Báo cáo " Two Frasnian/Famennian boundary sections in Vietnam " potx

VNUJournalofScience,EarthSciences23(2007)187‐193 187 TwoFrasnian/FamennianboundarysectionsinVietnam TaHoaPhuong* CollegeofScience,VNU Received02July2007 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 KimPass(CaoBang), NoongDePass(Nghe An), and SiPhaiPass, MaPiLenPass(Ha Giang).However,theF/Fboundarywith boththe linguiformisandtriangularis conodont zones that bracket the boundary hasonlybeenidentified 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 bedsS12/8andS12/9, within the lower part of Toc TatFormation(D 3 ‐C 1  tt). This part of the section is composed of thin bedded, stripped and colorful limestone interbedded with pure, thinandaveragebeddedgreyandpink‐greylimestone. 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 inagreyanddark‐grey, averagetothickbeddedlimestoneandclayeylimestonesequence thatbelongstotheXomNhaFormation(D 3 xn).HeretheF/Fboundaryhasbeendeterminedtolie betweenbeds9‐3band9‐4awithinbed9intheXomNhaSection. Keywords:Conodontfossils;LinguiformisZone;TriangularisZone;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,thereisaspecificfossillineage,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 CommissiononStratigraphy(ICS). Finding a GSSP that satisfies the required paleontologicalcriteriaisverydifficultbecause 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 facieschanges. The GSSP established the end of the FrasnianStageandbeginningoftheFamennian Stage(F/Fboundary)ofUpperDevonianattract attention in international stratigraphical circle, TaHoaPhuong/VNUJournalofScience,EarthSciences23(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 locationwereapprovedbythe ICS, placingthe boundarywithinthe UpperCoumiacquarryin 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,withinthe Palmatolepislineage,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 relativelynewconodontzonethatnowreplaces theupperpartofthenolongerusedgigasZone, 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 Famennianincludes rhenana, linguiformis, triangularis, crepida conodontzones,respectively(Table1). Our work on Upper Devonian sections from Viet Nam has concentrated on the F/F boundaryinterval[4,8‐11]. Asaresult,ab u ndant conodont assemblages have been found within this interval.However,Palmatolepistriangularis, thecriticalindexspeciesdefiningthebeginning of the Famennian was commonly found, while species typically used to define the linguiformis Zonearerelativelyrareandonlyfoundatafew localities (Table 1). Perhaps this is a result of localfaciescontrolontheenvironmentinwhich theseconodontslive.  2. Frasnian/Famennian section interval in Si PhaiPass 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 andchertwiththicknessofabout40mregarded byDangTranHuyen[2]astheuppermostpart 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 (15‐40cm)greenishandpinkishgreylimestone. Table1.Conodontzoneswithinthefrasnian/famennianboundaryinterval fromtheDongVanandXomNhasections Stage Internationalstandard conodontzones Conodontzonesdiscovered atDongVanSection Conodontzonesdiscovered atXomNhaSection rhomboidea rhomboidea crepida crepida  Famennian triangularis triangularis triangularis linguiformis linguiformis linguiformis rhenana rhenana jamieae jamieae  Frasnian hassi hassi hassi TaHoaPhuong/VNUJournalofScience,EarthSciences23(2007)187‐193 189 From limestone of thelower part of the Toc Tat Formation, the collected conodont assemblagebelongingtorhenanaandtriangularis zonesareasfollows[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;andPol.brevilamiformisOvnatanova. 2) Triangularis Zone containing Palmatolepis triangularis Sannemann; Pa. superlobata Branson &Mehl;Pa.clarkiZiegler;Pa.delicatuladelicatula Branson & Mehl; Pa. delicatula protorhomboidea Sandberg & Ziegler; Pa. cf. regularis Cooper; and Pa. clarki → Pa. minuta, Pa. minuta minuta Branson &Mehl.  