Ordovician conodonts from the Thong Pha Phum area, western Thailand

Ordovician conodonts from the Thong Pha Phum area, western Thailand

S. Agematsua,*, K. Sashidab, S. Salyapongsec, A. Sardsudc

aDoctoral Program in Life and Environmental Sciences, University of Tsukuba, Ibaraki 305-8571, JapanbInstitute of Geoscience, University of Tsukuba, Ibaraki 305-8571, Japan

cGeological Survey Division, Department of Mineral Resources, Rama VI, Bangkok 10400, Thailand

Received 11 March 2003; revised 24 March 2004; accepted 22 September 2004

Abstract

The Ordovician conodont biostratigraphy of a limestone sequence in the Tha Manao Formation, exposed in the Thong Pha Phum area,

western Thailand, comprises four Early to Middle Ordovician conodont zones. These are the Juanognathus variaviris Zone, the Walliserodus

comptus Zone, the Juanognathus jaanussoni–Histiodella holodentata Zone, and the Plectodina onychodonta Zone, in ascending order. These

conodont faunas are similar to those from the Midcontinent Province, North China, Australia, and Argentina.

q 2004 Elsevier Ltd. All rights reserved.

Keywords: Conodonts; Ordovician; Limestone; Thailand; Gondwana; Thong Pha Phum

1. Introduction

We have been studying Paleozoic and Mesozoic radio-

ralians from Thailand since 1990 in order to clarify the

geologic age of these radiolarian-bearing rocks and to

examine the taxonomic and paleogeographic significance of

radiolarian faunas (Sashida and Igo, 1999). During the Early

Paleozoic, thick limestone sequences that contain various

kinds of fossils were deposited on the continental shelf of

Gondwana (Metcalfe, 1999). Prior to the present study,

some literature indicated a fairly wide distribution of Lower

Paleozoic sedimentary rocks in western and southern

peninsular Thailand (Hagen and Kemper, 1976; Teraoka

et al., 1982). In July 2001, we undertook a field survey in the

Thong Pha Phum area of western Thailand in order to

establish the Lower Paleozoic conodont biostratigraphy. We

were fortunate to discover moderately well-preserved

Ordovician conodonts from limestone sequences exposed

in the hilly terrain near Thong Pha Phum. The conodont

faunas are similar to those reported from North America,

Argentina, North China, and Australia.

Here we present the Ordovician conodont biostratigraphy

for the Thong Pha Phum area and discuss the correlation of

1367-9120/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.jseaes.2004.09.009

* Corresponding author. Tel.: C81 29 853 4549.

E-mail address: [email protected] (S. Agematsu).

conodont faunas. We also briefly comment on conodont

paleobiogeography for the Early Paleozoic.

2. Tectonic and geologic setting of the Thong Pha Phum

area

There is general agreement that Southeast Asia com-

prises a complex assembly of allochthonous continental

blocks and fragments (e.g., Metcalfe, 1999). Based on this

fundamental concept, several models for the tectonic

framework of Southeast Asia have been presented (e.g.,

Hutchison, 1989; Metecalfe, 1999). Concerning the tectonic

division of Thailand, two principal allochthonous con-

tinental blocks, the western Shan-Thail Block and the

eastern Indochina Block are recognized (Bunopas, 1981).

According to Bunopas (1981), two fold belts, the western

Sukhothai and eastern Loei-Petchabun Fold Belts separate

the two blocks. The area between these two folds belts

consists of suture zones, the Nan-Uttradit to the north and

Sra (or Sa) Kaeo-Chanthaburi to the south (Fig. 1).

The Shan-Thai Block is an elongate continental block

trending north-south with a Precambrian basement. The

Lower and Middle Paleozoic sequences of this block

are Cambrian and Ordovician siliciclastic and carbonate

rocks and Silurian and Devonian fine-grained clastics

Journal of Asian Earth Sciences 26 (2006) 49–60

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Fig. 1. (A) Index map showing the study area and distribution of the

Ordovician limestones of Thailand (after Wongwanich et al., 1983). (B)

Seven stratigraphic belts of Thailand (after Bunopas, 1992). (C)

Generalized stratigraphic nomenclature of BS-2 (Bunopas, 1992).

Fig. 2. Index map showing the study area and the distribution of the

Ordovician rocks in western Thailand. Base map is from the Geological

Map of Thailand, scale 1:2,500,000 (Department of Mineral Resources of

Thailand, 1999).

