Study On The Paleo-Ocean Conditions Of Early Silurian Rhuddanian-Aeronian In Wuxi,Chongqing

The Early Silurian was the period of improved and recovered environment after Hirnantian Glaciation.Anoxic environment caused by regional transgression result in black shales widely distributed.Wuxi area of Chongqing is located in the southern margin of Daba Mountain,along the fold-thrust belt.The early Silurian black shale was associated with barite nodules,dolomite and bentonite.The existence of various special sediments and the complex tectonic environment since Indosinian restricted the understanding of the early Silurian paleo-ocean evolution.The research object of this study is Bailu section in Wuxi,Chongqing.Graptolite biozone division,barite and dolomite isotope analysis(barite barium isotope,oxygen isotope,sulfur isotope,dolomite carbon and oxygen isotope),black shale geochemical analysis(organic carbon isotope,total organic carbon,mercury content)were conducted based on paleontology,petrology,sedimentology and geochemical methods.And the paleo-ocean evolution process from Rhuddanian to Aeronian have been discussed,the sedimentary models of different stages have been established.The achievements and understanding are as follows.(1)The sedimentology and petrology characteristics of the Rhuddanian-Aeronian were cleared based on the outcrop geological survey of the Bailu section.In Bailu section,black shale is mainly developed,followed by siliceous shale,dolomite and bentonite,and accompanied by some barite nodules.The sedimentary environment is thought as deep-water facies of shelf.There are four different lithologic layers including(i)dense layer of black shales,(ii)dense layer of siliceous shales,(iii)dense layer of dolostones and barite nodules,and(iv)dense layer of bentonites.Barite nodules are syngenetic deposits,with several circles composing of barite crystals with different sizes.Dolomite and pyrite are visible in the center of barite nodules.Under the microscope,dolomite crystals of dolostones are seen to be with foggy center and bright edge.And with the scanning electron microscope(SEM),it shows that barite,celsianite and other barium minerals are developed in the thin-medium bedded dolostones.Thin layers of gray,light green or grayish white bentonites are found,in which altered biotite,quartz crystals and vitric pyroclasts can be seen under the microscope.(2)Paleontological strata and the boundary of Rhuddanian-Aeronian in Bailu section are clarified according to the identified graptolite species.A total of 11 graptolite species have been identified: Cystograptus sp.,Coronograptus cyphus(Lapworth),Atavograptus atavus(Jones),Atavograptus sp.,Rastrites pregerinus,Rastrites guizhouensis Chen and Lin,Psedorthograptus sp.,Paramonoclimacis chengkouensisa(Ge),Rastrites longispinus(Perner),Rastrites sp.,Paramonoclimacis sidjachenkoi(Obut and Sobolevskaya),distributed in Belts LM4-LM7 of Longmaxi Formation.Combining with the geochemical curves of this study section,the upper part of Coronograptus cyphus(Lapworth)(at 16.7m in the Bailu section)is defined as the boundary between Rhuddanian and Aeronian.(3)The Early Silurian paleo-ocean conditions revealed with dolostones and barite nodules are clarified according to geochemical analysis of barite nodules(i.e.,sulfur,barium and oxygen isotopes)and dolostone(carbon and oxygen isotopes).The results show that the barium isotope values of barite nodules vary relatively greatly(-0.41 to-0.12 ‰),the sulfur isotope values are as high as 66.74 ‰,and the oxygen isotope values are 19.10 to 21.06 ‰.The carbon isotope values of medium bedded dolostones and dolomites in the center of barite nodules are negative,ranging from-14.28 to-12.72 ‰,while the values of oxygen isotope are from-10.91 to-9.44 ‰.During the early Silurian,the study area was located at the continental margin,where upwelling current was active.A large amount of Ba brought by the upwelling current caused the anormaly of Ba content during the Aeronian.The extremely high sulfur isotope values indicate that barite nodules were formed in the sulfate-methane transition zone.Due to strong sulfate reduction in anoxic environment,contemporaneous interlayered dolostone and dolomites in barite nodules were formed.In summary,the deposition of barite and dolomite were influenced by original sedimentary environment,sulfate reduction zone and upwelling current in the study area.(4)Based on sedimentological characteristics and high-precision geochemical curves(of organic carbon isotope,total organic carbon and mercury content),the evolution of the Rhuddanian-Aeronian paleo-ocean conditions in Wuxi area are divided into six stages.(i)The first stage was the main enrichment period of organic matter,with high values of total organic carbon content and negative shift of organic carbon isotopes.It was the peak period of organic matter enrichment,caused by the rapid melting of glaciers and rising sea level shortly after the Hirnantian Glaciation.(ii)The second stage was the secondary enrichment period of organic matter,when the sea level was with stable highstand.The organic carbon isotope shows a relatively continuous positive shift.The primary productivity of this stage reduced because nutrients had been consumed,causing the gradually weakened trend of organic carbon burial.(iii)The third stage was the period of stable environment,indicating by the stable geochemical curves.It shows strong correlation between total organic carbon content and organic carbon isotope value,and abnormally enriched mercury content.This stable environment promoted the accumulation of organic matter and created suitable sedimentary conditions for the formation of barite and dolomite.(iv)The fourth stage was the period of frequent volcanic activity.Although tuffs(bentonite)were deposited,there is no obvious deviation of organic carbon isotope.And the volcanic activities had no obvious effect on the enrichment of organic matter.(v)The fifth stage was with Sedgwickii Event,a secondary glacial period after the Hirnantian Glaciation.In the deposits of this stage,organic carbon isotope shows positive and total organic carbon content fluctuates,responding to the global Sedgwickii Event.(vi)The sixth stage was the period of environment oscillatory recovery.During this period,there were no major geological event and abnormal enrichment of organic matter.The stable environment and special paleo-ocean conditions also promoted the formation of barite nodules.