房総半島富津砂州の形成に伴う完新世の貝類群集の変遷

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概要

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• Five marine molluscan assemblages are identified in the present shallow sea bottom around the Futtu Spit on the west coast of the Boso Peninsula and in the Holocene marine sediments below the spit.(1) A tidal flat assemblage characterized by Crassostrea gigas, Batillaria multiformis and Cyclina sinensis is present in the sediments on Pleistocene bedrock. (2) An inner-bay mud-bottom assemblage composed of Raetellops pulchellus, Macoma tokyoensis and Dosinella penicilata is found in the mud sea bottom deeper than 12m around the spit and in the muddy sediments below the spit from 20m to 30m in depth, which filled the depression of the basement. (3) An inner-bay sand-bottom assemblage dominated by Musculus senhousia and Tapes philippinarum is distributed in the sand sea bottom from 0 to 12m in depth to the north of the spit. M. senhousia often forms mats and covers the sea bottom. (4) An inland-sea sand-bottom assemblage consists of Mactra chinensis, Phacosoma japonicum and other outer sea molluscans. It is present in the sand bottom with a depth shallower than 15m and in the sand sediments which form the spit. (5) An inland-sea sand-and-mud bottom assemblage composed of fragments of Kellia porculus and Tonna luteostoma is found only in the westernmost core below 10m.These molluscan assemblages change with change in the sedimentary facies. From 10, 000y. B. P. to 5, 000y. B. P., the sea level rose rapidly, and the Pleistocene bedrock was submerged. The tidal flat assemblage occurred on tidal flats overlain by mud, and the inner-bay mud-bottom assemblage was present in the mud sediments. From 5, 000y. B. P. to the present, the sea level has dropped slightly and the spit has prograded westward. The inland-sea sand-bottom assemblage was present in the sand sediments which form the spit. This assemblage is also found in the shallow water to the south of the spit. After the spit was formed, the inner-bay sand-bottom assemblage appeared in the shallow water to the north of the spit.

著者

• 茅根 創

  地質調査所

• 茅根 創

  地質調査所海洋地質部

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