Analysis Of Sedimentary And Tectonic Environment Of The Late Cretaceous Xixia Basin

Late Cretaceous Xixia Basin, in the western of Henan Province is the key study object of this dissertation. Based on the plenty of field research in Xixia basin and the adjacent ones, analysis of thin section and data of calcite oxygen isotope, late Cretaceous stratum's petrologic character, sedimentary character and faults controlling the basin formation is analyzed. Stratigraphy and petrography Paleo-geography of Late Cretaceous Xixia Basin and adjacent basins are founded and the geotectonics is discussed.The results show that Gaogou Formation, Upper Cretaceous Series, is an alluvial fan, but there is some difference in sedimentary character and enivoment between the northern part and southern part of the Xixia basin. The northern part was deposited mainly by braided river, while the southern part was deposited mainly by mud-stone flow. Majiacun Formation, Upper Cretaceous Series, was deposited primary by braided river and secondary by meander. Sigou Formation, Upper Cretaceous Series, was deposited by meander and shallow lake. In the vertical stratigraphical series, there is a tendency that the sediments was transported from nor west to southeast and the sedimentary center wandered from west to east accompanying south.The southern and northern boundaries of Xixia Basin was controlled respectively by NWW-trending Xiguanzhuang-Zhenping-Guishan-Meishan shove fault and Mujiaya-Neixiang-Tongbai-Shangcheng shove fault. These two faults formed high angle Riedel (R') shears in the west of the basin and generated right steps of detral pull-apart basin. Syn-tectonic earthquakes in the basin dominated the river sedimentation to some extent. The different characters of faults forming these pull-apart basins and the developments of pull-apart basins lead the migration of sedimentation from nor west to southeast.Comprehensive the Late Cretaceous active faults in western Henan province and sedimentary characteristics of other Late Cretaceous small basins,we found that the Late Cretaceous active fault has obvious fractal features.Fractal dimension value is 1.2283 and the correlation coefficient is 0.999.The fundamental reason of fractal characteristics is primary and secondary fault of Qinling orogenic belt chenging occurrence in western Henan at Late Cretaceous and producing right steps of detral en-echelon faults which have fractal feature.R' fault generated by the en-echelon faults also has fractal characteristics.Composite pull-apart basin model can be used to explain the formation of pull-apart basin which was controled by single fault in the southwestern of Henan,this compound model has two kinds of circumstances.The first is compounded by two strike-slip fault.The second is compounded by a number of right steps of detral fault.The different levels of R' faults may also participate into the pull-apart process.The nature of the Late Cretaceous pull-apart basin in southwestern Henan province determined that these small basin have fractal feature in morphology,distribution and sedimentation.Small basins in southwestern Henan have right—lateral oblique slip arrangemen,these feature was determined by right—lateral oblique slip arrangemen of fault.The morphology of basin is converge westward and splay eastward. The basin general trend are not completely parallel to the regional tectonic line,but have a small-angle intersection with the regional tectonic line.All of Sangping Basin,Xiaguan Basin,Taohe Basin and Liguanqiao Basin have similar sedimentary characteristics with Xixia Basin in southwestern of Henan,they all have main controlling faults in the south part and sub-master faults in the north part. Deposition center parallel to the master fault in southern and near the southern fault. Lithology of sedimentary rocks and sedimentary facies have the feature that the grain size of sediments gradually becomes small from north and south sides to center of the basin,sedimentary facies gradually convert from alluvial fan to meandering river or shore-shallow lacustrine sediment.In addition,these basins have clearly variation regardless of lithology of sedimentary rocks or distribution of sedimentary facies.In the northern,the rock series of Sangping basin and Xiaguan Basin are mainly coarse clastic rocks. In the southern,the rock series of Taohe Basin and Liguanqiao Basin are mainly fine clastic rocks.The characteristic of Xixia Basin is range between the northern and southern basin.The sedimentary facies also reflect the change from the alluvial fan to lacustrine facies.General characteristics of change of lithology of sedimentary rocks and sedimentary facies are similar with that of those small basin. They all have fractal feature.This is apparently related to the fractal feature of the control basin fault.