Restoration Of The Primary Ordos Basin In Zhiluo-Anding Period

The Mesozoic Ordos basin is a residual interior cratonic basin that superimposed on the Palaeozoic large North China craton and experienced multistaged deformation by different styles. The discussion about the primary features of this basin in Mesozoic has been paid more attention. Various conclusions have been proposed. The Zhiluo-Anding period in middle Jurassic is an important stage in the development and evolution of Ordos basin. It's the main period for the hydrocarbon and coal seams forming and transforming, but the study about Zhiluo-Anding formations is very little. On the base of predecessor's work, taking integrated, dynamic and comprehensive analysis and the research theories about reformed basin as guidance, starting from the nowadays' remanent sedimentary entities and structural framework of the basin and its adjacent areas, by the systemic stratigraphic partition and compare, sedimentary facies analysis, multidisciplinary analysis of provenance, and the formation and evolution analysis of the adjacent tectonic units, the cognition about the sedimentary features of primary basin is acquired as a result.Effected by the intense and diversified denudation in later stages, the stratum distribution and thickness pattern of Zhiluo-Anding formations in Ordos Basin and it's synchronal stratum in adjacent areas altered largely. In the residual basin, Zhiluo-Anding formations distributed extensively, the thickness is from 100m to 750m, appearing west thicker than east generally. The synchronal stratum that can be compared sparsely distributed in the adjacent areas of Ordos basin, including Shanxi, Henan Provinces, the inner Mongolia, Ningxia autonomous regions, with thickness from 124m to 700m.Based on the analysis of great deal of outcrops and well logging data, taking the cycle compare as the main method, combine with the regional marker bed and the characteristic of lithology and color characteristics, the Zhiluo formation in the residual basin is divided into upper and lower members. The thickness of both the upper and lower members is consistent with the distribution pattern of the whole formation, and both show thicker in the west and thinner in the east.According to the analysis of the well sedimentary facies, regional sections compare, and combined with other methods, such as lithology identify, grain size analysis and geochemical, such sedimentary system types as the alluvial fan, braided river, meandering river, delta and lacustrine facies are identified in Zhiluo-Anding formations in residual Ordos basin. The regional mapping show, the lower member of Zhiluo formation include mainly braided river and meandering river facies, the delta facies developed locally; while the upper member include mainly fluvial, lacustrine and small scale delta facies. The sedimentary framework of Anding formation succeeded that of upper Zhiluo member, except the sedimentary range of lacustrine and the water depth are obviously increased. Zhiluo-Anding formations formed the second cyclic sedimentation after Fuxian-Yan'an period in Jurassic, experience the transition from warm-moist to drought-torridity. Zhiluo-Anding member usually lacks marginal facies, the depocencer close with the eastern boundary of denudation, and the obvious distributing pattern of sandstone thickness and ratio in residual basin, All of these provide foundation for seeking the sedimentary boundary of primary basin.On the basis of field survey and sedimentary facies analysis, Mawa formation in Yima shows characteristcs of alluvial facies. Mawa formation in Jiyuan shows alluvial, fluvial and lacustrine facies. Yungang formation in Datong is alluvial and fluvial facies of different type; Tianchihe formation mainly has characteristics of fluvial facies in drought-torridity environment. Yungang formation in Ningwu-Jingle basin shows succession from braided-river, meandering river to delta facies. Tianchihe formation in this basin is fluvial facies. The synchronal stratum of Zhiluo-Anding formations in middle and southern Shanxi province mainly show character of fluvial facies near provenance. Changhangou formation in Shiguai has extensive lacustrine facies. Zhiluo-Anding formations in north Helanshan is fluvial and lacustrine facies. Shiyanzi formation in Liupanshan is alluvial and braided river association.According to integrated provenance analysis of sedimentary facies, paleotectonic frameworks, paleocurrent, detrital statistics and other geochemistry methods such as rare earth, minor and major elements discrimination, sedimentary entities in Shanxi province, Helanshan and Liupanshan show close relationship with the residual Ordos basin, while the sedimentary entities in Yima, Jiyuan and Shiguai have poor unification with the residual Ordos basin. The difference about provenance position and type between Datong, Ningwu-Jingle basin and the residual Ordos basin indicates that there have additional provenanve joined in former areas.The systemic analysis of sedimentary palaeogeography, tectonic framework, fission-track dating and basin-mountain coupling shows, the large scale uplift of Lvliang mountain took place after later Late Crataceous-early Paleocene, there isn't uplift that dividing the sedimentary area into eastern and western section in Jinxi area during Middle Jurassic. Through analysis the age of magmatite and the regional tectonic setting, it is thought that Taihangshan has initially risen in Zhiluo-Anding period. The large scale uplift of both Helanshan and Liupanshan happened in Cenozonic.Combining the analysis of the sedimentary facies , provenance .nowadays' tectonic units of its neighboring area, the boundary of the original basin during the Zhiluo-Anding period is approximately concluded: The eastern borderline is to the west of Taihangshan; the northern borderline lie is in Cenozoic Hetao basin; the north-westerm borderline lie in Langshan-Bayanhaote basin-Kexueshan; the north-easterm borderline lie in the west of the Liupanshan; the southern borderline reach to nowadays' Weihe Cenozoic basin.Through the study on the distributing and transferring pattern of the depocenters and accumulation centers, and the determination of subsidence centers, it is show that the development and evolution of Mesozoic Ordos basin is affected by its neighboring plate tectonic actives' decreased and increased intensity with time, such as Pacific tectonic domain and Tethys tectonic domain, including their interaction. the subsidence of the basin is related to the convergence orogenic activity of Qinling Mountain and the twisted subsidence yielding in its foreland.