Geomorphic Development of the Tokoro River Lowland

Abstract

The Toko River Low]and, about 10 km long from south to north and 5 km wide from east to west, is located in the northeastern part of Hokkaido Island. The plain mainly consists of alluvial sediments of the Toko River, which are classified upper alluvial surface (Tosa surface) and lower alluvial surface. The upper alluvial surface is dissected in its northeastern part and composed of sandy sediments covered with Tokoro Volcanic Ash I-IV. The lower alluvial surface distributes along the Tokoro and Rai-Tokoro Rivers and its sediments are composed of sand and peaty silt. The geomorphological history of the Tokoro River Lowland in the latest Pleistocene and Holocene Periods is inferred as follows; (1) In the latest Pleistocene, former fluvial valleys were drowned by transgression and were filled with sandy and silty sediments. (2) In the early Holocene, sand spit or sand bar was formed at the mouth of embayment. (3) With progress of transgression, an embayment had extended to the southern part of the lowland. And very fine sediments such as clay and silt are deposited in the bottom of the lagoon. (4) A delta composed of sandy sediments was formed towards the northern part of the lowland around 5,500 y. B. P. (5) About 4,000 〜 3,OOO y.B.P., a slight regression caused dissection of the delta and formed Tosa surface. (6) After following transgression, the lower alluvial surface was formed covering with peat and silty sediments.