Identification of Guadalupian-Lopingian Boundary in Permian in Pelagic Chert Sequence and Correlation in the Worldwide
|School||China University of Geosciences|
|Course||Mineralogy,petrology, Mineral Deposit Geology|
|Keywords||Permian in Guangxi pelagic chert sequence Guadalupian Lopingian boundary conodont radiolarian Rare Earth Element(REE) pre-Lopingian crisis of endemic benthos and pelagic faunas|
The end-Permian Mass Extinction is the greatest and the most influencing event on biotic evolutionary history. Jin et al.(1995)considered: the end-Permian Mass Extinction comprised two distinct phases: the pre-Lopingian crisis event of endemic benthos and pelagic faunas that occurred in coincidence with global regression at the end of Maokou Stage; and an end-Lopingian event characterized by sharp declination of biomass and extinction of major fossil groups, and corresponding with a worldwide rapid flooding.The mass extinctive event between Guadalupian and Lopingian is the pre-Lopingian crisis event of endemic benthos and pelagic faunas. The division of Guadalupian-Lopingian boundary in Permian in this dissertation (i.e the basal boundary of Lopingian Series) is on the basis of this event.1、 The author get the following facts from the research of radiolarian biostratigraphy, conodont biostratigraphy, ecological evolution of radiolarian and conodont, Rare Earth Element (REE) geochemical characteristics and lithology characteristics from Guadalupian to Lopingian Series in Dachongling section in Guangxi.(1) On the basis of the analyses of characteristic radiolarian species and their stratigraphic ranges, totally six radiolarian zones were recognized in Dachongling section, namely, the Pseudoalbaillella longtanensis Zone, the Pseudoalbaillella globosa Zone, the Follicucullus monacanthus Zone, the Follicucullus scholasticus Zone, the Follicucullus charveti Zone, the Albaillella levis Zone in ascending order. The stratigraphic ranges of the six biozones are able to be compared precisely with those from Southwest Japan and the southwestern area of China.(2) Based on the stratigraphic distribution of characteristic conodont species in pelagic chert sequence in Dachongling section, we divided them into four conodont zones, namely, Jinogondolella granti Zone. Clarkina postbitteri hongshuiensis Zone, Clarkina postbitteri postbitteri Zone and Clarkina dukouensis Zone, in ascending order. The four conodont zones can be correlated with those in bathyal carbonate and lime-chert sequences identified by Jin et al.(2001) and Henderson et al.(2002). As a recent result, Jin et al.(2001) and Henderson et al.(2002) have formally proposed to establish the Global Stratotype Section and Point (GSSP) for the basal boundary of the Lopingian Series at the first occurrence of Clarkina postbitteri postbitteri Mei and Wardlaw in the Penglaitan Section in South China. In this paper, we identified the Guadalupian-Lopingian boundary (i.e the basal boundary of the Lopingian Series) on the basis of this standard.(3) Below the inferred Guadalupian-Lopingian boundary, the species of radiolarian genus Follicucullus have much higher diversity and abundance, but those of the genus Albaillella are very low. Upon the inferred Guadalupian-Lopingian boundary, the Follicucullus genus decreased intensively, but the diversity and abundance of genus Albaillella increased too much and Suborder Spumellaria Ehrenberg is dominant until the end of Dachongling Section.Theradiolarian ecological evolutionary process cross the inferred Guadalupian-Lopingian boundary is from a declined state to a recovery state.(4) Below the inferred Guadalupian-Lopingian boundary, conodont genus Jinogondolella occurred, but upon the boundary, all the genus Jinogondolella disappeared and the genus Clarkina took the dominant place of it. So, conodont genera show an abrupt change from Guadalupian to Lopingian Series, too. From the above, we can see that a abrupt change of ecological evolution of radiolarian and conodont occurred from Guadalupian to Lopingian Series.(5) An anomalous zone with the higher Ce/Ce* values in range from 1.42 to 1.80 (the background values are in range from 0.5-0.8, 2-3 times of the background values) occurred underlying the inferred Guadalupian-Lopingian biostratigraphic boundary. The Ce/Ce* values coincided with the process of the ecological evolution cross the inferred Guadalupian-Lopingian biostratigraphic boundary. Moreover, the Shale-normalized REE pattern of chert show that almost all of the samples have stable negative Ce anomaly below Dch45-4 ,but some high positive Ce anomaly occurred nearby the Guadalupian-Lopingian biostratigraphic boundary.