COMMENTARY:

USAGE OF STRATIGRAPHIC TERMINOLOGY IN PAPERS,

ILLUSTRATIONS, AND TALKS1

 

DONALD E. OWEN2

Department of Geology

Lamar University

Beaumont, Texas 77710

 

ABSTRACT: Some writers and speakers have problems with clear usage of stratigraphic terminology, a topic made more acute by the appearance of the complex 1983 North American Code and revisions in progress of the 1976 International Guide.  The basic categories of stratigraphic units are 1) material; 2) nonmaterial; 3) hybrid. Examples are the well-known rock (lithostratigraphic), time (geochronologic), and time-rock (chronostratigraphic) units, respectively.  Lesser-known categories include magnetostratigraphic, lithodemic, pedostratigraphic, unconformity-bounded, and diachronic units.

Both formal and informal stratigraphic units are recognized.  All words in formal units are capitalized.  Only the geographically derived name in informal units is generally capitalized.  Inadequate distinction between time and place words, both formal and informal, leads to unnecessary confusion.  Misuse of early versus lower and late versus upper is prevalent.

Publications such as lexicons and correlation charts are recommended as initial sources of stratigraphic information.  Naming, revising, and abandoning formal stratigraphic nomenclature is governed by specific rules laid down in the Code and Guide.

In illustrations of stratigraphic units, it is important to distinguish clearly between scales of time and position.  Strata are not measured in years, nor time in meters!

 

INTRODUCTION

 

Since the appearance of the new North American Stratigraphic Code [North American Commission on Stratigraphic Nomenclature (N.A.C.S.N., 1983)], it has become apparent that some writers and speakers have problems with clear usage of stratigraphic nomenclature. This is a continuing problem, made more complex because the 1983 Code has many more categories of stratigraphic units and much more detail than its predecessor [American Commission on Stratigraphic Nomenclature (A.C.S.N., 1961 and 1970)]. Stratigraphic nomenclature has recently undergone an expansion well outside the realm of sedimentary rocks into formal units that do not conform to the Law of Superposition. Also, authors working on areas outside North America should be advised that the International Stratigraphic Guide (Hedberg 1976) is being revised by the International Subcommission on Stratigraphic Classification (I.S.S.C.). Therefore, developments in international practice should be followed closely.

As a consequence of recent developments regarding the North American and international standards for stratigraphic terminology (Code and Guide as referred to in this article), it seems appropriate to update my previous attempt at practical advice on usage of stratigraphic terminology (Owen 1978). The purposes of this paper are 1) to summarize the currently available types of formal stratigraphic units and their usage; and 2) to review some informal conventions that are standard procedure in stratigraphic editing, but which are not written in the Code, Guide, or any other readily available source.

 

CATEGORIES OF STRATIGRAPHIC UNITS

 

In stratigraphic work, two basic categories of units are recognized: 1) material units, based on actual bodies of rock; 2) nonmaterial units, based on the abstract concept of geologic time. A ___________________________________________

          1 Manuscript received 29 August 1986; revised 6 October 1986

          2 Member, North American Commission on Stratigraphic Nomenclature, 1979-1985.

        Member, International Subcommission on Stratigraphic Classification.

combination of these two categories, a chronostratigraphic unit, is also commonly recognized. A chronostratigraphic unit is a body of rock that serves as the referent for the geologic time represented by that body of rock.

 

Well-Known Units

 

For most geologists composing most papers, the old "holy trinity" of stratigraphic units (Schenck and Muller 1941) (Table 1) that students are supposed to learn will suffice: 1) lithostratigraphic units (= material rock units); 2) geochronologic units (= abstract "time units")3; 3) chronostratigraphic (and biostratigraphic?) units (= hybrid time-rock units).

The status of the third group of units is slightly controversial. Separate terms for chronostratigraphic and biostratigraphic units are well established in official codes and guides, but a small group of dissenters maintains that chronostratigraphic units are unnecessary because they are based on biostratigraphic units (Weidmann 1970; Hancock 1977; Johnson 1979; Watson 1983; and Witzke et al. 1985). Most authors will find it easier to follow the practice of the establishment by adhering to the Code and Guide, which have been adopted by many publishers.

 

Table 1¾ Hierarchy of well-known stratigraphic units listed in decreasing order of rank.  Fundamental units, which are italicized, are the original, necessary rank within each category.  Other ranks are optional.

Geochronologic

Chronostratigraphic

Biostratigraphic

 

Lithostratigraphic

Eon

Eonothem

 

Supergroup

   Era   

   Erathern

 

   Group

      Period

      System

 

      Subgroup*

         Epoch

         Series

 

         Formation

            Age

            Stage

 

            Member; Lens; Tongue

 

 

Superbiozone*

               Bed; Flow; Laminae*

               Chron**

               Chronozone

   Biozone

 

 

 

      Sub-biozone

  * Recognized only in the Guide.

