DISCUSSION ON THE ASSEMBLAGE CHARACTERISTICS OF CLAY MINERALS IN THE JIANGHAN BASIN AND THEIR SEDIMENTARY ENVIRONMENT

Jianghan Basin is a non-marine faulted-down basin developed during Cretaceous time.During the period from Cretaceous to the end of Tertiary,the whole basin gradually contracted and the distributive areas of individual sedimentary sequences reduced from the area of the Xinjingzhui Formation over 24100km²,to that of the Qianjiang Formation over 10460km².Both of the two are of major source beds and exploratory targets.A set of unique characteristic clay minerals in these two formations are dominated by illites(90-900%)with a little chlorites(usually less than 7.7%),the unitary and stable mineral species would not be controlled by horizon and depth,neither would change with temperatures and pressures.What the principle factor on such an assemblage characteristics is the water body of the basin.Based on available data,it is proved that the Jianghan Basin had been a lacustrine basin controlled by transgression and regression.In addition,with the application of SEM,the determination of surface characteristics of quartz particles,and the mappings of the transgressive water body boundaries during the sedimentation of 43-42 sand bed in the Qianjiang Formation of the Qianjiang Depression are also made in the paper.