A FORMAL MODEL FOR THE GEOLOGICAL TIMESCALE AND GSSP

The ICS proposes formal chronostratigraphic units based on boundaries defined by a specific point in a sequence of rock strata in a unique and specific location (Global Standard Stratotype Section and Point, GSSP). Each stratotype shows evidence of a geological event deemed to correspond with the particular boundary. The age of the boundary may then be estimated through observations made on specimens retrieved from the section. However, correlation with other locations usually involves consideration of geological events not illustrated in the GSSP. This practice requires a clear understanding of relationships between events, stratigraphic sections, ages, and the elements of the standard geological timescale. We describe an information model for the geological timescale that formalizes these relationships, incorporating the principles underlying the GSSP.

The geological timescale is a temporal ordinal reference system (TORS), in which the standard named intervals (Phanerozoic, Paleocene, etc) correspond to component “ordinal-eras” of varying rank. An era may be composed of an ordered sequence of member eras of the next lower rank, but within a reference system only one such decomposition is permitted for each era. Such a system corresponds to a "constrained temporal-topology-complex". The boundary between eras is a “time-node”. The age of the node is the position of an associated time-instant. In the geological timescale this association is indirect, via observations made on specimens retrieved from the stratotype showing evidence of a specific stratigraphic event. Different stratigraphic events may be observed in other locations, having various relationships with the events associated with the GSSP.

A model for this system, expressed using the object-modeling notation UML, is based on a generic model for TORS given in ISO 19108, extended with classes for the stratigraphic elements described above. Use of UML provides a precise definitions of the components in the model and relationships between them. Furthermore, it also supports direct implementation in software such as Java, C#, and RDBMS schemas, using standard CASE tools. An XML encoding is also provided, defined using a W3C XML Schema conformant with Geography Markup Language, and thus compatible with many modern geospatial software systems.