Stratigraphy and its environmental implications of the Late Pleistocene Shuidonggou Formation in the western Ordos Block, North China

The Late Pleistocene sedimentary strata that is widely developed in North China records the signature of prehistoric human settlement. The Late Palaeolithic Shuidonggou Formation that is mainly distributed in the western Ordos Block of North China is best known for containing abundant palaeontological fossils and palaeolithic artefacts with an age span of similar to 35-20 ka B.P. However, its sequence and chronostratigraphy are poorly understood due to the lack of stratigraphic correlation of a uniform relative stratigraphy, hindering our understanding of the way of human adaptation to the environmental and climatic changes. In this article, analyses of high-resolution stratigraphy of Shuidonggou Formation complemented with chronostratigraphic framework were performed based on our field investigations, Optically Stimulated Luminescence dating and literature survey, to reconstruct environmental and climatic conditions in western Ordos Block in the Late Pleistocene. Stratigraphic correlations and chronological evidence indicate that the sedimentary units of the Shuidonggou Formation are similar to 35-10 ka B.P. in age, and three lithologic units of Shuidonggou Formation that is well approximated on a regional basis were identified: gravel layers that were deposited in a fluvial environment at the bottom, rhythm layers consist of sand layers, and clay layers deposited in a lacustrine environment in the lower part, and cycles composed of lacustrine silt and loess in the upper part. These units are defined by sedimentology, matching to lake levels. Continuous and widely developed clay and sandy clay about 3-10 m thick occur at various levels within river terrace II and littoral deposits located 10-20 m above the current lake level provide evidence for the existence of the high lake level/palaeo-lake in western Ordos Block between similar to 35 and 22 ka B.P. Palaeo-lake shrinking occurred after when lakeshore sand and gravel, as well as sandy facies of aeolian origin, started to accumulate in the study area in 22-17 ka B.P. The development of the palaeo-lake probably had been climate-controlled since the warmer and humid climate conditions suggested by the lower sequences corresponds to the Marine Isotope Stage 3 while the cool and dry climate were identified in upper sequences corresponds to the Last Glaciation Maximum. Besides, the human occupation in the Shuidonggou site was synchronous with the development of the palaeo-lake of the same time window. It is suggested that the evolution of the palaeo-lakes in the Late Pleistocene, together with the environmental effects of climate change, play an important role in the human occupation and survival strategies.