THE CRETACEOUS GEOLOGICAL HISTORY OF THE SICHUAN BASIN

On the basis of the division and correlation the strata and the character- istics of deposits and fossils, the author advances a different view on the the Cretaceous geologic history in Sichuan. The Ghengoiangyan Group and its cor- relatire strata are of latest Jurassic age, and the Jiading Group and its cor- relative strata are late Early Cretaceous to early Tertialy in age, the base of the latter being youunger than the top of the former. The latter is found to overlie the former in two drill holes in Guanghan and at a section of Long Q- uan, Chengdu. There is a lake basin in the Guanghan-Deyang-Zhongjiang area, while the other areas of Sichuan were denuded areas in the earliest Creta- ceous. From the late Early Cretaceous to middle Cretaceous, the sedimentary basin shifted towards the southwest, and a lot of eolian sandstone was deposited in the south but the fluvio-lacustrine deposition occurred in the west. In the Late Cretaceous, the sediments were of fluvio-eolian origin in the south but mainly of lacustrine origin in the west, intercalated with some salt deposits. As the Sichuan basin was affected by the first phase of the Yanshan- lan subcycle from the Late Jurassic to Early Cretaceous, its southeastern part rose owing to southeast-directed compression and the lake retreated to- wards the northwest. Owing to the effect of the second phase of the Yanshan- lan subcycle in the late Early Cretaceous to Late Cretaceous, the southwestern part of the basin sank, while the northeastern part rose. Therefore, a discon- formity was formed between the Jiading Group and the ChengQiangyan Group.