Research On Paleoceanography, Paleoclimate And Formation Mechanism Of Source Rock During Geologic Transition Period From Late Ordovician To Early Silurian In Southern Sichuan Province-Northern Guizhou Province,South China

The Ordovician-Silurian transition is one of the critical intervals in Earth history during which dramatic oceanic,climatic and biological turnover took place resulting in Mass Extinction.Under this background black shale of Wufeng and Longmaxi Formations deposited in Yangtze plate which is forced to be a semi-restricted shallow marine by Caledonian tectonic movement.Currently the black shales are getting more and more attention because they have been the key stratum of shale gas exploration.In this thesis I conduct the research chronologically on the stratum of Linxiang Period?corresponding to Katian?,Wufeng Period?corresponding to Katian?,Guanyingqiao Period?mainly corresponding to Hirnantian?and Longmaxi Period?corresponding to Rhuddanian?.During Late Katian tectonic movement resulted in a sediments turnover at the interface between Lingxiang and Wufeng Formations,that is Tectonic-Sedimentary Transition Boundary?TSTB?;and subsequently the Late Ordovician Glacial Maximum?LOGM?broke out.In the thesis the revolution of Paleoceanography and paleoclimate in front and rear of TSTB and LOGM are reconstructed through the research on sedimentary facies,paleogeography,paleoproductivity,paleo-redox,stable carbon isotope and Chemical Index of Alteration?CIA?.Besides,the geochemical characteristics and factors controlling the organic matter accumulation of black shales in Youyang section has been present.Firstly,the sedimentary facies and paleogeography of Linxiang Formation,Wufeng Formation,Guanyiqiao Formation and the bottom four meters of Longmaxi Formation are reconstructed,and moreover TOC contour maps of black shales of Wufeng and Longmaxi Formationss are made on base of lithofacies paleogeographic map.The lithofacies paleogeography of Linxiang Formation mapped with the method of dominating facies is divided into six palaeogeographic units including Qianzhong Paleouplifts,Chuanzhong Paleouplifts,tidal flat,carbonate shelf and shoal;and the lithofacies paleogeography of Wufeng Formation is classified into Qianzhong Paleouplifts,tidal flat,shallow shelf,deep-water shelf;and the the lithofacies paleogeography of Guanyinqiao Formation is classified into Qianzhong Paleouplifts,carbonate shelf,clastic shelf,tidal flat,and shore;and the lithofacies paleogeography of bottom four meters of Longmaxi Formation is classified into Qianzhong Paleouplifts,tidal flat,shallow shelf,deep-water shelf.The four lithofacies paleogeographic maps show that sedimentary facies belts controlled by old lands and Paleouplifts stretched along the west-east direction and got deeper from southwest to northeast in the study area.Besides,in order to acquire the distribution of organic matter we made Total Organic Carbon?TOC?contour map of study area which show that high TOC area?generally greater than 3%?coincides with deep shelf facies belts in Qijiang-Daozhen-Wulong area.This reveals that Qijiang-Daozhen-Wulong area not only can be designated as the optimal area of shale gas exploration but also be suitable for the research on the organic matter accumulation of the Wufeng and Longmaxi black shale in Southern Sichuan Province-Northern Guizhou Province,South China.Secondly,the primary productivity indices such as Alxs,Pxs,Ba_xs,Al/Ti and P/Ti are present to rebuild the evolution of paleoproductivity.In vertical the primary productivity varying frequently especially in at the TSTB and LOGM in Tianjiawan and Cangling sections indicate that tectonic movement and glacial event have an influence on primary productivity,and the primary productivity indices demonstrate that the primary productivity of Linxiang Formation,lower part of Wufneg Formation and the upper part of Longmaxi Formation is high relatively?Al/Ti ranges from 16.9 to 19?;and on contrast the primary productivity of the upper part of Wufeng formation,Guanyinqiao Formation and lower part of Longmaxi Formation is relatively low?Al/Ti ranges from 15.9 to 17.8?.Compared to the regions of elevated productivity in the modern equatorial Pacific?Al/Ti ranges from 26.7 to 110?the primary productivity in research area which is just close to Post Archean Australian Shale?PAAS??Al/Ti 16.7?and average pelagic clay in South Pacific Ocean?Al/Ti ranges from 0.34²0.1?is far below.Thirdly,the ocean paleo-redox in research area during Ordovician-Silurian Transition Period?OSTP?which consists of TSTB and LOGM has been reestablished by the indices of authigenic Uranium,U/Th?V/?V+Ni??V/Cr?Ni/Co.The paleo-redox indices such as V/?V+Ni?,V/Cr and Ni/Co change synchronously from Lingxiang to Longmaxi Formations of Tianjiawan and Cangling sections suggest that the paleo-redox condition of bottom water is almost the