| Globally,sedimentary processes record important features of regional tectonic activity,geomorphic evolution,climatic change and drainage evolution.Luojiawopeng Formation is a set of gravel accumulation strata in the eastern part of Songnen Plain,with five typical sections(Luojiawopeng section,Juren section,Niutoushan section,Lalin Airport section and No.81-3 hole section of Guanjiawopeng in Acheng County).Its weak research foundation and long-standing disputes clearly limit the deep understanding of the scientific issues related to the evolution of regional tectonics,geomorphology,climate and water system.Therefore,we choose two typical stratigraphic profiles of Luojiawopeng Formation(Luojiawopeng profile and Juren profile)as integrated research objects.Based on the analysis of sediment distribution and geomorphology,we adopt the methods of geomorphology,sedimentology,mineralogy and petrology,geochemistry and geochronology.The petrological characteristics,multiple environmental indexes and stratigraphic age of Luojiawopeng Formation are studied to establish detailed stratigraphic attributes.Finally,the early Pleistocene tectonic-geomorphology-climate-drainage evolution process of Songnen Plain was reconstructed.This study not only contributes to the division of the Quaternary stratigraphy and the reconstruction of the early Pleistocene regional environment in the Songnen Plain,but also reveals the regional tectonic uplift and the evolution of the Songhua River,and also provides an important scientific basis for understanding the coupling relationship between the major geological events of the early Pleistocene in the Songnen Plain.The results show that:(1)The ESR age of Luojiawopeng Formation is about ～1.2 Ma,which is seriously underestimated.Based on regional core and stratigraphic correlation,the stratigraphic age of Luojiawopeng Formation is determined to be 2.3 to 2.6 Ma,even older.Detrital zircon U-Pb ages indicate that the sediments of the two typical profiles of the Luojiawopeng Formation are mainly derived from the detrital material of the Zhangguangcai Range in the southeastern Songnen Plain.Elemental geochemistry and gravel characteristics reveal that the lithology of Luojiawopeng section is mainly felsic with a small amount of neutral and mafic components,while Juren section is mainly felsic parent rock.(2)Luojiawopeng section is a collection of weakly to moderately consolidated purplish red sand and gravel with local argillaceous and silty/sandy lenses.The gravel is highly weathered,unoriented,poorly sorted and rounded.The gravel composition is dominated by terrigenous clastic rocks and granite.The composition of heavy minerals is dominated by hematite and limonite,and does not contain unstable minerals.Juren section is a set of weakly consolidated brownish yellow sand gravel beds.The gravel has low weathering degree,poor sorting,good roundness and no orientation.The gravel composition is mainly terrigenous clastic rock,granite and quartz.Epidote and anatase are the main heavy minerals,followed by white titanite,and the content of other heavy minerals is low.(3)The geochemical composition and sedimentological characteristics of Luojiawopeng section reveal that sediments have undergone a strong chemical weathering process,which is mainly the product of the first cycle.Both the high degree of chemical weathering and the red character are formed during the depositional process and can reflect the paleoclimate environment in which the sediments were deposited.The section of Luojiawopeng section is interpreted as the accumulation of diluvial fan in hot and dry climate.The chemical weathering degree of Juren section is low,and the sedimentological,petromineralogical and geochemical characteristics reveal that this set of strata are lakefront or fluvial deltaic deposits.(4)Juren section and Luojiawopeng section are obviously different in sediment color,consolidation degree,sediment structure(gravel and matrix properties),stratigraphic structure(interlayer and lens),genetic types of sediments and geomorphological characteristics.Juren section may be earlier than Luojiawopeng formation,or even reach Cretaceous,so it can be used as a stratigraphic section of Cretaceous formation,rather than Luojiawopeng formation.(5)The discriminant diagrams reflect the tectonic background of active and passive continental margin as well as continental island arcs,consistent with tectonic setting characteristics in the transition zone between the Songnen Massif and the Jiamusi Massif.The main peak age of detrital zircon is about 175 Ma(137～220 Ma),which explains the subduction and collision process between the Songnen Block and the Jimusi Block during 150～200 Ma,and the final closure of the Mudanjiang Ocean after ～150 Ma.(6)The mountains around the Songnen Plain began to rise as early as～2.3 Ma.Environmental indicators such as sedimentary facies and geochemistry indicate that the Luojiawopeng formation has a relatively dry and hot climate,which is inconsistent with the global climate background in which cooling began in the early Pleistocene,indicating that there is a lag in the response of the Songnen Plain to global cooling.The early Quaternary river system in the Songnen Plain was dominated by short-distance intermittent rivers,with the main river system formed at ～1.6-1.2 Ma. |
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