甘肃陇中盆地会宁关公滩剖面磁性地层年代学研究

本文对陇中盆地会宁含小哺乳动物化石的关公滩剖面河湖相沉积序列进行了详细的岩石磁学和磁性地层学研究，磁化率各向异性结果表明关公滩剖面沉积物记录了典型的原生沉积组构，沉积物沉积以后几乎没有受到后期的扰动.最大磁化率主轴方向与现今的古水流流向基本一致，表明区域西高东低的地貌至少在中更新世早期已经形成.磁滞回线、等温剩磁获得曲线的矫顽力谱分析、三轴等温剩磁退磁曲线等岩石磁学结果显示：关公滩剖面河湖相沉积物的磁性矿物有磁铁矿、磁赤铁矿和赤铁矿，其中磁铁矿和赤铁矿是主要的剩磁载体，但其特征剩磁载体以赤铁矿为主.系统退磁结果显示，在大部分样品中，磁铁矿和赤铁矿均记录了几乎相同的特征剩磁的方向，整个剖面呈现单一的正极性.另外，关公滩剖面底部河湖相沉积物中产出丰富的小哺乳动物牙齿化石，经初步研究包括复齿鼠兔（Ochotonoides complicidens）、丁氏鼢鼠（Myospalax tingi）、方氏鼢鼠（Myospalax fontanieri）、变异仓鼠（Cricetinus varians）、大林姬鼠（Cricetinus varians）、复齿旱獭（Marmota complicidens）等六个种类，其中丁氏鼢鼠和复齿鼠兔是相对原始的种类，其时代跨越早更新世至中更新世早期，其他都是我国北方中、晚更新世常见的种类，该小哺乳动物化石组合性质表明其时代很可能为中更新世早期.因此，综合磁性地层学和小哺乳动物生物年代学结果，关公滩剖面记录了布容正极性时的沉积，剖面底部堆积及其哺乳动物群的时代应为中更新世早期.

Magnetostratigraphy of the fluvio-lacustrine sequence on the Guangongtan section in Longzhong Basin,NW China

The Huining area is located in a sub-basin of the Longzhong Basin where developed thick Neogene-Quaternary fluvio-lacustrine and aeolian sediments.Research on fluvio-lacustrine sediments in this area is relatively less due to its scattered distribution.The Quaternary fluvio-lacustrine sediments in Huining contain abundant micromammalian fossils.A solid age constraint of these fossils can improve our understanding of the diversity and evolution of terrestrial mammals during the Pleistocene.Here we present a detailed study on rock magnetism and high-resolution magnetostratigraphy of the Guangongtan section in Huining.#br#Small micromammalian fossils at the bottom of the Guangongtan section were examined to determine the approximate age of the fluvio-lacustrine sedimentary sequence. Six species were identified, including Ochotonoides complicidens, Myospalax tingi, Myospalax fontanieri, Cricetinus varians, Cricetinus varians and Marmota complicidens. Among them, Ochotonoides complicidens and Myospalax tingi have ages from early Pleistocene to early Middle Pleistocene, and other four species are taxa typical of middle and late Pleistocene in northern China. Therefore, these biochronologically useful species indicate an age of early Middle Pleistocene for the bottom of the Guangongtan section.#br#Anisotropy of Magnetic Susceptibility(AMS),thermal demagnetization of three-axis Isothermal Remanent Magnetization(IRM),hysteresis loop,and component analysis of coercivity distributions indicate that the magnetic minerals in the sediments are mainly magnetite, maghemite and hematite, and that magnetite and hematite serve as the dominant remanence carriers.However, the characteristic remanent magnetizations are primarily carried by hematite. AMS and systematic demagnetization results show a typical undisturbed primary sediment accumulation.An eastward paleocurrent direction which is concurrent with the present was discriminated from the orientation of the maximum susceptibility axis directions.This paleocurrent direction indicates that the current geomorphology was formed no later than the early Middle Pleistocene. Combining the magnetostratigraphic and biochronologic data,we suggest that the Guangongtan section correlates to the Brunhes chron.The bottom of the Guangongtan fluvio-lacustrine sedimentary sequence and associated small micromammalian fossils can be placed in the early part of Middle Pleistocene.