** Considered as non-hierarchical in the Code.

 

Lesser-Known Units

 

During the past few years, other types of stratigraphic units have been proposed, many of them in the 1983 Code (Table 2). Many of these categories are used almost exclusively by the special interest group that proposed them, but some have greater potential usage. Most of these lesser-known units are summarized here; however, the reader seriously interested in using these units should consult the appropriate literature.

 

Magnetostratigraphic Units.¾ These units are based on remnant magnetic polarity in rocks. In the Code, they consist of three types of stratigraphic units comparable to the "trinity" of well-known units: 1) magnetopolarity units, similar to lithostratigraphic units; 2) polarity-chronologic units, similar to geochronologic units; 3) polarity-chronostratigraphic units, similar to chronostratigraphic units (Table 2). The Guide formally recognizes only the first category; the other two are considered methods of recognizing geochronologic and chronostratigraphic units, respectively.

Lithic Units.¾ In addition to the familiar lithostratigraphic unit, which is restricted to generally stratified, tabular rock bodies conforming to the Law of Superposition, two other closely related lithic units are recognized (Table 2). One is the lithodemic unit, a generally unstratified body of intrusive, metamorphic, or highly deformed rock not generally conforming to the Law of ___________________________________________

          3 A quantitative geochronologic unit expressed in years is called a geochronometric unit.

 


Table 2.¾ Hierarchy of lesser-known stratigraphic units listed in decreasing order of rank. Fundamental units, which are italicized, are the original, necessary rank within each category. Other ranks are optional. Lithostratigraphic units, although well known, are repeated here to show corresponding rank with other lithic units

I.   Magnetostratigraphic units:

A.  Polarity-Chronologic*

B.  Polarity-Chronostratigraphic*

C.  Magnetopolarity

Polarity Superchron

      Polarity Superchronozone

      Polarity Superzone

  Polarity Chron  

        Polarity Chronozone

        Polarity Zone

    Polarity Subchron

          Polarity Subchronozone

          Polarity Subzone

II.   Lithic units:

A.  Lithostratigraphic

B. Lithodemic

C. Pedostratigraphic

Supergroup

     Supersuite

 

  Group

       Suite or Complex

 

    Subgroup**

 

 

      Formation     

         Lithodeme

     Geosol

        Member; Lens; Tongue

 

 

          Bed; Flow; Laminae**

 

 

III. Unconformity-related units:

A.  Unconformity-bounded (these units are being formally proposed)

      Supersynthem

        Synthem

          Subsynthem

B.  Seismic stratigraphic (these units are informal)

      Supersequence

        Sequence

          Subsequence

C.  Allostratigraphic (these units are formal)

      Allogroup

        Alloformation

          Allomember

IV. Diachronic units:

Episode

 

Diachron

  Phase

    Span

      Cline

* Recognized only in the Code.

** Recognized only in the Guide.

 

Superposition. Geologists working with these types of rocks are beginning to use lithodemic nomenclature (e.g., Taylor et al. 1984) although a hard core of stratigraphers valiantly opposes usage of stratigraphic nomenclature for unstratified rocks. Many formally named, unstratified rock bodies predate lithodemic nomenclature (e.g., Manhattan Schist).

The pedostratigraphic unit, a buried layer of soil with developed soil horizons, is the other type of lithic unit. Pedostratigraphic units of the 1983 Code are not the same as the now abandoned soil-stratigraphic units of the 1970 Code. Also, pedostratigraphic terms are not used for modern soils. Most pedostratigraphic units are upper Cenozoic, but older ones may be recognized (e.g., the Molas Formation, Pennsylvanian of southwestern Colorado, could be considered as a pedostratigraphic unit). Stratigraphic classification of ancient soils is a complex subject; the interested reader is referred to the 1983 Code (p. 850 and 864-865) and references cited there.

Unconformity-Bounded Units.¾ Perhaps the most debated category of stratigraphic unit recently is the unconformity-bounded unit, which has been given several different names (Table 2). Interestingly, many of our stratigraphic subdivisions that were defined early, including many of the systems at their stratotypes, are bounded by unconformities, although they are not formally classified on that basis.

The earliest proposal to recognize unconformity-bounded units formally was that of Chang (1975). The I.S.S.C., which has been discussing unconformity-bounded units since 1973, recently prepared a note (Salvador 1986) which is in press (G.S.A. Bulletin) and should be formally incorporated in the new Guide. The I.S.S.C. note thoroughly reviews the history of unconformity-related units, defines them, and proposes the basic unit called a synthem following Chang (1975). The synthem is defined as a purely unconformity-bounded unit that is independent of but may also contain other types of stratigraphic units such as lithostratigraphic, lithodemic, chronostratigraphic, biostratigraphic, and magnetostratigraphic.

