Permian - Triassic Conodont Biofacies Biological Replacement And Related Boundary Study

The Upper Permian and Lower Triassic arewell developed in South China and Tibet. Basedon the researches in the Late Permian-EarlyTriassic strata and fossils of the six sections(the Changxing section, the Shangsi section, theMaoritang section, the Hechuan section, thechaotian section, the Selong section), the conoconodont evolution, the conodont biofacies andprovinces, the quantitative conodont biostrati-graphy, the biotic alternation and the Permian-Triassic boundary have been studied in thispaper. The following remarks are worth mentioning.1. According to the conodont evolution andtheir distribution in Permian and Triassic inthe six sections, eleven conodont zones havebeen divided (Fig. 1). They are as follows (inascending order). (1) Gondolella subcarinata subcarinata—G.subcarinata elongata assemblage zone.(2) Gondolella subcarinata changxingensis-G.deflecta assemblage zone.(3) Hindeodus parvus zone.(4) Isarcicella isarcica zone.(5) Gondolella carinata zone.(6) Neospathodus dieneri zone.(7) Neospathodus cristagalli zone.8) Neospathodus waageni zone.(9) Parachirognathus geiseri-Pachycladinaobliqua assemblage zone.(10) Neospathodus homeri zone.(11) Neospathodus timorensis zone.2. as a result of detailed researches inPermian-Triassic conodont paleocology,lithological characterisics and theirvertical and horizontal distributions, werecognize five conodont biofacies (Fig. 2, Fig.3): Hindeodus facies (Tidal-Flat-Lagoon),Pachycladina-Parachirognathus facies (Subtidal),Gondolella-Hindeodus facies (Inner Shelf),Gondolella-Neospathodus (Outer Shelf), Gondolella-Xaniognathus facies (Basinal).3. Based on the presence of the conodontbiofacies in South China and Himalayan and onthe Stratigraphic range and the geographicdistribution of their elements, three conodont provinces (Himalayan province, Upper YangtzeProvince, Lower Yangtze Province) are recognizedin Upper Permian and Lower Triassic (Fig. 4).The geographic characteristics, the changesof the conodont biofacies, the relative dif-ferences or similarities of the three ConodontProvinces are discussed in detail.4. According to the changes represented bythe conodont biofacies in Late Permian-EarlyTriassic, two Transgression-Regression Cyclesare recognized in South China and Himalayan(Fig. 3): The first Transgression-RegessionCycle, early Changxingian to early LateCriesbachian and the second TransgrssionRegression Cycle, late Late Criesbachian tolate Dienerian.5. A continuous disappearance of thepalaeozoic faunal elements took place withinthe Upper Permian and the extinction rate ofold elements was much higher than the appearancerate of the new elements, in such a shortgeologic time span. Therefore, the extinctionof the organisms at the Permian-Triassic alter-nation is the greatest extinction in the historyof life. Fusulinids, Tetracorals, Tabulata,Trilabites, Non-fusuline foraminifers, Ammonoids,Brachiopods and Conodonts etc. extincted ordeclined rapidly in the end of the Changxingian.Only Glomospira, Geinitzina, Palaeolextularia,Crurithyris, Pseudogastrioceras, Gondolella etc. survived into the Griesbachian or higherlevel.6. After the extinction, biotic evolutionwas in a desolated period with a few survivorsof the Changxingian in Early Griesbachian.Faunas have low diversity and abundancy. OnlyOtoceras, Hypophiceras, Glyptophiceras,Towapteria began to appear in the period.In the Late Griesbachian, the faunas representedby Ophiceras, Claraia, Isarcicella began toflourish and grow radially (Fig. 5).7. A detailed research has revealed thatsome geological events such as crustal uplift,reginal regression, volcanic explosion, reversalof magnetic poles have happened during the latePermian. The geological events produced thecrises in the ecosystems and made a massextinction of faunas. In so far, the geologicalevents play an important role in phylogenesis.They have their effect in accelerating, inpushing forward the evolution. According to thefact that the biotic extinction and flourishare in keeping with the Transgression-RegressionCycles, we deem that the eustatic change of thesea level may be one of the important factorsin the Permian-Triassic biotic alternation.8. In recent years, a lot of work have beendone on the division of the Permian-Triassic boundary and brought forth four concludingopinions: (1) the base of the Otoceras bed.(2) the base of the Hindeodus parvus zone,(3) the base of the Ophiceras bed, (4) thebase of the Gyronites. In essence, as weconsider, the first boundary is an event-stratigraphy boundary, other boundaries arebiostratigraphical boundaries. The reasonengendered the four boundaries is differentfrom the standard and the priciple definedboundary.9. Based on the distribution of conodonts,Ammonoids and Bivalves, and using the method ofa graphic correlation to research in the bound-a(?)es of the base of the Changxing Formation,the base of the Otoceras bed, the base of theHindeodus parvus zone, the base of the Ophicerasbad in the ten sections (the Changxing section,the Shangsi section, the Maoritang section, theChaotian section, the Huaying section, theLiangfengya section, the Hechuan section, theXixiang section, the Selong section and theKashmir section) (Fig. 6, Fig. 7, Tab. 2, Tab.5, Tab. 4), in which, the Changxing section ischosen as Standard Reference Section, thefollowing results have been gainned: (1) thethree boundaries of the base of the Otocerasbed, the base of the Hindeodus parvus zone andthe base of the Ophiceras bed are separatelyconcordant except the Selong section and the Chaotian section. (2) except the base of theChangxing Formation, other boundaries inChaotian section and the four boundaries inthe Selong section are not coincident with theboundaries in the Changxing section, (3)According to the conodont distribution, exceptthe Chaotian section, the base of the ChangxingFormation in ether sections are not coincidentwith the base in the Changxing section. (Tab. 5).10. By the analysis for the boundaries, wedeem that it is suitable to put the Permian-Triassic boundary at the base of the Ophicerasbed. This boundary marks the beginning of a newperiod characterized by the emergence and radilgrowth of new biologic groups.11. Based on the research in the conodentbiofacies in South China and Tibet, it seemsthat South Tibet is a good area for reserarchin the Permian-Triassic boundary, because theInner Shelf was maintained from the EarlyGriesbachian to Late Griesbashian.