Stratigraphic Guide

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Chapter 3. Definitions and Procedures

A. Definitions

  1. Stratigraphy
    Stratigraphy, from Latin stratum + Greek graphia, is the description of all rock bodies forming the Earth’s crust and their organization into distinctive, useful, mappable units based on their inherent properties or attributes in order to establish their distribution and relationship in space and their succession in time, and to interpret geologic history.

  2. Stratum (plural=strata)
    Layer of rock characterized by particular lithologic properties and attributes that distinguish it from adjacent layers.

  3. Stratigraphic classification
    The systematic organization of the Earth’s rock bodies, as they are found in their original relationships, into units based on any of the properties or attributes that may be useful in stratigraphic work.

  4. Stratigraphic unit
    A body of rock established as a distinct entity in the classification of the Earth’s rocks, based on any of the properties or attributes or combinations thereof that rocks possess.

Stratigraphic units based on one property will not necessarily coincide with those based on another.

  1. Stratigraphic terminology
    The total of unit-terms used in stratigraphic classification. It may be either formal or informal.

    a. Formal stratigraphic terminology
    Uses unit-terms that are defined and named according to guidelines conventionally established.

    b. Informal stratigraphic terminology
    Uses unit-terms as ordinary nouns in a descriptive sense, not as a part of a specific scheme of stratigraphic classification.The use of informal terms in published documents is strongly discouraged.

  2. Stratigraphic nomenclature
    The system of proper names given to specific stratigraphic units.

  3. Zone
    Minor body of rock in many different categories of stratigraphic classification. The type of zone indicated is made clear by a prefix, e.g., lithozone, biozone, chronozone.

  4. Horizon
    An interface indicative of a particular position in a stratigraphic sequence. The type of horizon is indicated by a prefix, e.g., lithohorizon, biohorizon, chronohorizon.

  5. Correlation
    A demonstration of correspondence in character and/or stratigraphic position. The type of correlation is indicated by a prefix, e.g., lithocorrelation, biocorrelation, chronocorrelation.

  6. Geochronology
    The science of dating and determining the time sequence of the events in the history of the Earth.

  7. Geochronologic unit
    A subdivision of geologic time.

  8. Geochronometry
    A branch of geochronology that deals with the quantitative (numerical)measurement of geologic time. The abbreviations ka for thousand (103), Ma for million (106), and Ga for billion (milliard of thousand million, 109) years are used.

  9. Facies
    The term "facies" originally meant the lateral change in lithologic aspect of a stratigraphic unit. Its meaning has been broadened to express a wide range of geologic concepts: environment of deposition, lithologic composition, geographic, climatic or tectonic association, etc.

Caution against preempting general terms for special meanings

The preempting of general terms for special restricted meanings has been a source of much confusion. The preferable procedure is to conserve the original general meaning of a term and to seek a more precise and less ambiguous word for the special meaning.

B. Procedures for Establishing and Revising Stratigraphic Units.

The proposal of a new formal stratigraphic unit requires a statement of intent to introduce the new unit and the reasons for the action. A new unit must be duly proposed and duly described. This includes:

  • A clear and complete definition, characterization, and description of the unit so that any subsequent investigator can identify it.
  • The proposal of the kind, name, and rank of the unit.
  • The designation of a stratotype (type section) or type locality on which the unit is based and which may be used by interested scientists as a reference.
  • Publication in a recognized scientific medium.

1. Definition, characterization, and description

a. Name
(see section 3.B.v).

b. Stratotypes or other standards of reference
Gives the geographic location and geologic setting of the stratotype with an indication of accessibility, maps, and markers, both artificial and natural.

For units of the type for which it is impractical to use stratotypes as standards, reliance is placed on the accurate description and illustration of those features that constitute the diagnostic criteria of the unit.

c. Description ofunit at stratotype or type locality

d. Regional aspects
Geographic extent; regional variations in thickness, lithostratigraphy, biostratigraphy or other properties; nature of boundaries away from the type; criteria to be used in identifying and extending the unit over the area of its presence.

e. Geologic age

f. Correlation with other units

g. Genesis (where appropriate)

h. References to the literature

2. Special requirements for establishing subsurface units

The same rules of procedure used for outcrop sections apply to subsurface units established on the basis of exposures in mines, tunnels or from sections penetrated in wells. Stratotypes in well sections are designated by well depths and on well logs and in cores, if available. The following data are desirable for establishing subsurface units:

a. Designation of well or mine
The name of the well or mine and geographic location using conventional oil field or topographic nomenclature.

b. Geologic logs
Lithologic and paleontologic logs of the well or wells, and maps and cross sections of the mine, in written and graphic form with the boundaries of the new unit and its subdivisions.

c. Geophysical Logs and Profiles
Electrical and/or other wire-line logs and seismic profiles with boundaries and subdivisions of the unit shown at a scale adequate to permit appreciation of detail.

d. Depositories
A depository should be an institution with the proper curatorial facilities and assurance of perpetuity where the materials are available for study. The location of the depository for materials from the stratotype well, tunnel or mine should be given.

3. Naming of stratigraphic units.

The names of most formal stratigraphic units consist of an appropriate geographic name combined with an appropriate term indicating the kind and rank of the unit, e.g. La Luna Formation, except for some terms that were established in the early history of stratigraphy.

