山陰西部の鮮新－更新統都野津層群 : 模式地周辺地域における層位学的・堆積学的新知見と今後の課題

We divide the Tsunozu Group (ca.4.1-O.7Ma) into the Tsunozu-, the Shimanohoshi- and the Murogamiyama Formations, separated by a regional unconformity to each other. That between the former two formations, slightly angular, is called the 1-Ma unconformity . The Tsunozu Formation in the most part of the surveyed area comprizes brackish lagoonal siliciclastic deposits (marine deposits) and fluvial ones (non-marine deposits) which formed in the lower to lowest streams of north-trending many braided, rather small rivers, whereas in the northeastern part it consists mostly of the marine deposits. Dacite pyroclastic flows and volcanic breccias are locally contained in the upper marine deposits. The uppermost horizon of the formation is represented by coastal dune sands, suggesting the environmental change of depositional site toward coastal zone of an open sea. The Shimanohoshi Formation comprizes conglomerates and sandstones which deposited in the middle to lower portion of a north-trending large, wide river system developed on uplifed land due to the 1-Ma unconformity. It is characterized by a lot of volcanic materials, possibly relating to volcanism of the Oe-Takayama Paleo-Volcano. The Murogamiyama Formation represents a new transgression following the depositional time of the Shimanohoshi, consisting of coastal dune sands supplied from the east-northeast. The frequent transgressinos and regressions within the Tsunozu Group must have been related to global eustacy and/or local subsidence and uplift, and this is remained still open for further detailed researches as a significant task for the Tsunozu Group.