Middle Jurassic Rocks on the Ulakhan-Sis Ridge (Sakha Republic): Biostratigraphy, Lithological Features, and Depositional Environments

The age and depositional environments of Middle Jurassic terrigenous rocks crowning the Mesozoic section on the northeastern flank of the Nagondzha terrane within the Ulakhan-Sis Ridge are discussed. Based on our finds of retrocerams and the analysis of biostratigraphic data of predecessors, the Bajocian–Middle Bathonian age of these rocks is accepted. Three lithologically different sequences are identified in the section. Sandstones from the lower unit belong to feldspathic litharenites; from the upper sequences, to litharenites dominated by extrabasinal carbonate clastics (calclithites). The analysis of structural-textural features, mineralogical-petrographic composition, and distribution of trace elements in litharenites suggests that all these sequences were deposited in the shelf part of paleobasin associated with an active volcanic island arc, which presumably existed since the Early Jurassic on the Omulevka Craton terrane until the termination of the Kolyma–Omolon superterrane formation.