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Clay mineralogy analyses were carried out on Cenozoic sedimentary rocks collected from the southwestern margin of the Tarim Basin, northwest China, using X-ray diffraction (XRD) and scanning electron microscopy (SEM) methods. These investigations were aimed at determining the paleoclimate of the area in early Cenozoic times, especially during the late Eocene to early Oligocene. The proportion of clay mineral species and clay mineral indices, as well as the proportion of non-clay minerals, gypsum, and morphology of the clays showed that the paleoclimate of the Tarim Basin underwent a distinct change from warm and humid to cold and dry during the late Eocene to early Oligocene. The climate evolution derived from the clay mineralogical study in the Tarim Basin is consistent with the effects of Eocene-Oligocene climate cooling. Combined with the retreat of the Neo-Tethys Sea, cooling of global ocean temperatures reduced the moisture supply to continental interiors, leading to cooling and aridification in the Asian interior. (C) 2012 Elsevier B.V. All rights reserved.