S. Agematsu et al. / Journal of Asian Earth Sciences 26 (2006) 49–6050

and limestones. The Upper Paleozoic sequences of this

block are characterized by Upper Carboniferous to Lower

Permian glacio-marine diamictite and Lower Permian cool-

water faunas. Geological and paleontological evidence

suggest that this block originated in northwest Australian

Gondwana. The Indochina Block is also and elongated

stable block composed mainly of Precambrian rocks, but

with some Paleozoic shallow marine faunas and floras

probably deposited in a warm climate (Metcalfe, 1988).

Bunopas (1992) recognized seven longitudinal strati-

graphic belts, BS-1 to BS-5, and BI-6 to BI-7 from west to

east in Thailand. BS-1 to BS-5 covers the Shan-Thai Block

in Thailand. The Thong Pha Phum area is within BS-2

(Bunopas, 1992). Stratigraphic belts BI-6 and BI-7 of

Bunopas (1992) cover the Indochina Block in Thailand. The

following five Paleozoic stratigraphic units have been

recognized in BS-2 by Bunopas (1992): Cambrian Tarutao

Group, Ordovician Thung Song Group, Silurian to Carbon-

iferous Thong Pha Phum Group, Carboniferous Mae Hong

Son Formation, and the Permian Ratburi Group. The

Ordovician Thung Song Group was defined in Nakhon Sri

Thammarat province in southern peninsular Thailand

(Javanaphet, 1969) and has been officially applied to almost

all Ordovician limestone in Thailand (Bunopas, 1992).

Bunopas (1981) designated the Ordovician Thung Song

Group in western Thailand as the Tha Manao Formation that

yields early Middle Ordovician nautiloids. The Ordovician

limestone in the Thong Pha Phum area is conformable to the

underlying Cambrian quartzite and overlying Silurian to

Carboniferous sequence (Bunopas, 1981). The total thick-

ness of the Ordovician limestone is estimated as 450 m in

the Khwae Yai area (Bunopas, 1981) and at least 1000 m in

the Khwae Noi area (Hagen and Kemper, 1976). Hagen and

Kemper (1976) showed some localities of Ordovician

conodonts in areas along the Khwae Yai and Khwae Noi

Rivers. The studied sections are located about 5 km

northeast of Thong Pha Phum, near Khwae Noi River

(Figs. 2 and 3).

3. Lithostratigraphy

Ordovician conodonts were found in limestones from three

sections: Section I is at a roadside cliff along a logging road

from Ban Wara Kiang to the Khao Laem Dam. Section II is on

hillside, and Section III is along a small plantation road

(Fig. 3). Based on lithologic characteristics, field observations,

Fig. 3. Detail map showing the study sections and the distribution of limestones with sample locations in the study area.

S. Agematsu et al. / Journal of Asian Earth Sciences 26 (2006) 49–60 51

and previously known conodont biostratigraphy, rocks of

Section I are stratigraphically lower than those of Section II,

which are in turn lower than those of Section III (Fig. 4).

Fig. 4. Generalized lithostratigraphy of the studied sections in the Thong

Pha Phum area. Limestone classification is that of Folk (1959, 1962).

3.1. Section I

The rocks strike N 30–408W and dip 80–858 northeast.

Based on the sedimentary structures in limestone and

convex upward orientations of bivalve shells, this section is

a northeast-upward sequence. The sequence measured in

this section is as follows in ascending order; dark gray,

massive or partially finely laminated limestone (about

15 m); gray laminated limestone (about 15 m); pale gray

massive limestone (about 20 m); and calcareous sandstone

(about 10 m). Limestone in the lowest unit consists of

pelletal oosparite (Fig. 5-4) as seen in thin section.

Limestone classification based on microscopic observations

follows that of Folk (1959, 1962). Grains are mostly unclear

small ooids and peloids along with coarse- to fine-grained

quartz and bioclasts. Bioclasts include ostracods, bryozoans,

brachiopods, and their fragments. Gray laminated limestone

is pelsparite and contains peloids, lumps, and bioclasts

coated by some organic matter within sparry calcite cement.

