VNUJournalofScience,EarthSciences23(2007)187‐193 187 TwoFrasnian/FamennianboundarysectionsinVietnam TaHoaPhuong* CollegeofScience,VNU Received02July2007 Abstract. Conodont fossils belonging to Frasnian/Famennian (F/F) boundary interval have been discovered within several geologic sections in Vietnam, including Ngoc Lam, Xom Nha (Quang Binh), Kang Ka Pass,Nguom KimPass(CaoBang), NoongDePass(Nghe An), and SiPhaiPass, MaPiLenPass(Ha Giang).However,theF/Fboundarywith boththe linguiformisandtriangularis conodont zones that bracket the boundary hasonlybeenidentified at Si Phai Pass and Xom Nha Mountain. At Si Phai Pass, F/F boundary within the Dong Van‐Nho Que Section has been identified between bedsS12/8andS12/9, within the lower part of Toc TatFormation(D 3 ‐C 1 tt). This part of the section is composed of thin bedded, stripped and colorful limestone interbedded with pure, thinandaveragebeddedgreyandpink‐greylimestone. Exposure of F/F boundary is best observed in a cave mouth within Xom Nha Mountain. All four major F/F boundary conodont zones rhenana, linguiformis, triangularis and crepida have been found inagreyanddark‐grey, averagetothickbeddedlimestoneandclayeylimestonesequence thatbelongstotheXomNhaFormation(D 3 xn).HeretheF/Fboundaryhasbeendeterminedtolie betweenbeds9‐3band9‐4awithinbed9intheXomNhaSection. Keywords:Conodontfossils;LinguiformisZone;TriangularisZone;Frasnian;Famennian. 1.Introduction * Typical for all stratigraphic boundaries, at present fairly broad paleontological criteria have been used to define the F/F chronostratigraphic boundary. For each such boundary,thereisaspecificfossillineage,within a broader group of fossils that has the most important role in defining the boundary level. The concept of a Global Boundary Str atotype Section and Point (GSSP) is now used as the stratotype for each chronostratigraphic boundary defining each increment of geologic _______ *Tel.:84‐4‐8544423. E‐mail:phuongth@vnu.edu.vn time. These GSSPs are currently being established, must follow strict guidelines and first be approved by the International CommissiononStratigraphy(ICS). Finding a GSSP that satisfies the required paleontologicalcriteriaisverydifficultbecause of sedimentary hiatuses that often happen at the boundary, resulting in the removal of critical fossil zones. Such zones may also be missing, even in the case where sedimenta t ion is stable, due to a number of factors including facieschanges. The GSSP established the end of the FrasnianStageandbeginningoftheFamennian Stage(F/Fboundary)ofUpperDevonianattract attention in international stratigraphical circle, TaHoaPhuong/VNUJournalofScience,EarthSciences23(2007)187‐193 188 due to many important events, and sudden changes happened in organisms living at that time. Criteria defining the F/F boundary and locationwereapprovedbythe ICS, placingthe boundarywithinthe UpperCoumiacquarryin the southeastern Montagne Noire region, Southern France. The GSSP point agreed upon by the Subcommission on Devonian Stratigraphy (SDS) corresponds to the first occurrence of the conodont Palmatolepis triangularis,withinthe Palmatolepislineage,and defines the base of the Famennian [12, 13]. Conodont zonation places the boundary immediately following the end of the linguiformis Zone and at the beginning of the triangularis Zone [1]. The linguiformis Zone is a relativelynewconodontzonethatnowreplaces theupperpartofthenolongerusedgigasZone, with the rest of the gigas Zone now placed within the rhenana Zone. As a consequence of this change, the Upper Devonian for F/F boundary interval, from the uppermost Frasnian to the lowermost Famennianincludes rhenana, linguiformis, triangularis, crepida conodontzones,respectively(Table1). Our work on Upper Devonian sections from Viet Nam has concentrated on the F/F boundaryinterval[4,8‐11]. Asaresult,ab u ndant conodont assemblages have been found within this interval.However,Palmatolepistriangularis, thecriticalindexspeciesdefiningthebeginning of the Famennian was commonly found, while species typically used to define the linguiformis Zonearerelativelyrareandonlyfoundatafew localities (Table 1). Perhaps this is a result of localfaciescontrolontheenvironmentinwhich theseconodontslive. 