Clay minerals from the Lower Cretaceous to Middle Jurassic strata in the Bazhou depression,eastern Qilian folded belt

The properties of clay minerals from the Lower Cretaceous to Middle Jurassic strata in the Bazhou depression have been explored by means of thin section analysis,X-ray diffraction analysis and scanning electron microscopy.The results of the study have con-tributed to the classification of the clay minerals into three types:illite,mixed-layer illite/montmorillonite and chlorite.The types and contents of the clay minerals also vary with in-creasing buried depth.Three clay mineral zones may be delimited for the Lower Cretaceous to Middle Jurassic strata in the study area on the basis of the evolution of the clay miner-als.They are mixed-layer illite/montmorillonite zone,rapid conversion zone of mixed layers and illite zone.The evolution of clay minerals and formation of authigenic clay have record-ed most important diagenetic changes of sedimentary strata in the study area.