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 fo