THE SEDIMENTARY AND PALAEOGEOGRAPHIC CHARACTERISTICS OF THE UPPER ORDOVICIAN IN THE TARIM BASIN

The Late Ordovician epoch is an important period of sedimentation-structure transform in the Tarim Basin.On the palaeogeographic basis of being shallow in the west and deep in the east in the Cambrian-Middle Ordovician epoch,the north-south directional sedimentary framework of uplift alternating with depression was added,forming the diamictic continental shelf deposit system in the center and west and the deep-sea trough basin deposit system in the east of Tarim Basin.Because the tectogenesis was not harmonious,the vertical diamictic continental shelf deposit system in the center and west of Tarim Basin was different from the transverse one.The vertical variation is that there are many autogenic carbonate sediments in the earlier period and that terricolous sediments are dominant in the later period.In the early and middle periods,carbonate mild slope deposit of Qiaerbake Formation and carbonate platform deposit of Lianglitage Formation developed,forming three carbonate platforms in the center and north of Tarim Basin and in Bachu-Tangwangcheng area.In the later period,the Sangtamu Formation slope-tidal flat deposit with detrital rock being dominant developed.Transversely,in the deep depression areas away from the surrounding uplifts,the Querqueke Formation turbidite deposit with diamictic outer continental shelf deep basin facies developed in the east of the diamictic continental shelf and the static water mudstone deposit with diamictic outer continental shelf deep basin facies developed in Keping,Shaxi and Awati areas.In Bachu upheaval area with shallow water body and gentle sea-bottom slope gradient,the diamictic inner continental shelf deposit developed.The supercompensated deep-sea turbidite deposit developed in the eastern deep-sea trough basin.There still exists a slope-break zone with topography greatly varing between the central and western continental shelfs.Comparing with the outer continental shelf deposit,its turbidite sequence further developed and the coarse clastics obviously increased.