四川盆地宝塔组古水深重建——沉积相与微生物组构证据OA

The deposition of the Upper Ordovician Pagoda Formation in the Sichuan Basin(South China)corresponds to a critical period of transition from the Great Ordovician Biodiversification Event to the Late Ordovician mass extinction.The study of the paleo-water depth and evolution during the deposition of the Pagoda Formation provides significant insights into the sedimentary environmental evolution of the Sichuan Basin and global environmental changes during this period.However,it remains a subject of debate.We conducted sedimentological and microbial fabric analyses of limestones from the Pagoda Formation at sections including Haima(Xingwen,Sichuan),Qiaoting(Nanjiang,Sichuan),and Anwen(Qijiang,Chongqing)to reconstruct the paleo-water depth.The sedimentary facies of the Pagoda Formation and its underlying strata can be subdivided ascendingly into evaporative restricted platform facies,mixed open platform margin shoal facies,mixed restricted platform facies,and carbonate ramp facies,which have collectively recorded a transgressive process from shallow-water to deep-water environments during the depositional period.In the basal limestone of the Pagoda Formation,the identification of calcified cyanobacterial communities,including Girvanella,Subtifloria,and Proaulopora,along with their assemblages,indicate deposition within the shallow-water environment of the upper photic zone(～50 m water depth).In the middle-to-upper limestone intervals of the Pagoda Formation,these calcified cyanobacteria disappeared,while microbial boring fabrics associated with green algae(reticulate borings)and fungi(filamentous and spherical borings)are recognized.This shift suggests a significant increase in water depth to the low-light zone below the photic zone(>100 m),further supporting a rapid transgression-induced water deepening process on the Yangtze Platform during the Late Sandbian to Early Katian stages.These findings provide critical insights into the mechanisms driving global climate fluctuations and biodiversity changes during the Late Ordovician,highlighting their indicative significance for understanding the origins of these paleoenvironmental and biotic events.