地学雑誌
Online ISSN : 1884-0884
Print ISSN : 0022-135X
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三陸海岸・山田平野で得られたコア堆積物の特徴と年代
丹羽 雄一須貝 俊彦山市 剛松島 義章松崎 浩之
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127 巻 (2018) 1 号 p. 89-100

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 Along the Sanriku coastal area, discrepancies in crustal movements have been suggested between uplift on a timescale of 105 years and subsidence on a timescale of 101-102 years. Assuming the cause of these discrepancies and details of tectonic history cannot be clarified immediately, knowledge of the Holocene sedimentary succession with extensive radiocarbon dating data may provide a basic and important guideline for feature analyses of crustal movements in this area. Therefore, a study was carried out on a sediment core, YMD1, acquired from the lower reach of the Yamada Plain, central Sanriku coast. Core YMD1 was divided into three units on the basis of lithofacies, grain size distribution, and molluscan assemblages. Unit 1 (10,000 to 9000 cal BP), consisting mainly of peat or peaty silt, was considered to be marsh sediments. Unit 2 (9000 to 6000 cal BP), consisting of pebbles to silt, showing an upward finning succession with inner bay molluscan shells, was considered to be inner bay sediments. Unit 3 (after 6000 cal BP), consisting of silt to coarse sand, showing upward coarsening succession with marine molluscan shells, was considered to have been deposited in upward-shallowing marine environment after 6000 cal BP. Accumulation rate decreased from 3 to 1 mm/yr between 10,000 and 6000 cal BP, which indicates a landward retreat of the river mouth caused by an expansion of the inner bay environment, and increased to 5 mm/yr after 6000 cal BP, which reflects shoreline progradation. These features of core sediments clearly show that the sediment successions were formed in relation to the postglacial transgression and subsequent regression. Although sediment successions include possible intertidal sediments, data obtained from this study cannot allow for the exclusive interpretation that these horizons are of an intertidal origin. As a result, rate of crustal movement cannot be estimated in the Yamada plain at present. Further investigations are needed to estimate the rate of crustal movement on a timescale of 103-104 years based on analyses of a number of all-core sediments within the plain.

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© 2018 公益社団法人 東京地学協会
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