Reversal Of The Songhua River In The Late Early Pleistocene

Drainage evolution is the product of the interaction of tectonic,landform and climate,and plays an important role in the geodynamic system.Although the drainage evolution of East Asian has been widely concerned and made a lot of progress,however,the research on the evolution of Songhua River,the most important river in Northeast China is still weak,especially whether there is inversion in the middle-upper reaches is still uncertain.Sedimentary process is an important surface process,and it is important to explain the responses of rivers,lakes and aeolian sediments to the changes of tectonic-climate-landform-drainage.Therefore,the magnetic susceptibility(MS),element geochemistry and geochronology(paleomagnetism,OSL and ESR)of Quaternary sedimentary cores in Huangshan of Harbin were studied in this study.The results are as follows:(1)According to the lithology,sedimentary facies and MS characteristics of core sediments,the Huangshan core is divided into three sections:the Section?(0?30.4m):aeolian loess accumulates,and the MS changes periodically;Section?(30.4?62.3m):the MS fluctuates irregularly and has a large fluctuation range;Section?(62.3?101 m):the MS value is extremely low and the variation range is small.(2)Based on paleomagnetic dating,the chronostratigraphic framework of Huangshan core is established.The age of core bottom boundary is about 1.68-1.70 Ma,and the age at 62.3 m between the section?and?of core is about 0.94 Ma;By means of OSL,ESR and deep-sea oxygen isotope stack comparison,the age frame of the Section?(0?30.4 m)of the core is established,and the age of the bottom boundary is about 0.46 Ma.(3)The geochemical composition of elements shows a steady small change in the section?,but fluctuates sharply in the section?.The variation characteristics of a series of stable element ratios and provenance tracer indicators in the core are consistent with the element composition,which indicates that the sediment sources in the upper and lower sections of the core at 62.3 m have changed.(4)The comparison of Al₂O₃/Ti O₂ between core sediments and Songhua River modern river samples shows that the sediments in the section?of core have similar geochemical composition to the Songhua River Songyuan direction.However,the sediments in the section?have similar geochemical composition with the rivers in Jiamusi-Yilan direction.This indicates that the Songhua River system reversed in 0.94Ma,and the water system reversal was also supported by structural geomorphology and sedimentological evidence of gravel layer in T3 terrace of Tonghe.(5)Based on the analysis of the driving mechanism of tectonic-geomorphology-climate,we put forward the following drainage evolution model:in the early-middle period of Early Pleistocene,Songhua River was divided into two sections with the boundary of Jia-Yi watershed,and the middle-upper reaches flowed into Songnen paleolake from east to west,while the lower reaches flowed to Sanjiang Plain.At the end of early Pleistocene,the Jia-Yi watershed continued its tectonic uplift,the Songnen Plain and Sanjiang Plain on both sides continued to subside,the longitudinal gradient of rivers increased,and the rivers on both sides continued to trace their sources and erode,which caused the Jia-Yi watershed to be cut through in 0.94Ma.The lower Songhua River drainage invaded the middle-upper drainage,communicating the hydraulic connection between the rivers on both sides,and the Songnen paleolake water leaked out.The uplift of Songliao watershed in the middle Pleistocene forced Nenjiang River and the Second Songhua River to migrate and divert into Songhua River,and the area of Songnen paleolake shrank and finally dried up at about 0.46 Ma,gradually forming a modern Songhua River drainage pattern.