stratigraphic
units. American Association of Petroleum Geologists Bulletin, 53:
KOTTLOWSKI, F. E., 1969. Records of the Stratigraphic Commission for
1966–1968. American Association of Petroleum Geologists Bulletin,
ANDREWS, J., and JINGHWA HSU, K., 1970. A recommendation to the
American Commission on Stratigraphic Nomenclature concerning
nomenclatural problems of submarine formations. American
Association of Petroleum Geologists Bulletin, 54: 1746–1747.
WILSON, J. A., 1971. Records of the Stratigraphic Commission for
1968–1970. American Association of Petroleum Geologists Bulletin,
JAMES, H. L., 1972. Subdivision of Precambrian: An interim scheme to
be used by U.S. Geological Survey. American Association of
Petroleum Geologists Bulletin, 56: 1128–1133.
ORIEL, S. S., 1975. Application for amendment of Article 8 of code,
concerning smallest formal rock-stratigraphic unit. American
Association of Petroleum Geologists Bulletin, v. 59: 134–135.
ORIEL, S. S., 1975. Records of Stratigraphic Commission for
1970–1972. American Association of Petroleum Geologists
Bulletin, 59: 135–139.
ORIEL, S. S., and BARNE, V. E., 1975. Records of Stratigraphic
Commission for 1972–1974. American Association of Petroleum
Geologists Bulletin, 59: 2031–2036.
ORIEL, S. S., MACQUEEN, R. W., WILSON, J. A., and DALRYMPLE, G.B., 1976. Application for addition to code concerning magnetostratigraphic units. American Association of Petroleum Geologists Bulletin, 60: 273–277.
SOHL, N. F., 1977. Application for amendment concerning terminology
for igneous and high-grade metamorphic rocks. American Association
of Petroleum Geologists Bulletin, 61: 248–251.
SOHL, N. F., 1977. Application for amendment of Articles 8 and 10 of
code, concerning smallest formal rock-stratigraphic unit. American
Association of Petroleum Geologists Bulletin, 61: 252.
MACQUEEN, R. W., and ORIEL, S. S., 1977. Application for amendment
of Articles 27 and 34 of stratigraphic code to introduce
point-boundary stratotype concept. American Association of
Petroleum Geologists Bulletin, 61: 1083–1085.
SOHL, N. F., 1978. Application for amendment of Code of
Stratigraphic Nomenclature to provide guidelines concerning formal
terminology for oceanic rocks. American Association of Petroleum
Geologists Bulletin, 62: 1185–1186.
CALDWELL, W. G. E., and SOHL, N. F., 1978. Records of Stratigraphic Commission for 1974–1976. American Association of Petroleum Geologists Bulletin, 62: 1187–1192.
WEISS, M. P., 1979. Proposal to change name of commission. American
Association of Petroleum Geologists Bulletin, 63: 1986.
WEISS, M. P., and AITKEN, J. D., 1980. Records of Stratigraphic
Commission, 1976–1978. American Association of Petroleum Geologists
Bulletin, 64: 136–137.
HARRISON, J. E., and PETERMAN, Z. E., 1980. A preliminary proposal for a chronometric time scale for the Precambrian of the United States and Mexico. Geological Society of America Bulletin, pt. I, 91: 377–380.
JORDAN, R. R., 1982. Records of Stratigraphic Commission, 1978 –
1980: American Association of Petroleum Geologists Bulletin, 66: 238–240.
JORDAN, R. R., 1986. Records of Stratigraphic Commission, 1980 –
1982: American Association of Petroleum Geologists Bulletin, 70:
OWEN, D. E., and LASCA, N. P., 1987. Records of Stratigraphic
Commission, 1982 – 1984: American Association of Petroleum
Geologists Bulletin, 71: 353–355.
EMBRY, A. F., and LONGACRE, S. A., 1987. Records of Stratigraphic Commission, 1984–1986: American Association of Petroleum Geologists Bulletin, 71: 1434–1443.
FAKUNDINY, R. H., and LONGACRE, S. A., 1989. Application for amendment of North American Stratigraphic Code to provide for exclusive informal use of morphological terms such as Batholith,
Intrusion, Pluton, Stock, Plug, Dike, Sill, Diapir, and Body. American Association of Petroleum Geologists Bulletin, 73: 1452–1453.
FAKUNDINY, R. H., and LUNDIN, R. F., 1991. Records of Stratigraphic Commission, 1986–1988. American Association of Petroleum Geologists Bulletin, 75: 1275–1278.
CHANDLEER, F. W. and JORDAN, R. R., 1992. Records of Stratigraphic Commission, 1988–1990. American Association of Petroleum Geologists Bulletin, 76: 1933–1934.
61: EDWARDS, L. E., 1993. Application for amendment of North American
Stratigraphic Code to clarify relation of allostratigraphic and
lithostratigraphic units. American Association of Petroleum
Geologists Bulletin, 77: 909–943.
EDWARDS, L. E., and OWEN, D. E., 1996. Records of Stratigraphic Commission, 1991–1992. American Association of Petroleum Geologists Bulletin, 80: 1156–1159.
EASTON, R. M., BAARS, D. L., and COOK, D. G., 1997. Records of Stratigraphic Commission, 1992–1994. American Association of Petroleum Geologists Bulletin, 81: 1342–1345.
FERRUSQUIA-VILLAFRANCA, I., EASTON, R. M., EDWARDS, L. E., FAKUNDINY, R. H., and JONES, J. O., 2001. Application for amendment of the North American Stratigraphic Code concerning consistency and updating regarding electronic publishing. American Association of Petroleum Geologists Bulletin,
LENZ, A. C., EDWARDS, L. E., and PRATT, B. R., 2001. Application for revision of articles 48–54, biostratigraphic units, of the North American Stratigraphic Code. American Association of
Petroleum Geologists Bulletin, 85: 372–375.
EASTON, R. M., JONES, J. O., LENZ, A. C., FERRUSQUIA-VILLAFRANCA, I., MANCINI, E. A., WARDLAW, B. R., EDWARDS, L. E., and PRATT, B. R., 2005. Records of Stratigraphic Commission, 1995–2002. American
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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.
<A name="#fnote4>"4This article is modified slightly from a statement by the
International Commission of Zoological Nomenclature (1964, p. 7-9).
Remark (c) is from the advice of the Association of Earth Science
5Note that the initial letters Megaand Gigaare capitalized,
but that of kilois not, by SI convention.
6From the Greek demas, -os: “living body, frame.”
7Pluton – a mappable body of plutonic rock.
8Terminology related to pedostratigraphic classification is
summarized on pp. 187–188.
9From the Greek allo: “other, different.”
10Article 58e was revised in accordance with Article 21 in
1995. The revised remark is shown here. See Note 60 (AAPG Bulletin, v.
77, p. 909) and Note 62 (AAPG Bulletin, v. 81, p. 1342-1345) for
further details on the revision.