栃木県喜連川丘陵の谷壁斜面を刻む最終氷期後半の化石ガリーとその埋積過程 Hillslope Gullying and Filling during the Latter Half of the Last Clacial Age in the Kitsuregawa Hills, Central Japan
Many buried fossil gullies which were eroded in the past and subsequently buried by colluvial and eolian deposits including tephra layers, have been preserved on the upper parts of valley-side slopes of the Kitsuregawa Hills in the northern part of the Kanto Plain, Central Japan. This paper discusses burying processes of fossil gully formation and the climatic changes that induced the changes in geomorphic processes from gullying to filling on the hillsides in the Latest Pleistocene.<br>Observation of the facies and stratigraphy of deposits which fill the fossil gullies reveals the following features:<br>1. Three tephra layers, Ogawa Scoria (OgS, 22ka), Imaichi Pumice (IP, 12-13ka), and Shichihonzakura Pumice (SP, 12-13ka), are intercalated without disturbance in the deposits which fill up the fossil gullies; Kanuma Pumice (KP, 31-32ka) layer is not intercalated in them.<br>2. The fill deposits in the Last Glacial period, which are lower than IP layer, are divided into upper and lower at the horizon immediately below OgS layer.<br>3. The lower fill deposits mostly contain gravel, along with, in some cases, blocks of KP as secondary deposits.<br>4. The upper fill deposits are yellowish brown soft loam intercalating OgS layer, which resemble the same horizon of the eolian deposits on the terrace surface; in many places, however the upper fill deposits contain a small amount of gravel.<br>5. Distinct tephra layers are generally lacking on the upper parts of the valley-side slopes outside the fossil gullies, which are covered with only thin black soil.<br>6. The present ground surfaces of the fossil gullies do not show depressions or hollow forms.<br>The Above six characteristics are common in many fossil gullies. It seems clear that chief causes of the end of gullying and the change of sedimentary facies are not changes in eolian deposit supply but changes in climatic conditions.<br>Climatic change, from the interstadial to the stadial in the late Last Glacial, induced the end of gullying and the start of filling. During the interstadial of the Last Glacial Stage, the upper parts of valley-side slopes in the Kitsuregawa Hills underwent erosion, as a consequence of temporary rainfalls. After the fall of KP, the intensity of rainfall was reduced, but the gullyfilling deposits which had been supplied by active soil creep from the surrounding slopes and small failure on gully-side and gully-head slopes renined. Immediately before the fall of OgS, the intensity of rainfall subsided again, causing inactive failure of the gully-side and gully-head. Then the gully-fill deposits changed to eolian and soil creep origin.
第四紀研究 35(5), 359-371, 1996-12-01
Japan Association for Quaternary Research