This limestone intercalates with thin layers consisting of

clasts of bivalve shells, with diameters less than 2 cm. Silt-

sized quartz grains are also present. Pale gray massive

limestone is an arenaceous biosparite consisting of silt- to

dominant fine-grained quartz and bioclasts with sparry

calcite cement. Bioclasts contain fragments of crinoids,

bryozoans, and trilobites. Peloids and lumps are also present

in places. Calcareous sandstone consists mainly of silt-sized

to a dominant fine-grained quartz and minor amounts of

Fig. 5. (1) High-angle cross laminations of the gray to light gray limestone unit of Section II. (2) Lamination within arenaceous pelletal oosparite with silt-sized

quarts grains, ooids, and peloids in a sparry calcite cement. (3–6) Microphotograph of thin sections. (3) Arenaceous oolitic pelletal biosparite consisting of

peloids, silt-sized quartz grains, ooids, and various bioclasts in a sparry calcite cement, TPP-11. (4) Pelletal oosparite composed of quartz grains, ooids, and

peloids within sparry calcite cement, TPP-23. (5) Peloids within micritic matrix, TPP-13. (6) Micritic matrix of biomicrite, TPP-16.


peloids and bioclasts within sparry calcite and micritic

matrix. Bioclasts are less than 1 cm in diameter and include

trilobites and bryozoans.

3.2. Section II

Section II is located about 300 m northwest of Section I.

Rocks strike N 30–508W and dip 70–758 northeast.

The sequence includes, in ascending order: light gray to

gray limestone (about 35 m) and gray massive limestone

(about 20 m). The lower gray to light gray limestone is

characterized by high-angled cross laminations (Fig. 5-1).

This limestone consists of silt-sized quartz grains, ooids and

peloids in sparry calcite cement. These grains also form

laminations (Fig. 5-2). Bioclasts consisting of bryozoans,

crinoids, algae, and other fragments are commonly


surrounded by matrix. The upper gray limestone is an

arenaceous bioclastic pelsparite consisting of peloids,

bioclasts, and silt-sized quartz grains within sparry calcite

cement. Bioclasts are of bryozoans, crinoids and algae. The

diameter of bioclasts rarely exceeds several centimeters.

3.3. Section III

Section III is located about 800 m southeast of section I.

Rocks in this section strike N 45–558W and dip 60–808

northeast. The complete sequence could not be measured

due to soil cover. The sequence measured in this section is

as follows in ascending order; fossiliferous gray laminated

limestone (about 30 m); red to gray limestone (more than

50 m); gray massive limestone (about 12 m); and black

limestone (more than 80 m). The lowest limestone unit is an

arenaceous, oolitic, pelletal biosparite (Fig. 5-3) consisting

of peloids, silt-sized quartz grains, ooids, and bioclasts

coated with organic matter within sparry calcite cement.

Thin laminations consisting of coarse quartz grains and

bioclasts composed of fragments of bivalve shells, algae,

and bryozoans are frequently observed in the upper part of

this unit. The diameter of bioclasts commonly exceeds

1 cm. The red to gray limestone is a pelsparite consisting of

peloids, silt-sized quartz grains, and bioclasts within sparry

calcite cement. Bioclasts include fragments of crinoids,

ostracods, algae, and bryozoans. The gray massive lime-

stone is a biomicrite composed of bioclasts, the maximum

diameter of which frequently exceed 1 cm, and small

amounts of silt-sized quartz grains and peloids in a micritic

matrix (Fig. 5-5). The highest limestone unit is thickly

bedded (30 to 60 cm thick) biomicrite with fragments of

sponges, crinoids, ostracods, and cephalopods along with

silt-sized quartz grains (Fig. 5-6).

Dark gray to gray, thinly bedded (less than 10 cm thick)

sandstone crops out along a logging road. Field observations

suggest that this sandstone may be stratigraphically below

Section II.

4. Depositional environments

Based on field observations and thin section analysis, the

depositional environments of Ordovician rocks in the Thong

Pha Phum area are here inferred.

Limestones in Section I and II are mostly sparites, and most

of the limestones in these two sections contain well-sorted

quartz grains. These limestone characteristics may indicate

that they were deposited in basins under high-energy

conditions where terrigenous grains were continually pro-

vided (Wilson, 1975; Flugel, 1982). Some pelsparites in

Section I have thin parallel laminations with convex, upwardly

oriented bivalve shells. Furthermore, pelletal oosparite in

Section II has high-angle cross laminations (Fig. 5-1). These

sedimentary features indicate that the limestones were

deposited under strong currents. Therefore, the depositional

environment of limestones of Section I and II are interpreted

to be tidal bar or beach on the tidal flat or shelf (Pratt et al.,

1992; Tucker et al., 1990). The presence of calcareous

sandstones in the uppermost part of Section I, and dark gray to

gray thinly-bedded sandstones below Section III, indicates

abrupt influx of terrigenous grains.

