Abstract
Lower Cretaceous successions distributed in the Nariwa Area, Okayama Prefecture, have been redefined as the Hayama Formation. The type locality is Hayama Village, which is situated at the center of the study area. The Hayama Formation is characterized by stacking of fining-upward units which are composed of conglomerate, sandstone and reddish mudstone from bottom to top. The sandstone parts are poorly developed and often lack in the sedimentary cycles. The formation itself shows a fining upward succession as a whole and is divided into two members ; the lower Eda Conglomerate Member and the upper Sora Mudstone Member. The Eda Conglomerate Member is dominated by conglomerate parts of the units. On the other hand, the Sora Mudstone Member is mainly composed of reddish mudstone parts of the units. The boundary between these two members is not clear but gradational. The deposits of the formation are interpreted as fluvial deposits based on sedimentary facies analysis. Semi-arid type climatic environment is supported by the occurrence of carbonate concretions which are formed in the floodplain mudstone. Fission-track dating was carried out by analyzing a felsic tuff sample from the Sora Mudstone Member and 101±4 Ma indicating Albian age was obtained. Paleotopography was reconstructed by tracing out the bottoms of the Hayama Formation. Obtained contour map demonstrates a north to south trending valley, and imbrication of gravels indicate paleocurrents to the south. The valley was filled with 300 m-thick fluvial deposits of the Hayama Formation during Albian time.
