Upper Campanian-lower Maastrichtian sections of northern Rostov oblast: Article 2. Depositional environments and paleogeography

The study present the results of the integrated study on the Belgorod and Pavlovka formations (upper Campanian), Sukhodol Formation (Campanian-Maastrichtian), and Efremovo-Stepanovka Formation (Maastrichtian). Variations in lithological indicators and associated changes in the biotic assemblages were used to distinguish three stages in the basin evolution separated by hiatuses (Belgorod-Pavlovka, Sukhodol, and Efremovo-Stepanovka). This basin occupied the upland area in the north of the Paleozoic Donets Basin during late Campanian-early Maastrichtian times. Each stage was characterized by a specific depositional environment accompanied either by a decrease or by an increase in the terrigenous sediment supply from the Donets Basin and, possibly, Ukrainian Shield and sea-level and temperature fluctuations, as well as specific paleobiogeographic relations. During the Belgorod-Pavlovka stage, the basin was characterized by relatively deep-water environments, with warm waters and normal salinity, and predominantly carbonate sedimentation. The Sukhodol stage was marked by terrigenous sedimentation, a predominance of the agglutinated foraminiferal forms, and abundant radiolarians, which occurred during a marine regression and overall cooling. This stage corresponds to the global “Campanian-Maastrichtian boundary event.” The first half of the Efremovo-Stepanovka stage was marked by resumed carbonate sedimentation, warming, transgression, and deepening of the basin, which were replaced by a renewed regression at the end of this time interval.     

The first finding of reliable Jurassic radiolarians in the Crimea

Radiolarians of Leugeonidae Yang et Wang, 1990, which represent a morphologically distinctive group of spherical radiolarians of the Spumellaria order, were found for the first time in Crimea and reliably confirm the Jurassic age of the finding. The nodules, which host the Jurassic radiolarians, were collected by A.S. Alekseev in 1983 in the terrigenous sequence of the Lozovskaya tectonic zone. The radiolarian assemblage in the nodules includes Levileugeo ordinarius Yang et Wang, Triactoma jonesi Pessagno, Pseudocrucella aff. prava Blome, Paronella kotura Baumgartner, P. ex gr. mulleri Pessagno, and Praeconocaryomma sp. The Levileugeo genus is easily identified due to its unique hexagonal element, which is typical only of the Jurassic, in particular, Upper Bajocian–Lower Tithonian radiolarians.     

The Vendian Vorogovka Group of Yenisei Ridge: Chemostratigraphy and results of U–Pb dating of detrital zircons

Application of the method of isotope stratigraphy showed that the ⁸⁷Sr/⁸⁶Sr ratio in the least altered carbonate rocks of the Severnaya Rechka and Sukhaya Rechka formations in the Vorogovka Group of Yenisei Ridge ranges from 0.70813 to 0.70828, and δ¹³C varies from –0.7 to +1.8‰. Comparison of these data with similar data for the Late Precambrian sequences from other regions provides evidence for the Vendian age of the Vorogovka Group. This is supported by the results of LA ICP-MS U–Pb dating of detrital zircons from sandstone of the Severnaya Rechka Formation, which is the basal level of the group: the mid-weight age of the youngest zircon population is 584 ± 3 Ma.