ON THE PERMIAN/TRIASSIC BOUNDARY IN VIỆT NAM

ĐOÀN NHẬT TRƯỞNG¹, ĐẶNG TRẦN HUYÊN¹,
NGUYỄN XUÂN KHIỂN¹, TẠ HÒA PHƯƠNG²

¹Research Institute of Geology and Mineral Resources, Thanh Xuân, Hà Nội
²Hà Nội University of Natural Sciences, Nguyễn Trãi, Thanh Xuân, Hà Nội

Abstract: The Meishan D section (Changxing County, Zhejiang, South China) has been ratified as Global Stratotype Section and Point of the Permian/Triassic boundary. The P/T biostratigraphic boundary is defined by first appearance of Hindeodus parvus and the eventostratigraphic boundary - by mass extinction with lithological change. In Việt Nam, two Permian/Triassic boundary sections have been preliminary investigated and the conodont Hindeodus parvus has been found at one of them. Based on the mass extinction of Permian fauna and lithological change recorded from these sections, the eventostratigraphic boundary at both sections has been established. These are sections of potential for studying P/T biostratigraphic boundary in Việt Nam.

I. INTRODUCTION

The Permian/Triassic (P/T) boundary is one of chronostratigraphic boundary most paid attention by geologists, because it is not only the boundary between the Paleozoic and Mesozoic erathems, but also at this boundary there were great events of planet scale causing the mass extinction of about 90-95% of genera and species of the marine and land environments.

Griesbach (1880) is the first scientist discovering the ammonoid Otoceras in Himalaya and considered that it belongs to Triassic. Diener (1912) and Spath (1934) are the first scientists proposing the use of this fossil as marker fauna for determining the base of Triassic, and thus, the boundary between Paleozoic and Mesozoic. Later, this propose was accepted and the P/T boundary has been determined at the base of the Otoceras zone.

According to Yin Hongfu [6], to the 1984 year the base of the Otoceras zone was still largely accepted as the base of the Triassic system. However, in some former years as well as in the 90’ of the past century there still were some authors having doubts on the age of Otoceras by the following reasons:

1) There is not evidence on the higher position of the Otoceras zone in comparing with the Pseudotirolites zone (or Paratirolites zone) – the zone considered as uppermost Permian, because these zones belong to different paleo-biogeographic provinces. Therefore, there is not any locality where there is the succession of Pseudotirolites (or Paratirolites) – Otoceras zones in a same section.

2) The contact between the Otoceras beds and underlying ones is not continuous in all places where Otoceras was found.

3) The lower part of the Otoceras zone bears Permian features, because it contains clearly Permian ammonoids, conodonts and brachiopods.

Based on this, since 1986 Chinese geologists proposed to consider the conodont Hindeodus parvus (Kozur et Pjatakova) as marker fossil for the P/T boundary, in place of Otoceras. The major part of members of P/T Boundary Working Group (PTBWG) has been supporting this variant.

The fossil Hindeodus parvus has far superiority in comparing with Otoceras. It is a Tethysian fauna, while Tethys is the only basin having clearly uppermost beds of Permian. Hindeodus parvus is easily found in a same section over Pseudotirolites, Paratirolites and other Tethysian fossils characterizing Permian. It has rather large distribution, and has been found in China, Kashmir, Salt Range, Transcaucasia, Iran, Austria and Hungary. Particularly in China, it has been found in 19 areas of 9 provinces [6].

Figure 1. Location of studied sections

In 1996, 9 members of the PTBWG proposed to select the Meishan section as global P/T boundary stratotype with the base of Triassic determined by the first appearance of Hindeodus parvus at the bed 27c. From 1999 to 2000, the Meishan section passed through 3 tours of selection by the vote of the PTBWG (Triassic Subcommission, International Commission of Stratigraphy - ICS). Finally, in March 2001 the Executive Commission of International Union of Geosciences - IUGS voted the acceptance of Meishan D section (Changhsing District, Zhejiang Province) as global P/T boundary stratotype. The P/T boundary is determined at the base of the bed 27c, where the marker fauna Hindeodus parvus first appears in the evolutional succession Clarkina changxingensis à Hindeodus latidentatus à Clarkina meishanensis à Hindeodus parvus à Isarciella isarcica, corresponding to 251 Ma [7].

