长江上游水系变迁的河流阶地证据

以金沙江石鼓-巧家河段与川江宜宾-重庆丰都河段的阶地堆积物物质成分变化为着眼点研究了长江上游水系变迁。分河段对阶地堆积物重矿物成分进行时空对比后发现,金沙江下段沿岸各地的高位阶地重矿物成分差异较大,说明当时该地区尚未有贯通的金沙江,而可能是独立的小水系。白汉场、剑川、漾濞、元江等地古河谷堆积物样品重矿物成分的相似说明古金沙江可能是在石鼓、鸿文附近向南经过白汉场-剑川-漾濞盆地入红河的。川江各处高位阶地重矿物成分的差异说明当时该地区也存在分散的水系。金沙江下段以及川江贯通之后汇水东流,现代长江上游水系格局形成,并发育了重矿物成分相近的低阶地。

TERRACE EVIDENCE OF RIVER SYSTEM CHANGE IN THE UPPER REACHES OF CHANGJIANG RIVER

To understand the evolutionary history of the water system in the upper reaches of Changjiang River,river terraces in the lower reaches of Jinshajiang River(from Shigu to Qiaojia,sampled at Panzhihua,Yuzha,Longjie,Aoga,Malutang,Aqiao and Huadan) and the Chuanjiang River(from Yibin to Fengdu,sampled at Yibin,Luzhou,Hechuan,Lijiatuo,Guangyangdao,Changshou and Fengdu) were analyzed.Sand samples were collected within the gravel layers and the heavy mineral assemblage was analyzed by conventional method.Two series of river terraces developed respectively before and after the formation of the modern Changjiang River were recognized,with the former at relatively higher altitudes and the latter lower.This is indicated by their different heavy mineral compositions which represent the different lithological characters of the original drainage areas.Thus the develop history of the upper reaches of the Changjiang River is well reflected by the temporal and spatial changes of heavy mineral compositions at different sampling sites.The heavy mineral compositions from different sampling sites of the high terraces in the lower reaches of Jinshajiang River vary remarkably,suggesting that these terraces were derived from small and isolated rivers not yet linking to form the Jinshajiang River.The similar heavy mineral composition of fluvial sediments at Baihanchang,Jianchuan,Yangbi,and Yuanjiang shows that the ancient Jinshajiang River might flow southward across the Baihanchang-Jianchuan-Yangbi Basin and enter into the ancient Honghe River.The obvious difference of the heavy mineral compositions of high terraces of the Chuanjiang River also indicates that there were several independent rivers in that region at that time.The similarity of low terraces of Jinshajiang River and Chuanjiang River implies that by then the lower reaches of Jinshajiang River was connected with the Chuanjiang River,forming the modern river system of the upper reaches of Changjiang River.The evidence from terraces has shed new light to the study of river system change of the Changjiang River,which was previously only available from landform characters and tectonic movements.We believe that a better understanding will be achieved by further analyses of heavy mineral compositions of river terraces.