北海道北西部のサントニアンーカンパニアン推移部の化石層序学的及び古地磁気層序学的研究 Bio- and magnetostratigraphy of the Santonian-Campanian transition in northwestern Hokkaido, Japan

北海道北西部の白亜系上部エゾ層群の, 特にサントニアン-カンパニアン推移部について両階の境界を明らかにするため大型化石・微化石層序及び古地磁気層序を検討した。ここではサントニアン/カンパニアン境界はアンモナイトSubmortoniceras cf. condamyiあるいはMenabites mazenotiの初産出層準に置かれ, この層準はイノセラムスInoceramus amakusensis帯とI. (Playceramus) japonicus帯の境界付近, あるいは浮遊性有孔虫Marginotruncana pseudolinneianaの最終産出層準とGlobotruncana arcaの初産出層準との間に位置する。北海道中〜南部の白亜系エゾ累層群まで含めて検討すると放散虫及び石灰質ナノ化石層序, 放射性年代等も上記の対比に調和的である。この境界は古地磁気スケールの正帯磁気帯34の中にある。

In this paper, a study using several stratigraphic tools has been carried out to determine the more detailed features of the Santonian/Campanian stage boundary, with reference to the proposals of the Subcommission on Cretaceous Stratigraphy (SCS) of IUGS in 1983 and 1995. The integration of bio- and magnetostratigraphy of the Santonian-Campanian transitional zone of the Upper Cretaceous Yezo Group in northwestern Hokkaido, Japan, has clarified the following: the Santonian/Campanian stage boundary in the Japanese stratigraphic scale is located at the first occurrence of Submortoniceras cf. condamyi or Menabites mazenoti, which is situated close to the boundary between the Inoceramus amkausensis and I. (Platycerasmus) japonicus Zones, and located between the last occurrence of Marginotruncana pseudolinneiana and the first occurrence of the typical form of Globotruncana area. On the other hamd, the first occurrence of Aspidolithus parcus parcus is above the first occurrence of G. area, and the upper part of the I. (P.) japonicus Zone is correlated with the Cretaceous calcareous nannofossil zone CC18, based on the biostratigraphy of the Upper Cretaceous in south-central Hokkaido. A K-Ar age of 82.2±0.6 Ma was recently reported near to the horizon of the first occurrence of I. (P.) japonicus, which correlates with the well-known boundary age of 83.5 Ma. The boundary in the Japanese stratigraphic scale is situated in the marine magnetic normal polarity chron 34. Unfortunately, Marsupites, Placenticeras and Gonioteuthis, selected by SCS as definitions of the base of the Campanian Stage in 1995, are so far absent in the Cretaceous deposits of Japan.