SOUTHERN URALS – NEW OPPORTUNITY IN DEVELOPMENT UPPER PALEOZOIC TIME-SCALE

The International scale for upper Mississippian and Pennsylvanian has been proposed to include several stages established in the Russian Platform and S. Urals: Serpukhovian, Bashkirian, Moscovian, Kasimovian, Gzhelian, and Orenburgian. The S. Urals region is the type area for the Cisuralian (lower Permian), comprising the Asselian, Sakmarian, Artinskian and Kungurian stages. Regardless of the final outcome of the upper Mississippian and Pennsylvanian GSSP stage designations, the Russian sections will, at minimum, be critical reference sections for global correlation. Thus, the internationally accepted biostratigraphic definition of the upper Mississippian through Cisuralian time scale is linked directly to the southern Urals. Major marine fossils are numerous and well preserved in this region. A significant body of previous biostratigraphy in the region exists, and it provides a valuable framework for our proposed high-resolution studies. Recently several sections in the southern Urals has been proposed as a potential GSSP's for the most of the bases of the upper Mississippian and Pennsylvanian stages. The current status of the Cisuralian stages GSSP's is discussed in the current volume (see Chuvashov et al.). Numerous interstratified volcanic ash layers within well biostratigraphically constrained sequences were currently found in the late Paleozoic sections of the southern Urals making precise radiometric age control possible. Despite its critical role, the absolute age calibration of significant intervals of the late Paleozoic time scale remains poor. In the summer of 2001 over 60 volcanic ash layers were collected in S. Urals. In the majority of samples conodonts and radiolaria were recovered along with excellent and numerous zircons. Discovery of such conodont-zircon combinations in the Pennsylvanian and Cisuralian type sections in southern Urals provides an exceptional opportunity to develop a well constrained numerical scale and Graphic Correlation Composite Standard Section for the late Mississippian-Cisuralian geological time. With current NSF funded project as the first step we plan to date approximately 20 tuffs from Cisuralian sequence in the southern Urals. Isotopic compositions will be determined using isotope dillution B thermal ionization mass spectrometry (ID-TIMS) in the geochronology laboratory at MIT.