Biostratigraphy of Upper Jurassic of the Valy clay pit (Samarskaya Luka) by ammonites, ostracods and dinocysts

Abstract

The article presents results of a comprehensive biostratigraphic study of Upper Jurassic deposits of the clay pit near the village of Valy (Syzran district, Samara region). For the first time, a bed-by-bed description was made as well as a subdivision of the section into zones, subzones and biohorizons based on ammonites was established. Bauhini and Kitchini zones (Bayi subzone) are established in the Lower Kimeridgian, while the Upper Kimmeridgian is represented only by the Autissiodorensis zone. In the Lower Volgian, the Sokolovi and Pseudoscythica zones were recognized, while the Middle Volgian is represented by the Panderi zone. The age of the regionally developed unconformity at the base of the Trazovo Formation has been clarified. As in the sections located to the south of the studied section, this unconformity is located at the base of the Autissiodorensis zone, overlying different horizons of the Oxfordian and lower Kimmeridgian. For the first time for the Kimmeridgian of Central Russia in the marlstone band of the Kitchini zone (bayi biohorizon), solitary corals conditionally attributed to the genus Trochocyathus were found. Along with corals, other warm-water taxa (belemnites Hibolithes, rare ammonites Taramelliceras) were found in the same bed, suggesting deposition of this bed during the short-time warming event. 6 biostratigraphic units (zone and beds with fauna) were recognized by ostracods, along with 2 dinocyst-based units (assemblage and zone) which are compared with the stratigraphic subdivisions for these groups, previously proposed for the Kimeridgian and Volgian stages of the Russian Platform. The paleo-ecological analysis makes it possible to assume that sediments have accumulated in the warm shallow eutrophic basin with depths up to 50 m, with a gradual increase of eutrophy over time. At some levels, short-term episodes of severe shallowing or freshening are recorded by ostracods.