Research On Origin Of Permian Cherts From Continental Marginal Sea Of Eastern Paleo-Tethys Ocean

Chert,as a unique type of silica-rich sedimentary rock,has been preserved in stratigraphy from Precambrian BIFs to modern pelagic siliceous oozes.Chert may provide insights into ancient silica cycles in lithosphere,biosphere and hydrosphere,as well as dynamic process of basin deposition-fill.Through geologic time,the global Permian Chert Event is defined as Permian strata are biogenic silica-rich around the world.However,Permian cherts were deposited on continental marginal basins of Paleo Tethys Ocean are different in origin.Therefore,this thesis will discuss Permian cherts depositional response to paleogeograhy,tectonics,paleoceanography and paleoclimatology in continental marginal sea of Eastern Paleo Tethys.Thus,this thesis could provide vital clues for origin of Permian chert in Paleo Tethys archipelagic Ocean,compared with chert deposition in Panthalassa Ocean.The first research target of this thesis is to study the distribution and origin of Permian cherts across North China and South China,examining the petrologic,geochemical data of several typical Permian chert-rich sections,based on additional evidences from regional strata correlation and paleogeography.This research aims to clarify tempo-spatial distributions,sedimentary environments,origins and silica sources of typical chert sections.Chert deposition in North China occurred within marine-terrigenous strata from the Early Permian Taiyuan Formation and the Late Permian Upper Shihezi Formation.There is no chert deposition occurring within Permian continental strata in North China.Nodular chert occurs within the limestones of the Lower Permian Taiyuan Formation,which is a widespread marginal shallow-marine sedimentary deposit in North China.The chert samples from 2 sections of Jiaozuo and Yuzhou,are composed of cryptocrystalline quartz,chalcedony,minor calcite,and silicified calcareous fossils.Elemental analysis of chert samples show affinities with typical hydrothermal cherts.Isotopic evidences indicate that thecherts originate from hydrothermal silica,which precipitated within surface calcareous sediments,and transformed from opal to cryptocrystalline quartz and replaced carbonate during diagenesis.Volcanic ash beds in the Taiyuan Formation suggest that the hydrothermal chert was related to Early Permian tectonics along the northern and southern margins of the North China Craton.Faulting and volcanism resulted in ash deposition during marine regression,and supplied silica-rich waters for chert deposition during marine transgression.Late Permian bedded sponge cherts in North China,interlayered within mudstone beds of Upper Shihezi Formation,were deposited during rapid transgression periods.The north-bound transgression produced transient,sponge-hospitable deep-water basin,while sufficient volcanic-rich detrital input spurred surface sponge productivity.Only several sponge chert layers developed across North China,as the Late Permian transgression process is weak and limited.Permian cherts in South China are widespread distributed within various sedimentary basins.During Early Permian,most part of South China exposed as ancient landmass,Lower Permian Chuanshan Formation locally deposited without any chert deposition.During Middle Permian(depositional period of Qixia Formation),transgression led to submerging of previous landmass,which become widespread shallow marine platform,abundant nodular cherts deposited within limestone layers of Qixia Formation.Petrological and geochemical data from 4 sections of Chaohu,Susong,Songzi and Laibin support the biogenic model for nodular chert within Qixia limestone around South China.During Middle Permian(depositional period of Gufeng Formation),a broad transgression occurred and coincided with gentle tilting of the Cathaysia block,nodular cherts developed on carbonate platform,alternately,bedded(radiolarian)cherts were deposited within deep-water basin facies.Comprehensive analysis of chert samples from 6 sections of Chaohu,Susong,Songzi,Shizhu,Guangyuan and Laibin indicate biogenic cherts occurs in eastern region while hydrothermal cherts occurs in western region,divided by the boundary of Hubei and Chongqing.During Late Permian,the carbonate platform-basin tectonic frameworks of South China remain stable,while