42 Stratigraphic nomenclature and description STRATIGRAPHIC NOMENCLATURE AND DESCRIPTION HE ROCKS OF THE UNITED STATES are stratigraphic rank, and (3) is mappable. The unit may Tclassified for mapping purposes by a complex be formal: that is, it may be defined by standards of scheme of named material and temporal or geologic the presently (1983) accepted code, or may have been time terms. Most of the time terms (for example, defined under a previously accepted (1933, 1961, Cretaceous) were defined and first used by European 1970) code, or it may have been named and found! geologists and have since been accepted by geologists useful before the writing of any code and has gained in most other parts of the world. As parts of the acceptance through common usage. United States were mapped geologically, local Formal geologic names are those that have been divisions of rocks were recognized and named in defined according to the standards of the time in individual study areas. Lithology, rather than faunal which they were introduced. Units named and found assemblages, is the basis of this local classification, useful before 1983, the publication date of the most principally to aid in recognizing and mapping the recent code, are not discarded just because they were units so as to summarize the geologic history of each not defined according to current standards. Map wits area and evaluate the geologic resources. may also be informal and not defined by any stand- The stratigraphic units discussed in this chapter are ard. Any author using informal names should clearly classified into categories and ranks. The first category distinguish them from formal names. includes "material units" that are called lithostrati- graphic, lithodemic, allostratigraphic, magnetopolar- ity, or pedostratigraphic units in the North American GEOLOGICNAMES UNITS AND THE Stratigraphic Code (NASC) by the North American Commission on Stratigraphic Nomenclature (1983, p. 852). Lithostratigraphic unit is a term much used out- Because the Geological Survey examines various side the United States (International Subcommission aspects of geology throughout the United States, all on Stratigraphic Classification). Some lithostrati- its publications adhere to broadly uniform procedures graphic units may have faunal boundaries that coin- in dealing with the nomenclature and classification of cide with lithologic ones. Uncertainties in interpreting rock masses. The responsibility for this conformity is original definitions of lithologic units, or in correlating delegated to a group of Survey geologists assigned to them with units near or far, have led to many local one of the Geologic Names Unit (GNU) staffs in geologic names and to increasing numbers of units Reston, Va., Denver, Colo., and Menlo Park, Calif., each year. The second category is the "temporal under the technical guidance of the Geologic Names units" which are chronostratigraphic, geochronologic, Committee (GNC). geochronometric, polarity-chronostratigraphic, The Geologic Names Committee was organized in polarity-chronologic, and diachronic units. 1899 to consider all names of geologic formations and The term "geologic name" has evolved in its appli- other divisions of rock classifications, to determine cation by the Survey since the late 19th century. At whether the names comply with nomenclature used in that time the Geologic Names Committee (or GNC) previously published Survey reports, and to reconn- not only considered names for lithologic units and the mend policy on stratigraphic nomenclature. Members geologic age terms applied to them but also adjudi- of the committee are appointed by the Chief Geologist cated (1) the correct use of petrologic terms and and are responsible to him through authority dele- structural terms and their symbols, and (2) the apt- gated by the Director. The members of the GNC are ness of colors, patterns, and all other symbols used on chosen for their experience and knowledge in the geologic maps and reports published by the Survey. science. In addition to the chairman, the committee The term "geologic name" as used in STA is the currently consists of geologists chosen from the three name of a defined rock body, or the local name ap- centers (Reston, Va., Denver, Colo., and Menlo Park, plied to a mapped rock unit. The map unit is recog- Calif .). nized by its lithologic content (homogeneous or The Geologic Names Committee defines and heterogeneous) and its boundaries. It (1) is assigned a recommends policy and rules governing stratigralphic place within the geologic age sequence, (2) has a nomenclature and classification in all manuscript$;and Stratigraphic nomenclature and description 43 maps originating in the U.S. Geological Survey and names of the United States as they are used in concerning the geology of the United States. The publications. These records are available for reference actions of GNC apply whether these reports and maps at all times to all geologists inside and outside the are to be published by the Survey or by an outside Survey. A geologist who plans to name a forinal organization, and whether they result wholly or partly stratigraphic unit may reserve the geographic name from the work of Geological Survey employees. with one of the GNU staffs in Denver, Menlo Park, Except for papers intended as abstracts for talks or or Reston so that other geologists who may inquire for some papers in the Open-File Report series, every about the name can be informed of the first author's manuscript that is written by a Survey author and intention. If that geographic name is already reserved, that contains stratigraphic names is read and an author then has an opportunity to select another approved by a member of a GNU staff before its suitable name. Proposals for new nomenclature and publication is authorized by the Director. Technical revisions of existing nomenclature are made in problems, such as differences of opinion among reports that are published in recognized scientific Survey authors or nonconformance to Survey policy mediums as defined by the 1983 code. Proposals will or to the North American Stratigraphic Code, are be more quickly accepted if they are discussed with referred to the committee for recommended solution. peers who work for State surveys, academia, and The basis of the committee's action is the North industry as well as with those who work for the American Stratigraphic Code (the code) and its Survey during the preparation of a report. predecessors. The North American Commission on In reviewing manuscripts, GNU staff members Stratigraphic Nomenclature currently has more than depend on a file of annotated records, on the frame- 20 members chosen from 8 geological societies and work of policy and objectives as set forth by the GNC State and Federal surveys in North America: and its chairman, and on the North American Strati- American Association of Petroleum Geologists, graphic Code. Each staff member is expected to bring Geological Society of America, Geological Survey of significant departures from these guides to the atten- Canada, Geological Association of Canada, Canadian tion of the local committee members or to the chair- Society of Petroleum Geologists, U.S. Geological man of the committee so that problems may be Survey, Association of American State Geologists, resolved expeditiously at a local level. The full com- Asociaci6n MBxicana de Ge6logos Petroleros, Sociedad mittee formulates general policy and advises on Ge6logica Mexicans, and Instituto de Geologia de la specific nomenclatural problems. Universidad NacionAl Autdnoma de MBxico. Several editions of the code have been printed since the first TO THE one was prepared in 1933; the most recent edition, CONFORMANCE CODE published in 1983, is for sale by the American Asso- AND MODIFICATIONTO ciation of Petroleum Geologists, Box 979, Tulsa, OK 74101. Survey authors can get copies from one of the STRATIGRAPHIC UNITS GNU offices. Amendments and additions to the code Four codes (Ashley and others, 1933; American are proposed from time to time. They are published Commission on Stratigraphic Nomenclature, 1.961, as Notes or Reports, usually in the American Associa- 1970; North American Commission on Stratigraphic tion of Petroleum Geologists Bulletin. Before pro- Nomenclature, 1983) have been written to provide posed changes are adopted by the commission for guidance to scientists using stratigraphic terminology inclusion in the code, comments and discussions are and to provide the profession with standards for invited from the geologic profession. naming, defining, and classifying "rock units and their The classification and nomenclature of rock units in spatial and temporal relations" (North American Com- manuscripts resulting from cooperative investigations mission on Stratigraphic Nomenclature, 1983, p. 847). with State geological surveys, or other outside organ- Each code reflects advances in scientific knowledge izations or individuals, may follow the scheme of the and technology at the time of its preparation. Some cooperating organization, with an appropriate state- recommendations in the 1983 code are similar to ment of explanation. Any classification scheme used those drawn up in 1903 and published in the :24th must have been described in a published report or the Annual Report of the Director. Parts of the 1983 description must be included in the proposed report. code serve researchers who work with discontinuity- Although the Geologic Names Units do not judge bounded sequences