Brachiopods from the Hirnantian Stage at Shwe Thin area ...mynces.org/download/2018/ProceedingMNCES2018/53_Kyi... · the Ordovician-Silurian boundary beds in a wide region of the

692 The Second Myanmar National Conference on Earth Sciences (MNCES, 2018)

November 29-30, 2018, Hinthada University, Hinthada, Myanmar

1 Lecturer, Department of Geology, Meiltila University, e-mail: [email protected] 2 Demonstrator, Department of Geology, Kyaing Tong University 3 Assistant Lecturer, Department of Geology, University of Mandalay

Brachiopods from the Hirnantian Stage at Shwe Thin area, Pyin Oo Lwin

Township, Mandalay Region, Myanmar

Kyi Pyar Aung1, San Aye

2 and Thandar Aung

3

Abstract

The new section is located near Shwe Thin, 6.5 km northeast of Pyin Oo Lwin Township, Mandalay Region. The Hirnantian at this site is conformably underlying by the Katian (Upper

Ordovician) and overlain by the Rhuddanian (Lower Silurian), all of which are very

fossiliferous. The newly observed fauna consists of Dalmanella sp., Dalmanella testudinaria,

Hirnantia sp., Hirnantia sagittifera, Kinnella medlicotti, Kinnella kinelane, Paromalomena

sp., Plectothyrella crassicesta, Onniella? sp., Dysprorthis sp., Dysprosorthis cf. sinensis,

Leptaena sp., Paracraniops sp., Fardenia sp. and Aeginomena? sp. indicating a Hirnantian

age. The presence of the Hirnantia fauna at this new level indicates a significant cooling and

eustatic fall in the sea level resulting in a shallow subtidal environment. According to the

brachiopods and other related shelly fossils, the Myanmar fauna mentioned above shares a

very close relationship with those south Thailand and South China, all of which belong to

peri-Gondwanan paleoplate, i.e., the Sibumasu.

Keywords: Hirnantia Fauna, Brachiopod, Ordovician, peri-Gondwanan, Sibumasu

Introduction

The Hirnantian (latest Ordovician) crisis initiated major extinctions in virtually all the

well-known fossils groups of marine invertebrates, marking a significant interval of faunal

turnover at the close of the Ordovician. The Latest Ordovician extinction was thought to be

the second most severe extinction event in the history of Metazoan life exceeded only by the

end of Permian event (Rong et al., 2002). Mainly shallow-water Hirnantian marine biotas,

with the exception of those in the tropical zone, were brachiopod-dominated communities

associated with soft substrates. Hirnantia fauna (brachiopods) were abundant and dominant.

They evolved in many areas of the world during the Hirnantian (Rong and Harper, 1988;

Owens et al., 1991). This is mainly a cold/cool water facies fauna developed during the

Hirnantian Substage; it is of great importance for international correlation (Rong and Harper,

1988; Cocks, 1988; Rong and Harper, 1999).

However, accurate correlation of this shelly fauna with graptolite zonation remains

unsatisfactory. Wright (1968) first pointed out the possibility of diachronism of the Hirnantia

fauna. Rong (1979, 1984a) and Mu and Rong (1983) concluded that the brachiopod fauna

occupied different stratigraphical levels within the latest Ordovician in different places,

indicating its distribution to be diachronous. However, correlation of the lower and upper

boundaries of the Hirnantia fauna-bearing beds with graptolite facies is not precise.

Regressive sequences through the critical upper Ordovician in many parts of the world

usually lack graptolites facies and correlation of such sequences with graptolite zonation is

difficult (Fortey, 1989). It is essential to define the precise age of the latest Ordovician

extinction and thus the range of the glacial maximum.

The Shan Plateau, part of Sibumasu is a considerable number of continuous sections

across the Ordovician-Silurian boundary. They have yielded both graptolites and shelly

biotas, including abundant Hirnantia faunas associated with the trilobite Dalmanitina and

many other fossils (Cocks et al., 2002; Kyi Pyar Aung et al., 2017a, b). Correlations and

diacronism of the Hirnantia fauna is indeed of farther revision. The purpose of this paper is

The Second Myanmar National Conference on Earth Sciences (MNCES, 2018) 693


to update and document the range of the Hirnantian fauna in various localities of Shan

Plateau in Sibumasu. We report the discovery of the Hirnantia fauna in Shan Plateau. In

addition the find introduces new and interesting biogeographic and biostratigraphy data about

the Ordovician-Silurian boundary beds in a wide region of the Shan Plateau, with some

implications for the correlation within the northern Gondwana region. A detailed

reconstruction of the distribution of climatic belts on peri-Gondwana Sibumasu is also

attempted based on the known Hirnantian brachiopod assemblages.

Location

The new section is located near Shwe Thin, 6.5 km northeast of Pyin Oo Lwin

Township, Mandalay Region. The present area which forms part of the eastern Highland is

bounded by latitude 96º 30 N

to 96º 35

N and Longitude 21º 55

' 30

'' E to 22º 00

' E in one-

inch topographic map 93 C/9. This area is formerly known as Pa-Thin and renamed „Shwe

Thin‟ village. It is easily accessible throughout the year. The location map of the study area

and its environ is shown in figure (1).

