南西日本のペルム紀およびトリアス紀層状チャートの堆積サイクル

その他

Rhythm recorded in bedded chert sequence has been examined by compositional set of chert and mudstone beds, and a newly proposed fundamental unit of the bedded cherts is indicated. Thick chert bed with following thinner chert beds clearly appeared in the Upper Triassic bedded cherts, and some part of the Permian sequence. These chert beds and intercalating mudstone beds are set all together, and this set is repeated several times in the Upper Triassic. Stratigraphic recomposition of bedded chert sequence in this manner, 52 sets are recognized in the Middle to Upper Triassic, although those in the Middle Triassic are incomplete and unclear. Additional cycle formed by the first large set and following rather small three set can be distinguished in the Upper Triassic. This newly proposed diagram suggests that thickness of chert beds shows regular with some cyclic peaks, while thickness of mudstone beds gradually decrease. Mudstone beds also have some peakes correspond to those of the chert beds. In the Permian sequence, styles of set recognized in the Lower Permian and Upper Permian are different mutually, namely those of the Lower Permian are characterized by rhythmic and steady sets while those in the Upper Permian are senes of thinner chert beds and succeeding clear set of bedded cherts are repeated.
A set could be formed by dominant radiolarian blooming characterized by a thick chert bed and consequent drop of blooming indicated by some thinner chert beds. Two orders of cycle recorded in bedded chert sequence could indicate rhythmic change of nutrient supply or surface current velocity related to a circulation of the occean.