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