Comparative Investigations On The Sedimentary Facies And Fossil Communities Evolution Of Middle Permian Maokou Formation In Zigui And Jingshan Sections,Hubei Province

The end-Permian mass extinction(EPME)was the most severe extinction event during the Phanerozoic.Some views show that the rapid biota replacement during the end of Middle Permian(the GLB extinction)is likely to be the prelude of EPME.Foraminifers and fusulinidswere thought to be most suffered during the GLB event.Meanwhile,other taxa,e.g.,coral,bryozoan,brachiopod,bivalve,and ammonoid,also dramatically responded to the EPME,although it was asynchronous with that of foraminifera.There are many hypotheses for the causes of the GLB event,including global regression,the activity of Emeishan Large Igneous Province(ELIP),Kamuracooling,marine anoxia and methane leakage.However,those theories were deduced from different geological records in different regions,and therefore may be true under particular geological conditions at certain areas.Nevertheless,it is necessary to fulfill comparative studies among different sectionsin order to comprehensively understand the origin and process of this event.It is noteworthy that any geological record from a single section cannot account for the whole process of the complex GLB event due to the different geological settings of those sections.In South China,previous works mostly focus on the Middle Permian strata and volcanic events of ELIP and its adjacent regions.These areas have the advantage of extremely intense volcanic activity records,which provide more direct geological evidence for understanding environmental effects of Emeishan basalt province eruptions.However,the biotic crisis recordoften appears to be more serious than that in other regions.Threefore,more strata and fossil evidencesfar away from eruption areas are necessary in order to have a better understanding of the effects of Emeishan basalt province eruptions on the biotic crisis in entire South China.The main purpose of this paper is to analyzethe effects of the GLB mass extinction on the marine organisms and environments at Lianziya section in Zigui County and Lianshan section in Jingshan County of Hubei Province,both of which were located in the Middle Yangtze area and therefore less influenced by volcanic eventstform.Field investigations found that these two different water-depth sections display completely different geological recordseven they were situated in relatively stable tectonic background in the Yangtze carbonate platform,.During the deposition of the Middle Permian Maokou formation,the Lianziya section of Zigui was at a site of shallow upper shelf environment,while the Lianshan section of Jingshan was located at the lower part of continental shelf.Sedimentology and palaeoecology research of these two sections shed new light on exploring the evolution of biology and environment in different water-depth settings at the end of the Middle Permian.Previous researches focusing on this event tend to adopt chronostratigraphic and biostratigraphic methods,which provided important evidences for understanding the timing and process of the GLB event.However,the chronostratigraphic method becomesdifficult to carry on due to the lack of tuffs preserved in the sections far away from the volcanic origins.Biostratigraphic method may play important roles if there are sufficient fossil zones available for strata correlation,but it is usually difficult to make a detailed correlation for the evolution of organisms and environment in different sections when little fossils can be utilized for biostratigraphy or the fossil zones have too long durations,.By contrast,in some sections with great abundance of fossil and biodiversity,ecostratigraphic method displays its unique superiority and canmake up for the limitation of chronostratigraphic and biostratigraphic methods.Hence,this paper is managed to analyse the geological process of the Middle Permian event in marine environment of different water depthby correlating the microfacies and fossil communities between Lianziya section of Zigui and Lianshan section of Jingshan,Hubei Province.The main achievements of this thesis are as follows:(1)Three types of microfacies in Lianziya section have been identified,including high-energy shoal,low-energy shoal and mud shoal microfacies.In Lianshan section,there are 4 types of microfacies identified including low-energy shoal,low-energy shallow gentle slope,shallow underfilled shelf and deep-water basin microfacies.(2)Four types of fossil communities were found in Lianziya section via fossil community analysis,consisting of dasycladaceae-foraminiferal,Ungdarella-foraminiferal,phylloid and bryozoan community respectively.In Lianshan section,four completely different types of fossil communities were found.They are the permocalculus-foraminiferal,inozoa,trace fossil and radiolarian community in ascending sequence of the Middle Permian Maokou Formation.(3)Based on the analysis of sedimentary sequence and isotope data of Maokou Formation,we suggest the unconformity near the boundary between Maokou Formation and Wujiaping Formation in Lianziya section is approximately isochronal to the first occurrence of calcirudyte in Lianshan section,which may indicate a remarkable regression event.(4)Three stages of sedimentary evolution for the research area at the late Guadalupian have been established by the comparison of the microfacies in the two sections.During the first stage,algae-foraminiferal grainstone in both sections represents a stable shallow marine environment.Later on the occurrence of volcanic clast and intensified detrital influx reflects an active tectonic background.The regional differential uplift dominated the third stage.Lianziya area exposed and underwent erosion,while Lianshan area demonstrated subsidence basin with rapidly deepening water body.(5)Based on the comprehensive analysis and comparison of lithology sequence,sedimentary microfacies,palaeoecology and Carbonisotopic values in the upper Maokou formation of the two sections,this research proposes that the Lianziya section represents a regression process starting from sedimentary facies and fossil community near a subtidal shallow marine environment to a completely exposed old land that experienced a prominent weathering and erosion afterwards.On the contrary,the Lianshan area exhibits a transgression process starting from the sedimentation and fossil community in the lower part of subtidal zone to a deep-water basin community.These evolution processes both in sedimentary facies and fossil communities indicate that the Middle Permian great regression is not so universal as previously thought.This study suggests a conspicuous regression event happened at GLB in a shallow-marine platform(e.g.theLianziya section)while a sea-level rise process occurred almost at the same time in the deep-water section(e.g.Lianshan section).To summarize,the GLB event exerts different effects on the organisms and environment in different water depths and therefore volcanism or regression cannot act as a single reason for the mass extinction in that period.More comprehensive stratigraphic evidences and comparisons among different regions and different water depthsections are necessaryin order to completely understand this geological event.