?¹³C_carbStratigraphy,bio-and Lithofacies Of The Early Silurian In Northeast Guizhou,South China Block

Mixed depositional sequences of carbonates and terrigenous debris throughout the Ordovician-Silurian transition in Northeast Guizhou,Upper Yangtze region of the South China Block,regionally display extinction and recovery events in marine facies.Uneven stratigraphic lacunae of this time-interval are existences due to the brief expansions of the Dian-Qian-Gui Oldland and sea-level changes.The Daxichang Formation with a thickness of 129.7 m is dominated by terrigenous depositions and limited distribution in the Daxichang depression in Sinan.Graptolitic biozonations suggest that the age of the Daxichang Formation is from late Katian of Upper Ordovician to late Rhuddanian of Llandovery,Silurian.Bentic shelly fossil herein is extremely sparse.Ichnofossils of the upper Kation indicate instant inhabitation of opportunism taxa.Wrinkle structure as a kind of microbial induced sedimentary structure on silty substrata is the only reported anachronistic facies from the earliest Silurian unit in the South China Block.The Xiangshuyuan Formation in the Northeast Guizhou is tens meters thick and dominated by limestones interbedded with less siltstones and mudstones,suggests beginning of stable high-clarity marine-floor expansion in the Llandovery time.It significantly exhibits gradual recoveries of shelly fauna in bioclastic banks and coral-stromatoporoid reefs aftermath the latest Ordovician extinction events;meanwhile,depth and marine-floor substrate varieties were both considered as key roles in configuration of the benthic ecological profiles.Subfacies of the Xiangshuyuan Formation are respectively the Baisha Type in the near-shoal and Yinjiang Type in the off-shoal.Current biozonation of the formation is not idealized for high-resolution dating of the bio-and lithofacies.Limestone samples are available for measurements of the ?13Ccarb and ?18OCarb values from four localities;they are respectively from the Qiankou and Kuaizishan sections of Shiqian as representatives of the Baisha Type,Pugoucun of Shiqian and Daxichang of Sinan as representatives of the Yinjiang Type.Isotopic curves of the Xiangshuyuan Formation are firstly outlined with a positive excursion,which has a maximum shift value of 2‰,and is likely correlated to equivalent curves from the Laurentia and Baltic blocks.The profiles suggest that the Aeronian ?13Ccarb positive excursion event is potentially of a transglobal event.Correlation of the ?13Ccarb curves from four sections further shows that the Xiangshuyuan Formation is a diachronic unit in the Northeast Guizhou due to the different terrigenous impacts from place to place.Fossils identify from microfacies of the Xiangshuyuan Formation are additional of reconstructions of the historical biodiversity.Bryozoan,hyolitha,bivalve,ostracoda,sponge,calcareous algae,calcimocrobe are of the taxa which have never been documented in former literatures.Although most of these fine-grained fossils are hard to be named in genus and species ranks,they are exactly important intraclastic components of the limestones.Reef and no reef units are diverse in biotic structures due to the changeable turbulent,depth and clarity conditions on the epicontinental sea.Base on the bio-and lithological distribution patterns and ?13Ccarb curves,ecological successions of the Xiangshuyuan Formation are divided into five stages;they are:1)sparse occurrence of metazoanal frameworks;2)medium abundant habitation of metazoanal frameworks;3)biostrome formed by high abundant metazoanal frameworks;4)reef recovery constrained by sea-level rising;5)Coral-stromatoporoid reef recovery.Reefs of the Earliest Silurian were stenotropical niches with limited growth episode and narrow realm of the Baisha Type Xiangshuyuan Formation in the near-shoal belt.In contrast,Yinjiang Type Xiangshuyuan Formation in the off-shoal belt was deposited in deeper and calmer marine-floor where developments of biodiverisity and abundance of benthic fauna were likely constrained and is not idealized for the forming of bank and reef.