Occurrence of Palygorskite in Late Oligocene in Linxia Basin and Its Geological and Climatic Indicator

Palygorskite in the late Oligocene sediments at Dongxiang, Linxia, northwestern China was investigated by using X-ray diffraction (XRD) and scanning electron microscopy (SEM) . The results show that authigenic palygorskite occurs in the late Oligocene sediments, and trace amount of detrital palygorskite also can be found in the deposits. The authigenic palygorskite is present as silk-like aggregates with thin and long morphology, replacing the I/S mixed-layers along the plate edges, while the detrital palygorskite shows relatively straight fibers, occurring as a single particle in mixture with other fine-grained clay or covering on the surface of plated clay. Clay mineral assemblages of the lower section, the middle section, and the upper section of the late Oligocene sediments are illite, I/S mixed-layers, and kaolinite, illite and I/S mixed-layers, and illite, I/S mixed-layers, and chlorite, respectively, indicating the climatic evolution from humidity to aridity. The amount of palygorskite increasing from the lower to the upper section of the sediments reflects the similar evolution pattern with clay assemblage. Illite and I/S mixed-layers are ubiquitous throughout the late Oligocene sediments. However, the coexistence of illite, I/S mixed-layers, and kaolinite suggests different provenance of the clay. Palygorskite is a common clay mineral of desert soils and is characteristic of aeolian desert dust in the semi-arid and arid regions, and therefore, the occurrence of detrital palygorskite in the late Oligocene sediments implies the loess sedimentation from ~9 Ma in Linxia basin.