Research On The Rule Of Depocenter Migration And Evolution Of Late Triassic In The Ordos Basin

Based on integrating the previous research results and exploration data and guided by fundamental sedimentological theory and means,the author had deeply studied the development characteristics of sedimentary system and the location of depocencer and subsidence center of Yan-chang Formation of Upper-Triassic in Ordos Basin by comprehensively utilizing many theories and means,such as tectonical and stratum analysis, geophysics,geochemistry,basin analysis and ancient landscape restoration.Moreover,the author also had confirmed that meandering river delta sedimentary system was mainly distributed in northeast portion,and the braided river delta sedimentary system in southwest portion.Simultaneously,the author analyzed the plane exhibition characteristic of sedimentary facies of Yanchang Formation(C10-C1),and conformed the location of lake strandline and deep lake strandline in each period of Yanchang Formation.Based on analyzing the regional geologic background,light and heavy minerals, palaeocurrent direction and rock trace element characteristics,the author had concluded that there were four sedimentary source regions,such as northeast,southwest,northwest and south trending direction in Late Triassic period in Ordos Basin.Among them,the major proveniences were northeast and southwest trending,and secondly are northwest and south trending.Moreover,the provenances of each depositional stages of Yanchang Formation changed gradually and had good succession.In Ordos Basin,Yanchang Formation of Upper Triassic had four sedimentary evolutionary periods corresponding to lake basin,such as formation and enlarging period, largest extent period,returning period and the fading and dying period.In C10,C9 and C8 periods,the sedimentary systems had changed gradually from omnipresent fluvial facies into lacustrine transgressive facies,and in C7 period,the lake basin of Ordos Basin was in the largest extent period because the northwest and northeast trending faults began to activate strongly and entire basement sank violently,so for high subsidence rate,lake strandline extended rapidly,and the turbidit(?)es developed widely in southwest portion of Ordos Basin,In C6 period,the lake basin began to fade because of delta filling extensively,and in C4+5 period,the lake basin went on being in fading history except for the short-period of lake flooding when it appeared in northwest portion of the basin.In C3 period,the lake basin shrinked forward corresponding to the area of deep lake retiring to southeast portion,and in C2 period,fluvial sediments distributed widely,lake strandline shrinked rather more forward and the deep lake began to deplete.However,in C1 period,the main sedimentary system was delta plain in study area besides the short-term deep lake distributing in the area of from Zichang to Miaogou,but after the Triassic period,the lake of C1 Formation became fragmented because it had been eroded strongly.The depocenter of lake basin changed regularly because of being controlled by tectonic subsidence rate.In the view of the evolvement of lake strandline,the main axis of lake extended along the northwest-southeast trending during from C10 to C4+5 period,and lake strandline faded to northwest rather more greatly during from C3 to C2 period,but in C1 period,the main axis of lake had changed extending direction from northwest-southeast into approximately latitudinal trending.In Late Triassic period,subsiding cente(?)of Ordos Basin lay in the belt of Majiatan-Baibao-Yijun and the local subsidence region lay in the belt of Zhenyuan-Chongxin,southwest portion of Ordos Basin;but the depocencer lay mainly in the range surrounded by Zhengning,Heshui,Jiyuan,Wuqi,Zhidan,Taibai,Huangling and Huanglong,with stretching direction of from northwest to southeast and moving toward each side of northwest-southeast trending axis.The ranges of subsiding centers and depocencers were different.Their emigration and overlying had the control action on thermal evolution of source rock,and the overlying portion was of importance to study the distributing area of abnormal high values of of source rocks' thermal evolution.