Sedimentary Environments And Controlling Factors Of The Cambrian Xinji Formation,Lushan Area,Western Henan

The Early Cambrian sedimentary evolution of the southern North China margin has always been a focus in geosciences.The Xinji Formation,as a stratigraphic record at the bottom of Cambrian in the southern North China margin,it is important to analyse the sedimentary environment of Xinji Formation.This paper focuses on the Cambrian Xinji Formation(Shimengou Lushan,Western Henan)in order to analyse the sedimentary environments and controlling factors via methods of sedimentology,petrology,sedimentary geochemistry.The main conclusions are as follows:(1)The Xinji Formation of study area comprises a marine terrigenous clastic sequence,which includes 9 kinds of lithofacies: flat-pebble conglomerate phosphorite,massive sandstone,glauconitic(fine-grained)sandstone,glauconitic mudstone,hummock-stratified sandstone,parallel-stratified sandstone,dolomitic mudstone,calcareous pebbly sandstone,calcareous sandstone.The vertical evolution sequence of sedimentary environments of Xinji Formation is characterized by shallow marine storm flow to tidal flat and lagoon and washover fans.(2)The Xinji Formation in study area is affected by storms.The probability cumulative curve of grain size for the sandstones of Xinji Formation is characterized by steeply-dipping segment(1 ~ 2)pattern and steeply-dipping abrupt change pattern,and the C-M plot shows that the hydrodynamic mechanism comprises both traction and gravity flow,which are all consistent with storm depositional characteristics.(3)The geochemical characteristics for sandstones of Xinji Formation in study area show that the source rocks have obvious recycling characteristic;The primitive mantle-normolized spider diagram of trace elements is similar to the average upper continental crust and north American shale.The chondrite-normolized REE distribution patterns are all right skewed,with enriched LREE,stable content of HREE,moderrately negative Eu abnormity and weakly or non negative Ce abnormity;The provenance of Xinji Formation is mainly from the ancient sediments of the North China craton;The tectonic background of provenance is mainly displayed by passive continental margin,at same time,shows characteristics of active continental margin and continental island arc.The change trend of tectonic environment is transformed from activity to stability.(4)The depositional environments of Xinji Formation in Lushan area are closely related to the factors of paleogeography,paleostructure,palaeolatitude,and paleoclimate characteristics of the southern North China margin,which includes the basin consisting of depressions and uplifts,active continental margin,storm events at low latitude area,regional transgression and regression with the rising global sea level,and medium to relatively high temperatures.The study of sedimentary environments and controlling factors of the Cambrian Xinji Formation has important significance for us to understand the evolution of Early Cambrian sedimentary basin and palaeogeography study in the southern North China margin,which record and respond the early filling process of early Cambrian sedimentary basins in the southern North China margin.