Chronostratigraphy

- S.C. Finney

Although the systems, series, and stages of the International Chronostratigraphic Scale and the corresponding periods, epochs and ages of the Geologic Time Scale were basically established by the late 19th to early 20th century, their development was haphazard. And because they were generally defined on the basis of type sections or type areas, their boundaries were rarely defined with precision, resulting in greatly different interpretations of their extent between geologists, continents, and regions, and in substantial gaps or overlap between many successive units. To resolve this issue the International Commission on Stratigraphy (ICS) of the International Union of Geological Sciences (IUGS) was established and tasked with developing global standard chronostratigraphic units (systems, series, and stages) with lower boundaries defined by Global Stratotype Sections and Points (GSSPs), i.e. at precisely specified levels or points in specific stratigraphic sections. This process began in the late 1960s and is still in progress. It has required detailed study and correlation of stratigraphic successions worldwide. Specific international working groups were established to formalize system boundaries with the Silurian-Devonian boundary being the first one to be completed in the 1970s. But, two system boundaries (between the Triassic and Jurassic and the Jurassic and Cretaceous) still have not be formalized with GSSPs. In most instances, existing stages and series were retained, but their boundaries were investigated worldwide with the eventual goal being formal definition by GSSPs. For other systems, or parts thereof, existing classifications were found to be unacceptable, requiring the development of new stages and series.

From 1996 to 2004, I served as Chair of the ICS Subcommission on Ordovician Stratigraphy. During that time, I oversaw the final approval and ratification of the Cambrian-Ordovician boundary and the formal approval and ratification of almost all the new stages and series that needed to be established for the Ordovician System. I participated in developing GSSP proposals for some of the new stages and their GSSPs, but my primary contribution was leading an international teams of Ordovician stratigraphers who completed most of the work, studying candidate stratotype sections worldwide. The last GSSP to complete the Ordovician System, the base of the Middle Ordovician Series and the Dapingian Stage, was recently approved by the Ordovician Subcommission and ICS and ratified by IUGS. It was dedicated at a most elaborate dedication ceremony in July 2007 see photos). The completed international chronostratigraphic scale is shown in the accompanying figure.

Since 2000, I have also served as Vice Chair of ICS, a position that has given me considerable knowledge of and appreciation for the activities of all the subcommissions of the ICS working on all parts of the geologic column from Precambrian to Quaternary and including stratigraphic principles. Along with a group of colleagues recruited by Maria Cita, Chair of the ICS Subcommission on Stratigraphic Classification, I am contributing towards revision of the International Stratigraphic Guide, particularly the chapter on Chronostratigraphy. Considerable information on the activities of ICS can be found at its website at www.stratigraphy.com.

GPPS Dedication Ceremony pictures:

Some publications resulting from work on the Ordovician Subcomission and ICS include:

Mitchell, C.E., Chen Xu, Bergstrom, S.M., Zhang Y., Wang Z., Webby, B.D., and Finney, S.C., 1997. Definition of a global boundary stratotype for the Darriwilian Stage of the Ordovician System, Episodes, v. 20, no. 3, p. 158-166.

Finney, S.C., Bergstrom, S.M., Chen, X., and Wang, Z.-H., 1999. The Pingliang section, Gansu Province, China: Potential as global stratotype for the base of the Nemagraptus gracilis Biozone and the base of the global Upper Ordovician Series, Acta Universitatis Carolinae - Geologica, v. 43, no. 1/2, p. 73-75.

Bergstrom, S.M., Finney, S.C., Chen, X., and Wang, Z.-H., 1999. The Dawangou Section, Tarim Basin (Xinjiang Autonomous Region), China: Potential as global stratotype for the base of the Nemagraptus gracilis Biozone and the base of the global Upper Ordovician Series, Acta Universitatis Carolinae - Geologica, v. 43, no. 1/2, p. 69- 71.

Bergstrom S.M., Finney, S.C., Chen, X., Palsson, C., Wang, Z., and Grahn, Y., 2000. A proposed global boundary stratotype for the base of the Upper Series of the Ordovician System: The Fagelsang section, Scania, southern Sweden, Episodes, 23, no. 2, p. 102- 109.

Finney, S.C., and Ethington, R.L., 2000. Global Ordovician Series Boundaries and Global Event Biohorizons, Monitor Range and Roberts Mountains, Nevada, Geological Society of America Field Guide Series 2, p. 301-318.

Gradstein, F.M., Finney, S.C., Lane, R., and Ogg, J.G., 2003. ICS on Stage, Lethaia, v. 36, p. 371-378.

Melchin, M.J., Rong Jiayu, Gradstein, F., Koren', T.N., and Finney, S.C., 2004. Stability in stratigraphy, Lethaia, v. 37, no. 1, p. 124.

Finney, S., 2005. Global Series and Stages for the Ordovician System, Geologica Acta, v. 3, no. 4, p. 309-316. Available for free download in pdf format at http://www.geologica-acta.com.