Major Unconformities,Tectonostratigraphic Frameword,and Evolution of the Superimposed Tarim Basin,Northwest China

The Tarim basin experienced a complex tectonic evolutionary history from Sinian to Cenozoic.Eight large-scale and more than 20 subordinate unconformities defining tectonosequences of different protobasins formed in various tectonic settings have been identified within the Phanerozoic in the Tarim basin,their distribution determining the general characteristics of sequence stratigraphic framework of the basin.Tectonostratigraphic unit I(magasequence) consists mainly of the Sinian System,which formed in a rift or aulacogen setting and can been subdivided into two subordinate stratigraphic units(supersequences).Unconformity(T g9) between Sinian and Cambrian with surface karstification is regarded as a post-rift unconformity.Tectonostratigraphic unit II comprises the Cambrian and the Ordovician and can be divided into six subordinate tectonostratigraphic units,recording the tectonogeographic evolution of the prototype basins from Cambrian to Early Ordovician passive carbonate continental margin or cratonic depression and the Late Ordovician submarine to neritic retroarc foreland and cratonic depressions.The tectonic uplift related to the formation of the unconformity T g5-2 resulted in the remarkable change in basin tectonic setting from a passive divergent to an active convergent,with the development of the Tazhong(塔中) uplift,the Tangguzibasi(塘古孜巴斯),and the northern depression at the end of the Middle to the early Late Ordovician.The widespread angular unconformity T g5 formed by a relatively strong compressive deformation,which caused an abrupt tectonogeographic change of the basin from abyssal to a neritic setting in response to the collision and associated tectonic deformation of the North Kunlun(昆仑) orogenesis during the Late Ordovician to the Early Silurian.Tectonostratigraphic unit III is composed of the Silurian and the Lower to Middle Devonian and characterized by the development of fluvial or deltaic and clastic littoral and neritic deposits.Large-scale terrigenous clastic depositional wedges progradated from the north to south in the southeastern slope of the basin indicate the continuously shallowing and uplifting along the northern basin margin.Tectonostratigraphic unit IV includes the Upper Devonian,Carboniferous,and Permian and can be classified into two subordinate tectonostratigraphic sequences.The angular unconformity(T g3) at the base of the unit is the most widespread unconformity and the strong compression and uplift of the basin during this period has been suggested to be related to the collision of the Tianshan(天山) orogenesis and resulted in fundamental change in tectonic geomorphology with higher to the northeast and lower to the southwest.Tectonostratigraphic unit IV records another tectonic cycle from weak extension to compression in basin setting and is composed mainly of nearshore clastic deposits of embayment basin fills.From the Triassic,the Tarim basin evolved into a period characteristic of development of intracontinental depressions and marginal foreland basins and experienced several cycles from rapid subsidence to strong uplift and deformation,resulting in superimposition and reformation of differently orientated protobasins filled with a series of regional depositional cycles bounded by major unconformities and consisting of extremely thick alluvial and lacustrine deposits.