Палинологическая характеристика среднеюрских отложений из местонахождения Кулинда (Забайкалье): биостратиграфический и биофациальный анализ

Abstract

The Kulinda section belongs to the base of the Ukurey Formation in the Orlov Depression (Transbaikalia). The site has yielded numerous bones and associated integumentary structures belonging to the primitive ornithischian dinosaur. However, the age of the Kulinda deposits has not been investigated in detail so far. Palynological studies discussed in this paper clarify this question and provide the data on the palaeoenvironments. The Kulinda section is composed of siltstone and sandstone with admixture of volcanogenic material. Ten samples were collected from the bone-bearing deposits as well as the underlying and overlying sediments. The samples were treated by 15% hydrochloric and 30% hydrofluoric acids. Mineral and organic matters were separated by centrifugation in cadmium heavy liquid with specific gravity 2.25. Spore-pollen assemblage contains the species, which are important for Middle Jurassic stratigraphy: Podocarpidites rousei Pocockand, Eboraciato rosa (Sachanova et Iljina) Timochina. Their lowermost occurrences are revealed in the Bathonian in Western Siberia (Palynozone 10) and Kansk-Achinsk basin. The domination of Pseudopicea variabiliformis Bolchovitina and abundant Cyathidites are also characteristic features of the Bathonian of Western Siberia. Palynological zones in this region are calibrated against ammonite Boreal standard and foraminiferal successions that give the reliable age constraints. Spore-pollen assemblage from the Kulinda section shows that neighboring areas were occupied by coniferous forests with an admixture of pteridosperms, podocarpaceans and ginkgoaleans. More humid habitats near the lake were mostly occupied by cyatheaceous/dipteridaceous ferns. The climate was temperate and rater humid. The water genesis of deposits is confirmed by the presence of green algae Botryococcus and Leiosphaeridia. The age of the Ukurey Formation was broadly regarded as the Late Jurassic according to palaeontological data, isotope dating and its general position in the section. New palynological data show that its lower stratigraphic extend is wider, than it is was assumed earlier, and changed over the territory. The Bathonian age of the Ukurey Formation does not contradict to the general geological situation in Transbaikalia. Biofacial analysis confirms lacustrine genesis of the Ukurey Formation and evidences temperate warm and rather humid conditions on the nearby land areas.