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Paleoenvironmental reconstruction on the Mezu Formation, Minabe Town, Wakayama Prefecture, Southwest Japan
Bulletin of the Faculty of Education, Wakayama University. Natural science
The Mezu Formation distributed in the southwestern part of the Kii Peninsula, uncomformably overlies the Otonashigawa Group and the Tanabe Group. The Mezu Formation was first mentioned by Hatenashi Reseach Group (1971) . The early study on the formation was only the study on the gravel composition (Hatenashi Reseach Group, 1973). Therefore, this study attempts to clarity the stratigra- phy, the sedimentary environments, paleoclimate and the geologic age based on the sedimentary facies analysis and the fossil pollen analysis. The study area is Mezu area to the south of Route 42 in Minabe Town. In this study sedimentary logs were made for the sedimentary facies analysis. The Mezu formation mainly composed of conglom- erate, and subordinately of sandstone and mudstone lenses. Sediments in the study area were classified into 9 sediment types on the basis of sedimentary structure and the grain-size. Strata mostly dip gently, but steeply in some area. Fine-grained sediments in the study area are characterized by the abundance of coaly sediments, and the development of paleosols and the soft-sediment deformations. The Mezu Formation is classified into 3 members, namely, the Lower, the Middle and the Upper Members in ascending order based on lithofacies and the sedimentary facies. The Lower Member is 1 m in thickness and consists mainly of poorly sorted and massive conglomerate. The Middle Member is 14m in thickness and consists of trough-cross bedded conglomerate and sandstone. The Upper Member is 50m in thickness and consists of trough-cross bedded conglomerate and mudstone. Coaly sediments in the Mezu Formation yield fossil pollen and leaf fossils. Pollen zone is divided into 3 zones, namely, M-1 , M- 2 and M- 3 zones. M-1 zone corresponds to the Lower and the Mid- dle Members, and M-2 and M-3 zones correspond to the Upper Member. The geologic age of the formation assigned to the middle to late Miocene. Based on the sedimentary facies analysis and pollen analysis, the Mezu Formation is supposed to have been formed by braided rivers on the outer part of the alluvial fan. And the paleoclimate in the sedimentation stage of the formation was temperate comparatively.
Departmental Bulletin Paper
400 Natural Science