Mesozoic Sedimentary Evolution In Northeastern Ordos And Its Tectonic Implication To Taihang Shan Area

Ordos basin, locating in the western part of North China Block, is one of the largest hydrocarbon-bearing basins in China. It is famous for its simple architecture, gentle structure and stable subsidence. The basin is surrounded by orogenic belts, and to its east is the Taihangshan Mt. area. Tectonic evolution of Ordos basin could be divided into five stages, i.e. middle-late Proterozoic aulacogen, early Paleozoic marginal sea, late Carboniferous-middle Triassic large intracratonic basin, Mesozoic remnant cratonic basin and Cenozoic peripheral rift basin.The study area includes northeast Ordos basin and Taishangshan area. Triassic and Jurassic outcrop in both basin area (Yulin, Shenmu, Fugu, etc.) and mountain area (Ningwu, Datong, etc.). Whether they are deposited in the same basin and separated later by tectonism or they are initially deposited in different basins attracts people's great attention. Some researchers support the existence of a large Triassic basin or Jurassic basin. They infer that this large basin is not limited to the west of Yellow river and it could spread eastward to the west and north of Henan province and the middle part of Shanxi province. Nevertheless, divergence still exists in the timing and scope of the basin. In addition, though Ordos basin is adjacent to the west of Taihangshan area, a region underwent considerable tectonic event in Mesozoic time, it still accepts sediments of great thickness. It is suggested that these deposits should have recorded the tectonic movement in Taihangshan area.Combining sedimentary and structural geology methods together, this thesis studies the Triassic and Jurassic strata scattering in both basin and mountain area. The results show that: (1) the paleoenvironments in study area are braided river in late Triassic, river-delta-lake in early Jurassic-early middle Jurassic, and river-lake in middle-late middle Jurassic, respectively; (2) the paleocurrents flow to southwest or south throughout late Triassic to middle Jurassic, and the source area is attributed to Yinshan mountain that locates to the northeast of study area. Basin evolution includes three episodes, i.e. late Triassic, early to early middle Jurassic and middle-late middle Jurassic, among which are two regional uplifting stages. The lithology, genetic facies patterns of upper Triassic and lower-middle Jurassic in both Ordos basin and Taihangshan area are very similar and comparative, and thus they should be deposited in the same basin, indicating a"large Ordos basin"from late Triassic to middle Jurassic. Eastern boundary of the basin could extend to the east of Taihangshan in late Triassic, while in early-middle Jurassic the basin recedes westward and the boundary is somewhere between Taihangshan and Lvliangshan. Regional tectonic background for basin development is characterized by uplifting in the east and sinking in the west. Basin evolution records middle-late Jurassic orogenic process that spreads from east to west, indicating the initiation of plateau of east China in study area is no earlier than middle-late Jurassic.