Late Mesozoic Red Beds Ang Paleoclimate In Sichuan Basin

The Cretaceous has been thought to be a period of "greenhouse". But there have not been enough evidences from land sediment for this viewpoint. Sichuan basin is one of the most representatives in the Cretaceous terrestrial basin, in which a group of red clastic deposition with gypsum and desert had been deposited.If sediments of redbed, desert and gypsum represent products in a arid climate as a general knowledge. Whether Sichuan basin have been in a homogeneous climate of greenhouse in Cretaceous period or not. Then how did the climate change? Some people had studied paleoenvironment and paieoelimate of basinal edge by the research of molecular fossils and evolving of depositional system. But there is no enough compellent evidences inside basin for this climate. Based on clay minerals and component of rocks, this study anlyzes the climate evoluation of in Sichuan basin in the Middle Jurassic through early Paleogene.Both climate and tectonic index can be transfered by parametes of F/Q and L/Q. Our experimental results demonstrate that the climate had been strongly influnced the sediments in Middle Jurassic but the tectonics are the main controls afterwards in the Late Jurassic along the edge of the basin. In the south of the basin, climate is the dominant factor, which implies that tectonic doesn't extremely influence the deposition in the Middle Jurassic to early Paleogene.Proxies of clay minerals to paloeclmiate are the relative composition, crystallinity of illite, chemic index of illite. The result of experiments indicate that the main clay minerals in the redbeds are illite and smectite. Illite and smectite appear together in most samples. It implies that climate of the sourc area is mostly cool and arid. But there is only illite in sediments of Early Cretaceous and K/T boundary interval, which sugguests a warm and wet climate exisitance. Chemical index of illite of all samples are over 0.5, which indicate all clay minerals have been processed by strong chemical weathering. Consequently, climate in the basin had been in warm and wet.