In the Meishan section, last fusulinids were extinguished under the H. latidentatus - C. meishanensis zone (bed 25), while first macrofaunas characterizing Triassic (ophiceratids, Claraia wangi) do not occur yet together with Hindeodus parvus, but later, from the bed 28 upward, i.e. over the P/T boundary about 10 cm.

Table1. Zones of conodonts and ammonoids in the P/T boundary interval

	Conodont zones	Ammonoid zones
TRIASSIC	Zone Isarciella isarcica	Zone Ophiceras
	Zone Hindeodus parvus	Zone Otoceras	Upper Otoceras bed
PERMIAN	Zone H. latidentatus – C. meishanensis		Lower Otoceras bed
	Zone C. changxingensis yini Zone C. changxingensis	Zone Pseudotirolites - Pleuronodoceras

One can recognized that in the stratotype (as well as in many P/T boundary sections in South China) the extinction point of time of main Permian fauna groups and the appearance point of time of Triassic factors do not coincide one with another. Therefore, there are two boundaries for determination: 1) P/T boundary, considered as biostratigraphic boundary determined by the first occurrence of Hindeodus parvus (bed 27c); 2) Eventostratigraphic boundary determined by the mass extinction of characteristic Permian fauna groups (including fusulinids) and by the change (usually sudden) of the lithological composition (bed 25) [3-6]. The biostratigraphic boundary always lies over the eventostratigraphic boundary by a stratigraphic interval. The thickness of this interval changes in dependence to the deep- or shallow-water facies of sediments. In the Meishan and Baoqin sections (Zhejiang Province) this interval is of 19 and 15 cm in thickness (deep-water facies). In the Taiping and Heping sections (Guangxi Province) it is of 90 and 65 cm (shallow-water facies) [3, 5, 7].

Figure 2. Hindeodus parvus (Kozur & Pjatakova) x 200, found at Nhị Tảo section

In Việt Nam, the P/T boundary has been first touched upon in the studies of Đặng Trần Huyên [2]. It has been considered as coinciding with the boundary between the Đồng Đăng (P₃ dd) and Hồng Ngài (T₁ hn) formations. During the execution process of the Project “Stratigraphy of Upper Permian - Lower Triassic, forming conditions and related mineral resources in Bắc Bộ Region” some sections possible for the study on the P/T boundary have been discovered, among them the Nhị Tảo and Lũng Pù sections (Fig. 1) are most perspective ones. Especially, in the Nhị Tảo section members of the Project have been finding the marker fossil Hindeodus parvus. The boundary member of the two above sections are characterized by relatively homogeneous limestone of carbonate shelf facies, therefore, in the field it is difficult to distinguish the boundary between the two Đồng Đăng and Hồng Ngài formations. Investigations in these two sections realized at the beginning of 2004 have been allowing to describe in details these sections.

II. NHỊ TẢO P/T BOUNDARY SECTION

The Nhị Tảo P/T boundary section is situated near the cistern of the Nhị Tảo Village, in 13 km northwest of the Trà Lĩnh Townlet (Cao Bằng Province). The section is characterized by carbonate sediments of shelf facies containing in abundance benthonic faunas as follows (in ascending order):

Bed 1: organic limestone, dark-grey cherty limestone, 38 cm thick, containing the foraminiferas Reichelina sp., Nankinella sp., Neoendothyra sp., Dagmarita liantangensis, Frondina permica, Globivalvulina sp.1, Paraglobivalvulina gracilis.

Bed 2: dark-grey cherty limestone, 43 cm thick, containing the foraminiferas Dagmarita sp., Frondina sp., Cribrogenerina casta, Cr. guangxiensis.

Bed 3: dark-grey organic limestone, 8 cm thick.

Bed 4: dark-grey cherty limestone, 4.5 cm thick.

Bed 5: organic limestone, cherty limestone, 41 cm thick, containing the foraminiferas Reichelina sp., Nankinella sp., Baisalina pulchra.

Bed 6: organic limestone, cherty limestone, 25 cm thick, containing the foraminiferas Reichelina sp., Nankinella sp., Baisalina pulchra.

Bed 7: dark-grey argillaceous limestone, 10 cm thick, containing the foraminiferas Nankinella pulchra.

Bed 8: oolitic limestone, 12 cm thick, containing organic residues and the foraminiferas Reichelina sp., Nankinella sp., Neoendothyra sp., Dagmarita sp., Globivalvulina sp. 1, Baisalina pulchra, Hemigordius sp..

