| The distribution of mineral resource is controlled by the distribution of sedimentary strata in Zhiluo Formation of Middle Jurassic around the Majiatan-Huianbao area in the Ordos Basin.We can describe the characteristics and evolution of the sedimentary facies in the vertical direction by analyzing the spatial distribution characteristics and deposition of the strata of Zhiluo Formation in the study area,and the deposition process of Zhiluo Formation have been detailed studied,it can paly the guiding role in the exploration and development of mineral resource.Based on the field outcrops and combining with the drilling and logging data,the sequence and sedimentary characteristics of Zhi Luo Formation are divided and described.The lithology of Zhiluo Formation is divided into two positive cycles in the vertical direction,which can be further divided into four Members(ie,J2zI-J2Z4),the bottom of the two cycles are characterized by the thick sandstone,the bottom sandstone in Zhi 2 Member and Zhi 4 Member are known as "Gaoqiao Sandstone" and "Qilizhen sandstone",respectively.There are several types of clastic rocks developed in Zhiluo Formation.The stratigraphic structures contain parallel bedding,horizontal bedding,interlaced bedding,grading bedding and so on.There are few composition and content of palaeontological species in Zhiluo Formation,such as:Plant fossils,sporopollen and bivalve fossils.Logging facies identified by logging data can reflect the sedimentary facies types of Zhaoluo Formation.During the deposition of Zhiluo Formation,the western edge of the basin did not show a strong tectonic change at that time.The whole basin was stable sedimentation and the supply of debris was sufficient.The boundary of the basin was a steep slope,and the water carries a large amount of debris formed fan alluvial body in the piedmont.Because of the slope is large,the water system shows the characteristics of the braided river,river flooding Yan'an Formation,there exsits channel lag deposit and sandwiched with coal lines at the bottom of Zhiluo Formation.With the stable sedimentation of the basin,the accommodate space was increasing,but the erosion of the source area is weakened,and the supply of debris is reduced.The fine sandstone,siltstone and siltstone are the main body of the sediments.At the time of Zhi 3 Member deposition,the influence of the first stage of the Yanshan movement tends to be weak,and the basin shows the characteristics of flooding and develops a floodplain.Then the basin began to slow sedimentation,the depositional environment from the braided river transition to the meandering river,manifests a complete positive cycle in the vertical deposition,showing a typical river"binary structure" feature.In the mudstone preserved plant fossils show that the filicophyta leaves turned to thin slices,leaf membrane retreat.In Late Zhiluo Age,the area of the lake basin was the largest,and the sediment size was the thinest.During the development of the lake basin,experienced hot and dry weather.The paleo-flow data reflect the presence of detrital material in the northwest and southwest direction.Light mineral analysis shows that the debris is mainly from the stable block and the collisional orogenic belt.The heavy mineral data indicates that the parent rock type is composed of metamorphic rocks and acidic magmatic rocks,and there are three distinct regions of heavy mineral assemblage.The geochemical data shows that the parent rock types are mainly felsic rocks,and a small amount of ancient sedimentary rocks and basic rocks are mixed.The tectonic background of the source area is continental island arc wih the active continental margin.The weathered intensity of the source rocks in different regions is very different,due to the different transport distances of the sediments in different provenance areas.According to the zircon U-Pb age,the clastic sediments of Zhiluo Formation in the western Ordos basin are mainly from three different regions,which are the orogenic belts and crystalline basins in the northern margin of the North China Plate,the eastern Alxa Block in the northwest and the northern Qilian orogenic belt in the southwest.Based on the comprehensive sedimentary facies,paleocurrent,heavy mineral,geochemical data and zircon U-Pb age,it is deduced that one of the transport pathes of the clastic sediments in Zhiluo Age is that the granite,granodiorite,gabbro,diorite,granite gneiss,schist and gneiss from the northern margin of the North China Plate and the eastern Alxa Block subjected to long-term weathering and denudation,debris is transported under water,gradually deposited from north to south.There was a wide main channel development along Yinchuan,Majiatan-Huianbao area,Dingbian,and finally folw into the ancient lake in the vicinity atea of Wuqi.The other transport path is that quartz monzonite,black Mica granite,two mica granite,monzonite granite from the northern Qilian Orogenic Belt through weathering erosion effect,the debris were transported from the source area to the sedimentary basin by water,the main river is development along Pingliang,Qingyang,and ultimately enter into the ancient lake. |
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