琵琶湖の古環境変遷に関する新知見：烏丸深層ボーリングコアの堆積環境解析結果から

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  • New findings concerning the paleoenvironmental change of Lake Biwa based on the results of facies analysis for the Karasuma Deep Core
  • ビワコ ノ コ カンキョウ ヘンセン ニ カンスル シン チケン : カラスマ シンソウ ボーリングコア ノ タイセキ カンキョウ カイセキ ケッカ カラ

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As a result of sedimentary facies analysis for the Karasuma core drilled on the eastern shore of Lake Biwa, three major events could be recognized on the paleoenvironmental change over the past 1.8 million years (Masuda et al., 2021). (1) About 1.8 million years ago, this area was changed from a narrow valley to the lake delta environment in the rise of the stagnant water of natural dam. (2) About 1 million years ago, the depositional rate in this area began to increase. Large river deltas had moved from the east to fill the main valley above the basement, in addition to previous sedimentation by the small river that flowed in from the north. It established the South Lake as a drainage channel of Lake Biwa and the water area further extended to Katata area on the west coast. (3) After about 0.5 million years ago, the remaining basement ridges in the North Lake submerged and the lake area expanded. In the Karasuma area, the lake water has spread over the delta, and the deltaic environment developed again. The background of such environmental changes is the continuous rise of lake water level caused by the uplift of the western basement blocks by repeated seismogenic faulting. Thick strata in Lake Biwa was formed by sedimentation of large river deltas with much sediment yield and in addition by the tilting tectonics of uplifting of basement mountain side and subsiding of lake side at Biwakoseigan Fault Zones (Lake Biwa Western Sublacustrine Faults). Further, deposition of fluvial gravel beds in the upper part of the Karasuma core were controlled by precipitation changes with millennial cycle of the Late Quaternary glacial climate.

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