Abstract:

Clay mineralogy of the Late Pleistocene loess sediments in Tawan, Linxia, northwestern China，was investigated using X-ray diffraction(XRD)and scanning electron microscopy(SEM). The results show that mineral compositions of both the loess sediment and the palaeosol are illite, chlorite, mixedlayer illite-smectite, kaolinite, and minor palygorskite. The mineral assemblage of illite, chlorite, mixed-layer illite-smectite, and kaolinite suggests that the palaeoclimate of the source area changed from warm and humid to cold and arid. The Kübler index of illite crystallinity(IC), full width half maximum of the(001)reflection, ranges from 0.23° to 0.86°. It shows significant difference between the lithological layers. However, the IC curve presents two peaks, suggesting that the palaeoclimate of Late Pleistocene in Linxia experienced two cycles of warm and humid to cold and arid, and the late cycle was more humid and more arid comparative to the former one. Both authigenic and detrital palygorskite occured in the Late Pleistocene loess sediments, the stability of palygorskite indicates that the annual rainfall was around 300 mm during the Late Pleistocene.

Key words: clay minerals, crystallinity, palygorskite, Late Pleistocene, Linxia basin