Analysis On The Evolution Of Sedimentary Environments And Its Controlling Factors Of The Lower Part Of Bingmagou Formation In Yichuan,Western Henan

The study of Mesoproterozoic tectonic-sedimentary response and evolution has been the focus of Precambrian study in the southern margin of North China Craton.As the first set of clastic deposits overlying the volcanic rocks of the Middle Proterozoic Xiong'er Group in the southern margin of the North China Craton,the Bingmagou formation is of great significance for the study of the early Mesoproterozoic tectonic-sedimentary evolution events and the sea-land vicissitude in the southern margin of the North China Craton.On the basis of a great deal of field geological work,the sedimentary environment of the lower part of the Bingmagou formation during the depositional period was studied by using the sedimentology,petrology and geochemistry method.The results show that:(1)The lower member of the Bingmagou formation is mainly composed of a set of clastic deposits.Bottom up is mainly mauve boulder-purplish red fine conglomerate with thin layer of mudstone-mauve pebbly sandstone clip purple,gray green siltstone or mudstone-mauve coarse sand,medium sandstone folder purple red,gray green mudstone and siltstone interbedded.The lithology is gradually thinning.(2)During the deposition process of the lower part of the Bingmagou formation in Yichuan,alluvial fan was developed in the early stage,which was not affected by the stable water body,and formed the middle coarse conglomerate in the lower thick layer and block bed,and the stable water body intruded in the later stage,which was shown as retrogradation sequence.The sedimentary environment of alluvial fan-fan delta transition is shown as a whole.Fan root,fan and fan delta front subfacies were developed successively,and debris flow deposition,channel filling deposition,braided channel deposition,diffuse flow deposition,underwater distributary channel deposition and interchannel sedimentary microfacies were developed.The alluvial fan was gradually replaced by the fan delta,the sedimentation was dominated by the alluvial fan to the fan delta,and the early stage(the lower part of the Bingmagou formation)was dominated by the alluvial fan.Combined with geochemical analysis,the depositional environment in the upper part of the lower part of the Bingmagou Formation may be a continental fan delta and does not enter the marine environment.(3)There may be a large-scale uplift of the crust before the deposition of the Xiong'er Group on the southern margin of the North China Craton.This tectonic event led to strong weathering of the crystalline basement of the Xiong'er Group volcanic rocks and cratons.At this time,the southern margin of the North China Craton was high in the north and south,and the climate may be colder and drier.Seasonal and annual variations in surface runoff are significant.A large amount of debris was transported southwards and accumulated in the Wan'an Mountains area to form an alluvial fan at the bottom of the Bingmagou Formation.The stable water body continued to invade northward gradually,and the reduction gradually strengthened and the upper fan delta deposition developed.The evolution of sedimentary environment in the lower part of the Bingmagou Formation recorded the advancement of surface runoff to the south under large-scale tectonic movements before the marine deposition of the Wufangshan Group in the Middle Proterozoic Wufang Group in the southern margin of the North China Craton.Accompanied by the evolution of marine sediments from the south to the north.