Characteristics Of Stromatolites And Sedimentary Environment Analysis Of The Sinian Dengying Formation In Eastern Jiangxi And Western Zhejiang

Stromatolites are commonly found in the sinian dengying formation in eastern jiangxi province and western zhejiang province.These stromatolites are well exposed and have different macroscopic shapes,which are important signs for studying the sedimentary environment in this area.The characteristics of stromatolites were described in detail by field measurements,polarizing microscope and scanning electron microscope(SEM).The sedimentary environment of stromatolites was analyzed by the theories and methods of petrology,sedimentology and paleontology.Finally,the sedimentary environment and its changes during the dengying period in east jiangxi and west zhejiang were discussed.According to field measurements of jiangshan xindangwu section,jiangshan beijiao section and shangrao huangjiayuan section,it is found that stromatolites all appear in the upper part of dengying formation.According to the characteristics of stromatolites,it is divided into 5 categories: horizontal stromatolites,corrugated stromatolites,mound stromatolites,cone stromatolites(pyramid stromatolites and dome stromatolites)and columnar stromatolites(big columnar stromatolites,small columnar stromatolites,finger stromatolites).There are a variety of stromatolites in the xintang dock section,which are shown as horizontal laminae ? ripple ? dome ? cone ? columnar evolution.The stromatolites were developed in the beijiao section,and the stromatolites were columnar stromatolites.Only horizontal striated stromatolites are developed in huangjiayuan section.On the microscopic characteristics,stromatolite by rich algae laminae(laminated)with darker color and rich carbonate laminae(laminated color lighter)each other,the dark grain layer composed of algae and other microorganisms,micritic carbonate minerals and a small amount of crystal powder grade dolomite,bright color laminated by powder grade dolomite and a small amount of micritic carbonate minerals.However,there are obvious differences in laminae morphology,algal microbe morphology and micritic grade mineral content of all kinds of stromatolites.In super microscopic,there are obvious biological components in the stromatolites in the study area.Microbial mat,microbial membrane,EPS(extracellular polymer)can be seen in the microbial residue.Filamentous,rod-shaped cyanobacteria,and ellipsoidal sulfate reducing bacteria can be seen in the microbial calcification residue.It is shown that the stromatolites in the study area are biogenic and there are at least two microbial communities in the stromatolites,aerobic autotrophs and anaerobic heterotrophs.According to the analysis of the macroscopical,microscopic and ultra-microscopic characteristics of stromatolites,a comparative study with modern stromatolites was conducted.It is preliminarily believed that the horizontal striated stromatolites in this area developed in the supratidal zone,the corrugated stromatolites and hillock stromatolites developed in the intertidal zone,and the conical stromatolites and columnar stromatolites developed in the subtidal zone.Based on the measured petrological characteristics and sedimentary structural characteristics of the profile,referring to the stromatolite sedimentary environment indication.It is shown that the study area(southeastern margin of the lower Yangtze)in the late sinian period was located in the shallow Marine confined carbonate platform environment,and experienced the process of water deepening from the upper tidal zone to the intertidal zone to the lower tidal zone.This may be mainly controlled by the tectonic influence of tensile subsidence in the southeastern margin of Yangtze during this period.