The Prisayan Formation (Lower and Middle Jurassic) of the Irkutsk Coal Basin: New Data on Litho- and Phytostratigraphy

Abstract

Abstract —The demand for detailed bio- and lithostratigraphic research of the Prisayan Formation is significant because of the lack of its stratotype section. This leads to confusion about the meaning of the stratigraphic volume of the Prisayan Formation. On the basis of the results of a comprehensive research, a lectostratotype section that covers the whole section of the Prisayan Formation is proposed. The following three subformations are identified in the formation on the basis of lithological criteria: lower (Ust-Balei), middle (Idansk), and upper (Sukhov). Stratigraphic correlation of the Prisayan Formation with the coeval deposits of the West Siberian Plate is carried out. The common development trends of typical units of the flora of the Siberian paleofloral region are revealed in West Siberia and the Irkutsk Basin. The observed tendencies make it possible to compare the beds with Coniopteris murrayana, Sphenobaiera czekanowskiana from the lower subformation with the fossil plant assemblage of the Sharapovo Horizon; the beds with Coniopteris snigirevskiae from the middle subformation with the fossil plant assemblage of the Nadoyakh Horizon; and the beds with Raphaelia diamensis and Phoenicopsis irkutensis with the fossil plant assemblage of the Laidin Horizon of West Siberia. The Prisayan-1 palynoassemblage (PA) from the lower and middle subformations correlate with palynozones 5–7 of West Siberia. The Prisayan-2 PA corresponds to palynozone 8 of West Siberia. The paleobotanical and palynological data suggest that the stratigraphic volume of the Prisayan Formation is the upper Pliensbachian–Aalenian of the Lower–Middle Jurassic. This paper touches upon the history of accumulation of the Prisayan Formation, as well as the evolution of the paleorelief and vegetation.