In contrast, limestones in the middle and upper parts of

Section III consist of thick-bedded, black micritic

limestones with no coarse-grained quartz. These limestones

are thought to have been deposited under rather calm

conditions such as in lagoons (Wilson, 1975).

5. Conodont biostratigraphy

We collected more than 40 limestone samples from

various levels of the above three sections. Among them, 13

samples yielded conodonts. To date, 26 genera and 43

species have been identified. Based on the occurrence of

stratigraphically important species, we set up the following

four conodont zones; Juanognathus variabilis range zone,

Walliserodus comptus range zone, Juanognathus jaanus-

soni–Histiodella holodentata interval zone, and Plectodina

onychodonta assemblage zone, in ascending order (Fig. 6).

Representative conodont elements are shown in Fig. 7.

5.1. Juanognathus variabilis range zone

This zone has been established in Section I. The upper

limits of this zone are marked by the last occurrence of

J. variabilis Serpagli. This zone is characterized by

J. variabilis, Protopanderodus elongatus Serpagli, Pander-

odus cf. unicostatus Branson and Mehl, and Protopanderodus

primitus Cooper. J. variabilis has been described from the

lower Arenigian of the San Juan Formation in Argentina

(Serpagli, 1974), the Middle Ordovician of Malaysia (Igo and

Koike, 1967), the Arenigian Jefferson City Formation in

Jefferson City, Missouri, USA. (Ethington and Clark, 1971,

1982), and the Lower Ordovician Mystic Formation in

Quebec, Canada (Barnes and Poplawski, 1973). The occur-

rence of P. elongatus has been known from the middle

Arenigian of Argentina (Serpagli, 1974) and the Middle

Ordovician of Malaysia (Igo and Koike, 1967). Furthermore,

P. primitus has been reported from the Lower Ordovician in

Australia (Cooper, 1981), North China (An et al., 1983),

Argentina (Serpagli, 1974), and from the Middle Ordovician

in Malaysia (Igo and Koike, 1967). Therefore, the age of this

conodont zone is estimated to be lower to middle Arenigian.

5.2. Walliserodus comptus range zone

This zone is recognized in Sections I and II. The base and

top of this zone is defined by the first and last occurrence of

W. comptus (Branson and Mehl). W. comptus has been

reported from the Arenigian Jefferson City Formation in

Missouri (Branson and Mehl, 1933; Ethington and Clark,

Fig. 6. Lithostratigraphic column and stratigraphic distribution of conodont species in studied sections in the Thong Pha Phum area.


1971, 1982) and from the Lower Ordovician San Juan

Formation in Argentina (Serpagli, 1974). Other character-

istic species include Protopanderodus floridus Bagnoli and

Stouge, Drepanoistodus forceps (Lindstrom), Scandodus

brevibasis (Sergeeva), Ansella nevadensis (Ethington and

Schumacher), ramiform element A, and coniform element A

and B. P. floridus has been described from the middle

Arenigian in Sweden (Bagnoli and Stouge, 1997).

S. brevibasis has been reported from the late Arenigian in

Argentina (Serpagli, 1974) and North America (e.g.,

Ethington and Clark, 1971). A. nevadensis is known from

the Lower to Middle Ordovician in North America

(Ethington and Schumacher, 1969; McCracken, 1991).

The above faunal evidence suggests that this zone is middle

to late Arenigian in age.

5.3. Juanognathus jaanussoni–Histiodella holodentata

interval zone

This zone is recognized in Section III. The base of this

zone is defined by the occurrence of J. jaanussoni Serpagli,

and the top by Bergstroemognathus cf. extensus (Graves and

Ellison). This zone is characterized by ‘oistodontiform’

element A and B, Drepanoistodus arcuatus Pander,

Cornuodus longibasis (Lindstrom), Aurilobodus leptoso-

matus An, and H. holodentata Ethington and Clark. Among

these species, J. jaanussoni is known from the Arenigian

San Juan Formation in Argentina (Serpagli, 1974) and

Lower Ordovician of North America (Ethington and Clark,

1965). C. longibasis has been described from the middle to

upper Arenigian Horn Valley Siltstone in Western Australia

(Cooper, 1981). A. leptosomatus has been reported from the

upper Arenigian in North China (An et al., 1983) and the

lower Llanvirnian in several areas in Western Australia

(Watson, 1988). H. holodentata is known to occur in the

Middle Ordovician in areas of North America and northern

Europe (Ethington and Clark, 1982), and from the lower

Llanvirnian in North and South China (Wang et al., 1996)

and Australia (Nicoll et al., 1993). D. arcuatus is known

from the lower to middle Arenigian in North America

(Landing, 1976), the Arenigian San Juan Formation in

Argentina (Serpagli, 1974), and the Lower Ordovician in

several areas of Baltoscandina (e.g., Lindstrom, 1957,

1971). Based on the above faunal evidence, this zone may

indicate late Arenigian to early Llanvirnian ages.

