丹波帯の中・古生界の年代と構造形成(<特集>日本の中・古生界研究の発展と現状)

その他

It has been deciphered that the Tamba Group consists of two distinct suites of stratigraphic successions (and Type I and Type II) and that Type II is distributed in the synform, while Type I is developed in the antiform. Type II is composed of mainly Permian (partly Late Carboniferous) chert-greenstone formation and Early to Middle Jurassic mudstone formation which includes olistostrome and these formations are tectonically repeated within this suite. Type I suite is mainly composed of Triassic to Early Jurassic chert formation and Late Jurassic siliceous-black shale formation (including olistostrome) and they are regarded to be repeated tectonically within this suite Type I and II suites are regarded to be formed separately in two mutually different paleo-oceanic sub-basins and the Type II suite tectonically overlies the Type I suite as a nappe, prior to the upright-folding, probably Late Jurassic or Cretaceous. Synforms and antiforms in the Tamba Belt reveal a strongly asymmetrical distribution of the strata between northern and southern wings, concerning the lithologic feature, limb-dipping and width of the two wings. These lines of evidence led to the following hypothesis: Type I and II suites are composed of tectonically repeated several units which dip gently southward, in the course of construction of the "two-storyed structure". Recently, it has been clarified that the Permian clastic formations are distributed in a certain belt along the south of the Maizuru Belt. These formations which are called Oi and Katsumi Formations are different from the Tamba Group and its equivelent in the Tamba Belt, while they have certain a similarity with the contemporaneous strata of the Maizuru Belt. On the other hand, the Kato Formation, which is distributed south of the Oi Formation, has a certain similarity with the Type II suite so far as its age and lithology are concerned. The presence of the Permian clastic facies along the southern margin of the Maizuru Belt offers a new important information in connection with the examination of the relationshio between the Tamba and the Maizuru Belts.