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

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

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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 3xn). 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 Stratotype

Section and Point (GSSP) is now used as the

stratotype for each chronostratigraphic

boundary defining each increment of geologic

_

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 sedimentation

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,

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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, abundant

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

Famennian

triangularis triangularis triangularis linguiformis linguiformis linguiformis

Frasnian

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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 that 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 Formation (D3 xn). Until recently,

very few macrofossils have been found in this

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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.

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‐ 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).

subperlobata Branson & Mehl, Pa. delicatula clarki

Ziegler, Pa. delicatula postdelcatula Schulke,

Icriodus alternatus Branson & Mehl (X13, X14).

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 Subzone.

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 rare, but 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) was 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.

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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 (D3‐C1 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 (D3 xn). The F/F boundary is

placed in the middle of bed 9, between plates 9‐ 3b and 9‐4a in the section.

Paleontological characteristics 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

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Tiêu đề	Two frasnian/famennian boundary sections in vietnam
Tác giả	Ta Hoa Phuong
Trường học	Vietnam National University
Chuyên ngành	Earth Sciences
Thể loại	báo cáo
Năm xuất bản	2007
Thành phố	Hanoi

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