5.4. Plectodia onychodonta assemblage zone

This assemblage zone is recognized in the upper part of

Section III. This zone is characterized by the occurrence of

Ansella rigida (An), P. onychodonta An, Juanognathus sp.

B, J. sp. C. ‘B.’ rigida has been described from the upper

Arenigian to upper Llanvirnian in North China (An et al.,

1983). P. onychodonta has been reported from the middle

Llanvirnian in North China (Wang et al., 1996). This

assemblage zone may indicate a Llanvirnian age.

6. Correlation

Ordovician conodont faunas from the Thong Pha Phum

area generally sparse in terms of both species and elements.

Fig. 7. (1–3) P. primitus Cooper: (1) (a) aboral view, (b) lateral view, TPP-17; (2) (a) lateral view, (b) aboral view, TPP-28; (3) (a) lateral view, (b) aboral view,

TPP-18: (4, 5) J. variabilis Serpagli; (4) (a) lateral view, (b) aboral view, TPP-21; (5) (a) postero-lateral view, (b) aboral view, TPP-23: (6) D. arcuatus Pander;

(a) postero-lateral view, (b) aboral view, TPP-15: (7) P. elongatus Serpagli; (a) lateral view, (b) aboral view, TPP-22: (8, 9) W. comptus (Branson and Mehl);

(8) (a) aboral view, (b) lateral view, TPP-24; (9) (a) aboral view, (b) lateral view, TPP-28: (10) ramiform element C; (a) lateral view, (b) aboral view, TPP-18:

(11) A. rigida (An); lateral view, TPP-18: (12, 16) A. leptosomatus An; (12) (a) lateral view, (b) aboral view, TPP-15; (16) lateral view, TPP-13: (13) P.

onychodonta An; (a) lateral view, (b) aboral view, TPP-18; (14) J. jaanussoni Serpagli; (a) lateral view, (b) aboral view, TPP-11: (15) Bergstroemognathus cf.

extensus (Graves and Ellison); (a) lateral view, (b) aboral view, TPP-15: (17) Protopanderodus calceatus Bagnoli and Stouge; (a) aboral view, (b) lateral view,

TPP-17: (18) H. holodentata Ethington and Clark; lateral view, TPP-15: (19) Juanognathus sp. B; (a) aboral view, (b) lateral view, TPP-18: (20) S. brevibasis

(Sergeeva); postero-lateral view, TPP-25.


Table 1

Correlation of Ordovician conodont zones of the Thong Pha Phum area with those of North America, North China and Canning Basin in Australia

Stage Series North America, Midcontinent province

(Ethington and Clark, 1982; Sweet, 1984)

North China

(Wang et al., 1996)

Australia, Canning

Basin (Watson, 1988)

Thailand (this study)

Middle

Ordovician

Llanvirn C. friendsvillensis A. serratus Upper fauna ?

P. polystrophos E. suecicus–P.

onychodonta

Lower fauna P. onychodonta

H. holodentata T. tangshanensis

H. sinuosa

P. crytodens–H. altifrons–M. auritus

Lower

Ordovician

Arening M. flabellum–T. laevis P. paltodiformis J. jaanussoni–

H. holodentata

P. aranda–J. jaanussoni W. camptus

J. gananda–R. andinus J. variabilis

O. communis–‘M’.

Marathonensis

S. bilobatus ?


Therefore, it is difficult to analyze the faunas using relative

abundance methods. However, several species that have

short stratigraphic ranges and geographically restricted

distribution have been identified. We attempted to correlate

conodont faunas from the Thong Pha Phum area to other

areas in and outside of Thailand by using these species. The

correlation of Ordovician conodont zones with North

America, North China, and Australia is shown in Table 1.

6.1. Local correlation

Teraoka et al. (1982) reported the occurrence of Ordovician

conodonts from limestone of the Thung Song Group in

Tarutao Island, southern Thailand. They discriminated

Tremadocian to Arenigian conodonts from the limestone,

such as Drepanodus arcuatus, Paroistodus proteus, Proto-

panderodus recta, Drepanoistodus forceps, Paroistodus

parallelus, Acodus deltatus longibasis, Baltoniodus oepiki,

Scandodus brevibasis, and Prioniodus evae communis. The

Thong Pha Phum faunas share several species with the Tarutao

fauna. Although there was no description, if the identification

is correct, the Walliserodus comptus zone in the Thong Pha

Phum area is correlated with S4 of the Thung Song formation

in Tarutao Island.