Fig.1.TheF/FboundaryintheSiPhai MountainPassSection. Recently,inlimestonelayerstha toriginally are thought to  belong to the upper part of the rhenanaZone[10,11],wehavefoundthespecies Palmatolepis linguiformis (S12‐6, S12‐7, S12‐8 samples),thediagnosticspeciesforthelinguiformis Zone.Asaresult,itisnowclearthatwithinthis  section,theF/Fboundaryintervalincludestwo maindiagnosticconodontzones, thelinguiformis Zone (highest zone in the Frasnian) and the triangularis Zone (lowest zone in the Famennian).Thesezonescovertwo continuous layers (in beds S12‐8 and S12‐9) at the lowest partoftheTocTatFormation (Fig.1,2).  Fig.2.StratigraphiccolumnsoftheF/Fboundary intervalatSiPhaiPassandXomNhasections. 3.Frasnian/FamenniansectionintervalinXom NhaMountain The section is located just inside the front entranceofasouthwesternfacingcavenearthe hamletofXomNha(Fig.3,4).Carbonatelayers, 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 TaHoaPhuong/VNUJournalofScience,EarthSciences23(2007)187‐193 190 section.A minor but interesting Frasnian‐ Famennianassemblageofstromatoporoids and conodontswasfirstdiscoveredherebyNguyen 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 theaim to resolve the stratigraphicalsequence ofthesection.  Fig.3.ThepositionofXomNhaSectionintheWest ofQuangBinhProvince. The stratigraphic sequence and conodont composition in the limestone from the bottom tothetopofthesection(Fig.4,6)areasfollow: 1. Lowermost beds are composed of grey, darkgreylimestone,thickandmediumbedded (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.TheyincludeAncyrodellanodosaUlrich& Bassler; A. ioides Ziegler, Ancyrognathus triangularis Youngquist; Palmatolepis rhenana rhenana Bishoff; Pa. rhenana nasuta Muller; Pa. hassiMuller & Muller; Pa. jamieaeZiegler & Sandberg;andPa.foliaceaYoungquist(X1,X2). 2.Medium‐bedded (20‐75cm),finegrained limestone, intercalated with argillaceous limestone.Abundantconodontoccursfromthe rhenanaZonetocrepidaZone,andfrombottom totop.Theyareasfollows:  Fig.4.F/Fboundarylyinginthemiddleofbed9, XomNhaSection.  Fig.5.Crosssectionofbed9indicatingtheplates usedforconodontanalysis.TheF/Fboundarylies betweenplates9‐3band9‐4a. TaHoaPhuong/VNUJournalofScience,EarthSciences23(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, AncyrodellanodosaUlrich&Bassler(X4,X5,X6); Ancyrodella nodosa Ulrich & Bassler, A. ioides Ziegler,Palmatolepis boogaardiKlapper&Foster, Pa.rhenanarhenenaBischopff(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 foliaceaYoungquist,AncyrodellanodosaUlrich& 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, PalmatolepistriangularisSannemann(X9). ‐ Palmatolepis triangularis Sannemann, Pa.  subperlobata Branson & Mehl, Icriodus alternatus Branson&Mehl.(X10,X11,X12). ‐ Palmatolepis triangularis Sannemann, Pa. subperlobataBranson&Mehl,Pa.delicatulaclarki Ziegler, Pa. delicatula postdelcatula Schulke, IcriodusalternatusBranson&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, Ancyrolepissp.,(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 boundaryatXomNhaMountain: 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 tothelower rhenanaSubzone,whilethosefrom beds3to7belongto theupperrhenanaSub zone. 2.Inbed8,PalmatolepislinguiformisMuller– principalindexspeciesforthelinguiformisZone wasalsofoundtogetherwiththosefoundinthe rhenanaZone. 3. In bed 9, conodonts of the both linguiformis and  triangularis zones were found. Apartfromconodontsfound inthelinguiformis Zone, the principal index species for the triangularis Zone‐Palmatolepis triangularis was alsofound. 