Upper Cretaceous to Paleocene Hakobuchi Group, Nakatonbetsu area, northern Hokkaido : lithostratigraphy and megafossil biostratigraphy

Abstract

The uppermost part of the Yezo Supergroup in the Nakatonbetsu area, northern Hokkaido witnesses to the lithostratigraphy and magafossil biostratigraphy from the lower Campanian to Paleocene. It is subdivided into the Kotobuki Formation of the Upper Yezo Group (offshore siltstone), the Kamikoma (siltstone intercalated with fine sandstone layers), Hcitarozawa (sandy siltstone), Oku-utsunai (fine sandstone) and Utsunaigawa (sandy siltstone) Formations of the Hakobuchi Group in ascending order. The Hakobuchi Group is dominated by offshore sandy siltstone, in contrast with the Ashibetsu, Oyubari and Hobetsu areas in central Hokkaido representing the westerly marginal shallow-marine sandy facies of the Yezo forearc basin along the Northwestern Pacific rim. The shallowest facies is represented by HCS sandstone of the lower parts of the Heitarozawa and Oku-utsunai Formations formed on storm-dominated lower shoreface. In terms of newly collected ammonites and inoceramids, we can establish three inoceramid zones with the geologic age as follows ; Kotobuki and lower part of Kamikoma Formations : Sphenoceramus schmidti Zone (lower Campanian), upper Kamikoma Formation : Inoceramus shikotanensis Zone (lowest Maastrichtian), uppermost Kamikoma to lower Heitarozawa Formations : Sphenoceramus hetonaianus Zone (upper part of lower Maastrichtian), upper Heitarozawa Formation : an unnamed ammonite zone above the inoceramid extinction horizon (upper Maastrichtian). A sharp erosional surface between the Heitarozawa and Oku-utsunai Formations in thought to be an unconformable sequence boundary. The surface itself was represented by ravinement surface and covered by transgressive lags containing Glycymeris shell beds of shallow-marine storm origin. The Oku-utsunai and Utsunaigawa Formations show a transgressive facies succession, consisting of the lag deposits, HCS sandstone, bioturbated silty sandstone and sandy siltstone. This facies succession and the previous micropaleontological control suggest that the strata of the uppermost Maastrichtian to Danian including the K/T boundary seem to have been eroded away.