Yezo Group

The Yezo Group is a stratigraphic group in Hokkaido, Japan and Sakhalin, Russia which is primarily Late Cretaceous in age (Aptian to Earliest Paleocene). 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 , 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, plesiosaurs and marine turtles. Dinosaur remains are among the fossils that have been recovered from the group. These include a partial cervical vertebra and right hand from the therizinosauridParalitherizinosaurus, from Early Campanian from northern Hokkaido. Nipponosaurus is known from an unnamed unit of the group from Southern Sakhalin, probably late Santonian or early Campanian in age. Kamuysaurus, which is known from the early MaastrichtianHakobuchi Formation of southern Hokkaido, was also discovered in layers of this group.