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Dubki (Saratov region, Russia), the reference section for the Callovian/Oxfordian boundary

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Kiselev D. , Rogov M. , Guzhikov A. , Pimenov M. , Tesakova E. , Dzyuba O.

tom Vol. 4, no. 1

177-179

Efforts in the choosing of GSSP for the Cl/Ox boundary have led to distinction of some available sections with good succession of the ammonite faunal horizons in the vicinities of Saratov. One of the primary criteria for GSSP proposal is the good traceability across the world, and hence, the study of reference section (especially in the region with mixed faunas) in the terms of integral stratigraphy. Additional importance of the investigation of the reference sections for the Cl/Ox boundary is connected with wide distribution of the condensed facies and gaps perhaps due to the suggested sharp changes in climate of the Northern Hemisphere. Few variants of the infrasubzonal units for the Cl/Ox transitional beds of the Russian Platform were suggested recently. Here we are using the scheme of Kiselev and Rogov (2005). The Dubki section is located few kilometers north from Saratov and originally was a temporary quarry for the nearby road-building. Recently this section and/or ammonites from this section were briefly described by Mitta (Keupp & Mitta 2004) and by the present authors (Rogov & Egorov 2003). In addition, this section has recently been proposed as possible candidate for the Cl/Ox GSSP (Kiselev & Rogov 2004). Precise ammonite sampling permits establishing a succession of faunal horizons close to that of Northwest Europe (Fig. 1). Inclined boundaries between some horizons in the figure mean short transitional span, where index species (or both morphologies in lineage) co-occur. Only ranges of the few, most important taxa are shown. Strong Tethyan influence is fixed in the mojarowskii and baccatum horizons, while Boreal cardioceratids dominate in the other intervals. Among the Belemnitida two main groups are recorded. Belemnotheutids are generally scarce; they range from Henrici to Praecordatum subzones. Belemnitids are typically of Tethyan origin (Hibolithes). Boreal forms are common only in two restricted levels. Ostracods are numerous and diverse. However, they show wide oscillations in these two characters through the section, possibly reflecting sea-level changes. Remarkably, Infacythere dulcis is replaced by Nophrecythere oxfordiana at the Cl/Ox boundary as determined by ammonite biostratigraphy. Quite exceptionally, some levels in the studied section provided good samples for magnetostratigraphic studies across the Cl/Ox boundary. Lamberti Zone is chiefly characterized by normal polarity, whereas Mariae and Cordatum zones show dominant reverse polarity, corresponding to polarity structure of the M-35-M37 Chrons (Ogg 2004).