The Hakobuchi Formation is a

geological formation A geological formation, or simply formation, is a body of rock having a consistent set of physical characteristics ( lithology) that distinguishes it from adjacent bodies of rock, and which occupies a particular position in the layers of rock exp ...

Hokkaido is Japan's second largest island and comprises the largest and northernmost prefecture, making up its own region. The Tsugaru Strait separates Hokkaidō from Honshu; the two islands are connected by the undersea railway Seikan Tunnel. The la ...

Japan Japan ( ja, 日本, or , and formally , ''Nihonkoku'') is an island country in East Asia. It is situated in the northwest Pacific Ocean, and is bordered on the west by the Sea of Japan, while extending from the Sea of Okhotsk in the north ...

. It is the uppermost unit of the

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 centr ...

, being early

Maastrichtian The Maastrichtian () is, in the ICS geologic timescale, the latest age (uppermost stage) of the Late Cretaceous Epoch or Upper Cretaceous Series, the Cretaceous Period or System, and of the Mesozoic Era or Erathem. It spanned the interval from ...

in age. It consists of bioturbated

glauconitic Glauconite is an iron potassium phyllosilicate ( mica group) mineral of characteristic green color which is very friable and has very low weathering resistance. It crystallizes with a monoclinic geometry. Its name is derived from the Greek () m ...

sandstones Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates) b ...

, siltstones and conglomerates with coaly

mudstone Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Mudstone is distinguished from '' shale'' by its lack of fissility (parallel layering).Blatt, H., and R.J. Tracy, 1996, ''Petrology. ...

and minor

tuffite Tuffite is a tuff containing both pyroclastic and detrital materials, but predominantly pyroclasts. According to IUGS definition tuffite contains 75 to 25% volcanic (epiclastic) material. There are several classifications that define tuffite. ...

. It was deposited in a

continental shelf A continental shelf is a portion of a continent that is submerged under an area of relatively shallow water, known as a shelf sea. Much of these shelves were exposed by drops in sea level during glacial periods. The shelf surrounding an island ...

setting. It is noted for its fossil content with the invertebrates mainly consisting of bivalves and ammonites. With vertebrates including the

mosasaur Mosasaurs (from Latin ''Mosa'' meaning the 'Meuse', and Greek ' meaning 'lizard') comprise a group of extinct, large marine reptiles from the Late Cretaceous. Their first fossil remains were discovered in a limestone quarry at Maastricht on th ...

s '' Mosasaurus hobetsuensis'' and ''

Phosphorosaurus ''Phosphorosaurus'' ("phosphate lizard") is an extinct genus of marine lizard belonging to the mosasaur family. ''Phosphorosaurus'' is classified within the Halisaurinae subfamily alongside the genera '' Pluridens'', ''Eonatator'' and ''Halisauru ...

ponpetelegans''. As well the sea turtle '' Mesodermochelys''Hirayama R, Chitoku T. (1996). "Family Dermochelyidae (Superfamily Chelonioidea) from the Upper Cretaceous of North Japan". ''Transactions and proceedings of the Palaeontological Society of Japan. New series'' 184: 597-622
online, retrieved 28 July 2008
/ref> and the

hadrosaurid Hadrosaurids (), or duck-billed dinosaurs, are members of the ornithischian family Hadrosauridae. This group is known as the duck-billed dinosaurs for the flat duck-bill appearance of the bones in their snouts. The ornithopod family, which inclu ...

dinosaur ''

Kamuysaurus ''Kamuysaurus'' is a genus of herbivorous edmontosaurin saurolophine hadrosaurid dinosaur from Late Cretaceous Maastrichtian marine deposits of the Yezo Group ( Hakobuchi Formation) in the Hobetsu area near the town of Mukawa, Hokkaido in Ja ...

''.

References

{{reflist Upper Cretaceous Series of Asia Maastrichtian Stage