The Study On The Formation And Evolution Of The Yellow River Between Xiaolangdi And Sanmenxia

The study on the formation and evolution of the Yellow River is generally regarded as a key for understanding the landscape development and structural setting over the East Asia,attracting wide attention in earth sciences.Numerous previous works have focused upon this river,and obtained a series of advances and results.However,some views for the formation time and process of the eastward flowing Yellow River diverge still so greatly that we can not reach a consensus for this challenging issue until now.The Xiaoshan Mountains was deeply downcut by the Yellow River between Sanmenxia and Xiaolangdi,creating the narrow Sanmen Gorge.It links the middle and upper reaches of the Yellow River with its lower stream,thus providing good insight into its eastward flowing drainage pattern.On the basis of the geomorphic investigation,chronological sequence,and sedimentary component and fabric,here we re-construct the formation and evolution process of the Yellow River between Sanmenxia and Xiaolangdi.The following are our suggestions and conclusions.(1)Field investigations revealed that one planation surfaces and five fluvial terraces were formed along the Yellow River between Sanmenxia and Xiaolangdi.The planation surface extended westward to connect in parallel with the top surface of the basin fill.It can be correlated with the Tangxian planation surface dominating the middle Yellow River.The planation surface was strongly deformed across the Xiaoshan Mountain and then dropped its altitude gently,finally disappeared in the North China Plain.Along the eastern front of the Xiaoshan mountain,one gravel layer was accumulated on the depressions of the planation surface,which was also deformed to follow underlying surface.This gravel layer was covered immediately by Red Clay,being regared as one part of the planation surface.The terrace levels along the Sanmen gorge were also deformed convexly,pointing that the Xiaoshan Mountain was significantly uplifting relative to the Fenwei basin and North China Plain.(2)Based on the basal ages of the aeolian covers,and in combination with the previous work,a chronological sequence of ~3.63,~1.24,~0.86,~0.62,~0.13 Ma,and ~13 ka was established for the planation surface and terraces from T5 to T1,respectively.(3)The uppermost terraces(T5)at different sites can be correlated as the same level between Sanmenxia and Xiaolangdi,based on the comparative analysis for their sedimentary component and fabric.The fluvial sediments on this level are characterized by the current provenance and flowing direction,indicating that this terrace was formed indeed by the Yellow River.The gravel layer on the planation surface along the eastern front of the Xiaoshan Mountains presents the same flowing direction as the Yellow River,however,discrepant provenance suggests that it was not accumulated by the Yellow River.The different provenance and sedimentary setting between the upper Sanmen Formation,the uppermost terrace,the current Yellow River bed,and the gravel layer on the planation,imply that the Sanmen gorge had not been cut through by the river originating from the eastern front of the Xiaoshan Mountains,when the planation surface was formed.(4)On the basis of above results from the geomorphology,chronology,and sedimentary analysis,here,we re-constructed the formation and evolution of the Yellow River between Sanmenxia and Xiaolangdi.The different uplift occurring at >3.63 Ma brought about the subsiding Fenwei basin,which was fed by the local streams from surrounding uplifted regions.The lower Sanmen Formation was gradually accumulated into the basin,which is characterized by the gravel layer interbedded with clay.At the same time,a river system derived from the eastern front of the Xiaoshan Mountains,and flowing eastward to the North China Plain.Continuous erosion leveled previous landscape relief,creating the planation surface prior to the 3.63 Ma,over the middle reach of the Yellow River.Then,the Planation surface was uplifted.A new sub-basin therefore was formed within previous subsiding extend.It was fed immediately by local provenance,developing a set of upper Sanmen Formation,which is composed of sand,clay,and fine gravel.The relative uplift of the Xiaoshan Mountains resulted in headward erosion initiated by the river originating from its eastern front.The Sanmen gorge has been entrenched over this total length to create the through-going eastward drainage of the Yellow River between 3.63 and 1.24 Ma.An episode of drastic uplift initiating at 1.24 Ma forced the Yellow River to continuously downcut,developing a series of fluvial terraces.The modern spectacular down-cutting valley within the Sanmen gorge has been shaped gradually by the Yellow River since this uplift.