Quaternary deposits and palaeogeographic evolution in the Lixian depression,Dongting Basin,Hunan

The Quaternary deposits occur in a thickness of about 200 m in the Lixian depression,Dongting Basin,Hunan,and mainly consist of the early and middle Pleistocene and Holocene deposits.Apart from the clay on the top,the early and middle Pleistocene deposits are dominated by gravels,with silt,fine sand and clay in the central part of the depression.The Holocene deposits are made up of lacustrine-alluvial clay in the southern part of the depression.The Lixian depression was rifted and subsided during the early Pleistocene,and accepted the sediments in the channels and alluvial fans.The depression began to spread outwards during the middle Pleistocene,and became the shallow lake and alluvial fan environments from the central part of the depression outwards.During the middle and late stages of the middle Pleistocene,the basin-wide quiet lake environment appeared due to the spreading of the basin.The depression and its adjacent areas were subjected to the tectonic uplifting and erosion,resulting in the absence of the deposits during the later stage of the middle Pleistocene and the late Pleistocene.Till the Holocene,the lake deposits were developed in the southern part of the depression due to the lake level rising.