Late Pleistocene sedimentary features and the palaeoclimatic background in Hongsibao Basin

From the arc-shaped tectonic belt at the north-eastern margin of the Tibetan Plateau to the western part of the Ordos Plateau, there are widely developed Salawusu Formation, Shuidonggou Formation and loess of the Malan Formation. However, the facies transformation and sedimentary relationship between these three formations have always been under dispute. Based on detailed geological survey, the sedimentary relationship between these three formations can be discussed. Combined with the SEM(Scanning Electron Microscopy) analysis of the surface characteristics of the quartz particles and the content of the clay minerals, the time when the loess first deposited in the Hongsibao basin could be determined and the palaeoclimatic background could be discussed. According to this research, it is believed that:Salawusu Formation is overlaied by Shuidonggou Formation and Loess of the Malan Formation at the center and edge of the Hongsibao basin respectivley; The sedimentary environment of Shuidonggou Formation and Malan Formation is contemporaneous heterotopic facies, and the lacustrine sediments of Shuidonggou Formation were deposited at the center of the Hongsibao basin while the loess of Malan Formation was eposited at the relatively higher ground; The eolian loess first was deposited in the Hongsibao basin at the end of the age of Salawusu period characterized by the surface of the quartz particles eroded by hydraulic and eolian transportation, and there are developed localized columnar joints at the top of Salawusu Formation; there are developed warm-arid-warm-arid palaeoclimatic evolutionary process. At the middle of the second stage of the Salawusu Formation and the middle of Shuidonggou Formation, the content of illite and chlorite is relatively low while the content of the montmorillonite and kaolinite is relatively high, indicating that the palaeoclimate was mainly warm and moist. These two stages with warm and moist palaeoclimate correspond with the MIS 3 and MIS 5 stages of the SPECMAP. This research also can provide geological evidences for the evolutionary history study and comprehensive treatment of the ecological environment at the western part of Ordos Plateau.