Abstract:Formation of Quaternary sediment in the Three Gorges area is critical important for studying of the geological disasters, disaster events, soil erosion and climate changes. Shidaling section, nearly 5m sediment sequence deposited in eastern Xintan landslide, downstream of the Bingshubaojian Gorges, is discovered in recent geological survey. In this paper, based on the field investigation and profile measurement, combined with systematic grain size analysis, sediment formation process of the Shidaling section was studied. The results show that: 1) upper sediment of the Shidaling section is mainly composed of size fraction of < 50μm, average content of size fraction of > 50μm is 13.14% while that of size fraction of between 10 and 50μm is 50%, which is the basis grain size fraction of the aeolian sediments; 2) Grain size parameters, frequency curves and grain size distribution results (C-M、L-M and A-M figures) indicate that the upper Shidaling section is aeolian deposition while middle and lower Shidaling section is fluvial sediment, which showing a typical positive dual-texture. Chronology results and comparison with dating results of terrace reveal that the Shidaling section formed between late middle-Pleistocene and late Pleistocene, the upper section formed at 110-60 ka B.P.. Comparison with the contemporaneity sediment of the Yangtze River Basin, we propose that upper Shidaling section is Xiashu Loess, and is the western boundary of the Xiashu Loess. All those results demonstrate that the Bingshubaojian Gorges is the division of the Xiashu loess and Wushan loess, which is similar to the loess in Chinese Loess Plateau. Geomorphological speaking, the Bingshubaojian Gorges is a crucial area for division studying of regional climate and geo-environment. In the future, the section should be paid more and more attention.