New Outcrops of the Hottate River Retarding Basin in Yonezawa City, Northeast Japan.

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  • 山形県米沢市の堀立川遊水地に現れた地層
  • ヤマガタケン ヨネザワシ ノ ホッタテガワ ユウスイチ ニ アラワレタ チソウ

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An outcrop appeared around the Hottate River Pond excavation site in the southern part of Yonezawa City, Yamagata Prefecture. This outcrop consists mainly of an alternation of sandy mud and silty mud, including many wood and plant fragments and fossil cones of Picea bicolr Mayr. The 14C age of the sediments is about 18, 000 to 15, 000cal yrs BP, indicating that these sediments were deposited in the later part of the Last Glacial period. The volcanic ash intercalated in the upper part of the sediments is identified as the Asama-Kusatsu (As-K) tephra.<br>Almost all of the sediments were deposited under conditions of a back marsh and in the stream of the old Mogami river which flowed through the western area of Yonezawa City at this time. From 15, 000cal yrs BP, the old Mogami river changed to its present course through the eastern area of Yonezawa City. After this time, only few eolian sediments were deposited in the uppermost part of the outcrop.

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