大同盆地的磁性地层学研究

The Nihewan Basin has nurtured a wealth of ancient people and mammals, evidenced by the abundant Paleolithic sites and mammal fossils since at least the Pliocene. Therefore, the Nihewan Basin is one of the most important natural laboratories to understand the evolution of early humans (Homo sapiens) and of the Late Neogene mammals. Due to the lack of vocanic ash layers in the Nihewan Basin, magnetostratigraphy is a vital tool to construct a robust and detailed time framwork, which is indispensibale for the discussion of basin development and bio-evolution. Previous dating of the Nihewan Basin focuses Quaternary strata in the eastern sub-basin (Yangyuan Basin). The western sub-basin (Datong Basin) with less outcrops were largely ignored. However, the Datong Basin has well-preserved strata from the Pliocene and is therefore integral to establish the overall developing history of the Nihewan Basin. This study is to magnetostratigraphically date the DY-1 core sediments in the Datong Basin. Combined with 40Ar-39Ar ages of basalt as anchor points, we aim to estalish a high-resolution age model of the Datong Basin and furthermore acquire the overall age framework of the Nihewan Basin. Our results will provide the basis and evidence to understand the infilling history of the Nihewan Basin, and the evolution of early humans and mammals in East Asia.

泥河湾是我国发现旧石器古人类遗址和哺乳动物化石最丰富的地区之一，是研究东亚地区古人类进化、哺乳动物演化的宝库。详细而可靠的年代地层学框架是探讨泥河湾古湖的充填演化及其环境变化的前提。前人关于泥河湾盆地的年代学、地层学工作多集中在东盆地（即阳原盆地），对西盆地（即大同盆地）的研究十分有限。这主要是由于东盆地的露头较好、发掘的旧石器和化石较多的原因。然而，阳原盆地出露的地层主要集中在第四系，上新世地层十分有限。要想建立泥河湾古湖的整体年代地层框架和详细的演化历史，获得大同盆地的年代地层格架并完成东西盆地的对比就显得尤为重要。本项目拟选取大同盆地的DY-1钻孔岩芯，以高分辨率的磁性地层学和同位素年代学为手段，建立大同盆地上新世以来的年代地层框架。在此基础上，通过与阳原盆地的地层对比建立泥河湾盆地的整体年代地层学格架，从而为泥河湾盆地的充填和演化历史及其与古人类活动和哺乳动物演化的关系提供依据。

大同盆地位于汾渭地堑系的北段、泥河湾盆地的西部，赋存了连续沉积的新近纪-第四纪河湖相地层。建立大同盆地高分辨率的年代学格架是理解汾渭地堑系的拉张过程和机制的基础，也为探索古人类演化与环境、气候变化的关系提供线索。.本研究选择大同盆地的DY-1钻孔岩芯为对象，开展高分辨率的磁性地层学研究。结果表明，钻孔1048.53m深度之上的新生代沉积物中磁性矿物主要是磁铁矿和赤铁矿，少量样品含有胶黄铁矿。通过热退磁、交变退磁和混合退磁结合的手段，共获得652个层位的特征剩磁。利用特征剩磁的磁倾角建立的极性序列中包含25个正极性带和25个负极性带。通过与地磁极性年表的对比，我们得到了钻孔剖面的年龄框架：中更新统底界（即Matuyama/Brunhes界线）位于73.3m，第四系底界（即Gauss/Matuyama界线）位于274.8m，下上新统与上上新统的界线（即Gilbert/Gauss界线）位于372.5m，通过内插确定上新统底界为501.9m。DY-1钻孔岩心上部680~40m深度的地层记录了大同盆地7Ma以来的沉积，该年龄数据揭示出汾渭地堑系北段张开的时间不晚于7Ma。