A formal model for the geologic time scale and global stratotype section and point, compatible with geospatial information transfer standards Simon J.D. Cox* CSIRO Exploration and Mining, P.O. Box 1130, Bentley, 6102 WA, Australia Stephen M. Richard* Arizona Geological Survey, 416 W. Congress St., #100, Tucson, Arizona 85701, USA ABSTRACT A formal notation is used so as to provide a Structure of the Paper rigorous description of the various elements The geologic time scale is a complex data required to describe the structure and cali- This paper is structured as follows. An in- structure composed of abstract elements bration of the time scale in a manner that troduction and summary of key aspects of that represent time intervals and instants allows the logical consistency of the model stratigraphic methodology is provided in the and their relationships with speci®c con- to be evaluated. This is important, since al- ®rst section. Next, we brie¯y introduce infor- crete representations in the geologic record though stratigraphic methodology is one of mation standardization activities that provide as well as the observations made of those the most rigorously studied aspects of geo- the modeling framework and notation used in concrete representations. The International logical practice, it has evolved throughout this study. We then describe a general frame- Union of Geological Sciences' International the era of historical geology. There have been work for temporal reference systems and de- Commission on Stratigraphy guidelines signi®cant changes in best practice, in partic- velop a formal model for the geologic time recommends a very precise usage of the re- scale and its calibration within that frame- lationships between these components in ular in the shift from characterizing units to de®ning the boundaries between them. Nev- work. This conceptual model is used as the order to establish a standard time scale for basis for an XML implementation of the mod- use in global correlations. However, this ertheless, the time scale itself remains based on named units and eras. Other residues of el, presented using example documents de- has been primarily described in text. Here, scribing the International Union of Geological earlier practice remain visible in the descrip- we present a formal representation of the Sciences (IUGS)'s International Commission tion of the time scale, particularly where model using the Uni®ed Modeling Lan- on Stratigraphy (ICS) time scale with global agreement on the application of current prac- guage (UML). The model builds on existing stratotype section and point (GSSP) referenc- tices is incomplete. In this context, a rigorous components from standardization of geo- es. Some theoretical issues arising from the characterization of the relationships between spatial information systems. The use of a models are discussed. A summary of UML formal notation enforces precise de®nition the elements of the time scale, the evidence notation is given in Appendix 1. of the relationships between the compo- in the geologic record, and the application of nents. The UML platform also supports a speci®c procedures to effect numeric calibra- THE GEOLOGIC TIME SCALE direct mapping to an eXtensible Markup tion of the scale is essential. Language (XML)±based ®le format, which A secondary goal is to develop a machine- Units, Boundaries, and Stratotypes may be used for the exchange of strati- processable format for the exchange of in- graphic information using Web-service formation related to the time scale. The mod- The conventional geologic time scale is a interfaces. eling notation selected (Uni®ed Modeling reference system de®ned by a contiguous se- Language [UML]) is a commonly used soft- Keywords: time scale, stratigraphy, infor- quence of time intervals, each identi®ed with ware engineering standard, so the model can mation model, XML, UML. a name. These are recursively subdivided, re- be implemented easily on various standard sulting in a hierarchy composed of intervals INTRODUCTION platforms. In particular, an eXtensible Mark- of various ranks. The units in the scale are up Language (XML) document schema can ordered, so the relative temporal positions of Goals be derived directly from the model. The geologic objects and events may be recorded XML implementation supports lossless data or asserted, denoted by the names of units The goal of this paper is to provide an in- transfer using standard Web protocols, and it from the scale. formation model for the geologic time scale. provides a basis for formal encoding and As originally conceived, units of the geo- *E-mails: [email protected]; steve.richard@ computer processing of information that is logic time scale identify intervals, each cor- azgs.az.gov. the basis for de®ning geologic time scales. responding to the time during which a partic- Geosphere; December 2005; v. 1; no. 3; p. 119±137; doi: 10.1130/GES00022.1; 6 ®gures; 1 table. For permission to copy, contact [email protected] q 2005 Geological Society of America 119 Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/1/3/119/3332362/i1553-040X-1-3-119.pdf by guest on 26 September 2021 S.J.D. COX and S.M. RICHARD ular sequence of rocks (stratigraphic unit) was plete time scale. However, a complete com- called global standard stratigraphic ages deposited. Historically, the units were chosen pilation is possible only based on sections that (GSSA). because, within the region where they were explicitly include boundaries. Implicitly, the Ages recorded using named units from the de®ned, they could be recognized through uni- shift from unit stratotypes to boundary stra- geologic time scale allow ordering without re- form lithological and biostratigraphic charac- totypes recognizes that, although the time quiring numeric values. However, while the teristics, which correlated with a relatively scale is based on time intervals covering the assignment of numeric values is not necessary consistent geological environment in a single domain, its logical consistency is contingent for use of the time scale, it is convenient to period. The representative object or prototype on a globally recognizable, unambiguously or- calibrate the time scale against a time line. For in a stratigraphy de®ned in this way is a type dered sequence of events. those boundaries de®ned by a stratotype, it section for the geologic unit of interest, for- provides a locality from which specimens may mally called a unit stratotype. This approach Governance and Calibration be collected, the age of which may then be supports stratigraphic practice in which as- measured using quantitative techniques. When signment of strata to a unit is based on the the material in the stratotype is unsuitable or The guidelines of the ICS formalize this presence of characteristics that match the stra- insuf®cient for estimating the numerical age, practice (Remane et al., 1996). These are used ta to the type section for the unit. The basis then specimens from locations that can be cor- for the matching, or correlation, may be lith- by the GSSP project. The GSSP provides a related with the stratotype may be used in- ologic, paleontologic, or geochronologic, de- forum for the speci®cation of boundary stra- stead. The experimentally determined dates ®ning lithostratigraphic, biostratigraphic, and totypes used for correlation on a global scale. from such specimens provide an estimate of chronostratigraphic units. Local stratigraphic de®nitions will still be the chronologic position (numeric coordinate Construction of a consistent time scale us- used for local stratigraphic correlations, but on a time line) of the boundary. ing the unit stratotype approach depends crit- global stratotypes provide the ultimate refer- ically on stratigraphic completeness and the ence for inter-regional correlation. Local Formalization ability to geochronologically correlate bodies stratigraphic schemes must be related to the of rock globally. However, the corollary of a GSSP stratotypes through a coherent chain of This overview of the construction and cal- model based on continuity within units is that correlations in order to be connected to the ibration of the geologic time scale refers to a the boundaries between units correspond with global time scale. Alternatively, geochrono- variety of concepts, instances of which are re- changes in the geological environment and metric methods may be used for objects with- lated to each other in various ways. Effective discontinuities in the stratigraphic record. in those parts of the time scale where chro- use of the time scale for stratigraphic corre- Such discontinuities, and the incongruity of nometric values for the boundaries have been lation requires that these concepts and their lithostratigraphic and biostratigraphic units accepted. interrelationships be precisely understood. with chronostratigraphic units, result in incon- Physical changes in the rock record ob- There is a signi®cant body of literature that sistent and ambiguous correlation in the vicin- served at the boundary horizon are inferred to describes the system, an introduction to which ity of chronostratigraphic unit boundaries. result from a geologic event or events. Ideally, is provided by Walsh et al. (2004) and refer- For these reasons, the complementary ap- these are manifested globally by similar or re- ences therein, in the ICS guidelines (Remane proach to speci®cation of the time scale is lated physical changes in sediment deposited et al., 1996), and in the overview to the In- now preferred, focusing on the boundaries at the same time. Correlation of the boundary ternational Stratigraphic Chart (International rather than the intervals. Within this model, horizons by correlation of physical changes in Commission on Stratigraphy, 2004). the focus is on a representative point within other stratigraphic sections is the basis for es- However, the model for the construction the geologic horizon corresponding to the tablishing the relative age of strata throughout and calibration of the geologic time scale has boundary of interest, formally called a bound- Earth.