(6) Lithology characteristics: before the pre-Lopingian crisis event, lithology is mainly thin to extremely thin radiolarian-bearing chert; after the pre-Lopingian crisis event, the lithology become extremely thin clay chert or chert clay, intercalating white tuffaceous chert and claystone, and clay content increase too much, which indicate that the sediment was affected greatly by the terrigenous matter. Lithology characteristics change greatly through the Guadalupian-Lopingian boundary.2 > The above-mentioned facts draw the following conclusions( 1 ) The conodont zones in Guadalupian/Lopingian boundary stratigraphy in bathyal carbonate and lime-chert sequences responds well in the pelagic chert sequence in Dachongling section.(2) The author identified Guadalupian-Lopingian biostratigraphic boundary at the base of Dch46-C in Dachongling section according to the correlation of the four conodont zones in Dachongling section with those in bathyal carbonate and basin lime-chert sequence by Jin et al.(2001) and Henderson et al.(2002).(3) Guadalupian-Lopingian biostratigraphic boundary located between the upper radiolarian Albaillella levis Zone on the basis of the correlation of radiolarian zones with the co-occurring conodont zones.(4) The abrupt change of ecological evolution of radiolarian and conodont through Guadalupian to Lopingian, and the geochemical anomalous zone with the higher Ce/Ce* values underlying the Guadalupian-Lopingian biostratigraphic boundary, and the great change of lithology through the G-L boundary may indicate the pre-Lopingian crisis event of endemic benthos and pelagic faunas.(5) The Guadalupian-Lopingian event stratigraphic boundary was divided at Dch45-1 in Dachongling section at the beginning of the pre-Lopingian crisis event.(6)The author consider it is the tectotic activity that resulted in the pre-Lopingian crisis event that occurred in coincidence with global regression at the end of Maokou Stage from the analysesof ecology, geochemical characteristics and lithology. This viewpoint agree with that of Jin(1995), moreover, the author draw other two conclusions: the first one is that the event occurredvery abruptly in the beginning, which resulted from the abrupt change of radiolarian ecologicalcharacteristics; the second is that the event continued a long time because the anomalous zonewith the higher Ce/Ce* values continued a long time.3> The innovative points in this dissertation and the main unsolved problems andsuggestionsThe innovative points in this dissertation:(1) Guadalupian-Lopingian biostratigraphic boundary and event biostratigraphic boundary is first identified in pelagic chert .sequence in this dissertation.(2) It’s the first time to approximately divide the Guadalupian-Lopingian boundary in the radiolarian zones on the basis of the correlation of co-occurring conodont zones.(3) The pre-Lopingian crisis event is first discovered to respond well in pelagic chert sequence.(4) It’s the first time to do an elementary research of the process of the pre-Lopingian crisis event in pelagic chert sequence.Main unsolved problems and suggestions:Although the author have made some achievement in the research of Guadalupian-Lopingian boundary in Dachongling chert section, there still leave some unsolved problems.(1) The other conodont zones in the middle and upper of Lopingian have not been found in Dachongling because the sedimentary basin closed earlier, therefore , I suggest that we find a better chert section in the future in Guixi basin.(2 ) The interval of Rare Earth Element samples is relatively wide. In the key layers, samples should be tested for geochemical characteristics bed by bed and more items should be analysed in order to establish more integrated geochemical stratigraphy from Guadalupian to Lopingian Series. All these will do benefit to research the stages of the pre-Lopingian crisis event.(3) The character and causes of the pre-Lopingian crisis event needed to be researched further, for example, the research of Mylankovitch cycles might discover if the character and causes of the pre-Lopingian crisis event related with the rhythm of the earth, and the research of volcano activity in this epoch (there are some tuff layers near the Guadalupian-Lopingian boundary in Dachongling section) might discover if the pre-Lopingian crisis event was affected by the volcano activity.(4) A further research should be done that the detail number of the families and genus and species of radiolarian extinction and origin as well as times of radiolarian extinction and process of biological recovery through Guadalupian to Lopingian in Permian.