SEDIMENTOLOGY AND BASIN EVOLUTION OF THE UPPER PERMIAN, SICHUAN AND WEST HUBEI PROVINCES

The late permian Strata, Sichuan, Which are a kind of sequence of mixed siliciclastic and carbonate sediments, were deposited during the period of relative tectonic quiescence on the Yang Zi Platform which was adjacent to lower relief tectonic highland of Kang Dian (Yunnan and the west part of Sichuan Provinces) .During the early period of the Late Permian, Sichuan and the west part in the central-west part of Sichuan, the carbonate ramp in the east part of Sichuan, the open marine basin in the west part of HuBei.The coal-bearing basin in the central-west part of Sichuan was developed into the coal-bearing siliciclastic sediments that contained continental and marine materials named Long Tan Fromation.The terrigenous materials were entered into the basin from the west part of lower relief tectonic highland o Kang Dian.The influx of the materials, however, was not great enough to shut out carbonate productior completely.Therefore, the carbonate ramp in the east part of Sichuan consisted of medium bedded and dark gray chert limestone with some silicious shales, which are called Wu Jiaping Formation.In the open marine basin of the west part of Hubei, there was a deposition of argillaceous limestong and silicious shale. By the late period of the Late Permian, transgressive rate was increasing and the sea was expanding from the carbonate ramp to the coal-bearing basin. Only minor siliciclastic materials poured into the basin.At he same time, the coal-bearing basin was begining to migrate toward the west pat of Sichuan ba sin in which sediments deposited are named Xin Wen Formation (P₂²) .The carbonate sediments called Changxing Formation (P₂²) overlain on the coal strata of Longtan Formation. At the north-east pan and east part of Sichuan basin, Changxing limeston are overlain by the Dalong Formation (P₂³) consisting of limeston and silicious shales which are a part of deep water facies. It can be concluded that transgression is a major geological event according to the correlation of the Upper Permian' s strata, the migration of coal-bearing scries, the distribution of clay minerals in the shales of the coal-bearing units and the climbing on the ramp for organic build-up from cast to west.The characteristic of dcpositional cycles in the coal-bearing series shows that there are some smaller events of transgression-regression during the large transgrcssive period.The upper- part of each cycle shows the small transgressivc scqcnces and its lower part contains the small regressive sequences. The transgressive and regressive sequences are quite clear in the coal series. However the eustatic sea-level changs caused by transgression and regression are hard to recognize in the limestone of Wujiaping Formation and Changing Formation. Perhaps the changes can be revealed by the crosional surfaces in organic build-up and the interbeddings between shales and limestones in the Wujiaping and Changxing Formations. With the increase of transgrcssive rate and the contract of coal-bearing basin, carbonate materials were rapidly produced.The processes of carbonate deposition effectively and rapidly turned a carbonate ramp in the Wu Jiaping period into a carbonate platform in the Changxing period. Consequently, the coal- bearing basin diminished toward the land ( the rate of the basin diminution is about 200m/ 1000yr.), and the open marine basin in the west part of Hubei displayed a feature of starved basin.Their dipositional rates in the various areas are different.The coai-bearing basin, for example, is about 0.6036cm/1000yr., the open marine basin in the west part of Hubci 0.5004cm/ 1000yr., the Wujiaping carbonate ramp 0. 888cm/ 1000yr., and the Changxing carbonate platcform 1.4728cm / 1000yr..The depositional rate of the Changxing carbonate plateform is 2.5 times as much as the rate of the coal-bearing basin, and 1 times as much as the rate of Hubei open marine basin. Therefore, it can be concluded that the Late Permian Lithofacics model of Sichuan basin and the west part of Hubei results from the changes of sea level and the