Geomorphic Development of the Ota River Lowland, Southern Part of the Fukuroi City, Shizuoka Prefecture, Japan

Abstract

The Ota River Lowland is located in the middle part of the Tokai region, Central Japan, extending about 20km from north to south and 12km from east to west. The Lowland is mainly composed of muddy deposits which are carried by the Ota River.The purpose of this paper is to clarify the formative process of the Ota River Lowland. This study is based on the analysis of data from boring, the results of radiocarbon dating, the distribution of archaeological remains, the distribution of the Jori grid-system, and old maps.The sediments of the Ota River Lowland are divided into six units: the basal gravel layer (BG), the lower silt and clay layer (LC), the middle sand layer (MS), the middle silt and clay layer (MC), the upper sand layer (US), and the upper silt and clay layer (UC). The basal gravel layer is considered to represent the river bed gravel in the Last Glacial Age. The lower silt and clay layer was deposited under a low salinity environment and is divided into silt and clay. The silt layer contains a small quantity of shell and the clay layer contains a little humus. The middle sand layer is contrasted to the Holocene Basal Gravel Bed (HBG). The HBG was named by Endo et al. (1982), and indicates the basal gravel layer of the Holocene deposits. The middle silt and clay layer contains a large quantity of shell which shows that the depositional environment was affected greatly by marine water. The upper sand layer is the marine deposits, containing a little biotite, pyrite and magnetite, and is supposed to represent the deposit which is carried by the Tenryu River. The upper silt and clay layer is composed of humic silt and clay, and is considered to have been deposited in a sedimentary environment of backmarsh.In conclusion, the geomorphic development of the Ota River Lowland during the latest Pleistocene to the present is summarized as follows.1) In the latest Pleistocene, the Ota River Lowland was developed in the former fluvial valley formed by the lowest sea level of the Last Glacial Age. Afterward, the former fluvial valley was drowned by transgression, and was filled with silt and clay (about 20, 000-10, 000 years ago).2) About 10, 000 years ago, the sea level dropped because of the cold climate episode and the fluvial sand accumulated in the drowned valley again.3) In the early Holocene, the sea level rose rapidly. As a result, the Ota River Lowland became a bay. After the culmination of the post glacial sea level rise, the sea level remained stable. At this time, the sediments of the Tenryu River drifted into the bay of the Ota River Lowland, and closed the bay mouth (about 10, 000-6, 000 years ago).4) The closed bay became a brackish, swampy lagoon (about 6, 000-1, 900 years ago).5) In the late Yayoi period, the sea level rose again and the sea water flowed into the middle part of the Ota River Lowland (about 1, 900-1, 700 years ago).6) By the seventh century, the Jori grid-system was enforced and the sea level dropped again. However, the water area was left in the southern part of the Ota River Lowland as before (about 1, 700-1, 300 years ago).7) The water area disappeared owing to the Hoei earthquake in 1707 (about 300 years ago).