Early workers in seismic stratigraphy (Vail et al. 1977) borrowed the term sequence of Sloss et al. (1949) and used it as a depositional sequence recognized on a seismic section by its bounding unconformities (or correlative conformities, in places). This unit has also been called a seismic sequence. Sloss (1963) viewed the sequence as a major lithostratigraphic unit bounded by unconformities, but Vail et al. (1977) emphasized its chronostratigraphic significance. Sequences are still treated informally.

The allostratigraphic unit, a discontinuity-bounded lithic unit of sedimentary rock, was first recognized formally in the 1983 Code. Although this unit was proposed and has been used primarily by geologists working with upper Cenozoic deposits (e.g., Morrison 1985), it was realized that allostratigraphic units could be of any age and that they could be recognized on seismic sections. Allostratigraphic terminology closely parallels lithostratigraphic terminology (Table 2).

A variety of unconformity-bounded units is now available to choose from. Authors should take advantage of this wide choice of potentially useful stratigraphic units and should not be confused by the diversity of concepts and usages. The debate over which type (or types) of unconformity-bounded unit that gains general acceptance during the future should be interesting to follow.

Diachronic Units.¾ The 1983 Code formalizes, for the first time, the diachronic unit, a temporal unit that consists of the unequal time spans represented by types of material units (Table 2). Although it has long been recognized that many lithostratigraphic and some biostratigraphic units are time-transgressive (diachronous) from place to place, no formal terminology has been available to indicate this diachroneity. Some would question the need for a formal diachronic unit. Apparently, geologists working with the Quaternary have found diachronic units, as well as allostratigraphic units, useful. It appears too early to tell if diachronic terminology will spread into pre-Quaternary units, but someday we might have the Tapeats Diachron, for example, as the diachronic unit corresponding to the Tapeats Sandstone, the classic time-transgressive formation of the Grand Canyon region. Time will tell!

Other Units.¾ New categories of stratigraphic units are formalized from time to time, and others are abandoned when they seem to serve no practical purpose after a trial period. For example, the geologic-climate units of the 1970 Code are not mentioned in the 1983 Code. However, formal recognition of hydrostratigraphic units (e.g., aquifers) is currently being considered by N.A.C.S.N. Geologists wishing to petition N.A.C.S.N. should follow the procedures outlined in its bylaws (Owen et al. 1985).

 

CAPITALIZATION: FORMAL VERSUS INFORMAL NAMES

 

General Rule

 

Many writers seem confused about the conventions for capitalization of stratigraphic names, but speakers never encounter this problem except in preparing slides. The rule is simple: All words in every formally named stratigraphic unit begin with capital letters except for the specific name in a biostratigraphic unit. This rule has been in effect since the 1961 Code. All of the ranks of stratigraphic units listed in Tables 1 and 2 are formal, except for the seismic stratigraphic units and the unconformity-bounded units (which are being proposed for formalization by I.S.S.C.). For example, the Whitewater Arroyo Shale Tongue of the Mancos Shale, in northwestern New Mexico, is included in part of the Cenomanian Stage, which was deposited during the early part of the Late Cretaceous Epoch. However, in some instances, stratigraphic units are used informally. For example, a widely recognized subsurface lithostratigraphic unit in the Arkoma Basin is known as the Spiro sandstone (note the initial lowercase letter in sandstone), an informal basal Atokan unit not to be confused with the Spiro Sandstone (note the initial uppercase letter in Sandstone), a formal Desmoinesian unit recognized on the surface in the same area. Such duplication of the geographic part of stratigraphic names should be avoided for obvious reasons, but it does occur all too frequently.

 

Geochronologic and Chronostratigraphic Units

 

The most troublesome instance of capitalization problems of stratigraphic names usually involves geochronologic/chonostratigraphic units, especially in subdivisions of Cenozoic Epochs/Series and throughout the Precambrian. Many Phanerozoic Period/System and Epoch/Se­ries names and some boundaries have been formally proposed and agreed upon by appropriate international organizations (Table 3), with exceptions, especially regarding Cambrian subdivisions. Therefore, authors may write Middle Ordovician, Late Cretaceous, and so on, with some confidence. Also, some Epochs/Series and nearly all Ages/Stages that have formal given names are capitalized (e.g., Cincinnatian, Llandovery, and Eocene Epochs/Series and Maysvillian, Frasnian, and Danian Ages/Stages). However, be aware that vague terms like late Paleozoic and middle Cretaceous are informal. Also, the grouping of Ages/Stages within the Cenozoic, such as lower Eocene and late Miocene, are all informal, because they have never been formally defined and these groupings are awkward because they occur at a rank intermediate between Epochs/Series and Ages/Stages. Such intermediate ranks are generally not used in subdividing Mesozoic and Paleozoic Epoch/Series¾they are divided directly into named Ages/Stages (e.g., the Upper Devonian Series is divided directly into the Frasnian and Fammenian Stages, in contrast to the Miocene Series, which is informally divided into lower, middle, and upper parts, and each part contains two formally named Stages). Modifiers of Ages/Stages (early Maysvillian, upper Aptian, etc.) are generally informal.

