広島県君田地域に分布する備北層群 : 非海成堆積物の堆積環境について

その他

The Middle Miocene Bihoku Group is sporadically distributed in the Chugoku-district，southwest Japan．Non-marine sediments are recognized of the base of the Bihoku Group in many regions．In the Miyoshi and Shobara regions，it is called“Shiomachi Formation”．
In the Kimita region，non-marine sediments of the base of Bihoku Group are described sedimentologically，and five sedimentary facies are recognized on the basis of facies analysis．The sedimentary facies indicate sedimentary environments such as talus，alluvial fan，fan-delta，and saline swamp．
The mud，muddy sand and sand interleaved in conglomerate beds were analyzed for C，N，S contents．The results indicate that the sedimentary environment of the muddy sand including lignite in Facies C was a swamp under fresh water，and the sand including lignite in Facies E was deposited in a saline swamp such as a mangrove swamp．
As a result of facies analysis and C，N，S element analysis，we consider that the basal conglomerate of the Bihoku Group formed a alluvial fan at the first stage，and changed to fan-delta by relative sea level change．
In Kimita area，the non-marine sediments of the conglomerate bed should be separated from the non-marine“Shiomachi Formation”，considering the palaeoenvironmental differences．