河北阳原盆地中更新世湖相地层顶部文石的发现及其科学意义

位于河北省西北部的阳原盆地在第四纪期间长期被泥河湾古湖所占据，堆积了巨厚的第四纪河湖相地层，其中夹有多层化学沉积。文中对广泛分布于盆地西端河湖相地层顶部的白色化学沉积进行了深入研究，通过野外调查和年代测试、电子显微镜扫描观察、X射线粉晶衍射、常量和微量元素分析，以及硅藻和介形类化石鉴定等一系列实验手段，确认北梁地区地层中的灰白色化学沉积主要由文石组成，其年龄大致在270 ka前后，属中更新世晚期，是在当时比较温干的气候环境下，湖泊逐渐萎缩、湖水不断咸化、文石类矿物大量析出的结果。该地区的泥河湾古湖于中更新世晚期消亡。本研究首次发现了中更新世晚期的化学沉积，在时代上明显区别于以前发现的晚更新世晚期化学沉积。该化学沉积的发现为探讨泥河湾古湖消亡过程和原因提供了直接的沉积学证据，对于重建我国北方中更新世晚期的古环境也具有重要的科学意义。

The discovery of aragonite sedimentary on the top of lacustrine sedimentary in the Middle Pleistocene in Yangyuan Basin,Hebei Province and its scientific significance

Yangyuan Basin located in the Northwest of Hebei Province is a Cenozoic rift basin. It was occupied by the palaeolake Nihewan for a long time during the Quaternary and filled up by a thick succession of lacustrine and fluvial sediments. Several chemical sedimentary layers in the basin had been found previously and the chronologic results suggested that they belonged to the late Pleistocene. Recently, a new chemical sedimentary layer was found in Beiliang section located in 16 km southwest of Yangyuan County by field investigation, which widely distributed in the top of lacustrine and fluvial sediments in western basin. In order to discuss the sedimentary processes and environmental history of the palaeolake Nihewan, the sampling of this section was performed in much detail, and the samples were analyzed by OSL dating, U series dating, SEM, XRD, XRF, and diatom and ostracoda fossil identification. Its U series dating shows that the age of grayish white chemical sedimentary layer is about 270 ka, much older than that of the chemical sediments found before. Mineralogical data of this grayish white chemical layer reveal that it was mainly composed of aragonite(71%) and ankerite (12%). Ostracoda identification shows very low diversity (only two ostracod species) and very high abundance. These evidences suggest that the chemical sedimentary layer was formed under a relatively warmer and dryer climate. The palaeolake had shrank rapidly and became a salt lake during that stage, which led to a large number of minerals like aragonite to be precipitated. The disappearance of the palaeolake Nihewan was during the late Middle Pleistocene at Beiliang area. This result would be of a great significance for reconstruction of paleoenvirenment in northern China during the late Middle Pleistocene.