Reconstruction of Original Sedimentary Province of the Jurassic in the Northwestern China

The Jurassic strata are widely distributed in the northwestern China.The Early to early Middle Jurassic strata are major coal bearing strata in the northwestern China that consist of sedimentary products of warm humid climte,which are easily traced and correlated regionally.The late Middle to Late Jurassic strata are mainly composed of red beds,interbedded conglomerate and light gray sandstones deposited in the alluvial and fluvial environments,showing paleoclimate changed from humid to dry features.In recent years,hydrocarbon exploration and researches in the Jurassic of NW China found the following facts:many present basin margin areas often lack basin marginal coarse grain sediments of the Jurassic,while present tectonic uplift belts separating sedimentary basins are not present in some periods of the Jurassic or even whole Jurassic.Thus,it is necessary to reconstruct original sedimentary province of the Jurassic basins in NW China. Reconstruction of sedimentary province of the Jurassic in NW China is based on marginal sedimentary facies analysis,stratigraphic contact relationship analysis,paleocurrent system analysis,and tectonic/structural framework analysis.Marginal sedimentary facies analysis focuses on identifying alluvial fan and fluvial depositional systems that define sedimentary extent of basins.Stratigraphic contact relationship analysis is study on relationship between the residual Jurassic strata and underlying or overlaying strata because the relationship reflects depositional setting of the Jurassic strata and transform extent after the Jurassic deposition.Paleocurrent system analysis can help recognize relationship among remade basins at present.Identification of source area,shape and boundary of basins can be carried out on the basis of paleocurrent distribution.Tectonic/structural framework directly or indirectly affects deposition,sometimes are main factor to control sdimentary province and distribution of depositional systems.The Jurassic deposits of NW China are obviously controlled by tectonic/structural framework.For example,there are three subsiding zones in the Qaidam Qilian sedimentary province during the Middle to Late Jurassic deposition,which show similar sedimentary features.Thus,study results of data rich subsiding zones can help predict distribution of depositional systems of little data subsiding zones. Reconstruction of sedimentary province of the Jurassic in NW China is based on the four methods stated above combined with distribution of residual Jurassic outcrops and depositional systems.The results show that the original sedimentary province of the Early to Middle Jurassic is much larger than the residual sedimentary province of the Early Middle Jurassic at present,for example,the Junggar,Turpan hami and other basins are one large sedimentary province;whereas the original sedimentary province of the Middle to Late Jurassic is no big change compared with the residual sedimentary province of the Middle to Late Jurassic.