Abstract:

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.

Key words: Yangyuan Basin, late Middle Pleistocene, chemical sedimentary, palaeolake evolution