Abstract

The Tianshan range lies down on ~2800km in Central Asia and separates two desertic basins, the Tarim to the south and the Junggar to the north. Its summits (>7000m) resulted of a later Cenozoic reactivation induced by the India-Asia collision. This range is a natural barrier to the atmospheric circulation in Central Asia and has yielded an impact on the climatic evolution during the Neogene. Its morphologic evolution is due to both tectonic and climate, at several time and spatial scales. Sediments from erosion are trapped in the basins. The north Tianshan piedmont is composed of fault bend-fold anticlines incised by rivers, which exposes outcrop of Cenozoic fluvio-lacustrine series derived from the erosion of the range. From a multidisciplinary study (facies, organic matter-OM and heavy minerals-HM) on Jingou He, Kuitun He and Ebi Nor sites, we reconstructed palaeoenvironmental and palaeoclimatic evolution since 25Ma. We have shown that OM composition in sediments is not linked to sedimentary environments. Moreover, palynofacies study has revealed global climatic changes, as aridification in Central Asia at ~15, 8 and 3 Ma, and regional changes on northern piedmont. The results confrontation with the magnetic study reveal climatic variations on northern piedmont, due to either regional tectonic (~20.5, 15.5, 11.5 and 4.5 Ma) or climate (~20.5; ~17.5; ~13.8; ~9-8.8; ~4.5 and ~1.2 Ma). In accordance with palaeoenvironmental and palaeoclimatic reconstructions in Junggar basin during the Neogene, we may conclude 1) palynofacies study revealed high resolution environmental changes, despite poor OM contents, 2) parameters confrontation permitted to propose tectonic or climatic origin and 3) correlation between study sites showed that climate variations are on regional scale or more.