Abstract
Continuous occurrence of international planktonic foraminiferal index species was recognized in the Middle to Upper Turonian successions in the Hobetsu area, Hokkaido, Japan. It enables us to assume the stratigraphic position of the upper limit of the Helvetoglobotruncana helvetica Zone, an important planktonic foraminiferal zonal boundary. Helvetoglobotruncana praehelvetica, Marginotruncana pseudolinneiana, and Praeglobotruncana gibba, which are also internationally recognizable species, occur associated with Helvetoglobotruncana helvetica. This association indicates that stratigraphic top of the Kajiyanosawa route should be assigned to close to the top of the total range of Helvetoglobotruncana helvetica. Correlation of the carbon isotope fluctuation through the Kajiyanosawa route with that of well-studied sections supports our chronostratigraphic interpretation from planktonic foraminiferal biostratigraphy
Concurrence of genus Archaeoglobigerina with Helvetoglobotruncana helvetica in the Middle Turonian of this area demonstrates that the timing of the first appearance of Archaeoglobigerina is older than previous known.