2. Frasnian/Famennian section interval in Si PhaiPass This interval is belonging to the longer section from Nho Que River to Dong Van (Dong Van District, Ha Giang Province). It extends from the public cistern of Si Phai Village to Si Phai Pass. The lowermost part of this interval is characterized by grey limestone interbedded with argillaceous shale, thin to medium bedded limestone, siliceous limestone andchertwiththicknessofabout40mregarded byDangTranHuyen[2]astheuppermostpart of Si Phai Formation. This part of section contains numerous tentaculites Homoctenus and conodonts belonging to varcus, cristatus, disparilis, and transitans zones.Overlying on the above mentioned rocks are the first limestone layers of the Toc Tat Formation that composed of thin bedded stripped, variegated, interbedded with medium bedded limestone (15‐40cm)greenishandpinkishgreylimestone. Table1.Conodontzoneswithinthefrasnian/famennianboundaryinterval fromtheDongVanandXomNhasections Stage Internationalstandard conodontzones Conodontzonesdiscovered atDongVanSection Conodontzonesdiscovered atXomNhaSection rhomboidea rhomboidea crepida crepida Famennian triangularis triangularis triangularis linguiformis linguiformis linguiformis rhenana rhenana jamieae jamieae Frasnian hassi hassi hassi TaHoaPhuong/VNUJournalofScience,EarthSciences23(2007)187‐193 189 From limestone of thelower part of the Toc Tat Formation, the collected conodont assemblagebelongingtorhenanaandtriangularis zonesareasfollows[11]: 1) Rhenana Zone containing Palmatolepis subrecta Miller & Youngquist; Pa. ljaschenkoae Ovnatanova; Pa. jamieae Ziegler & Sandberg; Pol. uchtensis Ovnatanova & Kuzmin; Pol. lodinensis Polsler; Ancyrodella nodosa Ulrich & Bassler;andPol.brevilamiformisOvnatanova. 2) Triangularis Zone containing Palmatolepis triangularis Sannemann; Pa. superlobata Branson &Mehl;Pa.clarkiZiegler;Pa.delicatuladelicatula Branson & Mehl; Pa. delicatula protorhomboidea Sandberg & Ziegler; Pa. cf. regularis Cooper; and Pa. clarki → Pa. minuta, Pa. minuta minuta Branson &Mehl. Fig.1.TheF/FboundaryintheSiPhai MountainPassSection. Recently,inlimestonelayerstha toriginally are thought to belong to the upper part of the rhenanaZone[10,11],wehavefoundthespecies Palmatolepis linguiformis (S12‐6, S12‐7, S12‐8 samples),thediagnosticspeciesforthelinguiformis Zone.Asaresult,itisnowclearthatwithinthis section,theF/Fboundaryintervalincludestwo maindiagnosticconodontzones, thelinguiformis Zone (highest zone in the Frasnian) and the triangularis Zone (lowest zone in the Famennian).Thesezonescovertwo continuous layers (in beds S12‐8 and S12‐9) at the lowest partoftheTocTatFormation (Fig.1,2). Fig.2.StratigraphiccolumnsoftheF/Fboundary intervalatSiPhaiPassandXomNhasections. 3.Frasnian/FamenniansectionintervalinXom NhaMountain The section is located just inside the front entranceofasouthwesternfacingcavenearthe hamletofXomNha(Fig.3,4).Carbonatelayers, mainly limestone, at this section belong to the Xom Nha For mation (D 3 xn). Until recently, very few macrofossils have been found in this TaHoaPhuong/VNUJournalofScience,EarthSciences23(2007)187‐193 190 section.A minor but interesting Frasnian‐ Famennianassemblageofstromatoporoids and conodontswasfirstdiscoveredherebyNguyen Huu Hung et al [5]. Therefore, for detailed study on this boundary interval, numerous limestone samples were collected bed by bed for conodont study with theaim to resolve the stratigraphicalsequence ofthesection. Fig.3.ThepositionofXomNhaSectionintheWest