Tectonic-Depositional Setting And Proto-Type Basin Evolution Of The Cambrain And Ordovician In Tarim Area

Protro-type basins refer to the product of geodynamic environment in a period of geological time. The research of protro-type basins contributes to reveal the tectonic-depositional environment and its evolution during the formation of basins. Tarim Basin has undergone the cycle of extension and convergence during Cambrian–Ordovician, which was the key tectonic change in Tarim Region. At present, Cambrian–Ordovician thick carbonate rock fillings is becoming the favorable horizon for petroleum exploration and development in Tarim Basin. As a result, restoration of Cambrian–Ordovician protro-type basin in Tarim region is not only important for tectonic-depositional environment, but also has guiding significance to petroleum exploration of Lower Paleozoic carbonate rock.According to the latest drilling, logging, field outcrop and seismic data, Cambrian– Ordovician stratigraphic system was rebuilt. Based on regional unconformities, we finished the sequence stratigraphic division. Combined analysis the characteristics of peripheral Cambrian-Ordovician outcrops with the study of orogenic belt and paleo plate, we restored peripheral tectonic environment. And the evolution stages of the basin were also analyzed by the tectonic subsidence characteristics. From point to line, from line to face, the tectonic-sedimentary environment of Tarim Basin in different Cambrian–Ordovician periods was rebuilt, and basin-mountain coupling relationship was also revealed.(1) Tarim plate experienced the drift from north to south with rotating counterclockwise during Cambrian – early Ordovician. These movement pattern lead to the form of North Kunlun ocean, North Altun ocean and South Tianshan rife valley. The tectonic setting of Tarim area was extension during this period. Tarim plate quickly moved to the north from the late of late Ordovician with rotating clockwise. The change of the direction of motion and rotation led to the close of North Kunlun ocean, North Altun ocean and the formation of South Tianshan ocean. The tectonic setting of Tarim area was compression from the south side and extension from the north side. At the end of Ordovician, the South Tianshan ocean began to subduct both to north and south. The tectonic setting of Tarim area was compression both the south side and the north side.(2) Tarim area experienced a major transgression-regression cycle during the Cambrian, including two sub-cycles: rapid early Cambrian transgression to middle Cambrian regression and late Cambrian slow transgression to the end of Cambrian regression, respectively corresponding to two stages of sedimentary evolution: The Western Tarim craton depression experienced evolution from shore land face-deep water continental shelf- restricted plantform facies of early Cambrian to evaporate platform facies, restricted platform facies of late Cambrian to the exposure of the end Cambrian. Eastern Tarim cratonic marginal depression experienced evolution from deep-water siliceous mudstone basin facies, deep water clay-bearing carbonate basin facies to deep-water carbonate basin facies.(3) Tarim Ordovician basin experienced two transgressionâ†' regression cycle: early Ordovician transgressionâ†'middle Ordovician regression and late Ordovician transgressionâ†'the end of Ordovician regression, with the sedimentary evolution as follow: The Western Tarim craton depression experienced evolution from restricted platform face-open platform of early Ordovician, open platform inner shoal face-platform depression face-platform basin of middle Ordovician, submerged platform face-open platform face- carbonate and debris mixed stacking shelf face-platform basin faceturbidite basin face of late Ordovician to clastic coastal facies–shelf face of the end of Ordovician. The Eastern Tarim craton marginal depression experienced evolution from deep water carbonate basin facies, deep water black shale basin facies to turbidite facies.(4) Cambrian- Ordovician prototype basin of Tarim is divided into five tectonic units, including South Tianshan, West Kunlun, Altyn, Western Tarim craton depression and the eastern Tarim cratonic marginal depression. The part of south Tianshan mountain region experienced rift-stretching, ocean basin expansion, reduction of ocean basin. Northern Tarim developed through continental slope, passive continental margin, active continental margin. Western kunlun mountain and altun region have experienced the expanding, subduction and closed of ocean basin. The part of south Tarim have evolved through passive continental margin, peripheral foreland basin and then developed with filling and subducting. The west craton depression and the east cratonic marginal depression compound the whole Tarim cratonic basin. In Ordovician the west was significant atrophy because of uplifting, while the east was subducted due to large-scale filling.(5) The structure of Cambrian- Ordovician Tarim basin experienced the change from east-west differentiation structure of platform-basin to north-south differentiation structure of uplift-depression. the basin evolution can be divided into the following four stages:1) Early Cambrian- Ordovician, the stage of stable carbonate platform construction under the extension background.2) Middle Ordovician –the early of Late Ordovician, the stage of carbonate platform break up under the compression background.3) Middle and late of Late Ordovician, the stage of sediment filling under the terrigenous clastic injection background.4) The end of ordovician, the stage of north-south differentiation structure of uplift-depression finally form under the compression background.The evolution of Cambrian- Ordovician Tarim basin has a very good coupling relationship with the peripheral orogenic belt, especially early Paleozoic orogeny effect of the West Kunlun and Altun area paly a significant role.