早更新世晚期松花江水系袭夺:地球化学和沉积学记录

The capture of the Songhua River system in the late Early Pleistocene:geochemical and sedimentological records

水系演化重建是恢复区域构造历史及古环境变化的重要方法。松花江作为中国七大水系之一,其水系演化的相关研究目前还比较薄弱,尤其是第四纪松花江中-上游是否存在流向反转尚无准确结论。沉积物是河流地质过程的直接产物,是水系演化研究的关键。为此,本研究对哈尔滨荒山钻孔岩芯(HS)进行了磁化率、古地磁和元素地球化学组成分析。结果表明,岩芯62.3 m(0.94 Ma B.P.)处,沉积物的岩性、磁化率及元素地球化学组成均发生显著变化。岩芯沉积物岩性在62.3 m上下发生明显变化,是河流沉积两个沉积旋回的转折点。62.3 m以下地层的磁化率极低,基本为0,且变化幅度极低,元素地球化学组成则急剧波动,并表现出与现在松花江哈尔滨段下游水系(岔林河、蚂蜒河和牡丹江)相近的地球化学组成;然而,62.3 m以上地层的磁化率突然升高(99.673×10-8 m3 kg-1),并具有周期性的高低变化。元素组成呈现稳定的小幅变化,并表现出与拉林河、嫩江和松花江吉林段相近的地球化学组成。0.94 Ma B.P.沉积物物源的变化表明河流流向发生了显著变化,松花江中上游河段流向曾在此时发生反转。地球化学记录的水系反转也得到了河流阶地地貌和沉积学证据(砾石的粒径和排列方向)的支持。早更新世早-中期,以佳-依分水岭为界,松花江中上游(肇源-依兰河段)河流自东向西流入松嫩湖盆,松花江下游向东流经三江平原;早更新世晚期,受构造-地貌-气候耦合作用的影响,佳-依分水岭持续抬升,而三江平原一侧不断下沉,佳-依分水岭两侧河流发生溯源侵蚀,最终导致佳-依分水岭在0.94 Ma B.P.被切穿,松花江中上游水系被下游水系所袭夺,河流流向发生反向,自西向东流经依佳峡谷进入三江平原,现代松花江水系逐渐建立。本研究有助于我们加深对松嫩平原水系演化的理解,为探索东亚水系演化的一致性和区域差异性提供研究证实,同时对松花江流域自然资源禀赋的调查乃至国土空间开发利用具有重要实践意义。

Drainage evolution and reconstruction is an important method toretrieve regional tectonic history and paleoenvironmental changes.As one of the seven major river systems in China,the research on the evolution of Songhua River is limited,especially the question if there was a reversal of flow direction in the middle and upper reaches of the Songhua River in Quaternary has not been accurately addressed.Sediment is the direct product of river geological process and a key to the study of river system evolution.Therefore,the magnetic susceptibility,palaeomagnetism and elemental geochemical composition of Harbin Huangshan drilling core(HS)were analyzed in this study.The results show that the lithology,magnetic susceptibility and elemental geochemical composition of the sediments change significantly at the core 62.3 m(0.94 Ma B.P.).The lithology of the core sediments changed significantly around 62.3 m,which was the turning point of the two sedimentary cycles of river deposition.The magnetic susceptibility of the strata below 62.3 m is very low,basically 0,with a very low variation range,while the element geochemical composition fluctuates sharply,showing similar geochemical composition to that of the lower reaches of Harbin section of Songhua River(Chalin,Mayan and Mudanjiang rivers).However,the magnetic susceptibility of the strata above 62.3 m suddenly increases(99.673×10-8m3kg-1)and presents a period high.With low variation,the element composition shows stable and small variation,and shows similar geochemical composition with the Jilin section of the Lalin,the Nenjiang and the Songhua rivers.The change in sediment source at 0.94 Ma B.P.indicated that the flow direction of the river changed significantly,and the flow direction of the middle and upper reaches of the Songhua river reversed at this time.The geochemical record of water system inversion is also supported by river terrace geomorphology and sedimentological evidence(gravel size and alignment direction).In the Early-Middle Pleistocene,the upper and middle reaches of the Songhua River(Zhaoyuan-Yilan reach)flowed from east to west into the Songnen Lake Basin,and the lower reaches of the Songhua River flowed eastward through the Sanjiang Plain.In the late Pleistocene,the Jia-Yi watershed continued to rise under the influence of tectonic-geomorphological-climatic coupling,while the one side of the Sanjiang Plain continued to sink and the rivers on both sides of the Jia-Yi watershed began to retreat.Source erosion eventually led to the Jia-Yi watershed being cut through at 0.94 Ma B.P.,the upper and middle reaches of the Songhua River being attacked by the lower reaches of the river system,and a reversal of the flow direction of the river.The river flowed from west to east through the Yijia Gorge into the Sanjiang Plain,and the modern Songhua River system was gradually established.This study will help deepen our understanding of water system evolution in the Songnen plain and provide research confirmation for exploring the consistency and regional difference of water system evolution in East Asia.At the same time,it is of great practical significance to investigate the background of natural resources in the Songhua River basin and even to develop and utilize the territorial space.