ofQuangBinhProvince. The stratigraphic sequence and conodont composition in the limestone from the bottom tothetopofthesection(Fig.4,6)areasfollow: 1. Lowermost beds are composed of grey, darkgreylimestone,thickandmediumbedded (20‐60 cm) with dimish layer surface, 1.45 m thick. Conodont comes only from the upper part of beds, belongs to the lower part of the rhenana Zone, and are numbered as 1 and 2 in Fig.3.TheyincludeAncyrodellanodosaUlrich& Bassler; A. ioides Ziegler, Ancyrognathus triangularis Youngquist; Palmatolepis rhenana rhenana Bishoff; Pa. rhenana nasuta Muller; Pa. hassiMuller & Muller; Pa. jamieaeZiegler & Sandberg;andPa.foliaceaYoungquist(X1,X2). 2.Medium‐bedded (20‐75cm),finegrained limestone, intercalated with argillaceous limestone.Abundantconodontoccursfromthe rhenanaZonetocrepidaZone,andfrombottom totop.Theyareasfollows: Fig.4.F/Fboundarylyinginthemiddleofbed9, XomNhaSection. Fig.5.Crosssectionofbed9indicatingtheplates usedforconodontanalysis.TheF/Fboundarylies betweenplates9‐3band9‐4a. TaHoaPhuong/VNUJournalofScience,EarthSciences23(2007)187‐193 191 ‐ Palmatolepis rhenana rhenana Bishopff, Pa. jamieae Ziegler & Sandberg (X3); Ancyrodella nodosa Ulrich & Bassler, Pa. hassi Muller & Muller, Pa. boogaardi Klapper & Foster, Pa. juntinaensis Han, Pa. hassi Muller & Muller, AncyrodellanodosaUlrich&Bassler(X4,X5,X6); Ancyrodella nodosa Ulrich & Bassler, A. ioides Ziegler,Palmatolepis boogaardiKlapper&Foster, Pa.rhenanarhenenaBischopff(X7). ‐ Palmatolepis linguiformis Muller, Pa. rhenana rhenana Bischoff, Pa. rhenana brevis Ziegler, Pa. rhenana nasuta Muller, Pa. gigas Miller & Youngquist, Pa. subrecta Miller & Youngquist, Pa. hassi Muller & Muller, Pa. juntinaensis Han, Pa. ederi Ziegler & Sandgerg, Pa. eureka Ziegler & Sandberg, Palmatolepis foliaceaYoungquist,AncyrodellanodosaUlrich& Bassler, A. ioides Ziegler, Homoctenus sp. (X8); Pa. rhenana rhenana Bischoff, Pa. rhenana nasuta Muller, Pa. gigas Miller & Youngquist, Pa. subrecta Miller & Youngquist, Pa. linguiformis Muller, Ancyrodella nodosa Ulrich & Bassler, PalmatolepistriangularisSannemann(X9). ‐ Palmatolepis triangularis Sannemann, Pa. subperlobata Branson & Mehl, Icriodus alternatus Branson&Mehl.(X10,X11,X12). ‐ Palmatolepis triangularis Sannemann, Pa. subperlobataBranson&Mehl,Pa.delicatulaclarki Ziegler, Pa. delicatula postdelcatula Schulke, IcriodusalternatusBranson&Mehl(X13,X14). ‐ Palmatolepis triangularis Sannemann, Pa. triangularis → crepida, Pa. triangularis → tenuipunctata, Pa. subperlobata Branson & Mehl, Pa. delicatula platys Ziegler & Sandberg, Pa. werneri Ji & Ziegler, Pa. delcatula postdelicatula Schulke, Pa. weddigei Ji & Ziegler, Pa. minuta loba Helms, Pa. quadrantinodosalobata Sannemann, Pa. regularis Cooper, Pa. crepida, Ancyrolepissp.,(X15,X16,X17,X18,X19). From the above mentioned conodonts and their location in the section, the following remarks can be maked for defining F/F boundaryatXomNhaMountain: 1. Seven lowermost beds in the section (from sample X1 to X7, Fig. 4) contain conodonts belonging to the rhenana Zone. The conodonts in the first and second beds belong tothelower rhenanaSubzone,whilethosefrom beds3to7belongto theupperrhenanaSub zone. 2.Inbed8,PalmatolepislinguiformisMuller– principalindexspeciesforthelinguiformisZone wasalsofoundtogetherwiththosefoundinthe rhenanaZone. 3. In bed 9, conodonts of the both linguiformis and triangularis zones were found. Apartfromconodontsfound inthelinguiformis Zone, the principal index species for the triangularis Zone‐Palmatolepis triangularis was alsofound. 