Geomorphological features of landslides distributed in upper stream of Naka-river, Tokushima, Japan

In this study, Geomorphological features of landslide distributed in the upper stream of Naka-river, Tokushima, are studied with air-photograph interpretation and map reading. The number of landslide in investigation area is 0.29 number/km2 in Chichibu belt, 0.34 number/km2 in Kurosegawa belt, 0.17 number/km2 in Sambosan belt and 0.09 number/km2 in Shimanto belt. The most number of landslide is Kurosegawa belt and the least number of landslide is Shimanto belt. Landslides show tendency to become smaller in equivalent coefficient of friction as a large scale landslide. Direction of landslide in Kurosegawa belt has a lot of trend of NNE to NE and in Chichibu belt has a lot of trend of ENE to E. Some 63% is in under 50 meters of height from river bed to end of landslide body, but some 23% is in over 100 meters. Deepening of river beds proceeds in transverse valley at the Sakasyukito-river in Kurosegawa belt, while ingrown meander grows in the Naka-river that flows in Sambosan belt and Shimanto belt. Distribution of landslides in these areas is related to the effect of deepening and lateral erosion of river

ヨシノガワタニ ミマチョウ ナカウエ ノ ドチュウソウ カラ ミイダサレタ コウド ヘンセイガンレキ

A condensed bed of the gravels derived from high-grade metamorphic rocks has firstly found in the lower Pleistocene Dochu Formation, from the Nakaue outcrop, Mima-cho in Tokushima Prefecture. The metamorphic gravels are garnet amphibolite, spotted amphibolite, serpentinite and coase grained piedmontite-quartz schist. The content of the metamorphic gravels is nearly 10 percent. Comparison with the modern original gravels, based on the mineral composition and rock texture on the slab, the metamorphic gravels were identified with the Sambagawa Metamorphic Rocks. Among them, garnet amphibolite and spotted amphibolite are high-grade metamorphic rocks that the modern distribution is restricted only in the Higashi Akaishi-yama district, central Shikoku, and the amphibolite gravels are rarely found in recent Yoshino-gawa riverbed around the Mima-cho area. The Nakaue outcrop and gravels of high-grade metamorphic rocks were described, and the provenance of the gravels was discussed with tectonic and geomorphological respects

シコク サンチ カラ ケンシュツ サレタ テフラ ニ モトズク サンチ シャメン ノ チョウキ アンテイセイ

Formative processes of mountain slope in Shikoku Mountains are discussed based on tephrochronology. We carried out identification of tephra layers in colluvial deposits at mountain slope, Shikoku Mountains, Tokushima Prefecture. These tephras named locally Ohyohchi, Nakaue-1 and -2 tephras. Ohyohchi tephra and Nakaue-1 tephra are correlative with AT tephra from South Kyushu, and Nakaue-2 tephra is correlative with K-Ah tephra from South Kyushu, in terms of the similarities of petrography, refractive index of volcanic glass. The occurrence of these tephra layers shows that K-Ah tephra overlies colluvial deposits including AT tephra. Thus, recurrence interval of slope failure was estimated to be more than ten thousand years

ヨウケツ ギョウカイガン ノ フウカ ニヨル ガンセキ ブッセイ ノ ヘンカ

Temporal changes in colour and physical properties due to weathering during 240 ka were examined using welded tuff gravel in fluvial terrace deposits with a known emergence time that were distributed in Miyazaki Plain, Japan. The time between the age of each terrace formation (8, 90, 120 and 240 ka) and the present was assumed to be the weathering period. The results of the measurements are as follows: (1) effective porosity increases as the weathering period increases; (2) the a*, b*-values of colour induces increase as the weathering period increases. These results suggest that (1) the increase in pores in gravel that occurs as the weathering period increases is caused by the dissolution of biotite; (2) iron concentration and the formation of goethite occur in older gravels. Dissolution of biotite and increasing pore volume play major roles in the weathering of welded tuff

