Study On Quaternary Stratigraphic Division In Fangezhuang Area,Tangshan City,Hebei Province

Tangshan City,Hebei Province,located in the northern margin of the North China Plain depression belt,is controlled by the NE-trending Douhe fault and Guye fault,forming three sedimentary units:Jinggezhuang sag,Tangshan uplift and Kaiping sag.The study area is located in Kaiping depression,which is controlled by Guye fault,Dachengshan fault and Xujialou-Wangxizhuang fault.The whole terrain is flat and continuous,with rivers and miniature oxbow lakes.Influenced by the ancient Luanhe River and Shiliuhe River,alluvial and diluvial sediments have been developed in large area since Quaternary.In order to study the Quaternary stratigraphic division and environmental change in Fangezhuang area,according to the field investigation,borehole core,¹⁴C dating,optically stimulated luminescence dating,paleomagnetic,geochemical characteristics and micropaleontological analysis,the lithostratigraphy,chronostratigraphy,On the basis of the previous research data,this paper puts forward a fine division scheme of the Quaternary strata in this area,determines the division basis,defines the bottom boundary and environmental evolution,and enriches the content of the Quaternary research in the plain area of Hebei Province.According to the paleomagnetic study and the lithostratigraphic characteristics,the Quaternary and Neogene boundary in Fangezhuang area is buried at 255.43 m and the age is 2.588 Ma.The Quaternary stratigraphy is divided into five lithostratigraphic units:Yangliuqing Formation（Qp₁y）,Ouzhuang Formation（Qpo）,Yangjiasi Formation（Qp₃-Qh₁y）,Gaowan Formation（Qh₂g）and Qikou Formation（Qh₃q）.The Ouzhuang Formation（Qpo）is further divided into four segments.The Yangliuqing Formation（Qp₁y）is mainly composed of trigonometric deposits containing ostracods,gastropods and charophytes.The Ouzhuang Formation（Qpo）is mainly composed of meandering river facies and lacustrine facies,in which ostracoda,gastropoda and charophyte fossils are contained in the first member,gastropoda and gastropoda fossils are contained in the second member and the third member.The Yangjiasi Formation（Qp₃-Qh₁y）is a set of alluvial-pluvial fan deposits containing gastropoda fossils.Gaowan Formation（Qh₂g）is dominated by inundation facies and contains gastropod fossils.The Qikou Formation（Qh₃q）is a set of edge-floodplain sediments with a small amount of lacustrine facies sediments.The bottom boundary of the Lower Pleistocene is divided into thick yellowish-brown-red and variegated clay layers,corresponding to the Songshan reverse polarity zone.The bottom boundary of the Middle Pleistocene is divided into thick layer brown-brown and light gray clay silt with light yellow medium thin layer fine silt,which is 73.95 meters deep,corresponding to the Burong positive polarity zone.The bottom boundary of the Upper Pleistocene is divided into grayish brown,yellow silty clay and interbedded medium fine sand,with a depth of 32.2m and an age of 0.128Ma,corresponding to the Brong positive polarity zone.The bottom boundary of the Holocene is divided by the appearance of bright gray clay powder with rust stained development,buried 12 m deep,and the age is 10 ka B.P.,corresponding to the Brong positive polarity zone.According to the analysis of weathering coefficient,V,Ba and Th/U curves,the values were low in the Pliocene,representing the high values in the lower part of the Yangliuqing Formation in the early Pleistocene,and then decreasing.There is an obvious extreme zone near the boundary between Yangliuqing Formation and Ouzhuang Formation,which corresponds to the change of climate environment about 2 Ma.There are obvious changes in every member of the Ouzhuang Formation,near the boundary of the Ouzhuang Formation and the Yangjiasi Formation.Si O₂/（Al₂O₃+Fe₂O₃）、Ca O/Mg O、Fe₂O₃/Fe O、Fe₂O₃/Mn O、Rb/Sr.The change characteristics of the curves are basically the same,and the change trend of the curves is inversely related to the weathering coefficient.According to the above analysis and the regional pollen data,the Fangezhuang area is located at the margin of Luanhe alluvial fan in the early Late Pleistocene.In the late Late Pleistocene,the climate changed from cold and slightly wet to cold and dry,braided river and Shunzhi river developed well,and the flow variation rate was high,and more buried river course was formed.In the early Holocene,the climate was mild and slightly wet,and the hydrodynamic conditions of braided rivers were weak.In the middle of Holocene,the climate was still warm and humid,with a brief cooling.With the increase of amount of precipitation,the lateral erosion of the river is strengthened,and the river migrates,and many oxbow lakes are formed.According to borehole lithology¹⁴C dating,opto-luminescence dating,paleomagnetism,stratigraphic geochemistry and characteristics in Fangezhuang area,this paper uses multiple stratigraphic divisions such as petrostratigraphy,chronostratigraphy and biostratigraphy.Based on previous research data,this paper determines the basis and mark of Quaternary stratigraphic division in the study area,establishes quaternary stratigraphic framework and analyzes the evolution of sedimentary facies and sedimentary environment.