Charcteristics analysis of magnetic fabric in Xiashagou section and the environmental changes of the Nihewan Lake

Nihewan Basin is one of a series of well-developed East Asian Cenozoic basins in Hebei Province, North China, which are rich sources of mammalian faunas and Paleolithic sites. During the past decades, detailed magnetostratigraphic dating was conducted on the Nihewan Formation and associated mammalian faunas and Paleolithic sites, and the results have contributed significantly to our understanding of the chronostratigraphy of the Nihewan Basin. However, the information about the local regional environments in the Nihewan Basin is limited. In this study, 456 oriented samples were collected from the fluvio-lacustrine sequences of Xiashagou section (thickness 260 m) for detailed rock magnetic and magnetic fabric studies. We try to rebuild the environmental evolution process of Xiashagou area and then discuss the evolution of the paleolake, and provide some new instructions for the environmental changes in Nihewan Basin. We selected 12 typical samples to measure hysteresis loops，isothermal remanence magnetization acquisition and its back-field demagnetization characteristics. The results show that the magnetic minerals in the fluvio-lacustrine sequences are mainly Pseudo-Single Domain (PSD) magnetite particles and a small amount of hematite. The results of magnetic fabric are typical for a primary sedimentary magnetic fabric, which show that the original sedimentary fabric has been preserved in the Xiashagou deposits.

The magnetic foliation (F) of sediments was found to be larger than the magnetic lineation (L).  F and anisotropy degree (Pj) and the flow velocity function (Fs) in the magnetic fabric parameters are more obviously sensitive to environmental changes. There is no long-term stable flow direction in Xiashagou area. According to magnetostratigraphic age framework of fluvio-lacustrine sequences in the Xiashagou section and the top age of fluviol-acustrine sediments in the Haojiatai section, the age of the magnetic fabric parameters and magnetic susceptibility change curves in the Xiashagou section is from 2.7 Ma to 0.3 Ma. The analysis of the lithology and the periodic changes of the parameters of the Xiashagou section showed that the environment of the lake has undergone several significant changes during the period of 2.7～0.3 Ma，which occurred at 2.4 Ma，1.9 Ma，1.3 Ma and 0.9 Ma. So the environmental evolutions of the Nihewan paleolake are divided into five stages.