兰州地区新生代最老的风成沉积

The Oldest Eolian Deposit of Cenozoic in Lanzhou Area

黄土高原西部风成沉积的研究,是近年古气候研究的重要问题之一,但对新生代风成沉积底界的认识仍存在争议。渐新世晚期发育于兰州地区的咸水河组的岩性特征与黄土高原的风尘沉积有相似的特征,对其进行系统的沉积相和沉积环境研究对认识黄土高原西部早期风尘沉积有重要意义。本研究对兰州以北厚约916m的凤凰山剖面进行了岩石磁学、粒度、成份分析和电镜等多种分析,初步结果表明,咸水河组大部分黏土没有水平层理,其粒度概率分布曲线、概率累积曲线、组成特征和磁化率、元素含量及扫描电镜与典型黄土、古土壤和红黏土非常相似,为风成沉积,是目前发现的兰州地区新生代最老的风成沉积。这套黏土的色调和磁化率等环境代用指标与黄土高原的第四纪黄土接近,指示了第三纪早期一个低温干旱的地质环境。咸水河组中出现的砂砾石层,以颗粒大、分选差、一般无水平层理或层理不明显为特征,其粒度概率曲线、累积概率曲线与典型河流沉积物非常相似,是河流沉积物,指示了多期河流发育,可能与青藏高原的隆升有关。

The study of eolian deposition in Western Loess Plateau is one of important problems in resent paleoclimatic studies,but the age of the Cenozoic eolian deposition is still in dispute.The lithologic features of Xianshuihe Formation developed in Late Oligocene in Lanzhou area are similar to those of eolian deposition in Loess Plateau,and it is significant to study sedimentary facies and depositional environmental systematically for understanding of the early eolian deposition in Western Loess Plateau.This study was carried out the rock magnetism,grain size,composition analysis and electron microscope analysis of Fenghuangshan Section which is 916m thick,north of Lanzhou.The results preliminary showed that：Most of the clay is non-horizontal bedding;the distribution curves,probability cumulative curves and composition characteristics of grain-size,susceptibility,elemental content,morphology of quartz grains of them are similar to those of typical loess,paleosol and red clay samples.So they are eolian deposit,which is the oldest eolian deposit of Cenozoic found in Lanzhou Area presently.The environmental proxies such as colour and magnetic susceptibility show that this clay is much close to Quaternary Loess of Loess Plateau,which indicates that there was low temperature and dry geological environment in Early Tertiary.Sand and gravel layers appearing in Xianshuihe Formation are generally sorted poorly with non-horizontal or unconspicuous horizontal bedding,and the grain-size distribution and probability cumulative curves of them are similar to those of typical fluvial deposit,so they are fluvial deposits which may be related to the uplift of the Qinghai-Tibetan Plateau.