Establishment of utility ranking for fossil datum planes in Upper Cretaceous sequences based on carbon isotope stratigraphy.

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(1)About two kilometers of detailed plan map(1/1000) along Horotate River in Kotanbetsu, Tomamae Town, Hokkaido, Japan was drawn based on measurement and careful investigation of macro fossil occurrences. Plan maps in 1/5000 scale were also made in the Oyubari, Hobetsu and Haboro areas. Mudstone samples were collected during the survey and the localities are indicated within these plan maps.(2)Along Horotate River, macrofossil did not show abundant occurrences, however, continuous occurrence of regionally important age indicative species, namely Inoceramus hobetsensis, was identified. It enabled us to delimit the stratigraphic distribution of this species in detail.(3)Carbon isotope analysis on majority of samples from Horotate River sequence have been suspended because of machine troubles at an institute where we have expected to analyze them. Samples from Oyubari and Hobetsu areas and a part of samples from Horotate River sequence were analyzed at Indiana University, U.S.A.Each carbon isotope profile for each area showed internationally recognizable pattern that enables us to calibrate biohorizons of macrofossil within the framework of international correlation.(4)Planktonic foraminifers from Hobetsu area include variety of age indicative species. Among them, continuous occurrence of Helvetoglobotruncana helvetica addresses stratigraphic range of internationally significant foraminiferal zone. Four other cosmopolitan species also support stratigraphic position of upper limit of H.helvetica Zone in this area.(5)Continuous effort for establishment of detailed chronostratigraphy for the Cretaceous in Hokkaido will be carried out after this project. Such data collection will open the way for utilizing the Yezo Group as an international chronological standard succession.