Depositional environments and their changes analyzed from the Karasuma Deep Core on the side of Lake Biwa, Shiga Prefecture

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  • 琵琶湖岸，烏丸コアの堆積相解析と堆積環境の変遷
  • ビワ コガン,カラスマ コア ノ タイセキソウ カイセキ ト タイセキ カンキョウ ノ ヘンセン

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The changes of the depositional environment during these 1.8 million years were clarified for the Karasuma Deep Core of 915 m long stored in the Lake Biwa Museum, Shiga Prefecture based on sedimentary facies analysis. The following transition images were obtained: (1) About 1.8 million years ago, the water of a natural dammed lake flooded into a narrow tributary valley with talus cones, and as the water level rose, the valley lake changed to a drowned lake. Approximately 1.6 million years ago, the continuous rise in water level gradually changed the elongated body of water into a wide lake, and small rivers flowed into it. (2) Around 1.35 million years ago, shallow lakes expanded further as the water level continued to rise, and rivers carrying a large amount of sand from the granitic area of the basin developed a sandy delta. (3) Since about 1 million years ago, the amount of muddy sediment from rivers has increased and creating a muddy deltaic environment. The possible cause is the progradation of eastern big river deltas. (4) About 750,000 years ago, rivers gradually began to carry coarse-grained gravel, and sedimentation proceeded to change to a lakeshore plain environment. This change may have been strongly influenced by the river basin expansion and the tectonic uplift of the mountains. This river is the predecessor of the modern Yasu River, and from this period to tens of thousands of years ago, the river flowed into the Karasuma peninsula area. (5) Approximately 550,000 years ago, this area changed to a lake delta environment again, and around 400,000 years ago, the water level increased and the amount of sediment carried by the river increased, and a large gravelly lake-delta developed. (6) Around 100,000 years ago, this point changed to the fluvial channel and floodplain environment of the Yasu River, and the river moved northward in tens of thousands of years ago, resulting in the current lakeside lowland environment. Then, the thick strata of the Karasuma Deep Core were formed by the repeated delta movements of forward and backward with the lake level change.

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