Features Of Sedimentary Facies Of The Haojiatai Area In The East Of The Nihewan Basin In The North China

Nihewan Basin is located in Yangyuan County Zhangjiakou,Hebei Province.The Cenozoic filling in the basin has long been considered the standard layer in early Pleistocene of northern China,which occupies an important place in the Quaternary geological studies in China.And the Taiergoudong section is located in the east of the Nihewan Basin in the North China.The main strata of this section contain the Pleistocene and Pliocene.Pleistocene Haojiatai and Nihewan Formations underlie the malan loess and palaeosol,which exposed at the top of the Taiergou section.with 9.4m in thickness,and overlie the Pliocene Yuxian Formation.Sedimentary textures and lithofacies coevally suggest that the Haojiatai and Nihewan Formations should be dominated by the delta plain and delta front subfacies deposits,with minor prodelta deposits.The delta plain subfacies consists of distrubutary channel,interdistributary bay,natural levee,marsh and freshwater lake microfaces.They are characterized by lenticular coarse-grained sand and gravel deposits with large tabular cross-bedding and basal erosion scouring structure,yellowish-brown medium-fined sandy lenticles with calcareous concretions,and dark clay with abundant carbon clasts.The delta front deposits,which consist of subaqueous distributary channel,subaqueous interdistributary bay,river mouth sand bars,and end bar of branching channel micro facies,are characterized by basal erosion scouring structure,lenticularly medium-fined sand deposits with gravels,fine-grained sand deposits with small tabular cross-beddings,and well-sorted fine-grained quartz sands.The prodelta was less developed relatively,which mainly consists of light-gray clay,green and gray clay with calcareous concretion.The Yuxian Formation contains shore and shallow-lake subfacies deposits.The shore-lake deposits contain clay,silty sand and minor sand-gravel deposits with various colors;they are characterized by mud crack and slumping structure.The shallow-lake deposits,however,consist of silty sand and clay deposits,with lenticular sand and mud interlayers locally.Paleocurrent data indicate that the source of the Pliocene Yuxian Formation should be located in the NE of the Nihewan basin,and the Pleistocene Nihewan Formation and Haojiatai Formation should be derived from the NE-NW of the Nihewan basin.The bottom 15m sediments of the Nihewan Formation is not only derived from the NE source area,but also some sediments should be derived from the SSW source.It indicates that a southward-inclined paleogeograph to the Nihewan Basin during the Pliocene and Pleistocene.The significant changes of sedimentary facies and paleocurrent that ocurred between the Pliocene and Pleistocene provide more available basis for the lower boundary of Quaternary research in Nihewan.