|
Over recent decades, with exploration of the oceans, with ice cores from Greenland and Antarctica and with all the high resolution methodologies now available for integrated stratigraphy, there is a tendency in several milieus to abandon the use of stages for the Plio/Pleistocene. We do not agree with this approach and consider the use of standard stages in the Plio/Pleistocene to be as appropriate as for older parts of the stratigraphic column.
In the hierarchy of chronostratigraphic units (erathem - system - series - stage), stage is the unit of lowest rank, and of widest and longest usage. Stage is the only chronostratigraphic unit defined on the basis of a typological criterion: units of higher rank are usually defined on the basis of hierarchic principle. Many stages were defined in the Old World, in the early days of stratigraphy, deriving their names from the locality where they were originally defined (e.g. Maastrichtian from Maastricht) based on their marine fossil content.
The distinction between global stages and regional stages arises from the application of the Global Stratotype Section and Point (GSSP) concept, (the "Golden Spike"), as prescribed by the International Stratigraphic Guide (Hedberg, 1976) and further developed by Cowie (1986) and Remane et al. (1996). In pre-Hedberg times, stages were usually loosely defined and the many names proposed by various authors created controversies and confusion. Nowadays, a proposed GSSP must be approved first by a Working Group, then by the voting members of the Subcommission (one for each system), then by the voting members of ICS and finally ratified by IUGS. It is a long process, leading to precise definition of a marker horizon in a measured section, with multiple criteria for worldwide correlation.
All stages defined in pre-Hedberg times were regional and few are ratified in the modern international Geological Time Scale (GTS). Regional stages defined nowadays serve a different purpose: to provide a time scale in remote areas or in areas that underwent geological histories different from that of the classical areas (e.g. New Zealand regional stages). However, proponents of new regional stages are requested to correlate them with the global stages of GTS.
In the Plio/Pleistocene, the Pliocene Series has three standardized stages: Zanclean, Piacenzian and Gelasian (oldest to youngest), all defined by GSSPs in the central part of the Mediterranean, considered to be the classical area. No stages are yet formalized in the Pleistocene Series (subdivided into Early, Middle and Late Subseries) but a redefinition of the Calabrian Stage (= Early Pleistocene Subseries) has been submitted for publication. Others are planned. With just one name, with a precise and well controlled time significance and multiple criteria for global correlation one can identify a defined time interval in both terrestrial and marine environments, independent of latitude, bioprovincialism and historical evolution.
|
