Clay Mineralogy and its paleoclimatic significance of the Oligocene-Miocene sediments in the Gerze Basin, Tibet

Abstract

This study collected the early Oligocene to middle Miocene sediments from the Gerze Basin of Tibet, and used X-Ray diffraction (XRD) and Scanning Electron Microscope (SEM) to discuss their clay mineralolgy, clay indices, better understand the clay mineralogy and its paleoclimatic significance. The results show that clay minerals of the Gerze Basin sediments are mainly composed of illite and chlorite, with minor amounts of smectite and kaolinite, and their relative content varies along the section. Variations of relative contents and clay indices suggest that the Gerze Basin has experienced three-stage evolution of paleoclimate: I) high illite and chlorite contents, with fluctuant smectite and low (I+Ch)/(K+S) ratio, indicative of a dominant seasonal arid climate from the early Oligocene to late Oligocene; II) higher illite and chlorite contents and larger (I+Ch)/(K+S) ratio but absence of kaolinite, indicating a colder and drier climate from the late Oligocene to early Miocene; III) high illite and chlorite contents with fluctuant (I+Ch)/(K+S) ratios and occasional occurrence of kaolinite, suggesting that the climate became warmer and more humid compared with that of stage II in the mid-Miocene. These conclusions were also reinforced by the clay morphology, which suggests that physical weathering dominated in stage II, while relatively strong chemical weathering was dominant in stages I and III. Clay minerals of the sediments mainly consist of illite and chlorite, indicating that the source rock played a significant role in clay origin. It is inferred that global cooling and the enhancement of denudation and obstruction of northward moisture due to the uplift of the Qinghai-Tibet Plateau were responsible for the provenance of illite and chlorite under weak chemical weathering. Though the Qinghai-Tibet Plateau reached a certain elevation by the mid-Miocene, yet the mid-Miocene widespread warming might have largely impacted the Gerze climate.