A precise ammonite biostratigraphy through the Kimmeridgian- -Volgian boundary beds in the Gorodischi section (Middle Volga area, Russia), and the base of the Volgian stage in its type area

Abstract

A detailed study of the ammonite faunal horizons of the uppermost Kimmeridgian–Lower Volgian of the Middle Volga area (Russia) was undertaken at the most complete and well-known Gorodischi section. This section shows a complete transition from the Kimmeridgian to the Volgian and is a possible SSSP candidate for the Volgian Stage. Sixteen faunal horizons have been established between the top of the Eudoxus Zone and the top of the Lower Volgian. Revised descriptions of existing horizons (especially in the Kimmeridgian) are given and description of new horizons: cf. anglicum, aff. rebholzi, zeissi, ilowaiskii, cf. praecursor, sokolovi, pavida, "Franconites". Oscillations in the ratios of ammonites with different affinities from horizon to horizon reflect short-term changes in the climate and/or paleogeography. The presence of ammonites with Submediterranean affinities throughout the studied interval enables correlations to be made with the Tethyan ammonite succession. The evolution of the eudemic Subboreal lineage of Sarmatisphinctes has been analysed, and a succession of species through the complete Autissiodorensis Zone has been recognised. This comprises, in ascending order, S.cf./aff. subborealis, S.subborealis,S.zeissi, S.fallax and S. ilowaiskii. Two of the species described, Sarmatisphinctes zeissi and S.ilowaiskii, are new.