6.2. Comparison with North America

Based on investigations by Ethington and Clark (1982)

and Miller (1988), Lower to Middle Ordovician conodont

zones in North America are recognized as follows in

ascending order; Acodus deltatus–Macerodus dianae,

Oepikodus communis—‘Microzarkodina’ marathonensis,

Jumodontus gamanda–Reutterodus andinus, Protoprionio-

dus aranda–Juanognathus jaanussoni, Microzarkodina fla-

bellum–Tripodus laevis, Pteracontiodus cryptodens–

Histiodella altifrons–Multioistodus auritus, Histiodella

sinuosa, H. holodentata, Phragmodus polystrophos, and

Cahabagnathus friendsvillensis zones. The Juanognathus

variabilis Zone of the Thong Pha Phum area can be

correlated to the Jumodontus gamanda–Reutterodus andinus

Zone, because the occurrence of Juanognathus variabilis is

restricted to the middle Arenigian J. gamanda–R. andinus

Zone in North America (Landing, 1976; Ethington and Clark,

1982). Walliserodus comptus, which is the representative and

nominal species of the W. comptus Zone in the Thong Pha

Phum area, is known to occur from the O. communis–‘M.’

maratonensis to P. aranda–J. jaanussoni Zones in North

America (Ethington and Clark, 1982). So the W. comptus

Zone in the Thong Pha Phum area is correlated to J.

gamanda-R. andinus to P. aranda–J. jaanussoni Zones in

North America. J. jaanussoni is a characteristic species of the

P. aranda–J. jaanussoni Zone of North America. The

occurrence of H. holodentata is restricted to the H.

holodentata Zone of North America (Ethington and Clark,

1982). Therefore, the J. jaanussoni–H. holodentata Zone in

the Thong Pha Phum area is correlated from part of the P.

aranda–J. jaanussoni Zone to H. holodentata Zone of North

America. Although we could not obtain sufficient species for

international correlation from the plectodina onychodonta

Zone in the Thong Pha Phum area, this zone may be

correlated to the P. polystrophos and C. friendsvillensis

Zones in North America based on the stratigraphic

relationship with the lower zone. Further biostratigraphic

investigations of the uppermost part of the conodont zone in

the Thong Pha Phum area are needed.

