相模湾沿岸の砂丘の組成・時代などに関する若干の知見

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Sedimentological and geomorphological studies of the sand dunes are made at the coastal area along the Sagami Bay, Kanto District. An attempt is made to distinguish between beach and dund sands, to explain the distribution of textural parameters, mineral composition, shape and thickness of dune sands and their significance, and to classify chronologically the sand dunes. The main results are summarized as follows: 1) Sedimentological analysis of the dune sands in process of formation and the present beach sands are attempted to obtain the criteria for the recognition of the depositional environment and agency of ancient sands. The comparison of adjacent pairs of beach and dune sands from 8 localities in this area shows that it is generally possible to distinguish between the two as shown in Table 1 and Figs. 2-8. Namely, the dune sands are usually rounder than the adjacent beach sands and have a higher content of heavy minerals. As to the grain size distribution, the median diameter of dune sands is generally smaller than that of the bordering beach sands and the silt content of the former is larger than the latter. Moreover, the dune sands tend to be better sorted than the beach sands. These differences between beach and dune sands may be originated from that the dune sands are deposited as the result of selective transportation of on-shore wind from the adjacent beach. 2) Table 3 and Figs. 9-12 show the distribution of the textural parameters, mineralogical composition and roundness values of dune sands from 50 localities given in Fig. 1. It is obvious that the median diameter is largest in the vicinity of the mouth of the Sagami River and decreases towards the both sides of the river, especially towards the east. This tendency is similar to the median diameter distribution of the present beach sands which are thought to be transported by the eastery long shore current. The median diameter also decreases northwards from the coast, and the silt content and roundness values are higher in the northern area than in the south. Such a tendency may be caused by that the sand dunes situated at the inland are older than at the south, therefore, the weathering process is more remarkable at the former. 3) The coastal dunes of this area can be classified into four groups according to the arrangement of dunes, the thickness of weathered zone with A horizon, the condition of vegetable cover, the presence of volcanic ejecta and the radio-carbon dated drift-wood (Fig. 13). The sand dunes of A group are densely vegetated and has thick weathered zone about 1 m in thickness. They may be the oldest dunes in this area. This group is covered by the pumice fragments which erupted in A.D. 1707 from Mt. HOEI, a parastic volcano of Mt. FUJI. The potterey of Jomon type is also discovered on the some localities of this group. B group sand dunes have the weathered zone which is 30-50cm in thickness, so may be younger than A group. The drift-wood is discovered in the beach deposits which underly B group sand dune. Radiocarbon date of this wood is 1950±90 years B.P. (Gak-687). The pumice fragments in 1707 cover also this sand dunes. Sand dunes of this group, therefore, were formed between about 2000 years B.P. and 300years B.P. C group sand dunes at the southern part of B group have the very thin weathered zone. The sands of this group are intervened by the pumice fragments and black sands of scoria erupted in A.D. 1707. Accordingly, they are in process of formation at the time of this volcanic activity. The youngest sand dunes are D group which have no vegetable cover, weathered zone and volcanic ejecta. So that, the formation of this group began after the eruption in A.D. 1707 and has been still continued at present. 4) The thickness of dune sands and the altitude of the base beneath the dunes are obtained from the field observation and data of boring cores, using the above mentioned criteria of the distinction between the dune and beach sands (Figs. 15-18). The thickness of the dune sands varies from about 3 to 15 m and tends to increase towards the north. The alluvial beach deposits always can be recognized under the dunes. Their altitude lowers from the margin of the upland towards the shoreline at the eastern area of the Sagami River. This shows the successive lowering sea level during the progressive regression after the time of the post glacial transgression.
横浜国立大学の論文
1968-01-25

相模湾沿岸の砂丘の組成・時代などに関する若干の知見

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横浜国立大学 | 論文

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