Bed 9: dark-grey limestone containing organic residues, 8 cm thick.

Bed 10: dark-grey limestone, 10.5 cm thick, containing organic residues and the foraminiferas Reichelina pulchra, Nankinella orbicularia, Dagmarita chanakchiensis, D. lintangensis, Globivalvulina sp.1, Frondina sp..

Bed 16: grey, poorly dolomitic limestone, 10 cm thick.

Bed 17: grey, poorly dolomitic limestone, 18 cm thick.

Bed 18: grey, poorly dolomitic limestone, 6 cm thick.

Bed 19: grey, irregularly recrystallized, poorly dolomitic limestone, 14 cm thick, containing microgastropods.

Bed 20: grey, irregularly recrystallized, poorly dolomitic limestone, 14 cm thick.

The marker fossil Hindeodus parvus (Fig. 2), determined by Tạ Hòa Phương, was found in a limestone bed lying about 7 m above the bed 20.

For determining the P/T boundary in a certain area, it needs altogether obey the standards confirmed in the global stratotype, it means it is obliged to find conodont fossils in the evolutionary succession Clarkina changxingensis à Hindeodus latidentatus à Clarkina meishanensis à Hindeodus parvus à Isarciella isarcica for determining the first appearance of Hindeodus parvus. Although in the Nhị Tảo section the fossil Hindeodus parvus was found, but basing on this only finding we cannot determine the P/T boundary. Furthermore, in this level Đặng Trần Huyên [2] has been collecting the bivalve Eumorphotis multiformis, corresponding to the Claraia stachei zone, which is higher than the Claraia wangi zone. Therefore, one can say that the bed containing Hindeodus parvus at Nhị Tảo belongs to the H. parvus zone, but does not correspond to the base of Triassic system and does not create the standard for determining the P/T boundary.

However, in the Nhị Tảo section the eventostratigraphic boundary has been determined on the basis of the change of lithological and faunal characteristics. They happened between the beds 10 and 11 (Fig. 3). After the bed 10 all the Permian foraminiferas, such as Reichelina, Nankinella, Frondina, Dagmarita, etc., disappear in the section. On the lithological side, there is a clear change at this level: the section changes from limestone containing organic residues to dolomitic limestone. This change reflects an important event in the nature, annihilating in a short time and a large area many faunal groups, as well as changing the forming conditions of sediments. On the cause of this event, there has been being different hypotheses, including even cosmic strike [1], however, this problem will be touched upon in other occasion.

III. LŨNG PÙ P/T BOUNDARY SECTION

The Lũng Pù P/T boundary section lies in about 1.5 km west of Sảng Chải Village, about 7 km southeast of the Mèo Vạc Townlet (Hà Giang Province) (Fig. 4). It is characterized also by carbonate shelf facies, including following beds (in ascending order):

Fig. 5. Stratigraphic column and distribution of foraminiferas
in the P/T boundary section at Lũng Pù

Note: 1. Limestone, 2. Organic limestone, 3. Dolomitic limestone, 4. Cherty limestone,

5. Argillaceous limestone; RGSKĐT: eventostratigraphic boundary

Bed 1: grey limestone bearing organic residues, 9 cm thick, containing the foraminifera Nankinella sp..

Bed 2: grey limestone bearing organic residues, 13 cm thick, containing the foraminiferas Nankinella orbicularia, Hemigordius nanus, Frondina permica, Dagmarita sp., Cribrogenerina sp..

Bed 3: grey limestone bearing organic residues, 41 cm thick, containing the foraminiferas Nankinella orbicularia, Hemigordius nanus, Paraglobivalvulina sp., Dagmarita sp., Deckerella clavata.

Bed 4: grey limestone bearing organic residues, 41 cm thick, containing the foraminiferas Paraglobivalvulina sp., Gobivalvulina sp., Pachyphloia sp..

Bed 5a: grey limestone bearing organic residues, 5 cm thick, containing the foraminiferas Reichelina minuta, Dagmarita chanakchinensis, Nankinella sp..

Bed 5b: grey, fine-grained dolomitic limestone, 5 cm thick.

Bed 6: grey, fine-grained dolomitic limestone, 16.5 cm thick, containing microgastropods and the foraminifera Ammodiscus parapriscus.

Bed 7: grey, irregular-crystalline dolomitic limestone, 9 cm thick.

Bed 8: grey, rich in dolomite limestone, 17 cm thick.

