Abstract: This article describes the first findings of glendonites (pseudomorph after calcium carbonate hexahydrate ikaite, which crystallize at near-freezing temperatures) from the Bazhenovo Formation (West Siberia, Upper Jurassic–Lower Cretaceous). Glendonites originate from the depressed structures of the central part of the Frolovskaya Megadepression and the deepest areas of the West Siberian paleobasin. Glendonite findings have received a reliable stratigraphic reference and originate from the interval of the Ryazanian Regional Stage. Based on the stable isotopic composition, we propose ikaite crystallized and transformed during organoclastic sulfate-reduction in the bottom sediments. Ikaite crystallization might be caused by the penetration of bottom cold currents from the Paleoarctic basin due to sea level fluctuations during the Ryazanian Stage.