The Choshi Peninsula, located at the eastern margin of the Kanto tectonic basin, is composed of a series of late Pleistocene marine terraces except for a small older bedrock hill, Mt. Atago. Near Cape Inubo at the eastern tip of the peninsula, the last interglacial terrace is about 60m high, and the average rate of uplift is estimated to be 0.4m/1, 000 years. Holocene terraces are developed in the valley of the Tone River flowing eastward on the north side of the peninsula, and in the Takagami lowland in the Choshi Peninsula (Fig. 1). Excavations were carried out in these Holocene terraces in order to take systematic samples for reconstruction of environmental changes based on facies observation, 14C dates, and analysis of biological assemblages. Along the Tone River (Fig. 2), the Holocene terrace is well developed, and its upper limit of marine deposits ranges from 4.0 to 5.0m amsl. It is estimated that this surface emerged earlier than ca. 3, 500y.B.P. at the latest, judging from the presence of an archeological site of the late Jomon Period. Stratigraphic sections based upon excavation and bore hole are shown in Fig. 3. Takagami lowland, at an altitude of 9-10m amsl (Fig. 4), is overlain by a terrestrial peat bed up to 5m in thickness. The upper limit of the marine deposits, identified by facies observation and diatom analysis, is 4.8m amsl at the eastern margin of the lowland and is slightly lower northwestward (Figs. 5 and 6). In contrast with the unidirectional slope of the surface topography (Fig. 4), the basal topography beneath the Holocene marine deposits includes two buried valleys sloping in opposite directions (Fig. 5), and a former divide is found near Loc. 10. The eastward-sloping buried valley is called “Paleo Takagami Bay”. While the westward-sloping valley is filled with sandy deposits, Paleo Takagami Bay is filled with fine marine silt or clay deposited in a bay-head of a shallow drowned valley. 14C dates from Paleo Takagami Bay are 8, 380 and 10, 410y.B.P. for the basal marine deposits, 6, 520, 6, 960 and 5, 220y.B.P. for an analyzed marine clay, and six dates from the base of overlying peat deposit ranging from 6, 600 to 3, 580y.B.P. (Fig. 5 and Table 1). The transgression resulting in deposition of the marine sediments mentioned above is correlated with the post-glacial (Jomon) transgression. The emergence of Paleo Takagami Bay is considered to have taken place immediately prior to the deposition of the lowest part of the peat, that is, ca. 5, 000-5, 500y.B.P. Analysis of assemblages of molluscs, ostracods, diatoms and pollen from the longest core of Loc. 2 permits the reconstruction of the paleoecology of Paleo Takagami Bay during the Holocene (Fig. 7). Using molluscan assemblage as an example, a brackish water or shallow-marine assemblage from the intertidal zone is found in the lower part of the core. In the middle part of the core, an assemblage consisting of species common to a muddy bottom deeper than several meters predominates. The upper part of the core includes the shallow-marine assemblage characteristic of the base of the core as well as other assemblages characteristic of a shallow, sandy-bottom intertidal environment. Changes in water depth and environment deduced from the molluscan assemblages are concordant with those estimated from the analysis of diatoms, ostracods, and pollen. The upper limit of marine deposits obtained by different indices (Fig. 7) shows a good agreement, that is 3.3m amsl by facies observation, 2.3m by molluscan assemblages, 1.8m by ostracods, and 3.3m by diatoms. Three stages in the paleogeographic evolution of the Choshi Peninsula are depicted in paleogeographic maps in Fig. 8. Stage I is the early stage of the Jomon transgression.
This paper presents the following matters about the Fe-Ti oxide minerals contained in the 75 tephras, which are about 70% of marker-tephras sedimented on Oiso Hill, 25km east of Hakone Volcano. Examination of the content, occurrence and type of Fe-Ti oxide minerals clarifies the following features: 1) There is a positive correlation between the refractive indices of orthopyroxenes and the TiO2 content of titanomagnetite. 2) In many cases, the TiO2 content of ferromagnetic minerals enable us to identify each tephra. Then it is shown that the tephras, which can't be identified by similar TiO2 content, can be identified by the combination of the ratios of ferromagnetic minerals, Mn/Fe and V/Ti, V/Mn and Ti/Zn. Moreover others, which can't be identified, can be distiuguished by the content of ferromagnetic minerals, that of β-phase, and the ratio of attached minerals.
The plant macrofossil assemblage from terrace deposits, 18, 340±430y.B.P. by radiocarbon dating in Tado-cho, Kuwana-gun, Mie Prefecture in central Japan, is composed of many remains of subarctic and cool-temperate conifers, i.e., Larix leptolepis, Pinus koraiensis; cool-temperate and warm-temperate conifers, i.e., Tsuga sieboldii; some remains of cool-temperate deciduous broadleaf taxa, i.e., Fagus crenata; cool-temperate and warm-temperate deciduous broadleaf taxa, i.e., Betula grossa, Quercus serrata; many remains of marshy herbaceous taxa; etc. Judging from this fossil assemblage, it is presumed that there was a cool-temperate mixed broad-leaved deciduous and coniferous forest about 18, 000 years ago.