岡山県勝山－刑部地域の地質 : 堆積性およびダイアピル起源の混在岩

On the basis of the results of field surveying and microstructural study, we describe here the occurrence of chaotic rocks formed by diapiric processes, and the structural relationships between the Ohsayama serpentinite mélange, the Permian olistostrome, and the Sangun metamorphic rocks. The Ohsayama serpentinite mélange is distributed in the Osakabe area and was defined by Tsujimori (1998) as a tectonic mélange with a serpentinite matrix. It contains blueschist-facies schists with a metamorphic age (320 Ma) older than that of the Sangun metamorphic rocks (180 Ma), and is traceable to the north of the Permian olistostrome. The Permian olistostrome is distributed in the Katsuyama area and consists of submarine sliding deposits with mudstone, sandstone, alternating beds of sandstone and mudstone, and limestone blocks. In addition to these rocks, we newly found chaotic rocks of diapiric origin. These chaotic rocks are characterized by a predominant mud matrix and non-sorted clasts with sizes ranging from 1 mm to more than 10 m. The distribution of the chaotic rock body cuts the bedding planes of the other strata at a high angle. The clasts have irregular shapes, a zigsaw-puzzle structure, and disaggregated wisps. These features indicate that the clasts were broken and driven upward by overpressured fl uids. The Sangun metamorphic rocks, which are of a high pressure type and commonly show three generations of foliation, underlie both the Ohsayama serpentinite mélange and Permian olistostrome with a fault contact and form syncline trending E-W and plunging 7°to west at the northeast part of the surveyed area.