2015 年 121 巻 4 号 p. 147-152
Triassic bedded chert sequences in Jurassic accretionary complexes in Japan consist of centimeterscale rhythmic alternations of chert and shale beds that have been interpreted as recording astronomical cycles. Although the cyclostratigraphy was established in the Inuyama area of the Mino Belt, central Japan, the validity requires detailed cyclostratigraphic correlations to other Triassic bedded chert sequences in Japan. In this study, cyclostratigraphic analyses were performed on the Middle Triassic (Anisian) bedded chert sequence in the Tsukumi area of the Chichibu Belt, southwest Japan. The average duration of each chert-shale couplet is ca. 10 kyr, which differs to the ca. 20 kyr duration for the precession cycle during the Triassic, which was obtained by estimating the average duration of chert-shale couplets in the Triassic bedded chert sequence of the Inuyama area. The dominant cycles in a bed number series of thickness variations in the Middle Triassic chert beds are approximately 2-5, 10, 40, 200, 300, and 400 beds cycles, which partly differ from those of the Inuyama area.