テフロクロノロジー ニヨル トクシマケン カミヤマチョウ ノ コウネ ジスベリ ヒガシオオクボ ジスベリ ノ ハッセイ ジキ ノ スイテイ

In this study，tephrochronological ages of the Koune Landslide and the Higashiohkubo Landslide in Kamiyama Town，Tokushima Prefecture，are estimated. The Koune Landslide is divided into two landslides; the Primary Koune Landslide and the Secondary Koune Landslide. The Primary Koune Landslide is cut by main scarp and moving mass of the Secondary Landslide. The Secondary Koune Landslide is overlain by the Kikai-Akahoya Tephra and re-sedimentation of the Aira-Tanzawa Tephra， suggesting that the landslide occurred primarily between 7.3 ka and 26-29 ka. Therefore， the Primary landslide is older than the Secondary one. The Higashiohkubo Landslide is not overlain by the Aira-Tanzawa Tephra，but the Kikai-Akahoya Tepahra, suggesting that the landslide occurred primarily between 7.3 ka and 26-29 ka

シコク トウブ ヨシノガワタニ ニ ブンプ スル ドチュウソウ カラ ミイダサレタ テフラ

We carried out identification of tephra layers in the middle to lower Pleistocene Dochu Formation, Tokushima Prefecture. These tephras named locally Shiro tephra and Dochu tephra. Shiro tephra discovered at a construction site in Mima City, Tokushima Prefecture. Shiro tephra can be compared with Imakuma-II tephra, and Dochu tephra can be compared with Shishimuta-Pink tephra from central Kyushu, in terms of the similarities of petrography, refractive indices of volcanic glass and hornblende, and chemical composition of volcanic glass

カガワケン ニ ブンプ スル テシマイシセイ セキゾウ ブンカザイ ノ フウカ テイド ノ ヒョウカ

Weathering grade of Teshima stone, Miocene lapilli tuff, used in lithic cultural heritage in Kagawa Prefecture, Japan was evaluated by using colorimeter, magnetic susceptibility meter, moisture meter and schmidt rock hammer. The stone pagodas have been exposed to weathering for 200 - 600 years, and have developed surface erosion due to weathering. Basaltic rock fragments in Teshima stone are slightly weathered based on necked-eye observation with increasing weathering period. L*-value of color index increased with increasing weathering period, however a*- and b*-values of color index and magnetic susceptibility are constant with increasing weathering period. Increasing of L* value is derived from weathering of basaltic rock fragments in Teshima stone. The results of measurement of a*-, b*- values and magnetic susceptibility suggest that mineralogical weathering such as iron mineral concentration has not occurred. The hardness of excavated cave wall surface on quarry of Teshima stone using schmidt rock hammer decreases in the entrance of the cave. Weathering properties of Teshima stone are characterized by surface erosion and rapid decreasing of rock surface hardness

2004ネン タイフウ 10ゴウ ゴウウ ニヨリ トクシマケン デ ハッセイ シタ シャメン ホウカイ ニ エイキョウ スル ウリョウ チケイ チシツ トクセイ

Many slope failures and debris flows triggered by heavy rainfall associated with Typhoon 0410 (Namtheun) occurred in Nakagawa area, Tokushima, Japan on July 31st to August 1st, 2004. Extraordinary intense heavy rainfall was recorded in this area. Many slope failures and some landslides occurred at steep mountain slopes, which consist of accretionary complex. Distribution and density of slope failures are closely related to amount of total rainfall and soil water index, topographical features such as density of valley head and relative relief, and geological features such as crack density of rock masses

ワカヤマケン ニ ブンプスル チュウシンセイ ギョウカイガン ノ キュウガイ ニ ハッタツスル タフォニ ノ ブンプ ト ケイジョウ

Morphological features of tafoni on Miocene tuff cliffs have been studied in the Kozagawa area, Wakayama, Japan. Distribution of tafoni tends to develop on steep slopes characterized by massive fine-grained acidic tuff. Concave roof of tafoni has been formed with high decreasing rate of rock strength under high humid condition. This supports that concave roofs have been formed due to high rate of salt weathering. Piling up of rock blocks on the foot of such cliffs indicates that evolution of tafoni is closely related to rock fall from the cliffs. Development of tafoni makes a sort of canopies overhanging on steep cliff, and such unstable portions fall down as rock fall. In addition to development of tafoni, cracks within acidic tuff are also one of causes of rock fall. Such cracks occurred due to rotational moment of overhanging portion. Consequently, weathering rates of rocks due to salt weathering and spacing of these cracks are control the frequency of rock fall in these cliffs