引用本文: 高显丽, 李嘉皓, 刘平. 泥河湾盆地下沙沟剖面磁组构特征分析与古湖环境变化研究[J]. 第四纪研究, 2022, 42(2): 562-576. doi: 10.11928/j.issn.1001-7410.2022.02.19 高显丽, 李嘉皓, 刘平. 泥河湾盆地下沙沟剖面磁组构特征分析与古湖环境变化研究[J]. 第四纪研究, 2022, 42(2): 562-576. doi: 10.11928/j.issn.1001-7410.2022.02.19 GAO Xianli, LI Jiahao, LIU Ping. Characteristics analysis of magnetic fabric in Xiashagou section and the environmental changes of the Nihewan Lake[J]. Quaternary Sciences, 2022, 42(2): 562-576. doi: 10.11928/j.issn.1001-7410.2022.02.19 Citation: GAO Xianli, LI Jiahao, LIU Ping. Characteristics analysis of magnetic fabric in Xiashagou section and the environmental changes of the Nihewan Lake[J]. Quaternary Sciences , 2022, 42(2): 562-576. doi: 10.11928/j.issn.1001-7410.2022.02.19

泥河湾盆地是中国北方很重要的沉积盆地，但由于盆地范围较广，局部地区环境变化差异较大，目前对泥河湾盆地的环境变化的认识还较为局限。文章通过对下沙沟剖面（厚度260 m）的河湖相沉积物进行详细的岩石磁学和磁组构研究，以尝试恢复下沙沟地区以及盆地东部古湖环境的演化过程，希望能够对泥河湾盆地环境变化提供一些新的指示。研究结果显示，样品中磁性矿物以PSD的磁铁矿颗粒为主，部分样品中含有少量赤铁矿。下沙沟剖面的沉积物受后期扰动很小，属于正常的重力沉积。沉积物磁面理（F）比磁线理（L）更发育，磁组构参数中磁面理F、各向异性度P _j 和水流速度函数Fs对环境变化的响应更加明显，古水流分析结果显示下沙沟地区没有长期稳定的流向，环境变化情况复杂。根据下沙沟剖面河湖相沉积物的磁性地层学年龄框架，以郝家台剖面顶部河湖相沉积物的年龄作为该剖面的顶界，得到了下沙沟剖面2.7~0.3 Ma的磁组构参数变化特征。结合岩性和磁组构参数变化的阶段性特征，将下沙沟地区以及盆地东部古湖环境演化历史分为5个阶段：2.7~2.4 Ma，古湖早期逐渐扩张，于早更新世达到了古湖发育的最盛时期，水体动荡，略不稳定；2.4~1.9 Ma，环境变化主要受气候因素影响，下沙沟地区水动力频繁变化，古湖经历了一次扩张和收缩；1.9~1.3 Ma，下沙沟地区的沉积主要受控于河流作用，盆地东部主要受到构造因素的影响，古湖处于持续收缩的阶段；1.3~0.9 Ma，下沙沟地区环境变化剧烈，受中更新世气候转型的影响较大，古湖东部也较大范围的受到中更新世气候转型的影响；0.9~0.3 Ma，全球气候变化剧烈，对古湖东部环境也有一定的影响，但整体还处于水体较为稳定的湖泊环境，古湖并未开始消亡。

泥河湾盆地 下沙沟剖面 Abstract:

The Nihewan Basin where there are thick fluvio-lacustrine sequences, are rich sources of mammalian faunas and Paleolithic sites, which is a very important sedimentary basin in North China. 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 ancient lake, 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 Xiashagou section and the top age of fluvio-lacustrine 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 lake are divided into five stages.

2.7~2.4 Ma, this period was a Pliocene-Pleistocene transition. The magnetic susceptibility and frequency susceptibility values were relatively high. The magnetic fabric parameters F, Pj, and Fs were relatively low. The hydrodynamics intensity increased and then decreased slightly in Xiashagou area, which was generally relatively small. The water of the Nihewan lake was turbulent and slightly unstabled where the sedimentary environments of rivers and lakes alternately appeared. The lake was in the early stage of gradual expansion, reaching the heyday of ancient lake development in the Early Pleistocene.

