Inclined eastward and consisting of the Hetianhe, Hetianhedong, Tazhong paleouplifts and Bachu paleoslope, the central paleouplift belt in the Tarim Basin was a large composite paleouplift and paleoslope belt with complicated palaeogeomorphic features during the Middle to early Late Ordovician. A number of paleostructural geomorphic elements have been identified in the paleouplift belt and surrounding areas, such as the high uplift belts, the faulted uplift platforms, the marginal slopes and slope break zones flanking the paleouplift belt, the surrounding shelf slopes or low relief ramps, the shelf slope break zones and deep basin plains. They exerted great influence on the development of paleogeography of the basin. The marginal slopes and slope break zones flanking the uplift belt constrained the formation and deposition of the high-energy facies including reefal and shoal deposits during the Late Ordovician, which comprise the major reservoirs of the Lower Paleozoic in the basin. Toward the end of the Ordovician, the Tazhong paleouplift hinged westward and became a westward-dipped nose as the southeastern margin of the basin was strongly compressed and uplifted. The tectono-paleogeomorphic framework of the central northern basin during the Early Silurian and the Late Devonian to Early Carboniferous changed remarkably in topography from the initial low in east and high in west to high in northeast and low in southwest. The major paleogeomorphic elements developed in these periods included the strong eroded uplift high, the uplift marginal slope, the gentle ramp of the depression margin and the depression belt. The sandstones of the lowstand and the early transgressive systems tracts were deposited along the uplift marginal slopes and the gentle ramps of the depressions comprise the prolific reservoirs in the basin. The study indicates that the distribution patterns of the unconformities within the basin are closely related to the paleogeomorphic features and evolution of the paleouplift belt. From the high upli
LIN ChangSongYANG HaiJunLIU JingYanPENG LiCAI ZhenZhongYANG XiaoFaYANG YongHeng
As one of the major exploration objects of marine deposit in Tarim basin,Silurian has been paid more attention from oil/gas exploration and geologists.However,due to the widely deposit and later erosion,it is difficult to restore the original basin.The surrounding tectonic activity and provenance systems of Silurian Tarim basin have a lot of controversy.Aid of detrital zircons U-Pb dating data obtained from well drilling of Tabei(塔北) and Tazhong(塔中) areas and Sishichang(四十场) and Xiangyangcun(向阳村) outcrop profiles,integrated with other geological and geophysical data,the tectothermal evolution and provenance nature of Silurian deposit have been revealed.Zircons U-Pb dating shows Tarim basin has experienced 5-6 significant tectothermal events:3 500-3 000 Ma Paleo-Mesoarchean,around 2 500 and 1 800 Ma Paleoproterozoic,around 1 000 and 800 Ma Neoproterozoic,and 500-400 Ma Eopaleozoic tectothermal events.These tectothermal events reflected the evolution of Tarim microplates and Tarim basin,respectively,corresponded to the forming and spilitting process of Ur supercontinent,Kenorland,Columbia and Rodinia supercontinent.Difference between the samples of Tazhong and Tabei areas indicated that North and South Tarim microplates were different in Paleo-Mesoarchean,and later evolutions were more synchronous after Paleoproterozoic.Integrated with seismic data and outcrop interpretation,the U-Pb dating results also revealed that the surrounding tectonic activities were still very active during Silurian,and indicated different regions had different source systems.At Tadong(塔东) and Manjiaer(满加尔) depressions,major source systems came from Ordovician Altyn orogenic belts.At Tabei area and northwest of Tarim basin,major source systems came from recycling orogenic zone(the activity of South Tianshan(天山) Mountain) and Precambrian stable basement(local paleo-uplifts at north of Tabei).The Ordovician uplift and orogenic zone at the south of Tarim basin and Precambrian granite basement provided lots of
According to the different geometries and reflected characteristics in the seismic sections,the carbonate platform margin of the northern slope can be summarized as three basic depositional architectures in the Late Ordovician Lianglitage(良里塔格) Formation of the Tazhong(塔中) uplift.The type one mainly located in the west of the carbonate platform margin,and it showed obvious imbricate progradation from the interior to the margin of the platform.The type two was in the middle of the carbonate platform margin,which showed retrogradational stacking pattern in the same transgressive systems tract period,and the slope strata of the platform margin showed progradational sequence in the highstand systems tract period.The type three located in the east of the carbonate platform margin,and it showed the parallel aggradational architecture.The crossing well section along the northern slope of the Tazhong carbonate platform showed that the depositional thickness became thinner from the east to the west.The thickest belt located in the east of the platform margin,and became thinner rapidly towards the basin and the platform interior.These indicated that the paleogeomorphology of the Tazhong uplift was probably high in the west and low in the east during the period of the Late Ordovician Lianglitage Formation.According to the interpretation of seismic profiles and the computer modelling result,the depositional architectures of sequence O 3 l-2 showed aggradation,retrogradation and progradation from the east to the west of the carbonate platform margin during the transgression period.This meant that the accommodation became smaller gradually from the east to the west along the northern carbonate platform margin of the Tazhong uplift.The difference of the accommodation was probably caused by the difference of tectonic subsidence.Also,computer-aided modelling can be used to deeply understand the importance of various control parameters on the carbonate platform depositional architectures and processes.
