Yezo Group
The Yezo Group is a stratigraphic group in Hokkaido, Japan and Sakhalin, Russia which is primarily Late Cretaceous in age. It is exposed as roughly North-South trending belt extending 1,500 kilometres through central Hokkaido from Urakawa to Cape Sōya and Sakhalin from the south coast to Alexandrovsk-Sakhalinsky District. It consists of marine forearc basin sediments, typically turbiditic and bioturbated mudstones and sandstones with subordinate conglomerate primarily deposited on the continental shelf and slope of the ancient Yezo subduction margin. It forms a continuous depositional sequence with the Sorachi Group, which overlies the Horokanai Ophiolite. The sequence gradually shallows upwards with the terminal Hakobuchi Formation representing a fluvial-inner shelf environment.
Numerous fossils are known from the unit, mostly ammonites and bivalves, but also marine vertebrates such as mosasaurs and marine turtles. Dinosaur remains are among the fossils that have been recovered from the group, including those of an indeterminate maniraptoran consisting of a right metacarpal and manual unguals from the Early Campanian Osoushinai Formation from northern Hokkaido. Nipponosaurus, which is known from an unnamed unit of the group from Southern Sakhalin probably late Santonian or early Campanian in age. and Kamuysaurus, which is known from the early Maastrichtian Hakobuchi Formation of southern Hokkaido.