2.4~1.9 Ma, the magnetic fabric parameters F, Pj, and Fs changed frequently and slightly increased and then decreased during this period. The Nihewan lake experienced an expansion and contraction and the sedimentary environment of the ancient lake changed more frequently. The hydrodynamics in the marginal Xiashagou area was still constantly changing, and environmental changes were mainly controlled by the influence of climatic factors.

1.9~1.3 Ma, the ancient lake was still in the shrinking stage, and the intensity of sedimentary hydrodynamic changes. At this stage, the deposition in the Xiashagou area was mainly controlled by the rivers and the changes in the ancient lake environment were mainly controlled by structural factors.

1.3~0.9 Ma, the magnetic fabric parameters F, Pj, and Fs suddenly increased, which reached the maximum value of the entire section, and the oscillation amplitude was also the largest. During this period, the environment was affected by the climate transition in the Mid-Pleistocene. The environmental changes in the Xiashagou area responsed to this climate transition event, which also had a larger impact on the environment in the Nihewan area.

0.9~0.3 Ma, the magnetic fabric parameters F, Pj, and Fs were the lowest values of the entire section with almost constants. The sedimentary hydrodynamic force in the Xiashagou area was very small and stable. The global climate changing was severe, which had a certain impact on the environmental changes in the ancient lake, and the dying ancient lake did not start during this period.

Key words: Nihewan Basin Xiashagou section magnetic fabrics ancient lake environment Figure 2.

Rock magnetic properties of typical samples from Xiashagou section.(a~d)The change curve of magnetic susceptibility with temperature, where solid line represents the heating curve, and dot-dashed line represents the cooling curve; (e~h)Hysteresis loop; (i~l)Cumulative Log-Gaussian(CLG)curve of the IRM acquisition curve, where squares represent the experimental IRM acquisition curve, and thin solid line represents the IRM value of CLG curve component 1, dot-dashed line represents the IRM value of component 2, and thick solid line represents total IRM fitting value

Figure 6.

The lithology of Xiashagou section (a), the sample's low-frequency magnetic susceptibility (b), relative frequency magnetic susceptibility (c), magnetic fabric parameter change curve with depth(d~k), stereographic projection of maximum axis and minimum axis(l~p)and rose diagram of maximum axis deflection angle(q~u)of magnetic susceptibility in each segment, the magnetic polarity column ^{[ 14 ]} (v)of Xiashagou section and the geomagnetic polarity time scale(GPTS) ^{[ 55 ]} (w)

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Topography map of the Nihewan Basin and the location of the places mentioned in this article, and the color bar on the right indicates the altitude

Figure 2.

Rock magnetic properties of typical samples from Xiashagou section.(a~d)The change curve of magnetic susceptibility with temperature, where solid line represents the heating curve, and dot-dashed line represents the cooling curve; (e~h)Hysteresis loop; (i~l)Cumulative Log-Gaussian(CLG)curve of the IRM acquisition curve, where squares represent the experimental IRM acquisition curve, and thin solid line represents the IRM value of CLG curve component 1, dot-dashed line represents the IRM value of component 2, and thick solid line represents total IRM fitting value

Figure 3.

Day-plot of typical samples of Xiashagou section

Figure 4.

Relation diagram of magnetic fabric parameters of Xiashagou section samples

Figure 5.

Stereographic projection of maximum axis and minimum axis of the magnetic susceptibility of Xiashagou section. Where hollow dots represent the projection point of maximum axis K ₁ , and black dots represent the projection point of minimum axis K ₃

Figure 6.

The lithology of Xiashagou section (a), the sample's low-frequency magnetic susceptibility (b), relative frequency magnetic susceptibility (c), magnetic fabric parameter change curve with depth(d~k), stereographic projection of maximum axis and minimum axis(l~p)and rose diagram of maximum axis deflection angle(q~u)of magnetic susceptibility in each segment, the magnetic polarity column ^{[ 14 ]} (v)of Xiashagou section and the geomagnetic polarity time scale(GPTS) ^{[ 55 ]} (w)

Figure 7.

(a)Deep-sea oxygen isotope curve ^{[ 61 ]} , (b)Normalized loess-paleosoil median particle size ^{[ 62 ]} , (c, d)variation curves of low-frequency magnetic susceptibility, relative frequency magnetic susceptibility, and(e~i) magnetic fabric parameters during 2.7~0.3 Ma in Xiashagou section