西宁盆地晚始新世泥岩地层粒度特征及其古环境指示意义

摘要

西宁盆地较为完整地保存着一套新生代(约55～17 Ma)地层,盆地内的水湾村附近出露的晚始新世石膏-红色泥岩旋回沉积序列,是研究晚始新世气候演变的理想地质记录.关于这套地层的沉积环境存在不同的观点,尤其是关于其中泥岩地层成因的争论颇多,主要有洪积扇远端沉积、咸水深湖沉积和风积成因3种观点,差别甚大,至今没有统一的认识.沉积物的粒度可以用来反演其形成时的沉积动力及沉积环境,为了明确该段地层中泥岩的沉积环境,本研究选取水湾剖面代表性泥岩样品进行了系统的粒度测量及分析,结果表明,西宁盆地晚始新世泥岩沉积与黄土高原典型黄土-古土壤粒度特征类似,应为风积地层,而不是洪积扇远端或咸水深湖沉积.西宁盆地晚始新世风积地层的发现为盆地区域构造和古气候演化研究提供了新的研究视角.%Cenozoic sediments(about 55～17 Ma)in Xining Basin include a unique multi-cycle of mudstone and gypsum in Late Eocene(about 40～33.3 Ma).This section,termed Shuiwan section,is ideal in paleoclimate study.However,the origin of the mudstones is in debate.It is previously speculated to be distal alluvial fans,deep saline lacustrine facies or aeolian deposits.Sediment granularity reflects dynamics and environments when they deposit.In this study,samples including mudstones and gypsum of Shuiwan section were collected for systematic measurements of particle grain size,which is obligated for investigation of its sedimentary environments.Results showed that the grain size properties of mudstone were similar with those of typical loess-palaeosol of Chinese Loess Plateau(CLP),which inferred that the mudstones are likely aeolian deposits rather than distal alluvial fan or deep lacustrine sediments.This view potentially provides a new insight in regional tectonic and paleoclimatic evolution in Xining Basin during late-Eocene.