チチカカ湖・デサグァデーロ川 (ボリビア) の段丘地形と湖面変化

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It is well known that the large lake, named Balivian, had developed during the Pleistocene in the Bolivian Altiplano, since the study by Bowman in the beginning of this century. Although considerable attention has been paied to this Paleo -Lake, comparatively little is known about its cause of expansion and diminishing, chronology and even the precise distribution. From the geomorphological and stratigraphical survey of the lacustrine and river terraces in the valley of the Rio Desaguadero (Fig.1) where the southern part of the Lake Balivian had occupied, the following conclusions are drawn.<BR>A series of lacustrine terraces can be divided into four levels by means of the former shoreline topography, paleo-reddish soil, diatomite and pumice fall deposits. Distribution of each terrace is shown in the Figures of 2-1, 2-2 and 2-3. It is apparent that each lacustrine terrace indicates the different extent of the Lake Balivian during the different stages (Fig. 8). The maximum expansion of the Lake was recorded as the highest Tiwanaku (T) Terrace with the height of former shoreline of 3865-3870m above sea level. The T Terrace is composed of thick deposits, the lower part of which is characterized by the fanglomerates supplied from the large tributaries such as the Rio Mauri and the Rio Pontesuero in contrast to the well stratified lacustrine silts and clay of the upper part (Fig. 3). Of the succeeding lacustrine terraces, the Guaqui (G) Terrace is formed through the transgression into the forme r valley landforms dissecting the older terracetopographies (Fig. 5). The Machaca (Ma) Terrace and the Desaguadero (D) Terrace are probably strath terrace. The lacustrine terrace development has apparently controlled by the fluctuation of the Lake Balivian, which is given in the Figure 6.<BR>There are also four levels of fluvial terraces, which should be correlated with the lacustrine terraces each other, based on the stratigraphical (Fig. 3) and the geomorphological (Fig. 7) data.<BR>It must be stressed that the expansion of the fluvial depositional surface caused by the tributaries resulted in damming up the valley of the Rio Desaguadero where the coaser fanglomerate could not be transported. According to the stratigraphical data in the vicinity of western foot of the Cordillera Real near La Paz, the cyclic development of fanglomerate and glacial deposits can be established as shown in Figure 6. Consequently, it is quite possible that the Lake Balivian had developed under the environment of alluvial fan formation during the Interglacial ages.<BR>As the fluctuation of the Lake represents well an evolutional and chronological arrangement with the other data on paleo soil, glaciation in the Cordillera Real and fluvial activity, it should have been indirectly affected by the climatic changes. However, the stage of high lake level demonstrates the Interglacial age and that of low one the Glacial age. In this respect, the Lake Balivian is quite different from the so-called glacial lakes.
社団法人東京地学協会の論文

チチカカ湖・デサグァデーロ川 (ボリビア) の段丘地形と湖面変化

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