Graptolite zones in the Ordovician and Silurian of the Gorny Altai

Abstract

Summary The Ordovician and Silurian graptolite zonal scale, based on studies of the stratigraphic distribution of graptolite assemblages in Great Britain, is compared with the problems of using data in other regions where often no stratotypes for zonal subdivisions are presently defined. Assuming such local stratotypes in future we can note that the total graptolite scale, being a sum of sections with biostratigraphic graptolite zones distinguished therein, serves as an instrument for exact determination of geological time; and perhaps this scale does not need the distinction of stratotypes of zonal subdivision having as it does in each case exclusively chronostratigraphic significance. Zonal graptolite successions distinguished in different regions, and zonal subdivisions of the total graptolite reference scale, often have index species of the same name, and the same chronostratigraphic extent of zonal subdivisions. As a rule, stratotypes for provincial, regional and local graptolite zones are not distinguished, but some investigators offer characteristics of so-called standard sections where the whole successive set of zones with rich graptolite assemblages is given. According to a recommendation of The Stratigraphic Code of the U.S.S.R. (Stratigraphicheskii Kodeks S.S.S.R., 1977) stratotypes of regional biostratigraphic zones ought to be distinguished. In those cases where the world set of stratotypes is recognized in the same section of graptolite zones for the territory; a local reference section. The standard section necessary both for intra-regional correlations and for inter-regional ones can include either the full set of graptolite zones of a system, series, stages or only a limited number of zones. The Ordovician and Silurian of the Gorny Altai is used as an example of the above principles.