UPPER PALEOZOIC VOLCANIC ASH BEDS IN SOUTHERN URALS AND THEIR SIGNIFICANCE FOR THE GLOBAL TIME-SCALE

The International scale for Pennsylvanian has been proposed to include several stages established in the Russian Platform and Southern Urals: Bashkirian, Moscovian, Kasimovian and Gzhelian. The southern Ural Mountain region is the type area for the Cisuralian (lower Permian), comprising the Asselian, Sakmarian and Artinskian stages (Remane et al., 2000). Regardless of the final outcome of the 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 Pennsylvanian through Cisuralian (early Permian) time scale is linked directly to the southern Urals. Major marine fossils are numerous and well preserved in this region, making detailed multitaxa biostratigraphic control possible. A significant body of previous biostratigraphy in the region exists, and it provides a valuable framework for our proposed high-resolution studies. Numerous interstratified volcanic ash layers 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. Commonly cited time scales differ by as much as 14 Ma in the estimated age of the Pennsylvanian-Permian boundary, and vary by as much as 500% in the inferred duration of various stages. In the summer of 2001 we collected over 60 volcanic ash layers samples. 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 Pennsylvanian-Cisuralian geological time period. With current NSF funded project we plan to date approximately 20 tuffs from the southern Urals. Isotopic compositions will be determined using isotope dillution – thermal ionization mass spectrometry (ID-TIMS) in the geochronology laboratory at MIT.