Characteristics And Depositional Environment Of The Stromatolites Within The Second Member Of The Mesoproterozoic Longjiayuan Formation,Western Henan

Stromatolites recorded the life activities of early environment,cyanobacteria and microbial community,and their growth morphology was influenced by the sedimentary environment,which is of great significance for the restoration of palaeo-environment.The stromatolites of the second member of The Longjiayuan Formation of the Guandaokou Group of the Mesoproterozoic in western Henan are rich in morphological types and concentrated in distribution.The relationship between different morphologies of stromatolites and hydrodynamic force indicates different sedimentary conditions,but there are no previous detailed studies on this area.Based on this,This study conducted a systematic field investigation and petrological examination on the microbialite structure on the stromatolites of the Second Member of Longjiayuan Formation,studied the macroscopic morphology and microscopic characteristics of the stromatolites in detail.The spatial and temporal distribution and evolution of stromatolites in the southern margin of the North China Craton in the Mesoproterozoic were discussed,and the following insights were obtained:(1)The section of the second member of Longjiayuan Formation is continuous sedimentary,with a total of 15 layers of stromatolites exposed.According to the macro morphology,there are mainly horizontal stratified stromatolites,undulating stromatolites,mound stromatolites,conical stromatolites and columnar stromatolites.Through the microscope can be observed stromatolites have light and dark lamina,thrombolite,algal filaments,ooids and detrital materials.Microbe characteristics such as spherical crystal,filamentous body and coccal bacteria morphology can be observed by scanning electron microscope.(2)Different types of stromatolites are developed in different sedimentary environments,among which,Horizontal laminated stromatolites developed in the intertidal zone,undulating and mound stromatolites developed inintertidal high energy environment and subtidal low energy environment,cone and columnar stromatolites developed in the subtidal zone,it is considered that the stromatolites in the second member of Longjiayuan Formation were formed in tidal flat environment.(3)According to the macro morphology and micro characteristics of stromatolites,combined with the associated sedimentary structure,it is considered that the vertical sedimentary sequence of the second member of Longjiayuan Formation experienced the change of intertidal zone,subtidal zone and intertidal zone;Horizontally,according to the change of sea level rise and fall,the sedimentary model is established.(4)The microbiolites of the second member of Longjiayuan Formation are compared with the microbiolites of Jixian area in the northern margin of North China.The rock types,stromatolites and sedimentary environments of the two places are similar.This study is of great significance to the discussion of macro morphology of mesoproterozoic stromatolites,and provides reference value for understanding the potential relationship between proterozoic environmental evolution and microbiolites.This paper includes 25 pieces of figures,115 pieces of References.