兰州盆地永登剖面记录的第三纪沉积环境

对兰州盆地永登剖面第三系沉积进行了沉积学研究、并结合年代学研究结果分析了新生代以来沉积环境变化与气候变化。古新世末至早始新世沉积了一套扇三角洲相沉积 (细柳沟组 ) ,为具交错层理的砖红色砂岩 ,顶界年龄约 5 1Ma。兰州地区在 5 1MaB .P .经历了一次大的环境变化 ,气候由相对温暖湿润转为半干旱半湿润 ,气候炎热 ,湖盆由淡水湖转为盐湖 ,蕴示着由副热带高压控制的干旱带北移 ,兰州地区进入受行星风系控制的干旱带。早始新世晚期至早渐新世早期 ( 5 1～ 31.5MaB .P .) ,在近 2 0Ma时间内经历了微咸湖—盐湖阶段 (野狐城组 ) ,砂泥岩沉积中含有大量石膏夹层。早渐新世 ,兰州地区向湿润方向转化 ,兰州盆地由咸水湖转化为淡水湖 ,可能反映了东南季风逐渐形成 ,中国大陆由早先行星风系控制转化为季风控制 ,前期东南方向干旱带消失 ,西北干旱带出现 ,兰州地区相对早先湿润。早渐新世中期至中中新世 ( 31.5～ 15MaB .P .) ,兰州盆地在经历了三次大的沉积旋回后湖盆逐渐干枯 ,三个大旋回分别由河流相砂岩—微咸水滨湖相砂泥岩、泛滥平原相泥岩 ,河流相砂岩—淡水滨湖相砂泥岩—沼泽相、泛滥平原相泥岩 ,河流相砂岩—沼泽相、泛滥平原相泥岩组成。 2 0MaB .P盆地接受大量砂砾石沉积 。

Susceptibility variations of multiple origins of loess from the Ily Basin(NW China)

The magnetic susceptibility of loess from the Ily Basin,northwestern China shows maximum values in S0 paleosols but minimum values in other paleosols,the mechanism of which has been well debated.In this work,systematic magnetic measurements were made on a representative section from Neleke county.The results show that the loess horizons(L1,L2 and L3) have multi-domain magnetite grains of aeolian origin,S0 is characterized by production of pedogenetic ultrafine-grained ferrimagnetic minerals,and the other paleosols(S1,S2,and S3) are characterized by the formation of nonferrimagnetic minerals associated with waterlogging.The correlation between the low concentration of ferrimagnets,high paramagnetic content,high magnetic coercivity remanence,fine ferrimagnetic grain size and intensified pedogenesis suggest two competing processes of pedogenetic enhancement and pedogenetic depletion in the lower paleosols.Pedogenetic depletion dominates and is responsible for the low susceptibility.Changes in magnetic grain size distribution occur during pedogenetic depletion.The susceptibility variations are of multiple origins in the loess of the Ily Basin.Pedogenetic enhancement,pedogenetic depletion,and allochthonous input of magnetic minerals should all be taken into account to explain the variations of magnetic parameters.

伊犁黄土磁化率的增减及其成因

中国伊犁黄土磁化率一般在S0最高,但在其他古土壤层低,磁化率增减机制存在争议.对尼勒克县的一个剖面进行系统的磁学研究后发现,黄土层含有风成多畴磁铁矿,S0含有较多成壤成因的超细粒亚铁磁性矿物,其他古土壤层含有较多过湿条件下形成的非亚铁磁性矿物.非亚铁磁性矿物高含量、高矫顽力、亚铁磁性矿物低含量与较细的磁颗粒粒径对应于较高的成壤强度,揭示存在相互竞争的磁化率成壤增强过程和成壤削弱过程,以成壤削弱过程为主,导致磁化率降低.成壤削弱过程中,磁颗粒粒径发生变化.伊犁黄土磁化率变化成因多样,对磁学参数的解释要全面考虑磁性矿物的风成输入和成壤成因的磁性增强及磁性削弱作用.