The formal name of a biostratigraphic unit is formed from the names of one or more appropriate fossils combined with the appropriate term for the kind of biostratigraphic unit, e.g., Exus albus Assemblage Zone. (see Chapter 7 - Biostratigraphic Units).

a. Geographic component of names of stratigraphic units

i. Source
Geographic names should be derived from permanent natural or artificial features at or near which the stratigraphic unit is present.A name should be on standard published maps of the pertinent political jurisdiction.

Where such names are not available, the place from which the name is derived should be described and shown on an appropriately scaled map accompanying the description of the new unit.

Short names are preferable to long or compound names.The name of the stratigraphic unit should be exactly the same as the name of the geographic feature after which it is named.

ii. Spelling of Geographic Names
The spelling of the geographic component of the name of a stratigraphic unit should conform to the usage of the country of origin.

The spelling of the geographic component, once established, should not be changed.

The rank or lithologic component may be changed when translated to a different language.

iii. Changes in Geographic Names
The change in the name of a geographic feature does not affect the name of the associated stratigraphic unit nor does disappearance of the geographic feature require a new name.

iv. Inappropriate Geographic Names
A geographic name should not be misleading, e.g. London Formation for a unit in Canada, although a city with that name exists in Canada.

v. Duplication of Geographic Names
The name of a new stratigraphic unit should be unique in order to prevent ambiguity.The IUGS Lexique Stratigraphique International and national and regional lexicons contain lists of previously used names and inquiry to geological surveys and other regional organizations is recommended to discover previously used names not yet published in the lexicons.

vi. Names for subdivisions of Stratigraphic Units
If a unit is divided into two or more formal component units, the geographic name of the original unit should not be employed for any of the subdivisions.

b. Unit-term Component of Names of Stratigraphic Units
The unit-term component of a stratigraphic name indicates the kind and rank of the unit. A stratigraphic unit-term may differ in different languages.

c. Relation of Names to Political Boundaries
Stratigraphic units are not limited by international boundaries and should not differ across them.

d. Reduction in number of names through correlation
If correlation has established the identity of two differently named stratigraphic units, the later name should be replaced by the earlier, other considerations being equal.

e. Uncertainty in Assignment
If identification of a stratigraphic unit is in doubt, that uncertainty should be expressed in the nomenclature employed.The following conventions may be used:

  • Devonian? = doubtfully Devonian
  • Macoa? Formation = doubtfully Macoa Formation
  • Peroc-Macoa formation = strata intermediate in position (horizontally or vertically) between two stratigraphic units
  • Silurian-Devonian = one part Silurian, one part Devonian
  • Silurian or Devonian = either Silurian or Devonian
  • Silurian and Devonian = both Silurian and Devonian, but undifferentiated.

The name of the older or lower unit, if this distinction can be made, should always come first when two units are hyphenated or used in combination.

f. Abandoned Names
The name of a stratigraphic unit, once applied and then abandoned, should not be revived except in its original sense. Reference to abandoned names should indicate the original sense in which the name was used, e.g. "Mornas Sandstone of Hebert (1874)".

g. Preservation of Traditional of Well-Established Names
Traditional or well-established names that do not follow the above procedures and conventions should not be abandoned providing they are or may become well defined or characterized.

4. Publication

a. A Recognized Scientific Medium
Establishment or revision of a formal stratigraphic unit requires publication in a recognized scientific medium.

The main qualifications of a recognized scientific medium are that it is regularly published and reasonably available to the scientific public on request by purchase or through a library.

Abstracts, most fieldtrip guidebooks, dissertations, company reports, open file reports and similar media generally do not meet this requirement.

b. Priority
Publication of a properly proposed, named, and described unit has priority.

However, priority alone does not justify displacing a well-established name by one not well known or rarely used; nor should an inadequately established name be preserved merely because of priority.

c. Recommended Editorial Procedures
The editorial rules and procedures enumerated below apply to the English language. Rules of orthography of other languages may make these recommendations inapplicable.

i. Capitalization
The first letters of all words used in the names of formal stratigraphic units are capitalized (except for the trivial names of species and subspecies rank in the names of biostratigraphic units). Informal terms are not capitalized.

ii. Hyphenation
Compound terms for most kinds of stratigraphic units, in which two common words are joined to give a special meaning, should be hyphenated, e.g. concurrent-range zone, normal-polarity zone. Exceptions are adjectival prefixes or combining forms that are generally combined with the term-noun without a hyphen, e.g. biozone.

iii. Repetition of the Complete Name
After the complete name of a stratigraphic unit has been referred to once in a publication, part of the name may be omitted for brevity if the meaning is clear, e.g., the Oxfordian Stage may be referred to as "the Oxfordian", or "the Stage".

5. Revision or redefinition of previously established stratigraphic units

Revision or redefinition of an adequately established unit without changing its name requires a statement of intent to revise the unit, the reasons for doing so, and as much justification and documentation as for proposing a new unit.

Change in rank of a stratigraphic unit does not require redefinition of the unit or its boundaries, or alteration of the geographic part of the name.A stratigraphic unit may be promoted or demoted in rank without changing the geographic part of its name.