Геология

The Albian-Cenomanian rocks compose a single complex of terrigenous deposits over the Volga Region. Delineation of the boundaries of the Cretaceous system units generally tends to cause difficulties, accounted for by rare finds of the fossils. In the northern part of the DonMedveditsa dislocations, natural outcrops of the Albian and the Cenomanian beds in the Burluk river valley have been examined since the 1950s. The authors have earlier made some case studies on the Gordienki, Krasny Yar, Melovatka and Moiseyevo sections.

The record of a mandibular tooth plate of the chimaeroid fish Ischyodus sp. from the Lower Cretaceous deposits of the north of the Saratov Volga River Right Bank is described. The presumed age of the find is the Lower Aptian. This is the first recorded occurrence of chimaeroid remains in the Lower Cretaceous (pre-Albian) deposits of the Middle and Lower Volga regions, and the fourth record in deposits of this age in Eastern Europe.

The Late Cretaceous and the Paleocene representatives of the Aphrocallistes from the East European Province are known from the scanty skeletal fragments. The fragmentary preservation state of the fossils has brought about some contradictory characteristics of the genus and the idea of those sponges diversity. Colonial Aphrocallistes are regarded as the typical representatives of the Maastrichtian spongiofauna from the west of the European region.

The final, late Campanian – early Maastrichtian stage of development of echinoids Micraster and Isomicraster, was studied based on extensive material from the Upper Cretaceous sediments of the North Caucasus, Mangyshlak, Kopetdag, Volga and Transvolga regions. The changes in the complex of morphological characters in the youngest species, which are predominantly connected with variability in the structure of the oral surface of the test, considered in detail.

The Polyptycha sponges have been registered for the first time in the Upper Cretaceous beds from the Volga Region. The skeletons of those true colonies are peculiar for bushy morphotype complicated by anastomoses of the parent modules at their bending. The systematic position of the genus is disputable. This results from the incomplete understanding of the diverse skeleton morphologies in the Late Cretaceous hexactinellids and, correspondingly, from the lack of any standard criteria as regards the taxa of the “family” and the “genus” levels.

The relevance of the work is determined by the fact that Russian oil and gas companies are faced with the task of creating domestic information and measurement systems used in the process of drilling directional and horizontal wells.

The discovery of a tooth of the cow shark Hexanchus microdon (Ag., 1835) from carbonate deposits of the Karsun Formation in the ‘Krasny Oktyabr’ quarry (Volsk, Saratov Volga River Right Bank) is described. Based on the associated foraminifera complex, the stratigraphic position of the find was determined as Lower Maastrichtian, benthic foraminifera LC20 Falsoplanulina multipunctata Zone. It was calculated that the individual size of the described shark species H. microdon was 153–166 cm.

The results of petromagnetic and paleomagnetic studies of two sections of the Lower Syzran subformation (Paleocene) – Gremyachka and Klyuchi-2, located in the north of the Saratov Right Bank and represented by weakly magnetic silicites, are presented. Paleomagnetic data on sections, despite their low quality and fragmentation, are important for magnetostratigraphy, because they confirm the presence of a reverse polarity magnetozone (analogous to chron C27r and/or C26r) in the Lower Syzran subformation and its lateral stability.

A representative complex of the Turonian-Santonian carbonate-producing invertebrates has been revealed. It comprised benthic foraminifers, diverse echinoderms, brachiopods, calcareous and siliceous sponges.