| In comparison with the amount of study undertaken on the end-Permian mass extinction,the preceding Guadalupian mass extinction has received little investigation.Associated with dramatic changes in the marine and terrestrial environments of the Middle Permian,the genus-level crisis caused by the Guadalupian mass extinction ranks third in the Phanerozoic and has been proven to be a global biotic event.However,whether the cause of this extinction event is the habitat loss caused by global regression,or marine anoxia,acidification,poisoning and climate fluctuations related with the volcanic activity of the Emeishan Large Igneous Province are still debated.In addition,the more basic question about this event,whether the diversity decline occurred as an abrupt pattern during the Capitanian or as a protracted pattern during the Wordian-Capitanian remains to be further studied.The Guadalupian mass extinction led to the disappearance of a large number of photosynthetic and photosymbiont-bearing taxa.During the extinction event,reef carbonate production shut down,resulting in a second-order reef crisis in the Phanerozoic.Such a major transition should also be occurred in carbonate platforms,another important carbonate depositional system,but their appearance during the Guadalupian is still unclear.The growth and demise of carbonate platforms are closely related to contemporaneous biotic and environmental evolution which can participate in the production and accumulation of carbonate deposits by influencing carbonate factories.Carbonate platforms are good carriers of coeval marine biotic and environmental information.Study on the Guadalupian carbonate platforms is significant for further understanding the Guadalupian mass extinction and environmental changes behind them.Based on the analysis of sedimentary facies,biostratigraphy,sedimentary sequence,carbonate grain assemblages and foraminiferal morphogroups of 12 sections and 4drillcores in the southern Yangtze carbonate platform,this study recovers the sedimentary facies model and evolutionary history of the carbonate platform,as well as the paleo-ocean ecological conditions.On this basis,sequence stratigraphy correlation across the carbonate depositional regime,siliceous depositional regime and clastic depositional regime in South China was carried out.The sedimentary evolution model of the Yangtze carbonate platform during the Guadalupian was established.The controlling factors of platform evolution at that time were analyzed combined with the context of biotic and environmental changes.The following conclusions can be obtained:1.During the Guadalupian,eight sedimentary facies developed in the southern Yangtze carbonate platform: shallow-water platform facies,deeper-water platform facies,intraplatform depression facies,platform marginal shoal facies,platform marginal reef facies,upper slope facies,toe-of-slope facies,deeper-water basin facies.During the Roadian,the early Capitanian and the latest Capitanian,the southern Yangtze carbonate platform experienced three onlaps of siliceous,mud-rich deposits,corresponding to the commence and expansion of intraplatform depressions and the drowning of platform margins,respectively.From the late Roadian to Wordian,a large number of gravity flow deposits developed in the deeper-water facies zone.The massive carbonate breccias formed in southern Guizhou is the product of platform margin collapse and debris-flow deposits under the background of regression.During the late Capitanian,shallow-water platforms in the study area were exposed and manganese-bearing deposits were formed in the intraplatform depressions.2.During the Capitanian,the carbonate grain assemblages and foraminiferal morphogroups in the study area changed.In the platform interior,the carbonate grain assemblage changed from green algae-foraminifera to foraminifera-ostracod-brachiopod and sponge spicules/radiolarian-ostracod.In the platform margin,the carbonate grain assemblage changed from crinoid-foraminifera-peloid to thin-shelled-bivalve-ostracod.The foraminiferal morphogroups changed from exophytic,vegetative type,to endophyte,edible sediment debris type.These transitions reflect the weakening of ambient light,the reduction of oxygen concentration and the trend of seawater cooling.Meanwhile,the carbonate producers become heterotrophic-dominated.3.The sequence stratigraphy correlation of seven sedimentary transects shows that the Guadalupian sedimentary sequence in the southern Yangtze carbonate platform is traceable in the whole South China.The Yangtze carbonate platform experienced three phases of onlap by siliceous,mud-rich deposits,corresponding to three transgressive sequences(T1,T2,T3).The occurrence of massive carbonate breccias and carbonate mounds in the siliceous depositional regime,as well as the exposure of the shallow-water platform,indicate two regressive sequences(R1,R2)developed between the transgressive sequences.They comprise two and a half transgressive-regressive cycles which are consistent with the global glacio-eustatic changes during the Guadalupian,indicating that the sedimentary sequence of the Yangtze carbonate platform is controlled by global rather than local factors.4.The sedimentary evolution model of the carbonate platform in South China shows that the Yangtze carbonate platform was formed after the transgression T1 of Roadian age,and gradually decreased in size during the transgressions T2 and T3 of Capitanian age.The reduction of carbonate platform size is manifested as the commence and expansion of siliceous intraplatform depressions on the platform top(partial drowning of the platform top)and as an increasing onlap of siliceous basin towards the platform interior(retreat of platform margin).5.In the context of the global long-term regression during the Guadalupian,the partial drowning of the platform top and the synchronous retreat of platform margin imply insufficient supply of carbonate deposits,which should be resulted from the destruction of tropical,shallow-water and high-productivity carbonate factories on the platform top at that time.The massive disappearance of photosynthetic and photosynthesis-bearing taxa resulting from the Guadalupian mass extinction is directly related to changes in carbonate factories.The shrinking pattern of the evolution of the Yangtze carbonate platform can be a potential reference for further study of the Guadalupian mass extinction and marine environmental changes. |
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