In Precambrian nomenclature almost every geochronologic and chronostratigraphic subdivision should probably be regarded as informal, except for Archean and Proterozoic and their subdivisions into Early, Middle, and Late, which have been formalized as geochronometric units (Harrison and Peterman 1980, 1982). However, there is even some disagreement whether Archean and Proterozoic should be ranked as Eons/Eonothems or Eras/Erathems! Also, in the U.S., the geochrononietric units, Precambrian U through Z, used principally as map symbols by the U.S.G.S., are generally regarded as informal, even though upper-case letters are used. Roman numerals I through VI are used in much the same way in the U.S.S.R. Table 3 summarizes capitalization of geochronologic and chronostratigraphic units. Note that although most stratigraphic units, formal and informal, are conventionally divided into three subunits (e.g., Lower, Middle, and Upper Jurassic), others are conventionally divided into only two subunits (e.g., Lower and Upper Cretaceous).

In some situations, an author may wish to use terms that are generally regarded as formal in an informal way, usually because definitive data are lacking. For example, one might want to place a rock approximately in the upper part of the Cretaceous, without really knowing whether some part of it might be slightly older than the formally defined Upper Cretaceous Series. An author should clearly state that this inexact usage is intended, and thereafter the term upper Cretaceous (note the initial lower-case letter in upper) may be used. However, such usage is inappropriate for oral presentations, so that a nonambiguous statement such as "the upper part of the Cretaceous" is appropriate.

 


Table 3.¾ Summary of formal and informal usage of geochronologic and chronostratigraphic nomenclature.* Geochronologic modifiers in Roman type; chronostratigraphic modifiers in italics

Formal

Informal

Cenozoic

early, lower; middle, middle; late, upper

  Quaternary

early, lower; late, upper

    Holocene (Recent)

early, lower; late, upper

    Pleistocene

early, lower; late, upper

  Tertiary (Neogene + Paleogene)

early, lower; late, upper

    Pliocene

early, lower; late, upper

    Miocene

early, lower; middle, middle; late, upper

    Oligocene

early, lower; late, upper

    Eocene

early, lower; middle, middle; late, upper

    Paleocene

early, lower; late, upper

Mesozoic

early, lower; middle, middle; late, upper

  Cretaceous

middle, middle

    Late, Upper

 

    Early, Lower

 

  Jurassic

 

    Late, Upper

 

    Middle, Middle

 

    Early, Lower

 

  Triassic

 

    Late, Upper

 

    Middle, Middle

 

    Early, Lower

 

Paleozoic

early, lower; middle, middle; late, upper

  Permian

middle, middle

    Late, Upper

 

    Early, Lower

 

  Carboniferous

middle, middle

    Pennsylvanian or Late, Upper Carboniferous

 

    Mississippian or Early, Lower Carboniferous

 

  Devonian

 

    Late, Upper

 

    Middle, Middle

 

    Early, Lower

 

  Silurian

middle, middle**

    Late, Upper

 

    Early, Lower

 

Ordovician

 

   Late, Upper

 

   Middle, Middle

 

   Early, Lower

 

Cambrian

 

   Late, Upper

 

   Middle, Middle

 

   Early, Lower

 

Precambrian

early, lower; middle, middle; late, upper

  Proterozoic

 

    Late

upper; Precambrian Z and VI

    Middle

middle; Precambrian Y and V

    Early

lower; Precambrian X and IV

  Archean

 

    Late

upper; Precambrian W and III

    Middle

middle; Precambrian V and II

    Early

lower; Precambrian U and I***

* For  tables of generally accepted, formal Age/Stage names see one or more of the following: Salvador (1985); Palmer (1983) Harland et al. (1982); Odin (1982a, b; 1984). These references include isotopic ages of Age/Stage boundaries; however, they do not agree on all Age/Stage names or numerical ages of boundaries.

** The four references cited above formally recognize a two-fold subdivision of the Silurian; the U.S.G.S. formally recognizes three (Luttrell et al. 1986).

