BRAIDED-RIVER DELTAS OF MIDDLE TRIASSIC, KUQA DEPRESSION, TARIM BASIN

Braided river delta that braided river systems prograde into an inland lake were well developed in the Middle Triassic of Kuqa depression,Tarim Basin.Their characters are distinct and significantly different from the other types of deltas.The deposits of braided river deltas are mainly composed of pebbled sandstone and medium coarse sandstones.The bars of the braided channel and subaqueous distributary channel are the main bodies of deltas,and they are composed of many,finning upward lenticular sand bodies that pile vertically up each other.A single lenticular sand body has a thickness of 0.5～4m.Cross stratifications are abundant in the braided river deltas,but lateral accretion cross bedding resulting from lateral accretion of sandbodies are the most principal sedimentary structures and one of the most distinct character of braided river deltas.Because the braided river deltas are characterized by a high rate of coarse sediment input,they are less influenced by the waves of lake.The sandbodies of deltas are very excellent reservoirs in that they have a higher porosity,better permeability and a larger,stable thickness.