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The stratigraphy of the Upper Cretaceous deposits (Middle and Upper Yezo Groups) in the Obira area, northwestern Hokkaido, is described in this paper. The Upper Cretaceous sequences in this area are c...onsiderably thick as compared with those of other areas in Hokkaido. The main part is characterized by clayey and silty mudstones, but the basal and middle parts are rich in sandstone or conglomerate. The geological map and stratigraphic columnar sections of the Upper Cretaceous are shown in Figs. 2-3 and Fig. 10 respectively. The Upper Cretaceous deposits are fairly fossiliferous as listed in Tables 1-3, and can be divided into 9 Inoceramus Zones, namely I. concentricus nipponicus, concentricus costatus, labiatus, hobetsensis, teshioensis, uwajimensis, mihoensis, amakusensis and japonicas Zones (Fig. 11). These Jnoceramus indicate from Cenomanian to Upper Santonian in age. Species of Calycoceras, Vascoceras, Fagesia, Pseudaspidoceras, Mammites, Collignoniceras, Subprionocyclus and Barroisiceras (s. s.) are especially useful for the biostratigraphic subdivision and the international correlation. The lower part of I. hobetsensis Zone is characterized by such heteromorphs as Scaphites, Otoscaphites, Nipponites, Madagascarites, Eubostrychoceras and Hyphantoceras, and a small form of I. hobetsensis, and the upper part by a large form of I. hobetsensis and I. iburiensis. Lateral and vertical changes of sedimentary fades as well as contained fossils show that the Middle to Upper Turonian in this area is off-shore, labile shelf sediments of shallow to moderate depth and the Coniacian includes slump sediments (fluxoturbidite).show more
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