兰州—民和盆地河口群沉积相和岩石磁化率——祁连山白垩纪隆升的记录

研究白垩纪祁连山构造隆升过程对认识青藏高原形成及其环境效应具有重要意义,兰州—民和盆地连续出露的河口群沉积物较好地记录了早白垩世祁连山隆升过程。河口群从下向上由冲积扇、扇三角洲、辫状河、滨湖、滨浅湖、半深湖、三角洲7个沉积相体系组成;综合分析沉积相时空演化、砾石成分及古水流方向统计和岩石磁化率测量,揭示出早白垩世祁连山经历了3期隆升过程:早期(约143～123Ma)快速强烈隆升阶段,此时祁连山东段开始逐渐断陷下沉,盆地沉积了冲积扇至滨浅湖相沉积体系的砾岩、砂岩和泥岩,岩石磁化率值总体偏低,隆升速度较快;中期(约123～113Ma)稳定隆升阶段,湖盆面积达到最大,沉积物以半深湖相泥岩为主,磁化率值总体偏高,隆升速度较慢;晚期快速隆升阶段,约从113Ma开始,祁连山隆升速度加快,湖盆萎缩,沉积了以三角洲相为主的泥岩和砂岩,磁化率值总体偏低,直至109Ma以后,盆地回返,祁连山地区整体抬升遭受剥蚀。

The early Cretaceous tectonic uplift of Qilian mountains: evidence from the sedimentary facies and susceptibility of rocks of the Hekou group, Lanzhou-Minhe basin

The research on the Cretaceous tectonic uplift of the Qilian mountains is crucial in understanding the formation of the Tibetan plateau and its impact on the environment of Asia.The early Cretaceous nonmarine sediments of the Hekou group in the Minhe-Lanzhou basin(a subbasin of the Longzhong basin),north of the Tibetan plateau,have well documented the process of the tectonic uplift of the Qilian Mt.during the early Cretaceous.The Hekou group may be divided into eight segments and is composed of the conglomerates,gravel-bearing sandstones,sandstones,silts and mudstones with ca.4 000 meter in thickness.Seven sedimentary facies have been found,base on the analysis of sedimentary sequence from the bottom to the top,they are the alluvial fan facies,fan delta facies,braided fluvial facies,shore-lake facies,shore-shallow lake facies,semi-deep lake facies and delta facies.The analysis of the nature of sedimentary facies,the composition of gravels,the direction of paleo-current and the rock susceptibility measurement of the Hekou group suggest that the Qilian mountains had undergone three stages of tectonic uplift during the early Cretaceous.The early stage probably was lasted from ca.143 Ma to ca.123 Ma with the characteristics of fast and strong uplift;the basin was filled with the sediments characterized by the lower average value of susceptibility in the alluvial fan facies,fan delta facies,braided fluvial facies,shore-lake facies,shore-shallow lake facies from the bottom to the top.The intermediate stage spanned from ca.123 Ma to ca.113 Ma and the uplift was continued and smooth;the sediments with the higher average value of susceptibility deposited in the semi-deep lake,indicative of the basin extending to the maximum.The last stage took place since ca.113 Ma and the previous semi-deep lake shrank;the basin was filled with sediments having a relative lower value of susceptibility in the delta environment;it is argued that the Qilian mountains began fast uplift once again at ca.113 Ma and lasted out up to 109 Ma.