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1 2006

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A multi-proxy study of the Kimmeridgian/Volgian boundary beds in the Gorodischi section (Middle Volga area, Russia), the lectostratotype of the Volgian Stage

Rogov M., Schepetova E., Ustinova M., Price G., Guzhikov A., Pimenov M., Dzyuba O.

Volumina Jurassica

2006

Vol. 4, no. 1

208-210

The Gorodischi section is one of the most famous and well-studied sections among the Russian Jurassic. This section was chosen as lectostratotype of the Volgian Stage by Gerasimov & Mikhailov (1966). Following by the proposals of Cope (1996) regarding the acceptance of secondary standards, Zakharov (2003) suggested Gorodischi as Secondary Stratotype Section and Point for the Volgian Stage. In spite of the small thickness of Volgian rocks, recent studies show an absence of biostratigraphically significant gaps within the whole Volgian Stage (Kiselev & Rogov 2005). The ammonite succession consists of a mixture of Subboreal, Boreal and Submediterranean taxa, with an alternation of the dominant groups through the section, providing a highly accurate correlation of the Lower Volgian with the Tithonian Stage and Arctic Volgian. The succession of Neochetoceras has particular significance for the correlation of the Kimmeridgian/Volgian and Kimmeridgian/Tithonian boundaries, which is also marked by disappearance of aulacostephanids. Among the belemnites Boreal and Subboreal taxa are predominate with exception of the neoburgense horizon, rich in small Hibolithes. Nannofossil samples were collected from only part of the section, but changes in calcareous nannofossils permit the identification of the Boreal Zones N16-N17 (Fig. 1). Both sedimentologic, isotopic and petromagnetic data reflect rapid sea level fluctuation during the Kimmeridgian-Volgian transition. The character of the oscillations of the saturation remanent magnetization (Jrs) and growth of magnetic susceptibility after heating the rocks up to 500°C in air (dk) allows the recognition of three successive zones (I-III). The alternation of the light and dark clays reflects irregularities in the nannofossil versus organic matter abundance. Numerous features of the short gaps could be traced by the ammonite accumulations, sometimes associated with zonal phosphate nodules and in few cases by numerous belemnites. The frequency of condensed levels increases significantly from Kimmeridgian into the Volgian, simultaneously with a gradual coarsening of the terrigenous matter. The character of sedimentation as a whole reflects slow input of terrigenous rocks and oscillations in productivity of calcareous nannoplankton, controlled by climate change and eustasy. The changes in lithology testify to progressive shallowing of the sea basin and increasing of the sensitivity of sedimentation against sea level changes.