Evolution Of Sedimentary And Provenance Analysis In Middle Jurassic Of Mohe Basin

Mohe basin lies in the northwest of Heilongjiang Province, and covers the area of E121°07′～125°45′, N52°～54°. It belongs to Erguna terrane of Mongolia-Okhotsk orogenic belt in tectonic. Erguna terrane is one part of the Heilongjiang composite terrane, which is adjacent to Siberian plate in the north and west, to Bureya-Jiamusi terrane in the east and to Xing'anling terrane in the south. Mohe basin just locates at the collisional-suture belt of Siberian plate and Heilongjiang composite terrane,and is distributed in an east-west direction. The basin is linked together with Upper Amur basin in Russian, and the whole area is 38500km2. Mohe basin in the boundary of China accounts for 21500km2.The sedimentary and Provenance of the Middle Jurassic basin is studied thoroughly, and basin formation mechanics and sedimentary evolution in Jurassic are disscussed in the thesis.Mohe basin deposited Xiufeng formation, Er'shi'erzhan formation, Emuerhe formation, Kaikukang formation during Middle Jurassic period. Based on the investigation of outcrop in the field, the observation of core, and comprehensive analysis of logging and seismic data, the sedimentary characteristics have been proved. Mohe basin was a continental lake basin in Middle Jurassic, and Fluvial-fan delta-lacstrine sedimentary systems developed in the basin. Among them, Fluvial and fan delta mainly developed in Xiufeng formation and Kaikukang formation, Fan delta and lacstrine facies mainly developed In er'shi'erzhan formation and Emuerhe formation, the regularity of sedimentary evolution is that the grain size varied from grain-fine-grain.Based on comprehensive analyses of the data from field work, coring, well-logging and geophysical exploration, the planar assemblage characteristics of sedimentary facies of Major target stratigraphy-Xiufeng formation, Er'shi'erzhan formation, Emuerhe formationh had been analyzed, and the approximate lithofacies palaeogeography maps of Xiufeng Period, Er'shi'erzhan Period, and Emuerhe period in middle Jurassic had been compiled. The water deeper and deeper from south to north in Mohe basin in Middle Jurassic, and the tendency of variation of sedimentary facies displayed fan (delta) plain– fan (delta) front–shore lake– sub-deep lake from south to north. Er'shi'erzhan formation and Emuerhe formation is the prosperous period for development of lake in Mohe basin, and the dark mudstone in wide area had formed.Base on the analysis of the palaeocurrent of Mohe basin in Middle Jurassic, the provenances of the basin are from the south and the north. By analyzing the composition of sandstone, the heavy mineral assemblage and geochemistry of detrital rocks, Research indicated that the sources of the sediments in south were predominantly granites, metamorphic rocks and sedimentary rocks, which shared the similar component constitution with the northern part of Daxing'anling Mountains (Basement of Mohe basin); the source of sediments in north were predominantly granites ,metamorphic rocks, intermediate acidity volcanic rocks, intermediate basic volcanic rocks and sedimentary rocks, which shared the similar component constitution with the Early Mesozoic the active continental margin of the northern part of the Mongolia-Okhotsk orogenic belt and the southern margin of the Siberian craton. All of above suggested that the southern provenance of Mohe basin in Middle Jurassic may be the Daxing'anling Mountains, and the northern provenance was the northern part of the Mongolia-Okhotsk orogenic belt and the southern margin of the Siberian craton. The geochemistry characteristics of clastic rocks indicated that the tectonic settings of the two provenances are all the margine of active continental, which relatively accorded with the regional geological fact of this area and the analysis results of other people.Based on the comprehensive analysis of the predecessor's research results, According to the depositional characteristics, provenance characteristics and regional geology materials, Mohe basin in Middle Jurassic was considered to be a periphery foreland basin, which formation and evolution were associated with the Mongolia-Okhotsk orogenic belt. The evidences are: (1) the tectonic location of Mohe basin during Middle Jurassic periond was in good conditions for forming foreland basin; (2) the basin was asymmetrical, and the basement depth of northern is deeper than southern; (3) the deformation of the stratas was very intense, and the overthrust structures developed from the north to the south; (4) the provenance characteristics were apparently double source, and the north came from orogenic zones, the south came from craton areas; (5) looking from Mohe to Upper Amur, the basin had the sedimentary series of marine flysch to continental molasses, and the depositional center moved from the north to the south; (6) the materials in the boundary of Russia indicated that Upper Amur basin and Zaya Dep basin were foreland basins, Uda basin is hinterland basin. The two kinds of basin symmetrically were developed in the southern and northern Mongolia-Okhotsk orogenic belt.The characteristics that the sediments of Mohe Basin in mid-Jurassic developed from thick-thin-thick and two provenances, both north and south, can be explained well by the development of the peripheral foreland basin. From late Triassic to early Jurassic, Mohe Basin lied in the foreland arched area, which was denuded intensely to be the provenance for the Amer Basin without deposition. While the north-south napping continued, the foreland area and the depo-center migrated to south and the strata overlapped to the south, either.Sediments deposited in Mohe Basin since the Xiufeng period in mid-Jurassic, and the marginal facies including alluvial fan and fan(delta) developed in the south bonder of the basin. Foreland sequences started from Xiufeng Formation of Mohe Basin in mid-Jurassic. Mohe Basin underwent the obduction-napping and foredeep subsiding from Er'shi'erzhan periond , Emuerhe period to Kaikukang period in mid-Jurassic, and alluvial fan facies , fan(delta) facies and lack facies developed, and the characteristics were the thin-thick sequences. The depositional characteristics of foreland basin can be found in the model map of Er'shi'erzhan sedimentary facies. In late Jurassic, Siberian plate impacted and compacted to the Plaeo-Asian plate, the peripheral foreland basin was uplifted and a large nappe structure formed in the south of Mohe Basin.