ブータン王国ウォンディポダン周辺の河成段丘の地質と氷河湖決壊洪水堆積物

Abstract

Several fluvial terraces and other flat surfaces are well developed in Wangdi Phodrang District along Chang Chhu (Puna Tshan Chhu) in central Bhutan.The low terrace of 5 m to 18 m in relative height is widely distributed, preserving the river flat reliefs on the surface. The middle terrace of 20 m to 50 m in relative height is well developed from Wangdi Phodrang to the upper reaches. The high surfaces of 110 m in relative height are distributed only in Wangdi Phodrang Town and Bajo Monastery Hill, which is 2 km upstream from the former.Geology of the middle terrace is mainly composed of a well rounded and imbricated boulder bed with several sand beds of lesser amount.Geology of the high surface in Wangdi Phodrang Town is composed of two members : the underlying 50 m-thick massive huge boulder bed and the 60 m-thick mudflow deposits overlying the former horizontally. The huge boulder bed continues to the imbricated boulder bed of the middle terrace, suggesting that the mudflow forming the high surface is younger than the middle terrace.On the opposite bank of Wangdi Phodrang Town, there are huge mudflow deposits, which have flowed down towards Wangdi Phodrang Town. Judging from the stratigraphic relation with middle terrace deposits, flow direction, elevation, size, and lithology, the mudflow can be safely correlated with the mudflow at Wangdi Phodrang Town.Then, the mudflow may once have formed natural dam by covering the river flat gravel bed of the middle terrace. Estimated height of the dam is 1310 m a.s.l., and the crest was almost 1000 m wide.While Bajo Monastery Hill is composed of two geological members, the northern major portion of the hill is brown-colored massive sand of over 20 m thick, which are delta fan deposits of a tributary named Limte Chhu, while massive white sand distributed locally at southern downstream side of the hill is abutting on the former. Groundwater drilling on the middle terrace revealed that the well rounded boulder bed is thinly overlaid by white silty sand, which continues to the massive white sand at Bajo Monastery Hill. This evidently suggests that the massive white sand bed is younger than the middle terrace boulder bed.Judging from massive appearance without stratification and mixed occurrence with blocks and fragments of varved clay, the genesis of the massive white sand at Bajo Monastry Hill without doubt resulted from turbidity current of the Glacier Lake Outburst Flood (GLOF).When the GLOF flowed into the deep lake formed by the mudflow dam, the suspended sand, as well as transported blocks of varved clay, may have been settled in the lake. Concurrently, the GLOF may have destroyed the natural dam and drained the lake. Only a small portion of the GLOF sediments remained at the riverside stagnate concavity in the Bajo monastery area.The above-mentioned Quaternary geo-history constructed from field evidence is identical to the legend of the Wangdi Phodrang district.The legend is that an ancient lake was drained in one night by God, who was angry with the impious inhabitants. The upper reach of the lake in the legend is Punakha Town, which is at an elevation of 1340 m. Therefore, the size of the lake in the legend is as same as the size estimated by geology.The legend may suggest that the GLOF event was observed by ancient inhabitants settled in this area.