Heavy Mineral And Geochemical Characteristics Of The Huangshan Core In Harbin Indicate The Drainage Evolution Of The Songhua River Since The Early Pleistocene

The evolution of drainage system is the result of regional tectonic-climate-surface process.The research on the evolution of Songhua River system is still relatively weak,especially whether the flow reversal occurred in the middle and upper reaches of Songhua River in Quaternary is controversial.River sediments are the most intuitive product of river system development,which provides strong evidence for the formation and evolution of large river systems.The automated quantitative mineral analysis system TIMA(TESCAN Integrated Mineral Analyzer)has great application potential in source area identification and paleogeographic reconstruction.Therefore,in this paper,TIMA technique was used to analyze the geochemical composition of heavy minerals and whole rock minerals in the core sediments of Harbin Huangshan located in T2 terrace of Songhua River,and the results showed that:(1)There are significant differences in heavy mineral composition between the upper and lower sediments of the core 62.3 m.Sugilite appears for the first time in the strata above 62.3 m,while calcite,chromite,serpentine,pyrite and pyrrhotite only appear in the strata below 62.3 m.Most of the heavy mineral contents are significantly different in the upper and lower strata.Compared with the strata below 62.3 m,the contents of zircon,apatite,rutile,sphene,garnet and ilmenite decreased significantly in the strata above 62.3 m,while the contents of tourmaline,pyroxene,ferromagnetic minerals and fersilicite increased significantly in the upper strata.In addition,there are also significant differences in heavy mineral characteristic indexes such as ATi index.(2)The geochemical composition of the whole rock mineral elements is also significantly different between the upper and lower sections of the core 62.3 m.The overall change of formation elements above 62.3 m is relatively stable and the fluctuation range is small.The overall change of formation elements below 62.3 m is sharp and the fluctuation range is large.The contents of some elements changed significantly in the upper and lower strata,and the elements S,Ba,Pb,Th,Cr and Pr only appeared in the strata below 62.3 m.(3)The geochemical composition of core heavy minerals and whole rock minerals changed significantly at 62.3 m above and below the core,indicating the provenance change of core sediments.The provenance of the lower section came from Yilan direction,while the provenance of the upper section came from Songyuan direction.Therefore,the change of core provenance indicates the inversion of Songhua River system.(4)Combined with the fluvial-lacustrine strata found in Yilan,we propose a new model for the evolution of the Songhua River system : In the early Pleistocene,the Songnen River system and the Sanjiang River system were not yet connected.The Mudanjiang River in the upper reaches of the ancient Songhua River flows through Yilan-Tonghe-Harbin and injects into the Songnen ancient lake.The uplift of the Songliao watershed after 0.94 Ma B.P.forced the reversal of the flow direction in the middle and upper reaches of the ancient Songhua River,and flowed from west to east to the Tonghe-Yilan area to form an ancient lake.With the continuous flow of the Songhua River,the water level of the lake continues to rise,and the overflow of the lake cuts through the Jiayi watershed.The Songnen water system and the Sanjiang water system are connected,and the modern Songhua River drainage system pattern is established.