Figure (1). Location map of the study area.

Materials and Methods

The five key sections across Ordovician-Silurian boundary we examine in 2016 are

Panghsa-pye, Pinte, Shwe Thin (Pa-Thin), Nyannyintha, Linhpunlay (Fig. 2). We need to

examine the Hirnantia fauna of these sections in detail. So the Shwe Thin (Pa-Thin) section

containing the Hirnantia fauna with graptolite control are also briefly reviewed and compared

with Panghsa-pye section herein. This paper is based on the study of the Shwe Thin (Pa-Thin)

section along the road between Shwe Thin (Pa-Thin) village and Yebyantaung Waterfall. The

palaeontological studies have been undertaken on more than 100 samples (specimens) to

determine the name of macrofauna and the age of each studied interval by fluctuation in the

diversity and abundance of the whole macrofossils throughout the measured succession.



These allowed the identification of the palaeoenvironments and their vertical distribution. All

the sections were measured in road cutting.

Figure (2). Map showing the five Key (Hirnantia) sections of Shan Plateau Region.

Result

Stratigraphic Setting

The present area is essentially composed of the Paleozoic rocks ranging in age from

Ordovician to Devonian. According to the present stratigraphical understanding, the

Hirnantia fauna-bearing unit occurs together with overlying Graptolite-bearing shale

sequence, Panghsa-pye Formation. The uppermost unit of Ordovician, Hwe Mawng Purple

Shale Member has a typical lithology of buff, whitish shale-mud, and quartzitic bed

containing abundant brachiopods, trilobites and a few other fossil groups (shelly band) (Figs.

3-5). The name “Hwe Mawng Purple Shale Member” is now preferred to use because of its

realistic and more complete description.

As described in more detail below, the fauna of the shelly band near the base of the

graptolite-bearing sequence (Panghsa-pye Formation) consists of the following:

Brachiopods: Dalmanella sp., Dalmanella testudinaria, Hirnantia sp.,Hirnantia sagittifera,

Kinnella medlicotti, Kinnella kinelan, Paromalomena sp., Plectothyrella crassicesta,

Onniella? sp., Dysprorthis sp., Dysprosorthis cf. sinensis, Leptaena sp., Paracraniops sp.,

Fardenia sp., Aeginomena? sp.

As to age, on external criteria, the shelly band (Hwe Mawng Purple Shale Member)

near the base of graptolite-bearing sequence is overlain by deposits containing certainly Early

(Rhuddanian Zone), Llandovery graptolite, is Latest Ordovician (Hirnantian stage) (Fig. 6).



Figure (3). Outcrop nature of Ordovician-Silurian boundary showing Hirnantia fauna-

bearing unit (Hwe Mawng Purple Shale Member) overlies with the oldest

Silurian graptolite-bearing unit (Panghsa-pye Formation)

Comparison with Panghsa-pye Section

The brachiopods described from the Panghsa-pye Beds of Burma by Reed (1915), can

be identified in modern terms by Cocks and Fortey (2002) as follows: Kinnella medlicotti,

Onniella ?yichangensis, Aegiromena? planissima, Eostropheodonta hirnantensis, Coolinia

sp., Dalmanella mansuyi, Pseudopholidops sp., and Paromalomena mcmahoni and Trilobites

: Eoleonaspis shanensis, Mucronaspis mucronata. Thus of the eight brachiopods (Panghsa-

pye Bed) confirmed from Burma, all except Eostropheodonta and Kinnella have been

recovered from Thailand, and only Mirorthis, Cliftonia and Eospirigerina from Thailand are

not known from Burma. From the underlying Naungkangyi Beds of Burma, a brachiopod

fauna is known (Reed, 1915) that includes elements like Oepikina and Plectella which also

occur in apparently contemporary beds elsewhere in Satun Province of Thailand (Hamada,

1964).

Discussion

(1) The Hirnantia Fauna is a distinctive widespread brachiopod fauna in the terminal

Ordovician Hirnantian Stage. The Hirnantia Fauna, although widespread, is not a

cosmopolitan one because it is absent in the tropical belt. Recently, shells of the

brachiopods Hirnantia sagittifera (M‟Coy, 1851) and Kinnella kielanae (Temple, 1965)

were found in the claystones and siltstones of uppermost unit of Kyaingtaung Formation

(now called Hwe Mawng Purple Shale Member).

(2) The orthide brachiopod genus Kinnella, commonly represented by the species Kinnella

kielanae (Temple, 1965), is one of the diagnostic components of the Hirnantia fauna,

known from highest Ordovician strata worldwide. Prior to this study, no pre-Hirnantian

representatives of Kinnella were known, although it has been long realized that other

Graptolite-bearing unit

Hirnantia fauna-bearing unit

Latest Ordovician Unit

Earliest Silurian Unit



common or key genera of the Hirnantia fauna originated much earlier in the Ordovician

and continued well into the Early Silurian (Cocks, 19899; Rong and Harper1988).

