青藏高原东北缘临夏盆地新近纪风成沉积与古环境演变研究

Linxia basin is located at the junction of the northeastern margin of the Tibetan Plateau and the western margin of the Chinese Loess Plateau (CLP). Continuous sedimentary succession in this basin was deposited during the Cenozoic and provides an excellent opportunity to reconstruct the tectonic and climatic evolution of the northeastern Tibetan Plateau. Aeolian sediments are widely distributed in the northwestern China and are regarded as one of the most complete records of climate changes in the world. However, detailed investigation of aeolian sediments in the Linxia basin is currently restricted to the Quaternary and pre-Quaternary red clay sediments have only rarely been reported. Recently, a 160-m-long drill core was successfully obtained from a geomorphologically stable surface in the central part of Linxia Basin. High-resolution magnetostratigraphy determined that the late Pliocene-early Pleistocene sediments from the borehole have an age range of ~3.7-2.4 Ma. Further studies of lithology, sedimentation rate and grain- size distribution confirm an eolian origin of these deposits, which provides the first direct evidence of late Pliocene-early Pleistocene eolian loess–red clay sediments in the western CLP. In the present study, we are making efforts to carry out a further 400-m drilling project beneath the first 160-m drill core and to obtain the underlying red siltstone and mudstone sediments. Based on the previous sedimentological and magnetostratigraphic studies on the 3.7-2.4 Ma eolian sequences, this project plans to carry out detailed paleomagnetic measurements, provenance analysis and various climate proxies analyses for the underlying red siltstone and mudstone sediments of the section. This will not only greatly improve our understanding of the initiation of aeolian sediments in the Linxia basin, but also is of great significance in understanding the Neogene paleoenvironmental evolution of the northeastern Tibetan Plateau.

临夏盆地位于青藏高原与黄土高原的关键衔接带上，前期研究表明盆地中的新生代沉积地层不仅记录了青藏高原东北部的隆升过程，而且记录了该区季风和干旱气候的演化。风成堆积是我国西北地区最连续可靠的气候记录载体，但在临夏盆地明确鉴定的风成堆积及其气候记录仅限于第四纪。经过钻探取样及实验分析，我们最近在盆地东部成功发现3.7-2.4Ma的典型风成沉积；其磁化率和粒度变化在时间上与黄土高原中部同期红粘土可以进行很好对比，且在空间上有明显的梯度变化。这是首次在黄土高原西部发现晚上新世-早更新世的风成沉积，表明该地是研究临夏盆地新近纪风成沉积与古环境演化最理想的场所。项目拟在前期工作的基础上，对已获得岩芯的下伏地层进行进一步钻探取样，通过古地磁、沉积物成因和环境代用指标分析，揭示临夏盆地风尘堆积大规模出现的时间并提取其潜在的古气候信息，为黄土高原西部新近纪古环境演化以及风尘堆积时代和分布范围研究提供新的证据。

临夏盆地位于青藏高原与黄土高原的关键衔接带上，前期研究表明盆地中的新生代沉积地层不仅记录了青藏高原东北部的隆升过程，而且记录了该区季风和干旱气候的演化。风成堆积是我国西北地区最连续可靠的气候记录载体，但在临夏盆地明确鉴定的风成堆积及其气候记录仅限于第四纪。项目在前期临夏盆地晚上新世-早更新世风成沉积识别鉴定工作的基础上，通过对盆地东部天然露头剖面150m中新世泥岩和粉砂岩沉积物详细的粒度、岩石磁学、漫反射光谱以及同位素地球化学和粘土矿物种类等实验分析工作，揭示出该区中新世沉积不仅与第四纪黄土有着相似的Nd同位素组成，而且与天水、庄浪等地中新世风成红粘土粒度和磁性矿物种类组成以及铁氧化物含量的长期趋势变化特征也较为一致，因此支持临夏盆地东部地区中新世地层沉积主要为风成物质来源的观点。古环境代用指标的综合对比分析还揭示青藏高原东北部中中新世以来经历了17.3-14.5Ma的温暖湿润、14.5-12Ma的变冷干以及12Ma以后的温暖湿润三个气候演化阶段，其分别响应于中中新世气候适宜期及之后的大降温事件，表明全球冰量是控制青藏高原东北缘新近纪古环境演化的关键因素。上述结果为黄土高原西部地区新近纪古环境演化以及风尘堆积时代和分布范围研究提供了新的地质证据。