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
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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.
694 The Second Myanmar National Conference on Earth Sciences (MNCES, 2018)
November 29-30, 2018, Hinthada University, Hinthada, Myanmar
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).
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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
696 The Second Myanmar National Conference on Earth Sciences (MNCES, 2018)
November 29-30, 2018, Hinthada University, Hinthada, Myanmar
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
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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
698 The Second Myanmar National Conference on Earth Sciences (MNCES, 2018)
November 29-30, 2018, Hinthada University, Hinthada, Myanmar
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
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(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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