4. From bed 10 to bed 14, although fossils are ra re, bu t Pa. triangularis was found in each beds. 5.Beds15to19contain conodontsbelonging totheuppermostpartofthetriangularisZoneas wellasthewholecrepidaZone. TheseresultsindicatethattheF/Fboundary atXomNhaSectionislocatedwithinbed9.For a better definition of the boundary, a block containing bed 9 (33 cm thick) wa s cut into 12 smallplates(2.5cmforeach)withcorresponding labels from the lowermost to the uppermost plates: 9‐1a, 9‐1b ,9‐2a, 9‐2b, 9 ‐3a, 9‐3b, 9‐4a, 9 ‐ 4b, 9‐5a, 9‐5b, 9‐6a, 9‐6b (Fig. 5). These plates were used for conodont extraction, and in plates from 9‐1a to 9‐3b, different species of conodonts belonging to Frasnian were found, includingPa.hassiMuller&Muller,Ancyrodella nodosaUlrich &Bassler,Pa.linguiformisMuller, etc.However,fromplate9‐4a andupward,the individuals of Pa. triangularis Sannemann species were discovered, but the number of individuals of different conodont species decreased quickly. In the beds next to bed 9, along with Pa. triangularis Sannemann, Icriodus sp.andotherconodontsofFamennianagewere also found. Therefore, the F/F boundary has beenplacedbetweenplates9‐3band9‐4ainthe middleof bed9inXomNhaSection. TaHoaPhuong/VNUJournalofScience,EarthSciences23(2007)187‐193 192 Fig.6.DetailedstratigraphiccolumnoftheF/F boundaryintervalatXomNhaSection. 4.Conclusions The F/F boundary in Vietnam has been foundat2places:the SiPhaipassintervalwithin Dong Van‐Nho Que Section (Ha Giang), and inacavemouthnearXomNha(QuangBinh). AtSi PhaiPass,theF/Fboundaryhas been defined between beds S12/8 and beds S12/9, which are belonging to the lower part of the Toc Tat Formation(D 3 ‐C 1 tt). This interval is composed of thin bedded, stripped and variegated limestone beds interbedded with pure,thinandmedium beddedgreyandpink‐ greylimestone. The best locality for the F/F boundary is near Xom Nha in Xom Nha Mountain. The major conodont zones bounding the boundary, rhenana, linguiformis, triangularis and crepida zones, have been found in greyand dark‐grey, medium to thick bedded limestone and argillaceous limestone that belong to the Xom Nha Formation (D 3 xn). The F/F boundary is placedinthemiddleofbed9,betweenplates9‐ 3band9‐4ainthesection. Paleont olo gic al characteristic s and continuity of the sedimentary environment in the F/F Si Phai and Xom Nha boundary sections show that those sections are more close to the Coumiac stratotype in France [3] than to the Steinbruch Schmidt hypostratotype in Germany. In the last section, the change of the sedimentary environment related to the Kellwassereventisdistinctlypronounced[6]. Acknowledgements This paper was completed within the framework of Fundamental Research Program funded by Vietnam Ministry of Science and Technology. Theauthorwouldliketothankhis colleagues Nguyen Huu Hung, Doan Nhat Truong (Institute of Geology and Mineral Resources), Pham Huy Thong (Geological Mapping Division of North Vietnam), Nguyen Thuy Duong, Nguyen Thanh Lan (College of Science,VNU)fortheircooperationduringthis study. The author expresses his deepest acknowledgement to Prof. B.B. Ellwood (Louisiana State University, USA) for his help in editing the English text, and to Prof. Tong Dzuy Thanh (Vietnam National University, Hanoi)forimprovingthispaper. References [1] J.W.Cowie,W.Ziegler,J.Remane,Stratigraphic Commission accelerates progress, 1984 to 1989, Episodes12(1989)79. [2] Dang Tran Huyen, New data on the Devonian stratigraphy in Dong Van area, Journal of TaHoaPhuong/VNUJournalofScience,EarthSciences23(2007)187‐193 193 Geology142(1979)22(inVietnamese). [3] C. Girard, G. Klapper, R. Feist, Subdivision of the terminal Frasnian linguiformis conodont Zone, revision of the correlative interval of Montagne Noire Zone 13 and discussion of stratigrapphically significant associated trilobites, in Understanding Late Devonian and Permian‐Triassic Biotic and Climatic Event: Towards an Intergrated Aproach (Edited by D.J. Over, J.R. Morrow and P.B. Wignall), Elsevier, 2005. 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Criteria defining the F/F boundary and locationwereapprovedbythe ICS, placingthe boundary withinthe