The Infilling Sequence of Tertiary Strike slip Fault Basins in the Xiaojiang Fault Belt

The Xiaojiang fault belt is a principal fault of the strike slip fault system in the central part of Yunnan, in which a number of continental basins infilled with Upper Tertiary, Quaternary and modern limnic sediments have been developed. They occur in perfect order and special distribution, and their size, geometrical feature,infilling sequence characters and tectonic evolution all record the traces of strike slip tectonic activities. On the basis of detailed genetic stratigraphic, basin infilling sequence and depositional system tracts analyses with some basins, this paper reveals infilling sequence characters of strike slip fault basins. It should be pointed out that the tectonic movement plays a great role in the basin infilling including tectonic subsidence, strike slip movement and tectonic inversion. Genetic stratigraphic units are divided according to chrono stratigraphic interfaces. Each genetic stratigraphic unit must have closely genetic continuity, and possesses about synchronicity. Therefore, it is very important to establish chronostratigraphic interfaces in genetic stratigraphic division. After detailed analyses of many basin fill sequences in the Xiaojiang fault zone, three types of chronostratigraphic interfaces are defined, i.e. tectonic unconformity, transform interface of facies and broadly swamped event. Base on detailed genetic stratigraphic analyses of some basins,this paper reveals genetic stratigraphic evolution of small fault basins under the control of strike slip fault. In the Xiaojiang fault belt,most of coal basins are half graben fault basin and are controlled by marginal fault. During their development, their sedimentary infilling reveals eminent strike slip tectonic character. The basin's depocenters and sedimentation not only distribute along the marginal fault, but also appear clear transposition. Based on detailed study of sedimentary structure and sedimentary process, and analyses of genetic characters and well logging curves, the depositional systems of the basins are mainly alluvial fan system, delta fan system and lacustrine swamp system. On control of the Xiaojiang fault strike slip movement, the coal bearing sequences of fault basins comprise a complete depositional sequence cycle, mainly made up of both alluvial fan lacustrine swamp and alluvial fan fan front depression facies associations, and the vertical transformation of genetic stratigraphic units and the disposition of depositional systems are primarily controlled by the syndepositional structural framework and tectonic activities of the basin. During later period basin formation, the basin fill sequence was under a transpressional tectonic stress filled which brought about the basinwide structural reversion in appearance of en echelon folds, those fold axial traces oblique to the main trend of the strike slip fault zone. It shows the strike slip movement orientation of the basin' marginal fault.