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Panboreal and Boreal-Tethysian correlation of the Volgian Stage by ammonites
Mikhail A. Rogov

Building: Muséum d'Histoire Naturelle de Genève
Room: Amphithéâtre
Date: 2018-12-06 04:40 PM – 05:00 PM
Last modified: 2018-11-24

Abstract


The Volgian Stage, terminal stage of the Jurassic System, has been introduced by S.N. Nikitin (1881) for the european Russia but during the last decades it is became widely used throughout the Panboreal Superrealm. Strong restriction of latitudinal faunal immigration between different basins at the latest Jurassic leads to significant problems in Boreal-Tethysian correlation and also to high level of provinciality within superrealms. However, although zonal and infrazonal successions applied for different Boreal areas sometimes lacking any similarities in zonal index species (good examples are successions of the NW Europe, the Russian Platform and Northern Siberia, which has no common zones and subzones), substage boundaries are very well correlated throughout the Panboreal Superrelm. The lower boundary of the Volgian Stage is coincides with appearance of key virgatitid and dorsoplanitid genera (Ilowaiskya and Virgatosphinctoides respectively), which are showing independent disappearance of lappets, typical for their ancestral taxa. In the Subboreal and Submediterranean areas this boundary is also marked by extinction of aulacostephanids (Rogov, 2010; Gallois, 2011). Lower boundary of the Middle Volgian substage is marked by important evolutionary events (appearance of Zaraiskites, Pavlovia and Dorsoplanites) as well as by palebiogeographic changes, which lead to very wide distribution of early dorsoplanitids throughout the Boreal areas. In spite of strong differences in zonal and infrazonal subdivision of the Middle Volgian, its upper boundary is also very well traced, being marked by evolutionary turnover (appearance of Craspedites and Craspeditinae subfamily, which were common in the nearly all basins except NW Europe), and a little postdate the late Middle Volgian crisis (Rogov, 2013), marked by significant decrease of shell size, weakening of ribbing and changes in suture line patterns in Boreal ammonoid lineages as well as by long-term drop of Boreal ammonoid diversity. Boundary between the Volgian and Ryazanian stages cannot be precisely stated in the Russian Platform, the type regions of these units, due to regional unconformity, but easily traceable in other Boreal areas. It is marked by disappearance of Volgidiscus and appearance of typical Ryazanian genus Praetollia.

Boreal-Tethysian correlation of the Volgian Stage with Tithonian and lowermost Berriasian is only partially based on occurrences of Submediterranean ammonites in Subboreal regions and Subboreal Gravesia in Submediterranean areas. These ammonite records provided good background for precise correlation of the Lower Volgian in its type area with the Tithonian succession (Rogov, 2010). Above the Lower Volgian biogeographic segregation of ammonite faunas became very strong and lacking of any common ammonites in the Boreal and Tethysian areas prevents usage of these fossils for correlational purposes. Uppermost part of the Middle Volgian Scythicus Zone in Poland contains calpionellids typical for the Middle-Late Tithonian Boneti Subzone of the Chitinoidella Zone (Pszczółkowski, 2016). Uppermost Middle Volgian and Upper Volgian could be correlated with Tethysian succession only by means of magnetostratigraphy (Houša et al., 2007; Bragin et al., 2013; Schnabl et al., 2015). The lower boundary of the Volgian Stage is the only level which could be easily traced in the both Boreal and Tethysian areas by ammonites, but substage boundaries of the Volgian Stage lies far from well-correlated level in the Tithonian Stage and vice vera. Although diverse and numerous occurrences of Tethysian-derived ammonites in the Ryazanian Stage of the Russian Platform are well-known for more than one hundred years, their taxonomy and correlational potential still remains matters of controversies.