| 概要 |
The approach to the problems of the Alluvium in the Oita district has become possible by increasing knowledge on the subsurface geology based mainly on boring data, which have been accumulated through... efforts for the construction of the industrial area on the near-shore flat in the district. The terrestrial and submarine topography of the Oita district indicates that the Alluvium of the area is the complex deltaic sediments by the Oita and Ono rivers. The lighological analysis of subsurface data readily leads to the three-folded division of the Alluvium, the upper sand (Ia), .m iddle mud (lb) and lower alternation of sand and mud (II). A distinct pumiceous ash at the top of the lower bed (II) extends from the area of the up~stream to the ancient strand, the latter of which lies about 25 m below the sea level. This feature, together with -25 m conversion level of the submarine terrace of the north coast, suggests the sea-level stabilization at that level. Furthermore the compaction represented by N-number of argillaceous beds is remarkably different between lb and II, besides the thin boundary zone of higher compaction between lb and II suggesting the time-gap. Consequently the beds are grouped into the Alluvium I and II on the basis of the sedimentological and stratigraphical facts just mentioned. At the off-shore area between the mouths of the two rivers the Alluvium I shows. the thickness of 35 to 50m from in-shore to off-shore including Ia of the upper 9 to 17m and lb of the lower 25 to 45 m. The Alluvium I extends landwards into valleys with some marine tongues at the lower part. The Alluvium II consists of sand, muddy sand and mud, of which the arenaceous and argillaceous beds predominate respectively at the upper and the lower part. The Alluvium I is correlated to the Yurakucho bed of the Tokyo district, while the Alluvium II is considered to be equivalent in age to the Nanagochi bed of the same district. Although the inter-continental correlation of the Alluvium is one of the difficult tasks, the Alluvium I may tentatively be correlated to the beds of the Flandrian transgression and the Alluvium II may include the beds of the Allerod and some earlier phases. Boring data suggest the existence of the Newer Aso pyroclastics 40 to 50m below the sealevel at and off the shore line east of the mouth of the Ura river, the east tributary of the Oita river. The expected altitude of the Newer Aso pyroclasticsis, however, about 30m above the sea level at that area judging from the altitudinal curve of their distribution on the back land. Two possibilities may be postulated to accout for this contradiction. A fault after the deposition of the Newer Aso pyroclastic flow or a pre-Newer Aso terrace 80m below the lower middle terrace (Newer Aso terrace on land). The altitude of the basal gravel of the Newer Aso along the middle to lower course of the Oita river is quite similar to that of the recent alluvial plain at the same section. This suggests that the altitudinal curves of the fluvial plains of the ancient and recent rivers of the examined section represent practically the same part of the equilibrium curve and consequently the ancient Oita river of the Newer Aso phase is considered to have flowed into the sea at practically the same strand line as the present. Mr. KAMEYAMA (1968) pointed out that the Tosu loam, an equivalent of the Newer Aso, had beed deposited in North Kyushu under the environmental condition, which had been turning from submarine to subaerial. He put the stress on that the sea level at the Newer Aso phase might have been almost at the same level as at the present. These harmonious results conclude the impossibility of the deposition of the Newer Aso pyroclastic flow on the drowned pre-Newer Aso terrace 80 m below the lower middle terrace, if the former really existed. While the faulting after the Newer Aso phase seems very promising. Beppu Bay shows an asymmetric profile with a southwestwardly inclined basement, on which the delta and fan were built up respectively at the south and west. There is a conspicuous fault along the south boundary of the fan of Beppu with southeast extension limiting the southwest coast of the bay. This fault, the Asamigawa fault, cuts both the 180 m terrace and the 70 m one, which are respectively correlated to the upper upper terrace and lower middle one of the south hills of Oita. Accordin5ly it is likely that the fault with the head of more than 180m occured after the formation of the lower middle terrace. Faults of this kind are characteristic of the Oita district since Pliocene time and several fauting phases are distinguished along and north of the "Oita-Kumamoto tectonic line". The south coast fault is considered as an illustration of this tectonic character. This fault, in details, is not a simple extension of the Asamigawa fault, but of a combination of the Asamigawa fault of iWNW trend and another one of ENE trend with some modification by the faults of NS trend, judging from geological and geographical data including the submarine topography and asymmetric development of the ter r.a.ces on the Nyu and Tsurusaki Hills. On this framework the delta of Oita and the fan of Beppu were built up respectively at the south and the west border of the bay.続きを見る
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