6.3. Comparison with North China

Early to Middle Ordovician conodont zones in North

China have been established by Wang et al. (1996). They are

as follows in ascending order; Serratognathus bilobatus,

Paraserratognathus paltodiformis, Tangshanodus tangsha-

nensis, Eoplacognathus suecicus–Plectodina onychodonta,

and Aurilobodus serratus zones. The lower Arenigian to

lower Llanvirnian zones, the S. bilobatus Zone to the main

part of the P. paltodiformis Zone in North China share no

species with those of the Thong Pha Phum area. Therefore, it

is impossible to correlate the lower two zones of the Thong


Pha Phum area. Aurilobodus leptosomatus and H. holoden-

tata, which characterize the Juanognathus jaanussoni–

Histiodella holodentata Zone in the Thong Pha Phum area,

occur from the upper part of the P. paltodiformis Zone to the

T. tangshanensis Zone, respectively (An et al., 1983; Wang

et al., 1996). So the upper part of the J. jaanussoni–

H. holodentata Zone in the Thong Pha Phum area can be

correlated to the upper part of the P. paltodiformis Zone and

the T. tangshanensis Zone in North China. P. onychodonta,

which is the nominal species of the uppermost conodont

zones in the Thong Pha Phum area, is an index species of the

E. suecious–P. onychodonta Zone in North China. A. rigida,

which also characterizes the P. onychodonta Zone in the

Thong Pha Phum area, has been known to occur from the

T. tangshanensis to A. serratus Zones in North China (An

et al., 1983). To sum up these data, P. onychodonta Zone in

the Thong Pha Phum area is correlated from the

T. tangshanensis to the A. serratus Zones in North China.

6.4. Comparison with Australia

Watson (1988) recognized two Llanvirnian conodont

faunas, the lower fauna and upper fauna, in the Canning

Basin, Western Australia. Histiodella holodentata and

Aurilobodus leptosomatus (ZJuanognathus leptosomatus),

which occur from the upper part of the Juanognathus

jaaussonni–H. holodentata Zone in the Thong Pha Phum

area, have been reported from the lower fauna in Western

Australia (Watson, 1988). Therefore, the upper part of the

Juanognathus jaanusoni–H. holodentata zone in the Thong

Pha Phum area is correlated with Watson’s (1988) lower

fauna. P. primitus (ZP. nogamii), which commonly occurs

throughout the zones in the Thong Pha Phum area, and

Dorella rigida, one of the representatives of the Plectodina

onychodonta Zone in the Thong Pha Phum area, have been

known to occur in Watson’s (1988) upper fauna. Conse-

quently, the Protopanderodus onychodonta Zone in the

Thong Pha Phum area is correlated to the upper fauna by

Watson (1988).

6.5. Comparison with Argentina

Serpagli (1974) distinguished five Ordovician conodont

assemblage zones (A–E) in the San Juan Limestone of

western Argentina. Some species reported from this

limestone are also found in the Thong Pha Phum area.

Juanognathus variabilis, the nominal species of the lowest

conodont zone in the Thong Pha Phum area, and

Juanognathus jaanussoni, which is the characteristic

species of the J. jaanussoni–Histiodella holodentata

Zone in the Thong Pha Phum area, were first described

from the San Juan Limestone. These two species have a

stratigraphic range within Zones B–D in the San Juan

Limestone. Scandodus brevibasis, which occurs only from

the Walliserodus comptus Zone in the Thong Pha Phum

area, is known to occur only from Zone D of the San Juan

Limestone. Dreponoistodus forceps, which occur from the

Walliserodus comptus Zone in the Thong Pha Phum area,

have rather long stratigraphic ranges within Zones A to D

in the San Juan Limestone. To sum up these conodont

data, three conodont zones in the Thong Pha Phum area,

J. variabilis, W. comptus, and J. jaanussoni–H. holoden-

tata Zones are roughly correlated to Zone B, Zone B to D,

and Zone D of the San Juan Limestone, respectively.

6.6. Comparison with Malaysia

Igo and Koike (1967) reported Middle and Upper

Ordovician conodont faunas from the Lower Setul Lime-

stone of the Langkawi Islands, peninsular Malaysia. They

recognized three conodont zones, Scolopodus stauferi–

Scolopdus giganteus, Acodus similaris–Drepanodus altipes,

and Acodus mutates–Acontiodus hamari, in ascending

order. Only three species, Juanognathus variabilis, Proto-

panderodus elongates, and Protopanderodus primitus are

shared between faunas of the Langkawi Islands and the

Thong Pha Phum area. Therefore, the lower part of the

lowermost zone including J. variabilis and P. elongates, is

correlated with the J. variabilis zone of the Thong Pha

Phum area. The lowermost part of the Lower Setul

Limestone may be Lower Ordovician (Arenigian) in age,

although Igo and Koike (1967) concluded that the Lower

Setul Limestone was Middle to Upper Ordovician. Ordo-

vician conodonts are also reported from Perlis in the

northern part of peninsular Malaysia (Igo and Koike, 1973;

Metcalfe, 1980). This fauna is compared with the

S. staufferi–S. giganteus zone. However, it is impossible

to correlate because these faunas do not share species with

the Thong Pha Phum fauna.

7. Paleogeographic implications

Conodonts exhibit strong provincialism during certain

geologic ages (Bergstrom and Sweet, 1966; Barnes and

Fahraeus, 1975). Sweet et al. (1959) have suggested that

Ordovician conodont distribution can be separated into

North American and North Atlantic regions. This

provincialism distinguishes the North Atlantic and Mid-

continent Provinces (Barnes et al., 1973). The North

Atlantic Province includes northwestern Europe, the

British Isles and eastern Appalachian regions that extend

from Newfoundland to Georgia, in addition to Alabama

and western Nevada. On the other hand, the Midcontinent

Province includes the regions of western Appalachian

and Canadian Arctic Archipelago. Recent investigations

on Ordovician conodont paleobiogeography show that

Midcontinent conodont faunas have been known from

Siberia, North China, and eastern Australia (e.g., Stait and

Druce, 1993; Wang et al., 1996).

Ordovician conodonts from the Thong Pha Phum area in

Western Thailand consists of species that have been


reported from the North American Midcontinent area, North

China, Argentina, and Australia in addition to several

cosmopolitan species. The faunas of the Thong Pha Phum

area have links with those of the North American

Midcontinent area throughout the four conodont zones.