4. From bed 10 to bed 14, although fossils are ra re, bu t Pa. triangularis was found in each beds. 5.Beds15to19contain conodontsbelonging totheuppermostpartofthetriangularisZoneas wellasthewholecrepidaZone. TheseresultsindicatethattheF/Fboundary atXomNhaSectionislocatedwithinbed9.For a better definition of the boundary, a block containing bed 9 (33 cm thick) wa s cut into 12 smallplates(2.5cmforeach)withcorresponding 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, includingPa.hassiMuller&Muller,Ancyrodella nodosaUlrich &Bassler,Pa.linguiformisMuller, etc.However,fromplate9‐4a andupward,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.andotherconodontsofFamennianagewere also found. Therefore, the F/F boundary has beenplacedbetweenplates9‐3band9‐4ainthe middleof bed9inXomNhaSection. TaHoaPhuong/VNUJournalofScience,EarthSciences23(2007)187‐193 192  Fig.6.DetailedstratigraphiccolumnoftheF/F boundaryintervalatXomNhaSection. 4.Conclusions The F/F boundary in Vietnam has been foundat2places:the SiPhaipassintervalwithin Dong Van‐Nho Que Section (Ha Giang), and inacavemouthnearXomNha(QuangBinh). AtSi PhaiPass,theF/Fboundaryhas 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,thinandmedium beddedgreyandpink‐ greylimestone. 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 greyand 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 placedinthemiddleofbed9,betweenplates9‐ 3band9‐4ainthesection. 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 Kellwassereventisdistinctlypronounced[6]. Acknowledgements This paper was completed within the framework of Fundamental Research Program funded by Vietnam Ministry of Science and Technology. Theauthorwouldliketothankhis 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)fortheircooperationduringthis 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)forimprovingthispaper. References [1] J.W.Cowie,W.Ziegler,J.Remane,Stratigraphic Commission accelerates progress, 1984 to 1989, Episodes12(1989)79. [2] Dang Tran Huyen, New data on the Devonian stratigraphy in Dong Van area, Journal of TaHoaPhuong/VNUJournalofScience,EarthSciences23(2007)187‐193 193 Geology142(1979)22(inVietnamese). [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. [4] Nguyen Cong Thuan, Ta Hoa Phuong, Ne w dataontheageofthebaseofTocTatFormation in Ha Lang area, Cao Bang Province, VNU JournalofScience,NaturalSciencesandTechnology No.3 /XVIII(2002)87(inVietnamese). [5] Nguyen Huu Hung, Pham Kim Ngan et al., DiscoveringtheFrasnian‐Famennianlimestones (Upper Devonian) in Quy Dat area, Binh Tri Thien, Jo urnal of Earth Sciences 4/2 (1980) 27 (in Vietnamese). [6] E.Schindler,Event‐stratigraphicmarkerswithin theKellwasser Crisisnearth eFrasnian/Famennian  boundary (upper Devonian) in Germany, Palaegeography, Palaeoclimatology, Palaeoecology 104(1993)115. [7] Ta Hoa Phuong, New discovery of Devonian and Lower Carboniferous pelagic fossils in Dong Van area (Ha Giang Province, Vietnam), Proceedings of the Int. Symp. on Strat. Correl. of SoutheastAsia,Bangkok(1994)62. [8] Ta HoaPhuong,DoanNhatTruong,Preliminary studiesontheboundariesofFamennianstagein Vietnam,JournalofGeology,seriesB5‐6(1995)94 (inVietnamese). [9] Ta Hoa Phuong, Nguyen Huu Hung, Upper Devonian Frasnian/Famennian boundary in the XomNhalimestone,QuyDatarea,QuangBinh Province, Journal of Geology  238 (1997) 5 (in Vietnamese). [10] Ta Hoa Phuong, Devonian and Carboniferous stratigraphy in Dong Van Section, Ha Giang Province, Journal of Geology Special Issue (2000) 2(inVietnamese). [11] Ta Hoa Phuong, Devonian and Carbonifer ous conodontbiostratigraphyofDongVanSe ction ,Ha Giang Provin ce,Jo urnal of Geology, series B  19‐20 (2002)14(inVietnamese). [12] W. Ziegler, G. Klapper, Stages of the Devonian system, Episodes (International Geosciene News Magazine)2/8(1985)15. [13] W. Ziegler, C.A. Sandberg, The Late Devonian standard conodont zonation, Cour. Forsch. Inst. Senckenberg121(1990)1. . fossils belonging to Frasnian/Famennian (F/F) boundary interval have been discovered within several geologic sections in Vietnam,  including Ngoc. happened in organisms living at that time. Criteria defining the F/F boundary and locationwereapprovedbythe ICS, placingthe boundary withinthe