Turritella(中新世の円錐形貝化石)の配列からみた占環境の推定(予報)

Biostratinomy of the conical shaped shells，Turritella and other molluscans have been examined in the lower Middle Miocene（N8）Tatamigaura Formation in the Hamada area of Shimane Prefecture，Southwest Japan．The Tatamigaura Formation is mainly composed of muddy sandstones showing gradual deepening facies change．This formation occasionaly intercalates well sorted fine sandstone beds，one of which forms apparent Hummocky crossstratification（HCS）．Turritella on the HCS shows crowdly accumulated and their shell apex points dominantly both north and south directions．The shells on muddy sandstones deposited in probable fairweather，were sporadically scattered and their orientation indicates three directions，northeast，southeast and southwest．If this arrangement of shells was formed by current，probably very weak not to form any ripple marks，then the supposed direction of current is from southeast．This sedimentary environment could be under fairweather wave base，