Basin Evolution And Sediments Accumulation During Eopaleozoic In Ordos Continental Block

China marine oil and gas exploration has achieved significant breakthrough in Sichuan Basin, Tarim Basin, Ordos Basin and Qiangtang Basin which concentrated along Qinling–Qilian- Kunlun- tectonic belt of Thetys tectonic area. But Ordos Basin have not yet achieved a major breakthrough. Compared lower Paleozoic marine strata's to Upper Paleozoic and Mesozoic, which studied weakness. So it is necessary to resolve those basic geological problems, such as the evolution of Paleo-uplift and basin, the distribution and accumulation of sediments and the basin filling process et.al. Referring to the predecessors's research, guiding by the academic ideology of "structural control basin, basin control sediment, sediment control combination" and theories of sedimentology, paleontology, sequence stratigraphy, geochemistry and geophysics, based on outcrops , logging and seismic data, the character of sediments accumulation ,distribution and basin evolution in the early Paleozoic Ordos continental block were researched. The main content and achievements as follows:First, according to lithostratigraphy, biostratigraphy and chronostratigraphy of the lower Paleozoic in Ordos basin, the outcrops and drilling is carried out a systematic stratigraphic division and contrast. After research on the types of sequence boundary and classification standard, seven kinds of sequence boundaries are identified, sequence stratigraphy division plan of lower Paleozoic in Ordos basin is setted, the Cambrian and Ordovician can be divided into 5 Super sequences and 27 third-order sequences, and sequence stratigraphy framework was establishedSecond, by the outfield section, cores and well logging and seismic data, it is studied the sedimentary system development characteristics within the sequence framework. Six depositional systems are identified, they are the coastal sedimentary system, shelf depositional system, carbonate ramp depositional systems, carbonate platform depositional system, the continental slope depositional system and deep-sea basin depositional system; and it is analyzed in-depth the special sedimentary features, causes and significance of the eight categories, the phosphorus construction, mixed sediments, widespread oolitic shoal and wormkalk shoal, rich siliceous carbonate rocks, evaporites, reefs, deep water sediment and molassesThird, taking super sequence system tract as mapping unit, the tectonic sequence paleogeographic of lower Paleozoic in Ordos Basin were drew, which revealed the accumulation and distribution of sediments and the sedimentary evolution in different periods of early Cambrian, it was phosphorus deposition and mixed sedimentary stage during early Cambrian, in the late Cambrian was the stage of platform the widespread oolitic shoal deposits and wormkalk shoal; early Ordovician was the stage of the deposition of silica-rich carbonate ramp, Majiagou period was characterized by restricted carbonate platform environment, Late Ordovician was the rimmed platform and gravity flow developmental stage.Forth, after analysis on the dynamic background including the splitting of super continent Rodinia, paleo-Tethys Ocean, the formation and evolution of Paleo-Asian Ocean and the regional tectonic evolution of margin of Ordos Craton basin, the structure paleogeographic of North China continental block were drew systematic, the sediment construction and sequence structure diversity of 5 super sequences were analyzed deeply, which reveals the basin structure of Ordos Craton, everlasting Emeng ancient land developed in north edge early Paleozoic, southwest edge showed the structure of slop-upheaval-plat in background of passive continental margine during Cambrian, then it transformed into the structure of slop-upheaval-depression in background of active continental margin during Ordovician.Fifth, after discussion the mechanism of basin formation, it is brought forward that the splitting of supercontinent Rodinia and the open of Paleo-Tethys Ocean are the main forces of basin forming in Helan aulacogen and Jinshan aulacogen, which are they're in Qinliankun branch ocean basin and Craton formed in south edge of Ordos. During Cambrian, the north edge is in the condition of weak subduction due to the drifting north china continental block to east Gondwana, but the expanding of south edge Paleo-Tethys Ocean made the basin showed the character of stretching entirely, the slow subsidence inside of Craton mainly caused by the thermal subsidence and crust equilibrium compensation, and the subsidence mechanism of Craton basin edge connected with the lithosphere reduction in the background of stretch. In the end of Cambrian and the beginning of Ordovician, the sedimentation inside of Craton ceased due to the transforming of stretch field to stress field in north China continental block. During Majiagou period, the subsidence sedimentation took place inside Ordos Craton due to the remote stress of extruding both from south and north, further caused by the subduction of north Qilian Mountain and north Qinling Ocean, the sedimentation was significantly affected by the viscosity decrease of lithosphere and the crust nonuniformity of underlying pre-Cambrian Jinsha aulacogen, which was caused by pressure. The edge of Craton connected with the lithosphere flexure caused by plate subduction, which caused the sediments rate highly exceed the one of Craton interior. And during Pingliang to Beiguoshan period, middle caledonian movement improved the subduction and made north China Craton region rose quickly above the sea level, the rift basin only developed in southwest edge. In the end of Ordovician, north Qinling Mountain collided Ordos continental block obviously. Foreland flexure happened in south edge of Ordos, and there formed a relict sea with the character of foreland.Sixth, based on paleo-structure map , this paper analysized the forming evolution of paleohigh, found that paleohigh development controlled by four factors ,they were the base physiognomy of pre-Cambrian, plate physiognomy, global sea level change and diversity settlement of different structure part of plate. The reason why the area of upheaval reduce in Xinji to Zhangxia period were the upheaval-depression physiognomy inherited from pre-Cambrian base rift valley and the sea level rise in the low sea level background. The central upheaval rudiment late Cambrian and the entirely rise expose early Ordovician are caused by the transform stretch to convergence, sedimentation cease and sea level drop. The L shape upheaval is the Craton edge upheaval formed by the remote stress of convergence and diversity sediment. Finally, due to the plate colliding, basin inverted structuredly in late Ordovician and began to receive weathering and denudation for a long term.Seventh, based on predecessors'achievements, this paper integrated analyzed the evolution characters of Ordos Craton basin during early Paleozoic, revealed the coupling relationship among sea level change, basin sediment filling process, paleo-uplift evolution , tectonic event and paleo-climate, and constructed the basin prototype of different evolution stage. The structure-sediment pattern of Ordos Craton basin during early Paleozoic which was high in north and low in south, gentle in east and steep in west , upheaval in middle, were controlled by the splitting of supper continent Rodinia and first level global cycles during the forming of Pangea, and concretely controlled both by force system of north Paleo-Asian Ocean and south Paleo-Tethys Ocean. Everlasting Emeng ancient land developed in north edge of Craton due to the weak subduction of Paleo-Asian Ocean. So, the structure evolution in southwest edge were the main controlled factors of basin structure change, it experienced the developing process of slope-upheaval- flat in SS1-SS3,slope-upheaval-depression in SS4, gutter-arc-basin in SS5 and foreland basin in SS5 in Wilson structure cycle.