TRIASSIC PALEOGEOGRAPHY AND SEDIMENTARY FACIES OF THE KUQA DEPRESSION,TARIM BASIN

After the deposition of Late Permian thick-bedded molasses,the Kuqa depression began to enter a stage of development of a closed compressional flexural basin in the Triassic,thus ushering in the stage of development of a Mesozoic foreland basin,when the tectonic framework and style were very different from those in the Paleozoic.The Triassic climate in the basin was wet and hot and the sedimentary facies were very diverse.There are deposits of alluvial fans,fan deltas and braided deltas near the piedmont belt in the northern part.Towards the center of the depression,there are deposits of braided deltas and meandering rivers and lakes.Several coal beds are developed in backswamp deposits.Lakeshore and swampy carbonaceous mudstone and coal measures are major hydrocarbon source rocks of the Kuqa depression.In different phases of the entire Triassic evolution,thrust compressional stresses increased gradually and weakened gradually. The paleogeographic features in various phases were different.From the Early Triassic to Late Tiassic,the depocenter and subsidence center moved southward progressively,while the frontal uplift retreated toward the foreland continually.The Triassic frontal uplift and northern thrust belt were connected to the east of Luntai.This connected old land moved westward steadily from the Early to Late Triassic but the magnitude of migration was not large.In the Late Triassic it became a subordinate material source.