内蒙古二连盆地西南部第四纪沉积物中粘土矿物特征与古气候指示意义

CLAY MINERALOGY AND ITS PALEOCLIMATIC INDICATOR OF THE QUATERNARY SEDIMENTS IN THE SOUTHWEST OF ERLIAN BASIN,INNER MONGOLIA

通过对内蒙古二连盆地西南部第四纪地层剖面进行系统测制,结合古生物化石建立起该区第四纪以来相对完整的地层层序,并对剖面上所采集的粘土矿物样品进行X射线衍射分析,以及根据样品中的粘土矿物成分、质量分数对本区古气候演化进行探讨。结果表明:研究区早更新世-晚更新世中期沉积物中,大量出现高质量分数的伊蒙混层和蒙脱石,反映季节性冷暖、干湿交替的古环境气候;而晚更新世晚期地层中,伊蒙混层显著减少,伊利石增加,且总体含量高于高岭石,表明该区气候变冷且转为干旱;全新世地层中,高岭石、伊利石和绿泥石含量均有所增加,该时间段内化学淋滤作用仍然较弱,降雨量继续减少。总体而言,本区的古气候在第四纪经历了由干湿交替到温凉偏干的过程。与岩石地层、地貌特点组合等所反映的古气候特征及其变化较好的吻合。

The region of interest located in Wulanchabu League Siziwang and Damao area, southwest of Erlian Basin, Inner Mongolia lies between 41 °50′ -42°50′N and 111°07′ - 111°50′E. The studied area is mainly in hills land, low and middle mountain areas with the south parts a little higher than the north ones and the altitude as between 1400- 1600m. The section A located in the east of Damiao lies at 42°01′45″N, 111 °42′18″E with the thickness as 23.5m ; the section B and C located in the river terrace in the north of Hohnur lie at 42°39′05″N, 111°03′32″E with the thickness as 5.5m and 3.5m respectively; the last section D located in the south of Hohnur lies at 42°38′53″N, 111°13′37″E with the thickness as 3.4m. Based on stratigraphic correlation and paleontological fossils,the relatively complete sequence stratigraphy was formed since the Quaternary. 23 clay mineral samples collected in the four sections were characterized by XRD. The section is divided into three stages ： first,the mixed-layer illite-smectite is the dominant clay minerals,with mass fraction in the range of 71% -90%; secondly,the dominant clay minerals is illite,with mass fraction in the range of 8% -19%; thirdly, the mixed-layer illite-smectite and illite are the dominant clay minerals,with mass fraction in the range of 45% -50% and 28% - 34% respectively. This paper discussed the paleoclimate evolvement since the Quaternary by using the above analysis. The results show that the Early-Late Pleistocene sediments are mainly composed of mixed-layer illite-smectite and smectite. This reflects a midlatitude seasonal alternation of cold/hot and dry/humid paleoclimate. There are a sharp decrease in mixed-layer illite-smectite and an increase in illite content. Kaolinite content is minor,implying a change into the cold and dry climate in the late stage of Late Pleistocene. In Holocene sediments, contents of kaolinite,illite and chlorite are increased,due to the lack of the rainfall and the restricted chemical eluviation. By the research results, the palaeoclimate underwent an alternation from dry/humid one to warm/cool and moderately dry one in the Quaternary. Further,the characteristics of the lithofacies and landform coincide with the paleoclimate study.