关键词: 河流阶地;地层穿时性;玻璃陨石;旧石器;百色盆地

Abstract: Bose basin, which has a NW-SE strike and is ca. 100km long , lies in western Guangxi of South China. The basin formed in the beginning of the Tertiary, as defined by a series of lacustrine beds yielding coal. Influenced by the main episode of Himalayan movement a t the end o f the Eogene, the basin was uplifted causing erosion that was responsible for the nearly complete lack of Neogene deposits. During the end o f the Pliocene and Quaternary, new deposits were laid down and several erosional terraces were developed in the basin ( Fig . 1) . The present paper discusses some problems on the geomorphological evolution of the basin, especially TerraceⅣ ( TⅣ ) , which is the stratum yielding Lower Paleolithic stone artefacts.
1 Sequence of terraces
The Youjiang river, which originates from the Yunnan-Guizhou plateau adjacent to the Qinghai-Tibetan plateau, crosses the Bose basin from northwest to southeast and is associated with seven river terraces of differing elevation.
TⅠ and TⅡ , 10m and 15m above the river level, respectively, and 5- 7km in width, represent the main geomophological features of the present basin. TⅠ consists of yellowish-brown clayey , silty loam, and basal g ravel. TⅡ consists of red clay and basal gravel. TⅢ is the least developed in the terrace system of the Bose basin, especially in its eastern part, but can be seen in the western part of the basin where Bose city is built on it, as illustrated in the profile of Fig. 2-A. TⅢ is 30- 40m above the river level and consists of red clay and basal gravel. The red clay grades downward to sub-reticular mottled brick- red clay, and pebbles of sand and shale in the gravel have been strongly kaolinized.
TⅣ , situated typically 50- 100m above the river level, is of considerable interest because it yields stone artefacts of the Paleolithic period. It is well developed and distributed throughout the basin. It consists of two parts. The upper part includes latosols on the top and typical reticular mottled brick-red clay on the bottom. The low er part consists of a basal gravel. The latosols are poorly developed, support only scarce vegetation presently , and a re typically eroded to form badlands in which abundant stone artifacts and associated tektites have been found. TⅣ is often fragmented due to later faulting , and thus forms several platforms whose altitude above the current river level ranges from 25 - 100m, which gives the misleading impression of different terraces. How ever, geomorphological evidence of faults or the likely existence of faults could be recognized between separate platforms ( Fig. 2-B) . Therefore, it can be concluded that they belong in the same terrace.
On hills of higher elevation within the basin, three more platforms have been observed. These consist of gravel beds that unconformably overlie the Eogene beds, and dispersed pebbles and fragments of iron pan and tubercular iron-manganese, all situated at the top of the platforms, and colluvial slope materials consisting of red clay on the platform sides. These platforms ( TⅤ -TⅦ ) are 100m, 120m, and more than 170m above the river level, respectively. They may have developed at the end of the Pliocene and represent the earliest terraces in the basin ( Fig. 3-A) .
The terraces are typically associated with characteristic vegetation or other features, and can be named as follows according to these associations: TⅠ , rice terrace; TⅡ , village / town terrace; TⅢ , vegetable terrace; TⅣ , mango terrace; and TⅤ -TⅦ , forest terraces.
2 Stratig raphic diachronism of terrace deposition
The Laikui locality is situated on a convex bank of the river near Laikui village in Tianyang County. At that locality, TⅣ is 3km long and has been fragmented by faulting into four platforms o f different altitude above the riv er level( Fig. 3-B) . Stone artifacts were found on every platform and even in situ in the gravel. Were the stone artifacts from the different platforms made in the same period or during different periods? During the time of constant water flow at Laikui, the river deposited sand and gravel in the channel bed. During the overflow stage, water submerged the floodplain, which is represented by the present TⅣ terrace surface. From the channel bed to the floodplain, a succession of facies was deposited as follows: river facies of sand-gravel, bank facies of fine sand, floodplain facies of silt, loam, and clayey silt. These facies are laterally equivalent, and their interfaces represent a time-synchronous surface. Furthermore, the Laikui sediments were gradually eroded by the meandering river, removing sediment from the convex bank and depositing it on the concave bank. The resultinglitho stratigraphic sequence of reworked sediments ( and artifacts) consisted of gravel at the bottom and clayey silt a t the top. Artifacts found in the lower stratum, therefore, are not always older than the upper ( Fig. 4) . When early humans lived there, stone artifacts made by them were left not only on the floodplain ( terrace surface) but also in the river bed. Therefore, stone artifacts found in different strata of the TⅣ terrace at Laikui appear to represent the same period of time.V3 History of geomorphological developmentVAt the end of the Pliocene and beginning of Qua ternary, the Bose basin was uplifted slightly and intermittently, during which the earliest three terraces developed. In the following long , stable period, the basin subsided slightly, and the Youjiang river developed many meanders. The river 's lateral platform was continually reworked. During this period, climate was hot, vegetation was luxuriant and landform was flat. Apparently , this setting was suitable to early human occupation near the river and on the floodplain. About 0. 73 million years ago , there had been a tektite scattering event. The pieces of tektite were buried by later sedimentation associated with stone artifacts made by early man, who may have witnessed the tektite scattering. After then, the basin continued to be uplifted intermittently. With each uplift episode, the river cut terraces TⅢ , TⅡ , TⅠ , leading to the present geomorphological landscape.

Key words: Terraces, Stratum diachronism, Tektite, Lower Paleolithic, Bose basin