*** Roman numerals are widely used for Precambrian subdivisions in the U.S.S.R.

 

TIME WARPS AND PITFALLS-¾THE ENIGMA OF

TIME AND PLACE WORDS

 

Distinction between Time and Place Words

 

For some strange reason, a geologist who never refers to "upper Tuesday" or the "late peninsula of Michigan" in everyday life will readily take pen or microphone in hand and inform other geologists about the thickness of the Late Jurassic or the events that occurred during the Upper Cretaceous! Examples like these could fill the rest of this journal with enough left over to fill several editors' circular files. I shall spare the reader of all except my favorite example. Did you hear about the writer who managed to publish a whole paper about the events that occurred between the Late Jurassic and the Early Cretaceous (reference not cited)? Authors who are careless in usage of time and place words run the risk of implying that their carelessness may extend to data collection, analyses, and conclusions, as well.

Authors of most papers concerning ancient rocks are involved in reconstructing geologic history. Thus, events that occurred during a specific time interval in a certain paleogeographic location are being interpreted from evidence contained in the presently existing stratigraphic record. Therefore, an author should be extremely careful to differentiate clearly between the events (time-bounded) and the locations (space-bounded). In formal and even informal stratigraphic nomenclature, differentiation between time words and place words is fairly straightforward. Early and late clearly refer to time, and lower and upper clearly refer to location in space. Some confusion may occur with middle, because, unfortunately, it is used in both sets of nomenclature ("medial" has been suggested as the equivalent time term, but it is rarely used).

 

Exceptions

 

A few exceptions to the above rules on time and place words in stratigraphic nomenclature exist.

Lithodemic Units.¾ Rock bodies that do not generally conform to the Law of Superposition (lithodemic units) are referred to by geochronologic terms (early, middle, and late) rather than by chronostratigraphic terms (lower, middle, and upper), because lithodemic units generally depend on isotopic data for their ages (if known) rather than superposition. This convention generally applies to intrusive, highly metamorphosed, or highly deformed rock bodies. Tabular volcanic rock bodies do follow the Law of Superposition and may be treated much like tabular bodies of sedimentary rock.

The Precambrian.¾ Precambrian geochronometric units are rarely translated into equivalent chronostratigraphic units because they depend on isotopic ages for their definition. However, conventional chronostratigraphic units based on stratotypes can be recognized in the Precambrian, even though they may not be dated by biochronology. Stratigraphers of the Precambrian often debate the geochronometric method, based on isotopic ages, versus the chronostratigraphic method, based on stratotypes, of subdividing the Precambrian (e.g., Hedberg 1974). Conventional lithostratigraphic units such as the Belt Supergroup are defined in the Precambrian in exactly the same way as in the Phanerozoic. Historically, the term Series (now used only as a chronostratigraphic unit) was used as a high-rank lithostratigraphic unit in the Precambrian, but a valid lithostratigraphic term, such as Supergroup is necessary today.

Terraces.¾ A third exception is at the other extreme of the geologic time scale. Late Cenozoic terraces of all kinds are conventionally referred to by time terms (early, middle, and late) because the usual case is for older terraces to lie topographically above younger terraces. For example, it may be confusing to refer to an early Pleistocene terrace as lower Pleistocene because the early Pleistocene terrace is topographically higher than a late Pleistocene terrace.

 


Other Time and Place Words

 

After an author has mastered the correct usage of stratigraphic nomenclature, the correct usage of other time and place words may be even more difficult. Many of these other words may be used rather vaguely in both a time and a place context in everyday English, but in dealing with the complexities of reconstructing geologic history from the stratigraphic record, these words are best used in only one context to avoid confusion. Some common misusages follow.

Use of a term such as pre-Dakota unconformity implies that the Dakota Sandstone is a geochronologic unit. Use of pre-Cretaceous unconformity implies that unconformities are synchronous surfaces. One may use the term sub-Dakota (or sub-Cretaceous) unconformity, because it is a surface at the base of the Dakota Sandstone and the base of the Cretaceous strata (or preserved part of the Creta­ceous System). Do not be afraid to use the prefix super- (e.g., super-Jurassic unconformity); it is the opposite of sub-. Time words should be used with hiatuses. In the previous example, where basal Upper Cretaceous strata lie unconformably on uppermost Upper Jurassic strata, one could correctly refer to the Early Cretaceous hiatus, if referring to events that occurred during a time unrepresented in the local stratigraphic record. Also, a lithostratigraphic name should not be used in a time sense (e.g., "Dakota time") unless you want to imply that the boundaries of the lithostratigraphic unit are time-parallel. Few are!

Pairs of commonly used time and place words are included in Table 4. Careful attention paid to this rather short, incomplete list can improve clarity of writing and speaking and may help convince your audience that you know what you are talking or writing about.

