Unconformity between the Upper Maastrichtian and Upper Paleocene in the Hakobuchi Formation, north Hokkaido, Japan: a major time gap within the Yezo forearc basin sediments

Cretaceous sediments of the Yezo Group in the Yezo forearc basin-fill are widely distributed along the meridian zone of Hokkaido, Japan. The uppermost part of the Cretaceous sequence, namely the Hakobuchi Formation, ranges in age from Early Campanian to Late Maastrichtian and partly into Late Paleocene. It is subdivided into, in ascending order, the Kamikoma, Heitaro-zawa, Oku-utsunai and Utsunaigawa members in the Nakatonbetsu area, north Hokkaido. Recently, an unconformable sequence boundary was found between the Heitaro-zawa and Oku-utsunai members. A Lower Campanian and two Lower Maastrichtian inoceramid zones are established in the lower half of the Hakobuchi Formation. A lower Upper Maastrichtian ammonite fauna, composed of Anagaudryceras matsumotoi, Pseudophyllites indura and Zelandites varuna, can be recognized above the inoceramid extinction horizon in the upper Heitaro-zawa Member. Although there are no good index megafossils in the two overlying members, Glycymeris shell-beds of the basal Oku-utsunai Member overlie the Upper Maastrichtian Heitaro-zawa Member with a sharp, undulating, erosional surface. As Late Paleocene nannofossils were reported from the two upper members, the uppermost Maastrichtian and Lower Paleocene appear to have been eroded away at the unconformity. The K/Pg gap can be traced into the Oyubari and north Hobetsu areas of central Hokkaido and four drillholes offshore from Honshu in the Pacific Ocean. This might indicate large-scale subaerial erosion caused by a relative sea-level fall throughout the Yezo Basin.