大渡河流域河谷深厚覆盖层特征与发育分布规律研究

随着近年来对西部各流域水电梯级开发的全面实施，越来越多的钻孔资料揭露我国西南地区各流域普遍存在河谷深切和深厚覆盖的特殊现象。河谷深切和深厚覆盖层的存在，不仅大大增加了西南地区各流域水电开发的难度，同时也诞生出一系列新的科学问题。本文以梯级水电站较密集、河床勘探资料相对较丰富的大渡河流域为主要研究对象，通过系统分析整理该流域河床钻探资料，对该流域河谷深厚覆盖层的分布特征、物质组成与结构特征、形成时间等进行了较为系统地分析总结，并对其形成机理提出了新的认识。研究结果表明，除局部地段受构造隆升(大岗山、下尔呷)或构造断陷(冶勒)的影响外，大渡河流域95％的河段其河谷覆盖层厚度大于30m，河谷深切和深厚覆盖具有流域性、区域性特点。河谷覆盖层是由一套不同时期、不同成因类型的沉积物相互迭置的结果，由表及里宏观上可分为3层: 表层为现代河流相堆积；  中间主要为以冰水、崩积、坡积、堰塞堆积与冲积混合为主的加积层，厚度相对较大；  底部主要为古河床的冲积、冰水漂卵砾石层。覆盖层的形成时代一般在1～3万年前，基本与地质历史上末次冰期及冰消期时间相对应。为了更好地解释河谷深切和深厚覆盖具有区域性的特点，本文提出了全球气候变化和海平面升降引起河谷深切和深厚覆盖的观点: 冰期海平面的大幅度下降会引起区域性基准面大幅下降，河流比降加大，由此引起强烈下切，形成深切河谷；  冰后期海平面大幅回升，区域性基准面将大幅抬升，河流纵比降减小，水流的携砂能力减弱，冰期河谷深切所形成的固体物质开始大量沉积，并由此形成深厚覆盖层。

CHARACTERISTICS AND DISTRIBUTION OF THICK OVERBURDENS ALONG THE DADU RIVER VALLEY

Thick overburdens and incised valleys are almost seen along every river of South West China.Such a setting not only makes hydropower stations hard to build,but also brings out a series of new scientific problems.Based on borehole data mainly from the construction sites of hydropower stations,we studied the distribution,composition,structure and forming time of such overburdens along the Dadu River valley and approached their forming mechanisms.The overburdens are generally more than 30 meters thick and cover about 95% of the Dadu River valley.They consist of superimposed sediments of different origins formed at different times.They can be classified into three main layers in terms of forming time and origin.The upper layer is modem river deposit;the middle layer is relative thick,mainly consisting of ice-water deposit,colluvial deposit and deluvium as well as barrier dam accumulation;the bottom layer is formed by ancient river bed deposit and boulder rocks.The forming time is 10 to 30 thousands years ago,which largely corresponds to the last glaciation and then glacial melting.In order to understand the regional characteristic of thick overburdens and in incised valleys in West China,we proposed a view that they can be interpreted by global climate change and eustacy(sea level up and down) as a factor.