横浜付近の下末吉面の陸化過程
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概要
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The Shimosueyoshi terrace in and around Yokohama is a marine terrace in which a former shoreline is located at its western boundary. The terrace is composed of the marine Shimosueyoshi formation. Its original surface named “Shimosueyoshi surface” in this paper, is the depositional surface of the Shimosueyoshi formation in the middle and eastern zones of the area, and the wave-cut platform in the western zone near the former shoreline. The surface is conformably covered with the Shimosueyoshi Loam which is weathered materials of the air-fall ash. The pumice beds in the lower part of the Shimosueyoshi Loam have been dated as 120, 000-130, 000F.T.y.B.P. In this paper, the emergence of the Shimosueyoshi surface is discussed in detail through the author's investigation of the Shimosueyoshi Loam based on the tephrochronological method and the observations of the Shimosueyoshi formation.The lower members of the Shimosueyoshi Loam (SP0, SP1, SL2, SP2 in ascending order) are useful in indicating the positions of the shorelines at the time of ash fall. As shown in Fig. 3 and 8, SP0 rests immediately on the Shimosueyoshi surface at the former shoreline. The western zone of the Shimosueyoshi surface is capped with SP1 and the succeeding zones in the east are mantled with SL2 and SP2. Therefore, it is understood that the emergence proceeded eastward successively.The paleogeography at the time of the emergence (Fig. 9) is inferred by the observations of facies in the upper member of the Shimosueyoshi formation. At the transgression peak just before the fall of SP0, the wave-cut platform and sea cliff were formed at the former shoreline. Then, as the emergence proceeded the flat sea-floor of the wave-cut platform became the marshy plain in the western zone of the Shimosueyoshi surface. In the middle and eastern zones, the slightly undulating sea-floor had been formed by the accumulation of sand and silty sand. The seafloor emerged and changed into the marshy plain in the middle zone. But, there remained a bay at the hollow of the Shimosueyoshi surface. The bay was embayed by the two narrow ridges. The eastern one of them is a beach ridge running from NNE to SSW in the eastern zone.In this area, generally speaking, the shallow sea-floor emerged unless being covered with beach deposits or deltaic materials except the eastern zone of the Shimosueyoshi surface. Such process of the emergence resulted from the lack of the rivers which supplied sufficient materials for the development of beach ridges and vast deltas.