泥河湾盆地虎头梁剖面河湖相沉积物磁组构特征及其环境意义

MAGNETIC FABRICS OF THE HUTOULIANG FLUVIO-LACUSTRINE SEQUENCE AND ITS IMPLICATION FOR SEDIMENTARY EVOLUTION IN NIHEWAN BASIN

泥河湾盆地在第四纪环境变化和古人类演化研究领域占有重要地位。虎头梁剖面记录了中更新世以来泥河湾盆地逐渐消亡的过程。本文对虎头梁天然露头剖面进行了详细的沉积学、岩石磁学和磁组构研究。剖面沉积物中的磁性矿物以假单畴（PSD）的磁铁矿为主，还含有一定量的赤铁矿，磁组构类型为正常沉积磁组构。根据虎头梁剖面沉积相的划分和磁组构参数特征，将泥河湾盆地中-晚更新世晚期的演化过程划分为3个阶段：下部为第Ⅰ阶段（60~53m），中部为第Ⅱ阶段（53~12m），上部为第Ⅲ阶段（12~0m），包含3个完整湖侵旋回，每个旋回由下部灰黄色粗粒三角洲相到中部黄灰色滨湖相再到顶部灰绿色细粒湖相组成，相应的磁组构参数值由大到小的变化规律，中部第Ⅱ阶段推断由气候因素主导，上部第Ⅲ阶段可能主要由构造驱动，长期稳定的优势古水流方向以NW向为主，而东坡遗址时期的古人类可能生活在湖侵阶段的相对短期湖退的滨岸地带。

The Nihewan Basin is located in the transition zone between the North China Plain and the Inner Mongolian Plateau. It is filled with fluvio-lacustrine deposits called ‘Nihewan Beds’ and abundant mammalian fossils, besides there have a large number of Paleolithic sites. So, it is very important in exploring Quaternary environmental changes and early human evolutions. The pattern of final disappearing stage and evolution of the Nihewan Basin has been highly concerned for the past few years. The research not only help to understand the reason of the Nihewan Basin＇s disappear, but also to explore the evolution of the Northeast Asia early ancient human and their adaption to the environment. Situated at the center of Nihewan Basin, the Hutouliang section records the gradual disappearance of Nihewan Basin since Middle Pleistocene. Here we conducted a systematic sedimentary, rock magnetic and magnetic fabrics study on Hutouliang section. The section （40°10.100＇N, 114°29.699＇E） is on the south side of the Dadonggou, which is located at the Hutouliang Village, Yangyuan County, Hebei Province. Its thickness is about 60 meters and we collected 275 samples at 20 centimeters intervals. Besides the Dongpo site is located in the section of west 10 meters, which position corresponds to the depth of 17.7 to 17.9 meters of the section that we studied. According to the characters of the sedimentary facies, we divide the section into three stages. Stage Ⅰ：the lower part （60 to 53 meters）, stage Ⅱ：the middle part （53 to 12 meters）, stage Ⅲ：the upper part （12 to 0 meters）. In order to uncover the magnetic minerals of the section sediments, we select samples from the lower deltaic facies of yellowish-grey silt, the middle lakeshore facies of greyish-yellow sand and the upper shallow-lake facies of greyish-green clay. The three representative samples of the sedimentary environment are subjected to systematic and detailed rock magnetic analysis, which includes χ-T curve, hysteresis loop, FORC, IRM acquisition and DC demagnetization curves. We also conduct detailed AMS to all the samples. Rock magnetic analysis suggests that PSD-magnetite is the prominent magnetic mineral, and hematite is also present in this section. The analysis of AMS indicates the sediments of Hutouliang section is hardly disturbed by later period process. They belong to normal gravity deposition with classic characteristics of lacustrine sediments. The type of magnetic fabric is normal sedimentary magnetic fabric. Based on the division of sedimentary facies and characteristic of magnetic fabrics, the evolution of Nihewan Basin since the Middle-Late Pleistocene can be divided into three stages with 3 complete lake-transgression cycles. Every cycle is composed of the lower greyish-yellow coarse deltaic facies, the middle yellowish-grey lakeshore facies and the upper greyish-green fine-grained lacustrine facies. The magnetic fabrics shows parameters decrease upward correspondingly. The evolution in stageⅡ maybe is climately driven, while in stageⅢ it maybe is forced by tectonic movements. The prominent paleocurrent is NW. The early humans in the Dongpo period maybe lived in the shoreland in the short period of lake-regression.