Devonian System and Its Boundaries – The Last Remaining Issue to Be Resolved

The lower boundary of Devonian system was defined in 1977 as the first global GSSP. The system consists of seven stages with boundaries defined by GSSP, but there is still one major problem within the Devonian stratigraphy – the lower boundary of the Emsian Stage. The current basal Emsian GSSP was defined in the Zinzil’ban Gorge (southern Tian Shan) based on the FAD of conodont taxon Polygnathus kitabicus. However, the detailed correlations with Mauro-Ibero-Armorican, Rheno-Ardennan and Barrandian faunas revealed that the boundary lies well below the base of the classical Emsian in German sense. As a result, the GSSP correlates with the level in the lower part of the Praha Formation. But, the Pragian Stage was originally defined to correspond precisely to the Praha Formation (i.e. “the original Pragian”). Seven years ago, at the STRATI in Milan, the Subcommission on Devonian Stratigraphy decided to search an alternative section with stratigraphically appropriate level for the new basal Emsian GSSP. For several years, the SDS repeatedly called for suitable proposals to be submitted from any region, including southern Tian Shan. The most promising candidate sections outside the stratotype area are situated in regions of Perigondwana. In the last years, Lower Devonian successions in the Prague Synform were extensively studied for biostratigraphy and palaeontological, sedimentological, geochemical and petrophysical records. As a result, two proposals were produced recently - the prospective new stratotype (GSSP) and SABS (Standard Auxiliary Boundary Stratotype) for the basal Emsian boundary. The gracilis Event was selected as the critical marker for possible redefinition as it is stratigraphically much closer to the traditional boundary between the originally defined Pragian and Emsian stages than the present GSSP. Apart from the Prague Synform, no other proposals from other regions or other proposed solutions were submitted. The situation with the redefinition of the GSSP is very complicated. The problem of this boundary is very complex and thus, it is very difficult to solve this using standard procedures for defining the GSSP. The correlation marker works very well within Perigondwana, but in other regions it will be necessary to find correlation equivalents in the form of local correlation standards. The contribution will present the problems and possibilities of the future GSSP correlation. The research was financed by the Czech Science Foundation (21-21829S) and was conducted within institutional support RVO 67985831 of the Czech Acad Sci, Inst Geol.