 

Table 4.¾ Informal time and place words

Time

Place

late

upper

middle (medial)

middle

early

lower

young(er)

high(er)

old(er)

low(er)

post-

super-

pre-

sub-

after

above

before

below

when

where

while

whereas

sometime(s)

someplace(s)

often, frequent

abundant, common

occasionally

locally

during

in

 

LITHOSTRATIGRAPHIC NAMES

 

Introduction

 

The operational units for most geologists are the lithostratigraphic units, mainly groups, formations, members, and informal units. Lithostratigraphic units outnumber all the named geochronologic/chronostratigraphic units by several hundred times. In the U.S., approximately 40,000 lithostratigraphic units have been named, but no more than approximately 200 geochronologic/chronostratigraphic names are ever used. Little significant change in the methods of usage of lithostratigraphic nomenclature has occurred during recent years. Therefore, geologists should be proficient in using lithostratigraphic nomenclature. For those who may not be, perhaps the following paragraphs will be helpful.

 


Information Sources

 

Because of the sheer number of lithostratigraphic names, authors are occasionally bewildered about where to look for information on lithostratigraphic nomenclature, especially in geographic areas or parts of the stratigraphic column with which they are unfamiliar. A recently published large-to intermediate-scale geologic map or report, on the general geology of the area of interest, is usually a good place to start. Alternatively, for the U.S., the recently published set of 20 Correlation of Stratigraphic Units of North America (COSUNA) charts (A.A.P.G, 1983-1986) provides 570 stratigraphic columns throughout the U.S., excluding Hawaii (Childs 1985). For Canada, a similar, but older, set of four correlation charts was published by the Geological Survey of Canada (Douglas 1967, part C). These correlation charts for the U.S. and Canada essentially replace the series of 16 correlation charts published by the G.S.A. between 1942 and 1960, which may be useful for historical purposes. A list of stratigraphic names, useful because it is compiled by geographic area, is in Wilson et al. (1959). This list is noteworthy, although outdated, because it includes lithostratigraphic units in Mexico, Central America, Greenland, and some islands, as well as the U.S. and Canada. Swanson et al. (1981) provide a more up-to-date list, with references, but it is arranged by lithostratigraphic name and covers only the U.S.

If you already know the lithostratigraphic names in an area, but need historical data and examples of usage, the Lexicons of Geologic [Stratigraphic] Names of the U.S. (Wilmarth 1957; Keroher et al. 1966; Keroher 1970; Luttrell et al. 1981, 1986) are a good source. The Lexicons, except for the first two, cover only new names introduced since the previous edition, with very few exceptions. The 1986 Lexicon covers new lithostratigraphic names proposed from 1976 through 1980. For later data, the U.S.G.S. publishes periodic updates on changes in its usage of stratigraphic nomenclature in the Bulletin series (U.S.G.S. 1982, 1984, 1985). Some state geological surveys also publish catalogs of stratigraphic nomenclature. For Canada, the Canadian Society of Petroleum Geologists is publishing a Lexicon Series by geographic area. Three of the planned six volumes have been published (Christie and Embry 1981; Hills et al. 1981; Williams et al. 1986). There is no official organization that approves stratigraphic nomenclature for general use in the U.S., Canada, or Mexico.

Outside of North America, similar reference books on lithostratigraphic names exist for some countries and areas. A place to start is the Lexique Stratigraphique International, published by the International Union of Geological Sciences (I.U.G.S., 1956-1978), which may be outdated for many areas. Some national geological surveys and other organizations maintain files and attempt to regulate lithostratigraphic nomenclature. I am neither an expert on world stratigraphic nomenclature, nor a linguist, so I leave authors to their own resources on the international scene.

 

Changing Nomenclature

 

References.¾An author sometimes finds it necessary to change previously existing lithostratigraphic nomenclature. Specific rules for making changes should be followed to reduce confusion. The Code (Articles 3 to 30) and Guide (p. 15-44) contain these rules and should be studied before proposing changes in nomenclature. Even when no change is proposed for an existing lithostratigraphic unit, it is good procedure to cite the original reference naming the unit (see the Lexicons or equivalent) or a recent paper in which the usage you are following is well described. This advice is especially apt for controversial or poorly known units.

New Names.¾ If you must propose a new name, a detailed procedure must be followed. Casual mention of "the sand exposed at Cut and Shoot, Texas" does not formally name the "Cut and Shoot Sand."

The first problem encountered is usually the choice of a geographic name. You will probably find that many of your favorite choices have already been used and, therefore, are unavailable. Studying published maps of the area of interest should suggest potential names, and studying Lexicons and other literature should give you an idea of whether your potential names are already in use. Some writers have had to go to the extreme of formally naming geographic features in certain areas in order to have available stratigraphic names.

