Facies and paleoenvironment of the lower part of the Bushi Formation from Kaji Hill, on the western Kanto Plain

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  • 関東平野西縁,加治丘陵に分布する下部更新統仏子層下部の層相と古環境
  • カントウ ヘイヤ セイエン カジキュウリョウ ニ ブンプ スル カブ コウシントウ ブッシソウ カブ ノ ソウソウ ト コ カンキョウ

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Abstract

The Hanno Formation, Bushi Formation and Toyooka Formation are distributed on Kaji Hill, a hill on the western edge of the Kanto Plain, central Japan. The Lower Pleistocene Bushi Formation is composed of silt, sandstone, and conglomerate, as well as interbedded tephra. The authors carried out a study of the lower part of the Bushi Formation, dividing it into Units 1 to 5 in ascending order, based on erosional surface, facies, sedimentary structure, and fossils. And after examination of seven key tephra beds, the variations in facies of each unit were recognized. Unit 1 was deposited in tidal flat and swamp, Unit 2 were various sedimentary environments, Unit 3 was deposited in estuary and tidal flat, while Unit 4 was deposited by braided streams, the western area of Unit 5 has alluvial fan sediments, while on the east there are swamp or marsh sediments. Unit 2 is further divided into four subunits; 2-a to 2-d in ascending order. Unit 2 begins with braided river and swamp facies of Subunit 2-a; progresses to the estuary and tidal flat facies of Subunit 2-b; though the ravinement surface; then the Subunit 2-c -Gravel with rounded cobble- a transgressive lag, and finishes with littoral facies of Subunit 2-d. Therefore, Unit 2 was deposited under the strong influence of a sea-level change. The transgression which brought this was probably the first large-scale transgression after the deposition of the Hanno Formation. Based on the results of fission track dating of the tephra, and comparison of the strata at each location, the age of this transgression is about 2.0Ma.

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