Usolka section (southern Urals, Russia): a potential candidate for GSSP to define the base of the Gzhelian Stage in the global chronostratigraphic scale

Conodont species Streptognathodus simulator Ellison, 1941 has been proposed recently to define the Kasimovian-Gzhelian boundary in the global chronostratigraphic scale.The species distributed globally and traditionally has been used as a marker of the base of the Gzhelian Stage in the type sections in Moscow Basin and Urals. Recent studies of conodont taxonomy and biostratigraphy in southern Urals have established the chronocline with ascendant and descendant to Streptognathodus simulator species. Usolka section proposed here as a potential candidate for the GSSP (Global Stratotype Section and Point) to define the global Gzhelian Stage at the FAD of the Streptognathodus simulator within the chronocline Streptognathodus praenuntius Chernykh, 2005 – St. simulator Ellison, 1941 – St. auritus Chernykh, 2005. The chronocline recovered within 2.7 m of beds 4 and 5 at the Usolka section, with all three species described and properly figured. No obvious interruptions in sedimentation are recorded within the Kasimovian-Gzhelian transition there. Several volcanic ash beds are present below and above the proposed boundary, making radiometric calibration highly possible in the near future. Mode of preservation of conodonts with a CAI of around 1.0–1.5 provides excellent basis for the geochemical studies. Accessibility presently is adequate, and this exposure will be improved and maintained permanently for interested scientists. Future access will be guaranteed by means of legislative action to create a scientific preserve.