same in lateral.And in vertical the paleo-redox changed from oxic?including Linxiang Formation and lower part of Wufeng Formation?to sub-anoxic?middle-upper part of Wufeng Formation?,then to dysoxic?Guanyinqiao Formation?,then again to sub-anoxic?Longmaxi Formation?,and in particular the redox condition in front and rear of LOGM and TSTB has a sharp shift indicating that the tectonic movement and fiercely climate change play a crucial role in redox condition.Fourthly,High-resolution paleoclimate analysis has been done with the method of Chemical Index of Alteration?CIA?of the samples from Sd-1 drill cores.Our data show that:1)the CIA values of sediments commonly range from 50 to 65 in the top of Wufeng Formation,Guanyinqiao Formation,and the bottom of Longmaxi Formation,indicating a cold and dry climate during deposition;and the CIA values in the top of Linxiang Formation,Wufeng and Longmaxi Formations vary from 65 to 80,suggesting an warm and humid climate;and the CIA values?around 80?in the lower-middle parts of Linxiang Formation imply a hot and humid tropic climate.Besides,2)there are 7 cold and warm climate cycles including 2 small-scale climate cycles and 5 large-scale climate cycles;3)It is the first time to record 5 large-scale climate cycles in Late Ordovician Glacial Maximum?LOGM?which happens to hold the same view with Ghienne's report by sedimentological evidences in North Africa and West Asia;4)it is pulsating and gradually to enter into LOMG because of the 2 small-scale climate cycles before LOMG;5)a sharp drop of CIA values in the upper part of Linxiang Formation indicates the climate began to grow cool from Middle-Late Katian.In addition,6)it is a reliable and practical method with CIA value to reconstruct the paleoclimate in low latitudes where always lack of sedimentological evidences like tillite and glacial striae.Fifthly,carbonate ooids which mainly form in arid environments with a consistent elevated salinity is first discovered in the Guanyinqiao Formation?Hirnantian?firmly suggest that arid environment exist in Guanyinqiao Period.In addition,due to the discovery of the carbonate ooids,the formation mechanism of Guanyinqiao carbonate should be revised to the fact that the redox condition from sub-anoxic to dysoxic caused by the dropping of sea level,together with arid climate result in deposition of Guanyinqiao carbonate.Sixthly,the variations of carbon isotope(?¹³C)of the samples from Tianjiawan Section show that the depositional environment fluctuated during the Ordovician-Silurian Transitional Period?OSTP?.The first prominent shift of?¹³C during OSTP characterized by slowness occurred in the boundary between Linxiang Formation and Wufeng Formation,which may be caused by promoting of primary productivity owing to intensity of tectonic and volcanic activities.Besides,the second remarkable shift which usually is called Hirnantian Isotope Carbon Excursion?HICE?takes place in Guanyinqiao Formation.It is worth noting that the summit of the HICE in Yangtze area is always later than the HICE of other areas in the world.This maybe relating to semi-restricted ocean environment of Yangtze area during Hirnantian,which forced the local carbon cycling to appear and protected photosynthesis planktons from the strike of fierce environmental change of other areas in the world at the beginning of the Hirnantian so that the planktons would die only at the time when they were driven beyond the forbearance.In addition,the high TOC values on behalf of high rate of organic carbon burial triggered the HICE and LOMG.Last but not the least,on base of the research on the evolution of paleoenvironment and paleoclimate I evaluate the source rock by the method of Geobiofacies and residual organic matter?TOC?.The Geobiofacies including biohabitat types,paleoproductivity,sedimentary organic matter and buried organic matter suggest that upper-Wufeng Formation and Longmaxi Formation are excellent source rocks,and that lower-Wufeng Formation and Guanyinqiao Formation are second-rate source rocks,but Linxiang Formation is non-source rocks.Besides,the relationship between residual organic matter?TOC?and the paleoproductivity and the paleo-redox indicate that paleo-redox type controls the accumulation of organic matter.In conclusion,the genetic model of organic matter has been founded on base of the systematic research of evolution of paleoceanography and paleoclimate.The model reveals that:1)it is the sea level changing caused by tectonic movement and LOMG that play the most crucial role in the preservation of the Wufeng and Longmaxi black shales;2)the accumulation of organic matter was mainly controlled by redox conditions;3)however,such factors as paleoproductivity,clastic fluxes,fresh water inflows or a mixed deposition with a rapid sedimentary rate cannot be ignored due to their influences on organic matter enrichment and preservation.