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

Data on fossil assemblages and stratigraphic relationships of some marine Jurassic sequences of Eastern Transbaikalia were updated and summarized. The taxonomic composition of the Toarcian–Aalenian ammonite and bivalve assemblages 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. First data were obtained on microphytofossils from the Lower–Middle Jurassic marine deposits. 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 termination time 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.