Cyclostratigraphy of the Middle Triassic bedded chert sequence in the Chichibu Belt, Southwest Japan

Abstract:

Triassic bedded cherts from the Jurassic accretionary complexes in Japan consist of centimeter scale alternations of chert and shale beds. Previous studies have proposed that the rhythmical alternations of Triassic chert and shale beds represent astronomical cycles. Although cyclostratigraphy of the Triassic bedded chert sequence was identified in the Inuyama area of the Mino Belt, central Japan, the validity of its cyclostratigraphy requires detailed cyclostratigraphic correlations to other Triassic bedded chert sequences in Japan. In this study, we performed cyclostratigraphic analysises to the Middle Triassic (Anisian–Ladinian) bedded chert sequence in the Tsukumi area of the Chichibu Belt, southwest Japan. The average duration of a chert–shale couplet in the Middle Triassic bedded chert of the Tsukumi area is ~10 kyr. This duration is inconsistent with the ~20 kyr duration of the precession cycle during the Triassic, which was confirmed by estimated average duration of a chert-shale couplet in the Triassic bedded chert of the Inuyama area. The dominat cycles in a bed number series of thickness variations in the Middle Triassic chert beds show approximately 2–5, 10, 40, 200, 300 and 400 beds cycles. Given that the average duration of one chert–shale couplet is 10 kyr, these cycles correspond to approximately 20–50, 100, 400, 2000, 3000 and 4000 kyr periodicities. The periodicities of the Tsukumi chert are consistent with those of the Inuyama chert (approximately 40–60, 100, 140, 240, 400 and 4000 kyr). Previous paleomagnetic studies have revealed that the Middle Triassic bedded cherts in the Tsukumi area were deposited in the equatorial region (2.1°±5.2°S), whereas the deposition of the Inuyama cherts occur at relatively higher latitude (16.9°±10.2°N). If the interpretation that rhythmical alternations of chert and shale beds are paced by precession and eccentricity cycles is valid, the average duration of a chet–shale couplet from the Tsukumi area might reflect the semi–precession cycle (~10 kyr) in the equator area caused by biannual passage of the Sun. Further cyclostratigraphic analysises will requires to estimate the paleolatitudinally dependent patterns in the cyclicities of the Triassic bedded chert sequences in Japan.