New Paleontological and Stratigraphic Data on the Lower–middle Jurassic Strata of Marine Genesis in Eastern Transbaikalia

The data on fossil assemblages and stratigraphic relationships of some marine Jurassic strata of Eastern Transbaikalia were updated and summarized. The taxonomic composition of the Toarcian–Aalenian assemblages of ammonites and bivalves was revised for two key sections (Onon-Borzya and Bolshaya Kulinda) and a number of other sites. The new data expanded our knowledge about the previously little-known Toarcian belemnite and gastropod assemblages in the region. The first data on microphytofossils from the Lower–Middle Jurassic marine deposits were obtained. Based on the find of the bivalve shell Retroceramus ex gr. Popovi, the Upper Aalenian is established in the upper part of the Bolshaya Kulinda section previously taken as the Lower Aalenian. The ideas about the time of termination of the marine sedimentation regime in Eastern Transbaikalia are discussed in the context of the history of the gradual closure of the Mongol-Okhotsk Ocean.