Lithologic characteristics, biostratigraphy and geochemical analysis of the late Early to early Middle Triassic bedded cherts were described from the two locations, Ashimi section in Type I (T I) suite and Taki section in Type II (T II) suite of the Tanba Belt, Southwest Japan, and the sedimentary environments of siliceous rocks in eath suite is evaluated.
Bedded cherts in Ashimi section are composed of altemating beds of black to dark gray siliceous parts and dark gray to yellowish gray clay parts, and characteristically contain intercalated organic black mudstones. In two horizons in this section yielded the Spathian to Anisian conodont, Neopathodus homeri (Bender) . In contrast, bedded cherts in the Taki section producing the late Spathian radiolarians, Parontactinia sp. characteristically occurred from red cherts and shale partings. Moreover, some of the late Early to early Middle Triassic bedded cherts in T II unit also indicate the same color variation.
Lithologic feature described above suggests that the paleo-oceanic basins under the varied sedimentary environments (ex. oxic/anoxic environments) had occurred in the late Early to early Middle Triassic, and T I and T II units are representative of mutually different paleo-oceanic basins. This indicates that an anoxic sedimentary environment throughout P/T boundary had changed to oxic environment in oceanic basin of T II unit at the earlier stage than that of T I unit.