Stratigraphy of the Tokai Group and evolutional process of the Lake Tokai sedimentary basin

Ancient "Lake Tokai" was a big fluvio-lacustrine sedimentary basin in the late Neogene of Japan. Deposits of the Lake Tokai sedimentary basin are called the Tokai Group. The Tokai Group comprises rather compacted beds of mud, sand, and gravel with many intercalations of volcanic ash and lignite beds and ranges 7Ma (latest Miocene) to 0.8Ma (middle Pleistocene) in fission-track datings of volcanic ash layers. Based on the tephrostratigraphy, lower horizon of the Tokai Group crops out in further southeastern area, and upper horizon in further northwestern area. The basin territory shifted toward the northwest, and the history is divided into five evolutional stages. The basin territory had stretched in a NE-SW trend during the generation, extension, and culmination stages. The elongation axis, of the basin territory had changed from the NE-SW to NW-SE trend of the reduction and extinction stages in latest Pliocene time.