In North America, the Geologic Names Committee of the U.S.G.S. in Reston, Virginia, the Committee on Stratigraphic Nomenclature of the Geological Survey of Canada in Ottawa, and the Instituto de Geología of the Universidad Nacional Autónoma de Mexico keep records and respond to inquiries on stratigraphic names. Some state, provincial, and foreign surveys or committees also perform similar functions. Authors are responsible for researching previously used and available stratigraphic names and must not expect editors to do this for them.

After finding an appropriate, available name, the author must publish the following information on the new unit in a recognized scientific medium: 1) intent to name; 2) category and rank; 3) name and type locality; 4) stratotype; 5) lithologic description; 6) definition of boundaries; 7) historical background; 8) dimensions, shape, and geographic distribution; 9) age; 10) correlation; 11) genesis. The first eight categories are to describe the unit; the last three are useful inferences. These requirements are taken from the Code¾the Guide has similar requirements. Measured sections, maps, and other illustrations generally accompany the description of a new unit. Any attempt to name a new unit in a talk, abstract, open-file report, unpublished thesis, or inadequate publication is invalid according to the Code (Article 4).

Revising Names.¾ Believe it or not, revising (or abandoning) an existing lithostratigraphic unit requires attention to the same eleven topics listed in the preceding paragraph. Perhaps the intent is to discourage changes.

Most revisions of existing units involve changing boundaries. The Code (N.A.C.S.N., 1983, fig. 2) illustrates several different types of boundaries, but it is important to remember that boundaries of lithostratigraphic units are based on lithic change (even if gradational) and can be based on nothing else. Age boundaries, interpretations of depositional environments, and biological sequences are not factors in defining lithostratigraphic unit boundaries. Great care should be taken in establishing and revising boundaries of lithostratigraphic units¾boundaries should be chosen that are not just locally convenient, but recognizable over the entire geographic extent of the unit. Unfortunately, stratigraphers sometimes argue more about the boundaries than about what is between them! Authors are urged to be conservative in proposing boundary changes. If several different authors propose several different boundaries for the same unit, we are probably better off to start over with new names (see the following section).

Before going to the trouble of naming, revising, or abandoning a unit, authors should consider using informal nomenclature, especially if an innovative or controversial idea is involved. Unfortunately, debates about formal nomenclature tend to focus on name-calling rather than ideas.

Abandoning Units.¾ Geologists rarely go to the trouble of abandoning units, except in the process of revising and naming new ones. Thus, many units fade away from disuse. However, formal abandonment of lithostratigraphic units may be necessary in order to clean up nomenclature. To abandon a unit formally, the eleven categories listed above must be addressed.

Justifications for abandoning a formal unit include 1) synonymy or homonymy; 2) improper definition (e.g., defining a lithostratigraphic unit by chronostratigraphic criteria); 3) long-term disuse or obsolescence; 4) flagrant misuse; 5) impracticability. If a unit is formally abandoned, recommendations should be made for nomenclature to be used in its place.

Authors should be careful not to abandon units when they only intend to restrict the unit from their study area¾the unit may be valid and perfectly useful elsewhere. Also, just because two geographically separated units are found to be equivalent, it is not necessary to abandon one. There may be good reasons for retaining both names.

An abandoned unit must be completely abandoned, including its stratotype Also, abandoning a stratotype abandons the name of the unit as well. Reinstatement or reuse of an abandoned name for another stratigraphic unit is rare, but possible.

Changing Rank.¾ Changing the rank of a lithostratigraphic unit is a relatively simple process and does not require attention to the eleven categories listed above because neither the boundaries nor the geographic name of the unit changes. For example, a formation may become a member, and vice versa; a formation may become a group, and vice versa. Similar rank changes may also be accomplished easily in lithodemic, magnetopolarity, and unconformity-bounded units. Rank changes in geochronologic/chronostratigraphic units, particularly those of major rank, are much more troublesome and usually require submission to international committees, such as the I.U.G.S. International Commission on Stratigraphy and its subcommisions and working groups. For example, the June 1985 issue of Episodes, published by I.U.G.S., contains several articles on boundary problems and subdivision of chronostratigraphic units.

The main criterion controlling rank changes in material units is practicality. For example, if a group in one geographic area changes laterally so that it has no separately mappable formations as it passes into an adjacent area, then reduce its rank to formation in the latter area. An inconspicuous member of a formation in one area may become so prominent in a second area that it is raised to formational rank in the second area, but it retains the same geographic name. An established formation may be subdivided into new formations, and the established formation may be raised to group rank. Also, the same member may occur in two or more formations.

Some pitfalls to avoid in rank changing are 1) changing the boundaries of an existing unit; 2) using the same name for a rank and for one of its components (e.g., the Dakota Formation in the Dakota Group).