The relation between conodont faunas of the Thong Pha

Phum area and North China appears in the upper two zones

in the Thong Pha Phum area. The Argentina faunas are

related to those of the lower two zones and the lower part of

the Juanognathus jaanussoni–Histiodella holodentata zone

in the Thong Pha Phum area. The Thong Pha Phum faunas

also have similarities with those of Australia within several

Ordovician basins. The lower two zones in the Thong Pha

Phum area share several species with faunas from the

central part of Australia, such as Horn Valley Siltstone

(Cooper, 1981) and Coolibah Formation (Stait and Druce,

1993), while the upper two zones contain species that are

similar to faunas from the Canning Basin, eastern Australia

(Watson, 1988).

Ordovician conodont faunas have been reported from the

Shan-Thai Block area such as Tarutao Island, southern

Thailand, and Langkawi Islands, Malaysia. Teraoka et al.

(1982) noted that the fauna of Tarutao Island contained

several characteristic species similar to those in Europe and

Australia and correlated the fauna to the Emanuel

Formation of eastern Australia (McTavish, 1973). Although

the Thong Pha Phum faunas share several species with

Fig. 8. Paleogeographic reconstruction map showing the distribution of the

Midcontinent and North Atlantic Provinces during the Early Ordovician.

Paleogeography is after Metcalfe (1999).

the fauna from Tarutao Island, they do not contain species of

Baltoniodus and Prioniodus, which are characteristics of the

Tarutao fauna. Therefore, there is little affinity between

these faunas. The faunas of the Thong Pha Phum area have a

few common species containing Juanognathus variabilis

with the faunas of Langkawi Islands. However, it is difficult

to relate faunas of the other three zones in Thong Pha Phum

to the upper range of the Langkawi faunas because Thong

Pha Phum faunas do not share several characteristic species

with the Langkawi faunas such as H. holodentata,

Aurilobodus leptosomatus, and Plectodina onychodonta.

Recently, Ordovician conodont faunas correlative with

North Atlantic fauna of the same age have been reported

from South China by Wang et al. (1996). Furthermore,

investigations in the Tarim Basin and various areas in China

have suggested that conodont faunas referred to as the

Midcontinent fauna and North Atlantic fauna have alter-

nated between these areas throughout the Ordovician (e.g.,

Wang and Qi, 2001). The paleogeographic faunal transition

may have occurred during Ordovician times in areas of

northern Gondwanaland. Conodont provincialism during

Ordovician time is shown in Fig. 8.

8. Conclusions

Ordovician limestone, which is designated in the Tha

Manao Formation, is distributed in the Thong Pha Phum

area, western Thailand. We collected more than 40 lime-

stone samples from three sections of the Tha Manao

Formation located about 5 km northeast of Thong Pha

Phum. These sections are totally about 500 m in thickness.

The lower 360 m is mainly composed of sparry limestones,

which are thought to have been deposited under high-energy

conditions on the tidal flat or shelf. The upper 40 m of these

sections consists of micritic limestones. The depositional

environment of these limestones is interpreted to be calm

conditions such as in lagoons.

These limestones yield Early to Middle Ordovician

conodonts, which are classified into 26 genera and 43

species. The following four conodont zones are established;

Juanognathus variabilis range zone, Walliserodus comptus

range zone, Juanognathus jaanussoni–Histiodella holoden-

tata interval zone, and Plectodina onychodonta assemblage

zone, in ascending order. These zones are correlated from

the Jumodontus gamanda–Reutterodus andinus to Caha-

bagnathus friendsvillensis zones in North America and from

the Paraserratognathus paltodiformis to Aurilobodus ser-

ratus zones in North China. Early to Middle Ordovician

conodonts from the Thong Pha Phum area mainly consist of

species that have been reported from the North American

Midcontinent area, North China, Argentina, and Australia.

This faunal characteristic suggests the close relationship

between the Thong Pha Phum fauna and North American

Midcontinent fauna.


Acknowledgements

We are much indebted to the referees on the editorial

board, I. Metcalfe and Ed. Landing for critical reading of the

manuscript and useful suggestions. We thank H. Igo and T.

Koike for their continuous encouragement throughout the

course of this study. We also thank J. Lytwyn, Editorial

Assistant, Journal of Asian Earth Sciences for the improve-

ment of the manuscript. Our field survey was funded by a

grant under the Mombusho (Government of Japan)

International Scientific Research Program (Number

11440148 to K.S.) and also supported by the Department

of Mineral Resources, Thailand.

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