临夏盆地新近纪郭泥沟剖面磁组构特征及其环境意义

临夏盆地在东亚新生代地层、古气候、古生物研究方面占有举足轻重的地位.本文对盆地东部的郭泥沟剖面进行了详细的岩石磁学和磁组构研究，以揭示从早中新世到早上新世临夏盆地的沉积演化过程.郭泥沟剖面沉积物中的磁性矿物有磁铁矿、磁赤铁矿、赤铁矿和针铁矿，但剩磁载体以磁铁矿和赤铁矿为主.从上庄组和东乡组的褐红色粉砂质粘土到柳树组和何王家组的褐黄色粘土，赤铁矿含量呈现降低的趋势，与沉积物颜色变化一致.郭泥沟剖面沉积物磁组构类型为正常沉积磁组构.结合岩石磁学结果和磁组构参数特征可揭示临夏盆地早中新世-早上新世沉积的演化过程：早中新世上庄组为稳定湖相沉积，古水流方向为NNW，与南北向的大夏河方向一致；中中新世气候发生较明显的干湿波动，形成了东乡组的褐红色湖相粉砂质粘土夹粉砂、砂和青灰色泥灰质粘土条带，古水流方向主要为NNW，沉积过程主要受大夏河控制；中中新世晚期，受青藏高原构造运动影响，沉积相由湖相细粒沉积物转变为虎家梁组河流相砂砾层；同时，盆地的水动力条件也发生改变，晚中新世柳树组湖相沉积过程同时受南北向大夏河和东西向洮河控制，两个方向近垂直的河流共同作用导致柳树组内沉积各向异性度较低，面理和线理均不发育，磁化率最大轴偏角分布比较分散，磁组构确定的古流向为东西向和南北向；早上新世期间，由于受青藏高原隆升影响，沉积了何王家组下部的河流相砂砾层；受构造抬升影响，大夏河重新主导何王家组上部洪泛平原相沉积过程，水动力条件较为单一稳定，古流向主要为N向，与大夏河流向一致.

Anisotropy of magnetic susceptibility of the Neogene Guonigou section in the Linxia Basin and its paleoenvironmental significance

The Linxia Basin is located at the northeastern edge of the Tibetan Plateau. It is filled with late Paleogene to Quaternary fluvio-lacustrine and eolian deposits rich in mammalian fossils, which provide invaluable information about a series of major geologic and environmental events, such as Tibetan uplift, Asian aridification development and Asian monsoon evolution. In this study, we present the results of anisotropy of magnetic susceptibility (AMS) measurements of the Guonigou section in the eastern Linxia Basin, and try to probe into sedimentary processes of the Linxia Basin during early Miocene to early Pliocene. The Guonigou section with a thickness of 183.8 m consists of, from oldest to youngest, the Shangzhuang Formation of lower Miocene, the Dongxiang and Hujialiang Formations of middle Miocene, the Liushu Formation of upper Miocene, and the Hewangjia Formation of lower Pliocene. Mineral magnetic analyses, including temperature-dependent magnetic susceptibilities (χ-T curves), thermal demagnetization of three-axis isothermal remanent magnetization (IRM), hysteresis loops, IRM acquisition and demagnetization curves, and component analysis of coercivity distributions, suggest that a mixture of magnetic minerals, such as magnetite, maghemite, hematite and goethite, are present in the Guonigou sedimentary sequence. However, magnetite and hematite serve as the dominant carriers of remanence of the sediments. The component analysis of coercivity distributions reveals an up-section decreasing trend in the concentration of hematite.#br#There are two main rivers in Linxia Basin, including the north-south directed Daxiahe River and the east-west directed Taohe River.AMS measurements have been conducted on samples from the Guonigou section to determine the paleocurrent directions and to reconstruct sedimentary processes in the Linxia Basin during early Miocene to early Pliocene. The early Miocene Shangzhuang Formation, which is composed of reddish-brown silty clays, was deposited in a stable lacustrine sedimentary environment with little subsequent disturbances. The paleocurrent direction was NNW during the early Miocene, consistent with the Daxiahe River flows. The early Middle Miocene Dongxiang Formation, which is composed of the alternations of low-susceptibility sediments (coarse-grained silts, sands and/or marly silts and clays) and high-susceptibility sediments (brownish-red silty clays and silts), was deposited in alternating wet and dry conditions. The paleocurrent direction was mainly NNW (Daxiahe River dominated) with a small W component (Taohe River dominated) during the early Middle Miocene. The lake was closed in late Middle Miocene, when the basin was filled with fluvial gravels comprising the Hujialiang Formation. During the late Miocene Daxiahe River and Taohe River together transported sediments to the Guonigou section, comprising the Liushu Formation. The mutual effect of these two nearly perpendicular rivers produced a weak magnetic fabric in the Liushu Formation of lacustrine origin. The dominant paleocurrent directions were N-S and E-W. The sedimentary facies and paleocurrent direction changed in early Pliocene possibly due to the Tibetan uplift. The lower part of the Hewangjia Formation is mainly composed of fluvial conglomerates; and the upper part, mainly of floodplain clays with a north paleocurrent direction, which is dominated by Daxiahe River.