インド・パンジャーブ平原と北部タール砂ばくの地形学的研究

The author made a geographical field survey in Northwestern India as a member of the research team organized by the University of Hiroshima during the period from September 1972 to January 1973.<BR>The purpose of this paper is to elucidate the geomorphic history, tectonic movement and the sequence of climatic changes in the Punjab Plains and the northern part of the Thar Desert.<BR>The results obtained may be summarized as follows : <BR>1) The three physiographic divisions of alluvial plains are based on the characteristics of the surface features : dissected piedmont plain, alluvial upland and alluvial lowland (Fig. 1). The alluvial lowland is divided into two geomorphic units : active flood plain and abandoned flood plain. Abandoned flood plain in developed parallel to the river courses and is three metres higher than the active flood plain. It is often inundated by high floods. The alluvial upland is bordered by steep scarps, with relative heights of 1.5-12 metres above abandoned flood plain. Radiocarbon date on fossil shells suggests that the dissection of the alluvial upland has taken place within at most the last 4, 000 years.<BR>2) The flat alluvial plains of the Punjab in India have been formed by the deposition of the Indo-Gangetic alluvium, of which maximum thickness amounts to 300 metres above the basal rocks of the Siwalik Systems. The alluvial complex of the Pleistocene and Holocene age consists of fine to coarse sand, silt and clay. The recognition of paleosols and calcium carbonate deposits (kankar) allows the classification of the alluvial complex into three formations, which are upper, middle and lower alluvium.<BR>The rate of sedimentation of alluvial deposits are estimeted at 0.25-0.33 mm per year in Ludhiana district in the Punjab Malwa. The upper formation may be correlated with the Holocene based on this calculation (Table 1). The fossil shells mentioned above in the clay (UCL) of the upper formation constituting a top layer of alluvial upland give a radiocarbon date (GaK-5838) of 4, 300±150 years B.P. Radiocarbon dating appears to support this calculation.<BR>3) Deformed alluvium shows the folded structure with the axis of NW-SE direction about 40 km in length parallel to the axis of folded structure of the Siwalik Hills in the Punjab Malwa and Bist doab. It seems that this tectonic movement has been continuous since the middle Pleistocene. It may be evidence for the tectonic movement that the distribution of saline soils and drainage patterns of Whitebein River coincide with the axis of cryptsyncline.<BR>4) Riverine sand dunes are extensively distributed on the alluvial upland in the Punjab Malwa. These riverine sand dunes were formed along the channel scars of the Sutlej River. On the other hand, sand dunes develop very seldom in the Kurukshetra Plain. This may reflect the difference in geology of the drainage areas in the upper reaches of the Sutlej and the Yamuna River.<BR>The distributional patterns of sand dunes and channel scars show the following : the Sutlej River has been shifting toward NW direction and Yamuna River shifting toward SE direction. As a result of the shifting of the river course of the Sutlej or the Yamuna, Kalibangan and the other settlements in the Indus Culture along the abandoned river course deserted about 3, 700 years ago. These shiftings may be caused by the tectonic movement in the Punjab Malwa and the Kurukshetra Plain. The riverine sand dunes are divided into two systems : fixed sand dunes and active sand dunes. The active sand dunes have been formed on the fixed sand dunes by modern cultivation since the 19 th Century. The buried animal bone which was exhumed from a buried trench cutting into the fixed sand dunes gives a radiocarbone date (N-2233) of 435±115 years B.P. This fixed sand dune is overlain the active dund sand 3 metres thick.