Reexamination of ancient ripple mark designated as a prefectural natural monument, preserved in the Miocene gravel bed in Shimonohama Kami-cho, Hyogo Prefecture based on facies analysis

Abstract

Sedimentary-originated lineament referable to ancient ripple mark and designated as a prefectural natural monument is preserved in the base of the Miocene gravel bed of the Toyooka Formation assignable to the Hokudan Group in Shimonohama, Kamicho, Hyogo Prefecture. Facies associations show that most of the Miocene deposits in the Shimonohama area are thought to be fluvial, flood plane, debris flow, and volcaniclastic flow in their origin. Based on shape analysis, the lineament is interpreted to be referable to the gutter cast, a kind of flute cast formed under the bottom of channel of gravelly river, not to the ripple mark resulted from a wave movement as assumed previously.