| Abstract |
This paper describes the biostratigraphy of the upper part of the Upper Yezo Group (Upper Cretaceous) exposed in the Haboro and adjacent areas of northwestem Hokkaido to define the Santonian Stage in ...Japan by establishing a revised scheme of zonation. The Upper Yezo Group in the Haboro area is made up mainly of sandstone, siltstone and claystone, and is divided on the basis of predominant lithology into the Lower, Middle and Upper Haborogawa Formations. This lithostratigraphic division can be also applicable to the northerly adjacent Chikubetsu and southerly neighbouring Kotanbetsu and Obira areas. Some of relatively thick sandstone interbeds in the mudstone succession are laterally traceable as marker beds from the Chikubetsu area to the Obira area. Of these three formations, the Middle and Upper Haborogawa Formations which encompasses the so-called Santonian Stage are almost continuously exposed and yield abundant fossils. As a result of a biostratigraphic study of these two formations, the following four inoceramid zones were recognized; in ascending order, the Inoceramus uwajimensis-I. mihoensis Zone, the I. amakusensis Zone, the Platyceramus japonicas Zone, the Sphenoceramus orientalis Zone and the S. schmidti Zone. A subzone characterized by the association of S. sanrikuensis, S. cristatus sp. nov. and Inoceramus (Cordiceramus) kanmerai is discriminated in the upper main part of the P.japonicas Zone. On the other hand, the successions studied can be divided into two ammonite zones based on the Texanitinae, i.e. the Texanites collignoni-T. quinquenodosus Zone below and the Plesiotexanites kawasakii-P. pacificus Zone above. It is also suggested that the latter zone is subdivided into the subzones of T. amakusensis (lower), Protexanites (Anatexanites) fukazawai-P. (P.) bontanti shimizui and/or Submortoniceras cf. condamyi (upper). The T. collgnoni-T. quinque-nodosus Zone occupies the basal part of the I. amakusensis Zone, and is correlated with the Lower Santonian. The occurrences of Submortoniceras cf. condamyi, Menabites mazenoti and Globotruncana arca from the Platyceramus japonicas Zone indicate that this zone should be regarded as the Lowest Campanian. Consequently the zone of I. amakusensis corresponds precisely to the Santonian.show more
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