Middle Jurassic deposits of Ulakhan-Sis Ridge (Sakha Republic): biostratigraphy, lithological features and depositional environments

Abstract

The age and sedimentation conditions of the Middle Jurassic terrigenous deposits crowning the Mesozoic section of the northeastern flank of the Nagondzha terrane within the Ulakhan-Sis Ridge are discussed. Based on our own finds of retrocerams and analysis of biostratigraphic data of predecessors, we accept the age of the deposits as Bajocian–Middle Bathonian. Three lithologically different strata are identified in the section. Sandstones from the lower unit belong to feldspathic litharenites, and from the upper strata – to litharenites with a predominance of extrabasinal carbonate clastics (calklithites). Analysis of textural and structural features, mineralogical, petrographic composition and distribution of trace elements in litharenites suggests that all these strata had been accumulated in the shelf part of the paleobasin associated with an active volcanic island arc, which presumably existed from the Early Jurassic on the Omulevka craton terrane until the completion of the formation of Kolyma-Omolon microcontinent.