Sedimentary Environments and Source Rock Analysis of the Turpan Basin

Key words: 
• terrigenous sediments /
• sedimentary environment /
• paleocurrent analysis /
• Turpan Basin /

Abstract: The Turpan Basin is located in the eastern part of the Tian Shan and has a complex polycyclic sedimentary and tectonic history from the Late Permian to the Late Tertiary.Main stratigraphic boundaries follow in general tectonically induced sedimentary cycles,which are bounded by unconformities.More than 10 000 metres of clastic sediments were accumulated during the evolution of the basin.The sedimentary facies are abundant and the sedimentary environments changed with time.The oldest parts of the true basin fill are of Late Permian age.They and mostly nonmarine deposits and overlay unconformably Upper Carboniferous volcanic rocks and consist of lacustrine and fluvial dark mudstones,siltstone,sandstone and conglomerate.The Triassic strata consist of red coarse clasts in the lower part and dark,fine clastic sediments with coal and coal streaks in the upper part, indicating alluvial and lacustrine-fluvial environments with climate change from arid to humid during the Triassic.The Jurassic sediments form the most widely distributed and thickest part of the Turpan Basin,as well as the most important productive sequences of oil,gas and coal,The Lower and Middle Jurassic sediments consist of grey and green fine clastic coal-bearing rocks which were deposited in a lacustrine-swamp environment.The Upper Jurassic consists of variegated coarse clasts rocks,reflecting piedmontfluvial environment with arid climate.In the Cretaceous,the sedimentary range was more limited and the sedimentary environment belongs to a lacustrine system.The Tertiary strata are distributed through the entire basin and are the youngest of the basin fill investigated in this study.Conglomerate and sandstone are dominant in the lower part and deposited in a braided fluvial/alluvial environments,while mudstone and silty mudstone are dominant in the upper part and are a lacustrine deposition.Local interlaid gypsum beds and salt beds with red mudstone indicate a deposition in an arid climate and a local salt-water lake environment. Paleocurrent analysis reveals a complex pattern of sediment fluxes into the basin.Along with the tectonic variation,the source rock regions of the basin varied with time. From Permian to Jurassic,the Jueluotage Shan was the dominant source rock region for the Turpan Basin.The locally uplifted areas of the Bogda Shan(e.g.at Taoshuyuan)were only minor providers of source rocks .The paleocurrent directions were from south to north.The Qitai paleouplift was an important provenance area for the northern region of the basin,which leading to the transportation of sediments from north to south .Since the Latest Jurassic,the Bogda Shan gradually folded and uplifted,building another source rock region for the basin.The paleocurrent directions were from north to south.In the Cretaceous and Tertiary,the Bogda Shan was an important source rock region.This region was continually strongly uplifted in the Cenozoic age and the dominant source rock region for the basin with transport directions from north to south.In the Hami Sag,the Haerlike Shan was a continuously eroding region and an ever important provenance area,along with the Jueluotage Shan.The paleocurrent directions were always from north to south.