 

STRATIGRAPHIC ILLUSTRATIONS

 

In papers that treat several lithostratigraphic units or any complexity of chronostratigraphy, a stratigraphic illustration such as a columnar section, stratigraphic table or figure, correlation chart, or cross-section is generally necessary. In preparing these, note carefully that chronostratigraphic units are preferentially used in illustrations with lithostratigraphic units, especially where the vertical scale represents thickness, either linear or relative (Fig. 1). A column entitled "Age" should not be used with a vertical scale of thickness (a year is not 365 meters long). Unconformities and ordinary lithostratigraphic contacts are shown as lines separating adjacent, abutting lithostratigraphic units in such illustrations (Fig. 1).

In stratigraphic illustrations with a vertical scale representing geologic time, either millions of years before present (Ma) or relative time (e.g., Late Jurassic), geochronometric or geochronologic units are used instead of chronostratigraphic units (Fig. 2). In these illustrations, the ages, not the thicknesses of lithostratigraphic units, are shown. A hiatus ("time gap"), represented by an unconformity in the rocks, is illustrated as an actual gap in the illustration (Fig. 2), in contrast to a wavy line used for an unconformity (Fig. 1).

The simple suggestions in the above two paragraphs, if followed, should convince your audience that you know, graphically at least, that strata are not measured in years, nor time in meters.

 

     Fig. 1.¾ Names and thicknesses of stratigraphic                 Fig. 2.¾ Names and ages of stratigraphic units in the

    units in the Ficticia area, New Texico.                                 Ficticia area, New Texico.

 

NUMERICAL AGE DATA

 

In papers that deal with isotopic age or other quantifiable age data, a few minor, but troublesome, terminology problems regarding numerical ages can occur. First, according to the Code, use of the nouns "isotopic age" or "numerical age" instead of the nouns "date" or "absolute age" is recommended. Second, the term calibration should be used for the special form of designating chronostratigraphic boundaries in terms of numerical ages.

A recent change in the practice of abbreviating numerical ages should also be noted. The Code recommends the SI (International Standard) multipliers for numerical ages in years before the present (ka = l03; Ma = 106; Ga = 109). (Incidentally, the duration of an annum is a modern year and the present refers to 1950 A.D.!) Qualifiers such as "ago" or "before the present" are omitted after the above formal abbreviations, because duration from the present to the past is implied by their use. However, authors should remember that these formal abbreviations are not used for the duration of an interval of geologic time that does not extend to the present; in such cases, the informal abbreviations y., m.y., and b.y. are used. For example, the boundaries of the Late Cretaceous Epoch are calibrated at 95 Ma and 65 Ma (Odin 1982a,b; 1984), but the duration of the Late Cretaceous Epoch is 30 m.y.

 

CONCLUSION

 

With the appearance of the 1983 Code and the current revision of the Guide, formal stratigraphic nomenclature is undergoing significant expansion into nonstratified rocks. Authors should be aware of the distinction between material and nonmaterial stratigraphic units, and authors may need to consider some of the new, lesser-known stratigraphic units as well as the old, well-known ones.

Authors must strive for clarity, consistency, and correct usage of both formal and informal terminology because of the complex interrelations between time and space interpreted from the presently existing stratigraphic record. All of our geologic data ultimately comes from the geologic record; therefore, treat the source carefully.

 

ACKNOWLEDGMENTS

 

I am most grateful to Joshua I. Tracey, Jr., recently retired chairman of the U.S.G.S. Geologic Names Committee, for his detailed suggestions on topics for this paper and for his review of the manuscript, and to Amos Salvador, chairman of I.S.S.C., for his insight into I.S.S.C. matters and for his detailed review of the manuscript. Thanks are also due to the following reviewers, each of whom made significant improvements to the manuscript: Ashton F. Embry, chairman of N.A.C.S.N., Susan A. Longacre, vice-chairman of N.A.C.S.N., Robert R. Jordan, N.A.C.S.N. commissioner, Don C. Steinker of Bowling Green State University, Carolin Middleton of A.A.P.G., and Douglas D. Bergersen, a Lamar University student. Mention of the reviewers here does not necessarily imply endorsement by them or their organizations. Special thanks are also given to my most persistent critic, fellow editor Diane K. Sparks.

This work is dedicated to the memory of Steven S. Oriel, chairman of the N.A.C.S.N. Code Committee, who did not have an opportunity to review the manuscript.

 

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___________________________________________

Reproduced with permission of SEPM (Society for Sedimentary Geology) from:

Journal of Sedimentary Petrology, Vol.57, No.2, March, 1987, p. 363-372

Copyright © 1987, The Society of Economic Paleontologists and Mineralogists 0022-4472/87/0057-363/$03.00