Coordinated Mega- and Micro-Hiostratigraphy of the Upper Cretaceous in the Tombetsu Valley, Hokkaido

Abstract

The Upper Cretaceous(Campanian through Maastrichtian) deposits in the Tombetsu Valley, northern Hokkaido (Fig, 1) are much thicker and more fine-grained than the contemporary sandy and deltaic deposits of the Hakobuchi Group in central Hokkaido. The Tombetsu sequences are subdivided into five formations, from A to E in asending order (Fig. 2), in each of which sandstone demarcates its basal part and mudstone, fine-sandy siltstone and silty very fine-sandstone predominate in the main part. Formations A and B contain (Sphenoceramus) orientalis-Anapachydiscus(Neopachydiscus)naumanni and I. (S.) schmidti-Canadoceras sp, respectively and are referred to the Campanian. This is supported by certain species of planktonic foraminifera from A besides benthic species from A and B. Formations C, D, and E, which are characterized respectively by I. (Endocostea) shikotanensis, I. (S.) hetonaianusPachydiscus (Pachydiscus) flexuosus, and Inoceramus sp, nov.-Gaudryceras sp, nov. contain numerous benthic species of foraminifera, which in turn are mostly common with those from the upper part of the Nemuro Group in eastern Hokkaido. Spiroplectammina grzybowskii from D and E is noteworthy among others. On these lines of evidence Formations C, D, and E are referred to the Maastrichtian for the most part. Table 2 shows the full list of the foraminiferal species based on the samples from selected routes (Figs. 3 and 4) in this area. The Utsunai Formation which overlies unconformably E, contains planktonic foraminifera species (Table 3) of a late Eocene-early Oligocene age.