Late Pleistocene sedimentary environment changes of the Takatomi lowland in Gifu Prefecture, central Japan

Abstract

The authors collected numerous bore and all-core samples and analyzed widespread volcanic tephras and fossil diatoms to investigate the changes in the sedimentary environments of the Takatomi lowlands located in the northern part of Gifu City, and obtained the following conclusions. The Takatomi lowlands is composed of muddy lacustrine sediments and pumiceous fluvial sediments, called the Takatomi Formation. The Takatomi Formation is subdivided into the Takatomi basal gravel bed, the lower muddy bed, the pumiceous sand bed, and the upper muddy bed, from the lower to the upper levels. Widespread tephra called Aso-4 (87 ka), AT (26-29 ka), and K-Ah (7.3 ka) derived from volcanos in southern Kyushu were detected in the upper muddy bed. The fluvially rounded pumice particles obtained from the Takatomi pumiceous sand bed are correlated to On-Pm 1 and On-Yb tephra (about 100 ka) of the Ontake Volcano located in the upper part of the Kiso River (drainage) basin. The Takatomi basal gravel bed was deposited in an alluvial fan environment, and the lower and the upper Takatomi muddy beds were accumulated in a shallow freshwater lake environment. The Takatomi pumiceous sand bed is considered to have been transported by the Kiso River from the upper part of the drainage basin into the shallow freshwater lake of the Takatomi lowlands at about 100 ka, because no volcanic sediments are found in the drainage basin of the Toba River, which drains through the Takatomi lowlands today. The old Kiso River seems to have flowed westward through Seki City to the Takatomi lowlands, together with the Nagara River, and then turned to south to the lower Toba River. The Nishi-Fukase site, where the all-core bore deta were obtained, has been in a stagnant marsh environment with the accumulation of peat layers since 31 ka. The Nagara River probably flowed southward down from the Umehara Fault in the Takatomi lowlands, along the present course of the lower Toba River, until modern times.