Quaternary Strata in Shikoku Region

The Quaternary strata in Shikoku Region are divided as follows: 1. northern Shikoku; A: Kagawa Prefecture, Ehime Prefecture, B: eastern part of Toyo; C: central part of Chuyo, and D: southwestern part of Nan-yo, 2. central Shikoku along the Yoshino River; E: middle and downstream areas, and F: upper-stream area, and 3. southern Shikoku or Kochi Prefecture; G: southeastern part, H: central part, and I: southwestern part. In these nine subdivided areas, pre-Quaternary to Holocene strata are grouped as follows: 1. The Upper Pliocene to Lower Pleistocene sediments of the pre-terrace formations, located mainly in hill areas, are found in 1-A, 1-D, and 3-G. They include marine sediments distributed along Tosa Bay, and non-marine ones near inland rivers. The strata are conglomerates, sandstones and mudstones, some of which are intercalated with lignite and tephra. 2. The Middle Pleistocene is composed of Higher and Middle terrace deposits which consist of marine and non-marine terraces of higher and middle levels respectively. Higher terrace deposits are rather scarce, and they mainly occur in 1-D and 3-I. They are made of non-marine reddish-brown corroded gravels. The middle terrace deposits are mainly distributed in two areas: marine and non-marine. The marine areas are seen in coastal zones of 3-I, and partly at 1-C. Mud and sand with some marine molluscs and pollen are observed. Non-marine deposits are mainly found along the Yoshino and Shimanto rivers, and at 1-A. 3. The Upper Pleistocene Lower terrace deposits are mainly distributed in 1-A, 3-H and 3-I areas. They include Lower terrace deposits I and II, and are found near the cities of Matsuyama and Kochi. They are composed of fresh gravels underlying Onji (Akahoya) volcanic ash. 4. The Holocene sediments are mainly distributed along large rivers in the Alluvial Plains. They are divided into three parts in the whole Shikoku areas: Lower, Middle, and Upper parts. The Upper and Lower parts are characterized by the coarser facies of non-marine sediments, and the Middle one is characterized by the finer facies of marine sediments, intercalated with Onji (Akahoya) volcanic ash.