Kinnella first appeared at low latitudes, possibly in relatively shallow, warmer water epi-

continental environments and subsequently migrated to cooler water setting to become a

common element of the Hirnantia fauna.

Figure (4). Brachiopods from Pathin Section: A.E.F.G.I-K.M.U-W. Kinnella kinelanae

kinelanae, B.C.P.Q. Dalmanella sp., D. Dalmanella testudinaria, H. Hirnantia

sp., L.R. Onniella? sp., N.O. Paromalomena sp., S.X. Dysprosorthis cf. sinensis,

T. Kinnella medlicotti.

5 mm

3 mm

L

5 mm

A B

H G

F E

I J K

M N

D

P O

C

5 mm 5 mm 5 mm

5 mm 5 mm 5 mm 5 mm

5 mm 5 mm 5 mm

3 mm 5 mm 5 mm

T Q R S

W V X

5 mm 3 mm 5 mm

3 mm

3 mm

U

3 mm 3 mm 3 mm



Figure (5) Brachiopods from Pa-Thin Section: A-B.G.L. Kinnella kinelanae, C.

Plectothyrella crassicesta, D. Leptaena sp., E. Paracraniops sp., F.R. Hirnantia

sp., H. Onniella? sp., I.M.O. Dalmanella sp., J. Fardenia sp., K. N.W. Hirnantia

sagittifer, P. Dalmanella testudinaria, Q. Paromalomena sp., S-V. Kinella

medlicotti.

(3) As a characteristic taxon of the Shan Plateau Hirnantia fauna, Kinnella has been known

to occur in cool-water, siliciclastic-dominated depositional setting at the periphery of low-

latitude paleocontinents, as well as wider range of environments in association with

higher latitude Gondwanan and peri-Gondwanan terranes (Fig. 6) (Rong and Harper,

1988, 1999; Owens et al., 1991;Cocks and Torsvik, 2002). Most of the Hirnantian fauna

were interpreted to have inhabited relatively low energy settings below fair weather wave

base, with the fine-grained, soft, siliciclastic substrate (Rong and Happer, 1988, 1999).

3 mm

A B

H G F E

I J K L

D C

3 mm 3 mm 3 mm 3 mm

3 mm 3 mm 3 mm 3 mm

3 mm 3 mm 3 mm

3 mm 3 mm

3 mm

T

3 mm

W V

M N

P Q R S

O

3 mm

3 mm 3 mm

3 mm 3 mm 3 mm

U

3 mm



Figure (6). Stratigraphic correlation of northern Shan State including present area with

southern Shan State showing Ordovician–Silurian boundary ( Hwe Mawng

Purple Shale Member; Tanshauk Member)

(4) The Hirnantian deposits of the Shan Plateau are identified by their stratigraphical

position: below the oldest Silurian black graptolitic shales, and above the uppermost

Ordovician fossiliferous rocks.

(5) The basal contact of the Hirnantian rocks is sometimes an erosional surface above

various units of different ages. The most conspicuous deposits attributed to the

Hirnantian are the ubiquitous glaciomarine diamictites and other widespread light-

coloured quartzite units.

(6) The occurrence of the Hirnantia fauna is also shown to be diachronous in various places,

although more work on their precise correlation is required. Of them, most (Dalmanell,

Leptaena, Eostrophrodonta, Fardenia, Hindella) are long-ranging, cosmopolitan genera

whereas a few like Paromalomena, Plectothyrella are short-ranging and widely

distributed. The Cliftonia and Hindella fauna are typical of the cold water. There is no

obvious gap in strata from the upper Katian (Kyaingtaung Formation) suggesting that the

area was continually submerged by the sea. However, evidence for periods of non-

deposition can be observed. The black silty shales were developed as a hardground or

firm ground.

? ? ? ?

Southern Shan State

Wabya Formation

Panghsa-pye Formation

Nan-on Formation

Sitha Formation

Wunbye Formation

Lokepyin Formation

Not exposed

Nyaungbaw Formation

Kyaingtaung Formation

Linwe Formation

Northern Shan State Pyin Oo Lwin Present Area



(7) The topmost Ordovician Hirnantia stage in Shan Plateau is characterized by a variety of

lithologies and faunas which show the differentiation of the basin during substantial sea

level changes associated with the Gondwana glaciation (Fig. 7).

Figure (7). Palaeogeographic map showing the Gondwana and peri-Gondwana terrain within

the Latest Ordocician-Earlest Silurian (440Ma) (Cocks et al., 2002)

Acknowledgements

The authors thank to Acting Rector, Dr Ba Han, and Pro-Rector, Dr Kay Thi Thin, Meiktila University

for allowing undertaking this research work. Professor, Dr Zaw Min Thein, Head of Geology Department,

Meiktila University, is grateful for his warm encouragement and providing necessaries at the department.

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