岡山県大賀地域の非変成古生層と上部三畳系成羽層群との間の不整合の発見

Late Triassic strata of the shelf facies, called the Nariwa Group, are widely exposed in the Oga area, Okayama Prefecture. They clino-unconformably overlie the Sangun metamorphic rocks in the S. The Nariwa Group and the Sangun metamorphic rocks have hitherto been considered to be thrust over by the Oga Nappe on the N, which is composed of limestone dominant non-metamorphic Paleozoic strata. However, unconformity between the non-metamorphic Paleozoic strata and the Nariwa Group is discovered at 3 localities. The Nariwa Group forms a synclinorium and unconformably overlies the older strata both on the N and the S. Therefore, a post-Late Triassic large thrust sheet, called the Oga Nappe, does not exist. the Oga area has gone through the same tectonic history as the Maizuru Zone and the Mine area where Triassic strata unconformably cover the Paleozoic strata that had been disturbed during the Akiyoshi series of orogeny. -English summary