Sedimentary Features And Evolution Of The Late Paleozoic Isolated Bama Platform In The Youjiang Basin

The Late Paleozoic Ice Age（LPIA;332–255 Ma）was one of the most severe glaciations in the Phanerozoic,which greatly impacted the physical and chemical conditions in the ocean,ocean circulation mode,as well as paleoecology on a global scale.It has been suggested that far-field indirect proxies from tropical carbonate systems reflect the LPIA faithfully,and,to a large extent,contribute to new perspectives in our knowledge of the deep-time climate change.However,little attention has been devoted to the stratigraphical imprints of the LPIA on the equatorial eastern Paleo-Tethys（South China）.A number of isolated carbonate platforms of various sizes and characterized by similar sedimentological conditions developed in the Youjiang Basin during the Late Paleozoic,and their isolated character,which limits terrestrial input,as well as abundant and extensive exposures,make them a good archive for the investigation of the LPIA dynamics.This paper has taken the isolated Bama Platform in the northern part of the basin as an example.In order to reconstruct the evolution processes of depositional modes of the platform,systematic analyses of macroscopic sedimentary facies,microfacies,and foraminifer-based biostratigraphy were conducted.In combination with reconstructed relative sea levels,quantitative data on microfacies composition,carbon isotope stratigraphy,and sedimentary geochemical data,this paper mainly represents multiple responses of the Bama carbonate depositional system to the LPIA,and at the same time,discusses major perturbations in global and regional paleogeography,paleoceanographic physics and chemistry,ocean circulation mode,as well as the carbon cycle,with respect to the onset,development,and the icehouse–greenhouse transition of the LPIA.Accordingly,the following conclusions can be drawn:1.During the Late Devonian to Permian,the following depositional environments developed in the Bama Platform:proximal to distal,restricted platform,open platform,bioclastic/oolitic-peloidal shoals and/or stromatolite/sponge reefs,platform marginal fore-shoal/reef upper slope,lower slope–basin facies.It represents one of the most typical rimmed isolated carbonate platforms within the tropical realm.Regional tectonic evolution during the Late Paleozoic to Early Triassic is proposed here to have simply played a decisive role in the birth and demise of the Bama Platform.However,during its development,the distribution and evolution of these facies were mainly governed by the following causally linked factors:LPIA,biotic changeover,and eustacy.2.A late Visean major sea-level drop（more than 20 m）is recorded in the Bama Platform and can be traced worldwide.It has been suggested to be linked to the onset of the LPIA.A systematic difference inδ¹³C_carb values across the paleo-subequatorial realm is present in late Visean,probably as a result of the closure of the Rheic seaway.The closure might have given rise to reorganization of oceanic circulation,and subsequently played a vital role in exchanging heat and moisture between high-latitude Gondwana and the tropics,as well as in carbon cycling.This paper proposed an upwelling model based on analogies with changes in oceanic circulation induced by the closure of the Isthmus of Panama and restriction of the Indonesian Seaway.Quantitative microfacies analysis argues for an increase in nutrients availability,which can be also expected as a result of climate-driven upwelling.3.Seven cooling phases were identified in the Moscovian–Artinskian interval based on either subaerial exposure horizons and relative sea-level falls,or increasing trends inδ¹³C_carb.Distinctive transgressions recorded in the latest Gzhelian and late Sakmarian correspond to global sea-level rises and warming phases.The basal Kungurian is constrained by a pronounced positive excursion（+4.4 to+6.1‰）inδ¹³C_carb,incorporating a prominent drop in the relative sea-level in South China.Seven and a half third-order depositional cycles（0.93–1.41 Myr）were observed across the platform in the Kungurian.The K1–K6 cycles are interpreted as being of aquifer-eustasy origin in a greenhouse mode（10–30 m）,while the K7–R1 cycles are of glacio-eustasy origin in a icehouse mode（50–70 m or even more than 70 m）.Both have been suggested to be driven by long-period modulations of obliquity（¹.2 Myr）.This climate trasition from greenhouse to icehouse during the late Kungurian probably resulted from a high productivity and substantial burial of organic matter.4.Integrated carbon isotope stratigraphical,sedimentological,and geochemical data across the Bama Platform reveal that widespread anoxia near the sediment-water interface occurred in South China during the Kungurian.It prevailed at least in the upper slope settings during the early Kungurian and in both deep lower slope–basin and shallow shoal environments during the late Kungurian.High primary productivity,high sedimentation rate,and a relative sea-level rise were responsible for the late Kungurian anoxia（K7）,but these factors cannot account for the remaining anoxia at cycle boundaries in the early Kungurian.A hypothesis was proposed that divergent climate trends between the tropics and Gondwana during the Late Paleozoic icehouse to greenhouse transition might have played an important role in ocean stagnation and prevailing anoxia in and probably out of South China.The complete demise of the prevailing anoxia in South China resulted from a couple of coincident events during the latest Kungurian（K7/R1 boundary）,including vigorous ocean circulation,high atmospheric pO₂ concentrations,a sea-level drop,depressed primary productivity and low sedimentation rates.5.Three well-datedδ¹³C_org curves across the Bama Platform,tied to a high resolution basin-wide stratigraphic framework,are paired with their respectiveδ¹³C_carb data for the first time in this paper.Their comparison demonstrates that a prominent,rapid,and synchronous negative excursion inδ¹³C_org andδ¹³C_carb occurred in the early Kungurian（the herein proposed KCIE event;dated to ca.277.9–277.4 Ma）.A comparable shift is observed in other deeper-water marine sections in South China and two terrestrial sections in North China and eastern Australia.Based on carbon mass-balance calculations and concurrent geological records,we propose that the intensive volcanism of a large igneous province in northern Gondwana,combined with its induced voluminous methane-hydrates dissociation nearby and the ensuing worldwide conflagrations might have steered the KCIE event.The large production of greenhouse gases involved would have warmed the Earth and further resulted in ocean stagnation,anoxia,and acidification,all of which are in conformity with the documented sedimentological,geochemical,and palaeontological data.This major anomaly in the carbon cycle in the late Cisuralian is considered here to play a key role to promote the transition of the Earth to a greenhouse mode.