New data about Upper Jurassic formations of the Ulyanovsk-Saratov through and the northern part of the Peri-Сaspian Syneclise

Abstract

A detailed description of the Upper Jurassic formations of the south of the European part of Russia, mainly restricted by the Kimmeridgian and Volgian stages, is given. The characteristic of the reference section is provided for the Vechkusy Formation, and the neostratotype is proposed for the Novikovka Formation. The criteria for drawing the boundaries of local stratigraphic units (formations) and the features of strata distribution are considered in detail. It is proposed to change the stratigraphic volume of the Vechkusy, Novikovka and Trazovo Formations, refusing to align the boundaries of the formations with those of stages and substages, and instead to use most clear and widely traced lithological boundaries for defining formations. The Undory Formation, which ends the Upper Jurassic section in the region, includes intervals which differ significantly in their lithologies, and the Kashpir Formation is used here for the area characterized by siliceous sedimentation during the late Volgian. Oxfordian spongolites of the Ulagan Mt. that are sharply different from the coeval strata of adjacent regions ascribed to as Prieltonskaya Member. Despite the fact that isochronous lithological markers are present within the region under consideration, the boundaries of the formations are more or less diachronous, due to both the presence of sedimentary breaks and the temporal sliding of lithological boundaries.