Basin Analysis And Paleogeography In North Qilian Mountain To Beishan Area, Early Cretaceous

Based on the sediments of Early Cretaceous basins in the North Qilian toBeishan area, Gansu Province, the characteristics of rock sequences, sedimentaryenvironment evolution, stratigraphic correlation and especially the provenance ofbasins by using of the paleocurrents, gravel components, sandstone detritalcomponents and age of detrital zircons, which can provide the comprehensive andnewly information about provenance of basin, variation of sedimentary environment,basin evolution and paleogeography in the Early Cretaceous, the North Qilian toBeishan area.In vertical, the characteristics of depositional succession showed the rhythm ofcoarse-fine-coarse corresponding to sedimentary and palaeogeographical evolutionaryhistory in succession, i.e., from initial alluvial fan, and then fluvial, followed bylacustrine and fluvial environment finally. In plane, the depositional succession wasthick in south and thin in north and the sedimentary center was located inChangma-Hanxia-Hongliuxia-Jiuquan area, which indicated the North Qilian toBeishan Basins existed in the Early Cretaceous.Two provenances were identified. The south provenance was composed ofmetamorphic basement and Neoproterozoic magmatic rocks from the Central Qilianterrance, the North Qilian volcanic arc (O), magmatic rocks origined from thecollision (S2-3-D) and the Early Mesozoic igneous rocks in the South Qilian. The northprovenance was composed of Paleoproterozoic magmatic rocks and deepmetamorphic rocks from the Dunhuang-Kuruktag terrance, which was located in thenortheastern of Tarim Block, the granites produced by the collision and post-collisionin the Paleozoic-Early Mesozoic, northwestern of North China. Besides, the evolutionof source showed the north provenance provided sediments to the Corridor in theinitial period, and then both provenances were existed in the early period, followed bymiddle stage, the south provenance became prior, while finally, the northwest andsouth supplied sediments together. By summarizing and comparing the invertebrates borning in the research area,we concluded that the Chijinqiao Fm. and Chijinbao Fm. were formed inBerriansian-Barremian, the Xiagou Fm. and Zhonggou Fm. were formed inBarremian-Albian. We also inferred the times of dinosaur bone beds discovered inMazongshan area, Yujingzi basin and Minhe basin in the Lower Cretaceous by usingthe reported palynoflora. The bone beds in Minhe basin was formed in Aptian-Albianor before that, whereas in Yujingzi basin, which was formed in Aptian-Albian and inAlbian in Mazongshang area.The mantle-derived basic lava, ageing in100-120Ma and developing around thenortheast of the Altyn Fault, was formed in intracontinental extensional background.Combined with the sedimentary features of both sides about the Altyn Fault, weproposed that the Altyn Fault was reactive in the Early Cretaceous and influenced theevolution of basins around it.In the Early Cretaceous, the research area was in regional extending backgroundwhich may be caused by thickening crust collapsed and regional mantle rising. Theevolution of basins underwent initial stage of rifted basin corresponding to ChijinqiaoFm. and lower part of Chijinbao Fm., the developing–stable stage corresponding toupper part of Chijinbao Fm. and Xiagou Fm., the shrink stage corresponding toZhonggou Fm.. In the respect of palaeogeography and paleoenvironment, the riftsedimentary areas were connected with each other and not separated by mountains,where many of vertebrates lived.