Sedimentary Environment And Tectonic Significance Of The Early Cretaceous Yijun Formation In The Southwestern Ordos Basin

Through the analysis of gravel group and the comparison of stratigraphic features in adjacent areas,the Early Cretaceous Yijun Formation conglomerate in the southwest of Ordos Basin was studied.It was found that the constitutive and hydrodynamic conditions of the conglomerate formation and the direction of the ancient water flow are obviously different in different areas of the study area.The macroscopic features are as follows:(1)The Early Cretaceous Yijun Formation Conglomerate in the early Early Cretaceous was a semi-arid alluvial fan in arid-semi-arid environments,with low weathering,and near the interior of the basin evolved into a braided river;gravel sorting and grinding which is a trend toward better inside the basin,with multiple sources and rapid accumulation.(2)According to the difference of gravel group,the Yijun formation was divided into four regions,namely,Longxian-Qianyang District in the southwest,Fengxiang-Linyou District in the central and western regions,Yongping-Binxian District in the middle,and Qishan District in the south.The conglomerate in Longxian-Qianyang District in the southwest is mainly from the Paleozoic strata of the Longshan;the conglomerate in the Fengxiang-Linyou region of the Midwest is mainly from the North Qinling fold belt;the conglomerate from the Qishan District in the south is mainly from the underlying the Guandaokou Group strata in the Proterozoic;the conglomerate in the Yongping-Binxian area in the central part is mainly from the Beiqinling fold belt,and a very small amount comes from the source area of the Guandaokou Group in the Lower Proterozoic Guandaokou Group.(3)Through the vertical section of the conglomerate in the Fengxiang-Linyou area,the gravel group analysis shows that the gravel is relatively stable,and the conglomerate's gravel diameter and the shale's lens body can be divided into three phases.The vertical features are “fine grain size at the top and bottom,more sand and mud,coarser middle grain size,and less sand and mud content”,reflecting that in the early Early Cretaceous,the Northern Qinling region experienced at least three stages of uplift and erosion with different intensities.Therefore,combined with regional geological background,in the Early Cretaceous,the Paleozoic fold system in the southwestern region of the basin has already become a provenance area,while blocking the migration of the Beiqinling granite;while in the middle-western and southern areas,it is a single source provenance.Although the Weibei Uplift has seen its rudimentary shape in Qishan erea,it has not blocked the source material of the Beiqinling granite;the middle of the basin is the source area of the mixture,and the source material has moved farther away from the basin,suggesting that the Weibei Uplift has a smaller magnitude of uplift in the southern Binxian area.Therefore,it is believed that the Weibei Uplift of the Early Cretaceous has been uplifted in the southwest and the south,but the uplift amplitude has gradually decreased toward the east of the basin.Through the analysis of sedimentary environment,it is found that the main sedimentary environment of the remnant strata today is the fan-facies and fan-end facies of the alluvial fan,while the absence of the fan-facies of the alluvial fan indicates that the sedimentary boundary of the Early Cretaceous basin was at least today's Weihe area even reaches the north of the Qinling Mountains.