Bed 9: grey dolomitic limestone, 12cm thick.

Bed 10: grey dolomitic limestone, 13.5 cm thick.

Bed 11: grey, rich in dolomite limestone, 12 cm thick, containing microgastropods and the foraminifera Ammodiscus parapriscus.

Bed 12: grey dolomitic limestone, 9 cm thick, containing microgastropods and the foraminifera Ammodiscus parapriscus.

Bed 13: grey limestone, 9 cm thick.

Bed 14: grey, partially dolomitized limestone bearing organic residues, 9 cm thick, containing the foraminifera Ammodiscus parapriscus.

Bed 15: grey, fine-grained dolomitic limestone, 10 cm thick, containing microgastropods and the foraminifera Ammodiscus parapriscus.

Bed 16: grey, poor in dolomite limestone, 22 cm thick, containing microgastropods.

Bed 17: grey, irregular-crystalline dolomitic limestone, 21 cm thick, containing alga and microgastropod remains.

Bed 18: grey, poor in dolomite limestone, 14 cm thick, containing alga and microgastropod remains.

Bed 19: grey, poor in dolomite limestone, 13 cm thick, containing microgastropods and the foraminifera Ammodiscus parapriscus.

Bed 20: grey, poor in dolomite limestone, 27 cm thick.

Bed 21: grey, poor in dolomite limestone, 7 cm thick, containing microgastropods.

Bed 22: grey, poor in dolomite limestone, 6 cm thick.

Bed 23: grey, poor in dolomite limestone, 42 cm thick, containing microgastropods.

In the Lũng Pù section, the P/T biostratigraphic boundary has not been also determined, because conodont fossils have not been found yet, and only the eventostratigraphic boundary has been determined. It lies between the 5a and 5b bed (Fig. 5). The beds from 1 to 5a contain in abundance Permian foraminiferas, such as Reichelina, Dagmarita, etc.. They disappear from the bed 5b upward. On the lithological side there is also a change between the two 5a and 5b beds. The beds from 1 to 5a consist mainly of limestone bearing organic residues, while from the bed 5b upward there is only fine-grained dolomitic limestone; the contact surface between the two rock types is of suture form (Fig. 6). As in the Nhị Tảo section, the change between the two 5a and 5b beds in the Lũng Pù section is the imprint of an important event in the nature, causing the mass extinction of many organic groups and the change of forming conditions of sediments.

IV. DISCUSSION AND CONCLUSIONS

The two above described sections show that only the P/T eventostratigraphic boundary has been determined. It is also the boundary between the two Đồng Đăng and Hồng Ngài Formations. The posed problem is that, in Việt Nam (concretely, the two Nhị Tảo and Lũng Pù sections) is there the P/T biostratigraphic boundary? There are two possibilities:

1) There is a stratigraphic gap at the eventostratigraphic boundary. In this case there is only this boundary, as in the case of the Hồng Ngài section. There, beds of argillaceous limestone containing Claraia wangi, Glyptophiceras sp. belonging to the base of the Hồng Ngài Formation rests directly upon limestone containing Late Permian fusulinids of the Đồng Đăng Formation;

2) There was a continuous sedimentary deposition or inconsiderable gap at the eventostratigraphic boundary. In this case, over it there will be the biostratigraphic boundary. This lies in the basal part of the Hồng Ngài Formation, that means the P/T boundary does not coincide with that between the two Đồng Đăng and Hồng Ngài Formations.

Correlating with the Taiping P/T boundary section, Guangxi, China [3] situated in about 150 km northeast of Nhị Tảo in the same limestone bands extending from China to Việt Nam we notice that the Nhị Tảo and Lũng Pù sections have the same type of section with the same lithological and faunal composition. There is much possibility that the two Nhị Tảo and Lũng Pù sections belong to the above said second case, it means the P/T biostratigraphic boundary can be found over the eventostratigraphic boundary a certain interval in the section. The continuation of investment for more detailed study on this possibility is very necessary.

Acknowledgement. The paper is realized with the support of the Basic Research Program and the contribution of precious ideas from colleagues of the Division of Paleontology and Stratigraphy, Research Institute of Geology and Mineral Resources. The authors express their deep thanks.

REFERENCES

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Zone Isarciella isarcica

Zone Ophiceras

Zone Hindeodus parvus

Zone Pseudotirolites - Pleuronodoceras