nodular cherts and bedded cherts were deposited in platform and basin respectively.Geochemical analysis of chert samples from 6 sections of Chaohu,Susong,Songzi,Shizhu,Guangyuan,Laibin and Fusui reveal that hydrothermal cherts were restricted to Sichuan and Chongqing region of Upper Yangtze area,chert depositions within Dian-Qian-Gui region suffered hydrothermal activity and biogenic cherts were widespread in the rest area.The second research target of this thesis is to study the cyclostratigraphy of biogenic cherts-mudstone rhythms,conducting astronomical tuning of radiolarian chert-mudstone couplets from Chaohu,Lower Yangtze area,South China.This research aims to discover the imprint of millennium-scale Milankovitch signals,and unveil the paleoclimate evidence recorded in chert sequence.Based on the silica abundance variations through lithological sequence,the chert-mudstone thickness series could be transformed to 1-0 boxcar series.Spectral analysis of boxcar series from two sections reveals that 13 cm to 20 cm chert-mudstone cycles in the stratigraphic domain represent theoretical 32-kyr Middle Permian obliquity cycling,together with a hierarchy of other cycles with 12 cm,9 cm,7 cm,6.6 cm and 5.4 cm wavelengths.Tuning the 13 cm to 20 cm stratigraphic cycles to Earth's obliquity cycle periodicity indicates that the cm-scale cycles are precession-scale variations with a strong ~400 kyr amplitude modulation.Tuning to theoretical precession terms provides further support for the astronomical forcing of the chert sequence.The resulting astronomical time scale(ATS)assumes a Roadian/Wordian boundary age of 268.8 Ma for the onset of the first chert layer at the base of the sequence and ends at 264.1 Ma,for a total duration of 4.7 Myrs.We propose that monsoon-controlled upwelling contributed to the development of the chert-mudstone cycles.A seasonal monsoon controlled by astronomical forcing(i.e.,insolation)influenced the intensity of upwelling.Stronger upwelling increased radiolarian productivity in the surface ocean,increasing silica deposition.Glacio-eustatic oscillations from ice sheet dynamics in southern Gondwana modulated terrigenous mud flux to the basin.The two processes jointly contributed to the astronomical forcing of these tropical chert-mudstone sequences,and were characterized by comparably strong obliquity and precession responses.Subsequent diagenesis distorted the chert and mudstone layering,but not enough to destroy the original depositional patterns.The third research target of this thesis is to study the chert deposition responding toevent or secular geological process,reviewing the response mechanism of chert deposition to simultaneous variations in paleogeograhy,tectonics,paleoceanography and paleoclimatology in continental marginal sea of Eastern Paleo Tethys.In general,regional,variable and complicated paleogeographic framework controlled the tempo-spatial distribution of Permian cherts,and chert deposition only occurs within shallow carbonate platform,carbonate slope and deep-water basin.In North China,Early Permian chert deposition revealed contemporary tectonicmagmatic-hydrothermal activity,while Late Permian chert deposition suffered from the dual constraint of marine transgression processes and terrestrial volcanic inputs.In South China,nodular cherts occur during Qixia period of Middle Permian,were closely related to silica-rich,upwelling and current-developed Ancient Ocean which contributed to the wide range flourish of siliceous organism.The later chert deposition during Middle-Late Permian appears at different origins from Lower and Upper Yangtze area.Middle-Late Permian cherts in Lower Yangtze area are typical biogenic in origin,along with imprint of astronomical forcing within radiolarian chert-mudstone rhythms.The study of astronomical forcing of Gufeng chert sequences will be useful to unveil the paleoclimatology and paleoceanography recorded within other Permian chert-limestone or chert-mudstone sections.Middle-Late Permian cherts in Lower Yangtze area show strong evidences of hydrothermal activities.The Middle-Late Permian Emeishan basalt eruption from northwest edge of Upper Yangtze might spur the hydrothermal chert depositions.In that case,hydrothermal cherts would trace the depositional response of Emeishan big volcanic province and contemporary mantle plume.