Origin of cyclicity in Triassic‐Jurassic radiolarian bedded cherts of the Mino accretionary complex from Japan

摘要: Abstract The abundance of magnetic microspherules in a Triassic-Jurassic continuous sequence alternating chert and shale beds the Mino accretionary complex, central Japan, was measured systematically. Depending on time, extracted from change considerably minimum 0.9ppm/cm3 at latest Triassic (ca 208Ma) maximum 75ppm/cm3 late Early Jurassic 187Ma); however, is always higher approximately 10–100 (average 70) times than those adjacent bed any stratigraphic horizon. Such systematic difference reveals origin radiolarian bedded as cyclic-rapid accumulation biogenic SiO2 under extremely slow accumulative environments with probable aeolian dust origin. data for individual were obtained based microspherule biostratigraphy, i.e., ca 0.018g/cm2Ka lower 1.9g/cm2Ka beds. Duration time to make chert-shale couplet corresponds dominantly 15–20Ka interval 23 Ka) Upper cherts low paleolatitude, whereas 40–45 Ka 42 Lower ones which may been formed latitude Triassics before final accretion Asian continental margin. cyclicity correspond Milankovitch cycles have well documented deep-sea sediments.