掘削調査にもとづく伊豆半島南部における完新世相対的海水準変化

Abstract

Excavations by drilling, mechanised digger, and peat-corer were carried out in two coastal lowoands near the southern tip of Izu Peninsula on the Pacific coast of central Japan. The Holocene relative sea-level change was determined by sediment facies, 14C dating and environmental assesment of the fauna in Holocene deposits.Izu Peninsula is situated at the northern tip of the Philippine Sea Plate which is being subducted beneath the Japanese island of Honshu at the Suruga Trough along the west coast of the peninsula. Because the tectonic setting is different from other areas facing subduction zones along the Pacific coast, the record at coastal uplift in the Quaternary might also be expected to differ.In the lower reaches of the Okamo River, approximately 5m amsl (above mean sea level), a borehole reached the base of the Holocene marine deposits at-11.3m amsl. Sediment facies analysis and 14C dates of shell and wood samples from the core indicate the transgression reached this location ca. 7, 500yBP, and was followed by a gradual increase in water depth accompanied by deposition of silt and clay, which continued until ca. 4, 000yBP. Water depth shallowed after this time, judged by sandy deposits containing an intertidal molluscan fauna. The upper limit of marine deposits is about 2.0m amsl. Emergence above marine condition is inferred to be younger than ca. 3, 000yBP, because the youngest 14C date of mollusc shells (3, 860yBP) is -5.2m amsl and Kawagodaira Pumice of ca. 2, 800 to 3, 200yBP age is about -1m amsl. In addition, 14C dates of mollusc shells have been obtained from -8.0m amsl (6, 490yBP) and -7.0m amsl (5, 400yBP). The wide-spread Ah tephra of ca. 6, 300yBP occurs at -9.1 to -9.2m amsl. The time of culmination of the Post-glacial (Jomon) transgression is widely believed to have occurred ca. 6, 000yBP but here marine deposits of that age are at -7 to -8m amsl. In most coastal areas of Japan the paleo sea level of ca. 6, 000yBP occurs at an altitude of 2 to 3m amsl or higher, as the highest Holocene marine terrace or emergent shoreline.The upper limit of marine deposits estimated by the sediment facies and molluscan and diatom analysis at three locations beneath the terrace along the Okamo River range from 2-3m amsl. The time of emergence of the terrace inferred from 14C dates of paet, just above marine deposits, and of molluscs within the uppermost marine bed at these locations, is 3, 000-2, 600yBP.At an altitude of less than 5m amsl in the lower reaches of the Aono River, about 4km southwest of the Okamo area, bore hole stratigraphy and 14C dates of wood and mollusc shells show that the transgression reached this location at ca. 14, 000yBP at the earliest. The upper limit of marine deposits in this lowland and the time of its emergence are estimated at about 2m amsl and ca. 2, 000yBP, respectively. Sparse data in this area suggest the upper limit of marine deposits may be slightly lower and younger than those in the Okamo area. The marine deposits of ca. 6, 000yBP are buried about -9m amsl.The presence of uplifted late Pleistocene marine terraces in the southern part of Izu Peninsula indicates a general tendency of uplift at this area through the late Pleistocene period. The present study revealed, however, that the area was subsiding from at least about 6, 000yBP until uplift began at 3, 000-2, 000yBP. The emergence of the two lowlands may have been due to coseismic uplift because the long-term subsidence appear to have reversed and uplift occurred almost simultaneously in both lowland areas.