A Study Of Source Rock Features And Sedimentary Environmental Evolution During The Late Permian In Lower Yangtze Region,Southeastern China

The Permian-Triassic Boundary(PTB)was a special period in Earth history,characterized by the largest known mass extinction and significant environmental changes.The triggering mechanism and process of this famous geological and biological event have been studied for decades.In addition,a series of important mineral resources were formed within the sedimentary rocks of this age,such as the large volume of coal resource and marine source rock in South China.Therefore,intensive studies on the late Permian paleo-environmental features are needed due to its significance in either science or industry.The South China in the Paleo-Tethys has been an important area for the study of PTB because many well-preserved PTB sections ranging from deep water to shallow-water and to terrestrial settings outcropped in this area.Although these PTB sections contain valuable information for investigating the mechanism of Late Permian Mass Extinction(LPME)and other PTB problems,nearly all of these sections are loeated in the Upper Yangtze Region.In contrast,only a few relating sections were investigated in the Lower Yangtze Region except for the PTB Global Boundary Stratotype Section and Point(GSSP)Meishan section.Here,we reported three new PTB sections named Niushan,Caicun and Changqiao in the Lower Yangtze Region based on extensive field surveys.These sections are characterized by relatively continuous depositions in deep-water environment.In contrast,previous reported PTB sections were deposited in carbonate platform and contain sedimentary hiatus or condensed bed,which will obscure some geological information.As a result,these three sections may provide new cases to better study the PTB problems,including marine redox structure and environmental feature in deep water condition.In addition,with the development in petroleum exploration in the Lower Yangtze region,the distribution and formation mechanism of upper Permian marine source rock have attracted the attention of many petroleum geologists.Thus,these three new sections were studied in detail using multiple methods of sedimentology,petrology,mineralogy,and geochemistry.The major contributions of this work were summarized as follows.In the Lower Yangtze region,the Upper Permian Dalong Formation is mainly composed of black fine-grained rocks indicative of deep-water settings,including black shale/mudstone,chert,carbonate and volcanogenic claystone.Results of stratigraphic correlations and sedimentological analyses showed that the study area was subject to a complete sequence of marine transgression and regression and mainly deposited in marine,deep water,reducing,restricted and undercompensated environment.The Niushan section was a continuous deposition and contains the entire Changhsingian Stage and the Wuchiapingian-Changhsingian Boundary(WCB).Integrated organic carbon isotopic and U-Pb geochronological analyses were conducted to constrain its age and timing.U-Pb dating of magmatic zircons in volcanic ash indicated that the section includes the PTB.The organic carbon isotope curve of the section exhibits three positive excursions and two negative excursions that can be well correlated with other PTB and WCB sections worldwide,indicating that original PTB and WCB signatures was preserved.From bottom to top,the negative carbon isotopic excursion(NCIE)near the WCB(NCIE-1)at the bottom of the section may be ascribed to an increase in the productivity of organic matter;transgression favors the flourish of marine biosphere and results in the change of organic matter type from terrigenous to marine.Then,the two positive carbon isotopic excursions(PCIE-1 and PCIE-2)within the upper Permian Dalong Formation are possibly related to increasing input of terrigenous organic matter during a regression.PCIE-2 may also have been influenced by high marine productivity as well as blooms of green sulfur bacteria.Near the top of the section,the two NCIEs(NCIE-2 and NCIE-3)across the PTB tonsition may have been caused by widespread large-scale volcanic activity.The volcanogenic claystone widely deposited during the PTB period,in particular,over the Tethyan Realm,playing a key role to investigate the killing mechanism and associating geological events for the LPME.All fresh samples were collected from three deep-water marine sequences in the Lower Yangtze region.The PTB claystone(K-bentonite)are mainly composed of volcanogenic clays of illite and I/S mixed layers and phenocrysts of quartz,calcite,pyrite and magmatic zircon as well.Furthermore,their geochemical compositions are characterized by high K2O and low Na2O,enriching in the Large-ion Lithophile Element(LILE)but depleting the High Field-strength Element(HFSE).The whole-rock geochemical data of the claystones suggest that the source rock of the ash layers was intermediate-acidic rhyodacite.The PTB claystones were derived from silicic volcanism related to arc lavas,and are distinct from sediments derived from the Siberian Traps and Emeishan basalts.The claystones are different from other already-discovered claystones in the Middle-Upper Yangtze regions,implying chemically heterogeneous volcanism in South China near the PTB.We therefore inferred that all of the PTB claystone might generated from felsic magmas during volcanisms,which were derived from either eontinental eollisions around paleo_Tethys Oeean or subduction zone of Panthalassie Ocean along eastern margin of the Pangea Continent and South China,acting in multiple episodes.These intense volcanic activities in global scale as evidenced by pervasive distribution of the PTB claystone may play as key trigger for the biotic mass extinction at the latest Permian.For the Niushan section,high-resolution geochemical analyses including sulfur isotopes and trace elements were employed to explore the changes of oceanic anoxia,terrigenous input,and marine productivity across the entire Changhsingian stage in the Lower Yangtze Region,South China.Sedimentological observations and analyses of redox proxies(Mo/Al,U/Al,and V/Al,FeT/Al,?34Spy)showed that bottom water during the entire Changhsingian experienced transitions from episodic sulfidic(interval ?)to suboxic(interval ?),complete sulfidic(interval ?),and single sulfidic in dysoxic/oxic conditions(interval V).Detrital siliciclastic proxies including contents of Al2O3,?REE,Th,Nb,and Hf indicated enhanced siliciclastic influx across PTB age were started at the late Changhsingian.Multiple productivity proxies(TOC,Cuxs,Znxs)suggested the primary productivity may experience periods of stable low(interval ?),enhanced with strong fluctuation(interval ?),transient and sharp decline(interval ?),relatively high with narrow fluctuation(interval ?),and gradual decline with occasionally rise(interval ?)across the time interval of entire Changhsingian Stage.Compared with previous reports on the environmental changes in South China,we claimed that the deep water displayed similar evolutionary proeesses,although the Niushan section records more severe sulfidic benthic environment.The relationship between redox and productivity changes implied that widespread anoxia may be responsible for the LPME.In addition,the Dalong Formation is rich in organic matters with average total organic carbon(TOC)content of ca.2.0%.The type of organic matter is mainly ?2.and ?.The thermal maturity of most samples 1s over mature(Ro>1.7%).Therefore,the regional hydrocarbon exploration should be focused on natural gas(i.e.,shale gas).Based on the spatial distribution of Mesozoic-Cenozoic igneous rocks,we suggest that the over maturation of organic matter may be influenced by large-scale magmatism in South China during the Mesozoic-Cenozoic.Thus,in the areas with no/minor influence of magmatism(e.g.,the Jingxian-Guangde area),oil exploration potential can be considered.