The Early Cretaceous Basin Evolution Of Northern Qilian Mountain

Jiuquan Basin is surrounded by3sections of the north of Eurasian plate, the east of Pacific Plate and the southwest of the Indian plate. Late Jurassic-Cretaceous western Pacific oceanic plate subducted to the Eurasian continent, Mongolia Okhotsk Sea internal the Eurasian plate closed in the Early Cretaceous and formed collision orogeny, and the Northwest Territories Cretaceous mantle upwelling, all these aspects conspired to make the north of the early Cretaceous Hexi Corridor region in a extended environment. By measurement of western Jiuquan field outcrops in detail, this paper analyzed the Early Cretaceous sedimentary facies and depositional environment of this region, and studied the collected sandstone samples through indoor microscope and zircon dating, and the provenance of the basin, and eventually come to the dynamic evolution of the Jiuxi Basin in early Cretaceous.Western Jiuquan region developed the Chijinqiao Fm., Chijinpu Fm., Xiagou Fm. and Zhonggou Fm. in turn during the Early Cretaceous. The Chijinqiao Fm., both in northern basin of Chijinzhen and western of Hongliuxia, is a set of celadon-pale yellow, purple conglomerate and sandstone coarse clastic sediments. The Chijinpu Fm., in both the eastern basin of Xinminpu and western of Hongliuxia, is a rhythm interaction of coarse and fine clastic deposition, consisting of celadon-gray yellow, purple, gray and black conglomerate, sandstone, siltstone and mudstone. The Xiagou Fm. is a set of interaction of fine-coarse clastic deposition in eastern of the basin of Xinminpu, composed of gray, purple conglomerate, sandstone and siltstone composed of thick; while sets of fine deposition in west of Hongliuxia region, south of Hanxia region and Chang Ma, characterized by gray-green, gray-black clay rocks, calcareous mudstone, containing a large number of bivalve and gastropod fossils, and mafic basalt is produced in these regions. The Xiagou in the east of Xinminpu and south of Changma is a set of medium-coarse clastic sediments, characterized by reddish brown, purple, gray and yellow-celadon sandstone and conglomerate; while in the west of Hongliuxia and south of Hanxia, it is still sets of fine sediments, consisting of gray, brown shale and calcareous mudstone, etc.Early Cretaceous sedimentary sequence of Jiuxi area shows a overall coarse-fine-coarse cycle from the bottom up, the sedimentary system evolution shows initial alluvial fan-braided river-meandering river system, early alluvial fan-braided river delta system, middle braided river delta-lake system, and late alluvial fan-meandering river-lake system. During the Chijinqiao deposition period, the northern basin of Chijinzhen and western basin of Hongliuxia developed a set of alluvial fan- river depositional systems. During the Chijinpu deposition period, eastern basin of Xinminpu and western of Hongliuxia developed mainly a set of braided river delta deposition system. During the Xiagou deposition period, eastern basin of Xinminpu area was still the braided river delta deposition system, mainly the the edge sub-facies; while the western basin of Hongliuxia area turned to the the stable shallow lake deposits; southern basin of Hanxia and Changma area semi-deep lake, deep lake lacustrine depositional system, where the water was the deepest, and the deposition center lay; During the Zhonggou deposition period, eastern basin of Xinminpu and southern of Chang horse area devepoed into the alluvial fan-rivers deposition systems, western of Hongliuxia area remained the stable shallow lacustrine, and southern of Hanxia area was shallow-semi-deep lake lacustrine deposition system.During the Chijinqiao deposition period, the western basin of Hongliuxia area showed palaeocurrent of E-SE direction, and Chijinzhen S-W direction, the provenance of this period came from the west Dunhuang block Paleoproterozoic basement metamorphic and igneous rocks, and Beishan arae Hercynian collision and post-collisional intrusive rocks and metamorphic rocks. During the Chijinpu deposition period, the basin edge paleouplift activities lead to the palaeocurrent changes, from S-W to NE in Chijinzhen, and from SE to S-W in Hongliuxia, Xinminbao area mainly showed NE direction. During this period, the provenance came from both directions of north and south, the north is still Beishan area Hercynian intrusive rocks and metamorphic source region, and the provenance of Xinminbao from its southern Qilian or surrounding Silurian epimetamorphic sandstone, sandstone and limestone source rocks. During the Xiagou deposition period, the basin was under a stable state, the eastern margin of the basin Xinminbao inherited the palaeocurrent of N-NE, Hongliuxia showed a small-scale change, from S-W to NW, and Hanxia area palaeocurrent showed E direction. In this period its paleogeography was high in SW and low in NE, and its provenance turned to the south side, including the south palaeohigh, the Early Paleozoic North Qilian crack trap slot Close-mafic igneous rocks, as well as the ancient igneous rocks of Qilian orogenic belt. During the Zhonggou deposition period, the shrink and subsidence of the basin resulted in the palaeocurrent change again, east of Xinminpu area changed from NE to SW, the west of Hongliuxia remained W, south of Hanxia area became from E to W, the ancient basin geography turned to the trend of high in N and low in W, however, the source area still lay in the south side, including the Silurian grade metamorphic sandstone, sandstone, the North Qilian Island arc-mafic igneous rocks and collision granite, and the Early Mesozoic South Qilian magmatic rocks.The Jiuxi basin was under the extended environment in the Early Cretaceous, its rift evolution can be divided into four stages, namely the initial of the rift,the development of the rift, the largest rift and shrink of the rift, corresponding to theChijinqiao Fm., Chijinpu Fm., XiagouFm. and Zhonggou Fm. respectively. During the Chijinqiao deposition period, under the background of the global stretch, the lateral flow of lower part of the lithosphere and asthenosphere partial melting, or the mantle plume and deep thermal uplift, caused the initial rift of the basin; and it extended in the during the Chijinpu deposition period; the rift developed to the largest scale in the Xiagou deposition period, accompanying with the mafic basalt from the mantle, the water of the basin developed to the deepest and most extensive scale; during the Zhonggou deposition period, the mantle uplift cooling and heat sink back led to the basin depression stage, and eventually atrophy to end.