陸前高田平野の沖積層と完新世の海水準変化

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Sanriku Coast located northeastern part of Honshu-island is one of the typical “rias coast” in Japan. In this area, a small delta plains develop at each enbayment. The Rikuzentakata coastal plain, about 2km from north to south and about 2.5km from east to west, is larger one of these plains. The most part of the plain is lower than 5 meters above the sea-level. A beach ridge is recognized along the present shoreline, and a lagoonal area locates behind it. The Kesen river and the Hamada river flow on the western and eastern part of the plain, respectively (Fig. 1).The sequence of Alluvium along the shoreline is shown in Fig. 4. There are two buried valleys corresponding to each river. Excepting the Basal Gravel, which recognized on the buried valley floor about 10m in thickness, the thickness of Alluvium is about 28m. From their textural characteristics, Alluvium is devided into four layers as follows.(i) Lower Sand layer includes marine shells and shell-fragments, is considered as foreset bed sediments. Shellfragments collected at the upper part of the layer, were dated 7, 660±170 yr B. P. by radiometric method.(ii) Middle Mud layer, including a lot of marine shells, consists of silt or sandy silt. This layer is considered as bottomset bed sediments. At the period of Middle Mud sedimentation, the rate of sea-level rise had been superior to the rate of sediment supply. This period was estimated between 7, 500 and 5, 000 yr B. P.(iii) Upper Sand layer also includes shell-fragments. Sedimentary environment of this layer is considered as delta front (foreset bed). The relative rate of sediment supply during this period, exceeded the rise of sea-level gradually. In other wards, the sea-level rise became slower or nearly stable.(iv) Uppermost layer consists of fluvial deposits. At the primary time of this sedimentary period, the sea-level was slightly dropped (2m-4m) than the final sea-level which recorded at the top of Upper Sand layer (-2.7m a. s. l.). Several shallow valleys, dissected the Upper Sand layer, indicate above.From these, the Holocene sea level change curve was restored as the solid line in Fig. 7. On the other hand, annual subsidence about 0.8mm was calculated from the remeasurement of bench marks (Fig. 6) in and around the plain. If this subsidence have been assumed during Holocene, the sea-level curve would be expressed as the broken line in Fig. 7. The highest sea-level during Holocene is estimated at +1.3m a. s. l. and the age is considered about 5, 000 yr B. P.
東北地理学会の論文

陸前高田平野の沖積層と完新世の海水準変化

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