Lithostratigraphy of Permian marine sequences, Khao Pun Area ...
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The Island Arc (2000) 9, 173–187<br />
Research Article<br />
<strong>Lithostratigraphy</strong> <strong>of</strong> <strong>Permian</strong> <strong>marine</strong> <strong>sequences</strong>, <strong>Khao</strong> <strong>Pun</strong> <strong>Area</strong>,<br />
central Thailand: Paleoenvironments and tectonic history<br />
VICHAI CHUTAKOSITKANON, 1 PUNYA CHARUSIRI 1 AND KATSUO SASHIDA 2 *<br />
1 Department <strong>of</strong> Geology, Faculty <strong>of</strong> Science, Chulalongkorn University, Bangkok 10330, Thailand and 2 Institute<br />
<strong>of</strong> Geoscience, University <strong>of</strong> Tsukuba, Ibaraki 305-8571, Japan (email: sashida@arsia.geo.tsukuba.ac.jp)<br />
Abstract Geologic mapping and subsurface lithostratigraphic investigations were carried<br />
out in the <strong>Khao</strong> <strong>Pun</strong> area (4 km 2 ), central Thailand. More than 250 hand specimens, 70 rock<br />
slabs, and 70 thin sections were studied in conjunction with geochemical data in order to<br />
elucidate paleoenvironments and tectonic setting <strong>of</strong> the <strong>Permian</strong> <strong>marine</strong> sedimentary<br />
<strong>sequences</strong>. This sedimentary succession (2485 m thick) was re-accessed and re-grouped<br />
into three lithostratigraphic units, namely, in ascending order, the Phu Phe, <strong>Khao</strong> Sung<br />
and <strong>Khao</strong> <strong>Pun</strong> Formations. The Lower to lower Upper <strong>Permian</strong> sedimentary facies indicated<br />
the transgressive/regressive succession <strong>of</strong> shelf sea/platform environment to pelagic<br />
or abyssal environment below the carbonate compensation depth. The sedimentological<br />
and paleontological aspects, together with petrochemical and lithological points <strong>of</strong> view,<br />
reveal that the oldest unit might indicate an Early <strong>Permian</strong> sheltered shallow or lagoonal<br />
environment. Then the depositional basin became deeper, as suggested by the prolonged<br />
occurrence <strong>of</strong> bedded chert-limestone intercalation with the local exposure <strong>of</strong> shallower<br />
carbonate build-up. Following this, the depositional environment changed to pelagic deposition,<br />
as indicated by laminated radiolarian (e.g. Follicucullus sp.) cherts. This cryptic<br />
evidence might indicate the abyssal environment during middle Middle to early Late<br />
<strong>Permian</strong>; whereas, previous studies advocated shelf-facies environments. Following this,<br />
the depositional condition might be a major regression on the microcontinent close to<br />
Indochina, from the minor transgressive/regressive cycles that developed within a skeletal<br />
barrier, and through the lagoon with limited circulational and anaerobic conditions, on<br />
to the tidal flat to the sheltered lagoon without effective land-derived sediments.<br />
Key words: central Thailand, <strong>marine</strong> <strong>sequences</strong>, paleoenvironment, pelagic, <strong>Permian</strong>,<br />
Radiolaria, shelf, tectonics.<br />
INTRODUCTION<br />
The <strong>Khao</strong> <strong>Pun</strong> area, 4 km 2 in size, is located in the<br />
Kaeng Khoi District, Saraburi Province, central<br />
Thailand (Fig. 1). Physiographically, it consists<br />
<strong>of</strong> limestone mountains trending northwestsoutheast,<br />
covered by a dense forest.<br />
The Upper Paleozoic carbonate/clastic unit in<br />
this region has been visited by several groups <strong>of</strong><br />
geoscientists. One <strong>of</strong> the first studies was performed<br />
by Brown et al. (1951) who named this lime-<br />
*Correspondence.<br />
Accepted for publication 20 December 1999.<br />
© 2000 Blackwell Science Asia Pty Ltd.<br />
stone and the other <strong>Permian</strong> limestones throughout<br />
the country as the Ratburi Limestone. Borax<br />
& Stewart (1966) worked on the Paleozoic stratigraphic<br />
correlation <strong>of</strong> northeastern Thailand.<br />
Tittirananda’s (1976) PhD dissertation was on the<br />
stratigraphy and paleontology <strong>of</strong> the limestones<br />
along Highway no. 21. Hinthong et al. (1985)<br />
systematically compiled the geology <strong>of</strong> the entire<br />
1:250 000-scale topographic map sheet <strong>of</strong> ND<br />
47–8 (Changwat Phra Nakhon Si Ayutthaya).<br />
Wielchowsky & Young (1985) studied lith<strong>of</strong>acies in<br />
the <strong>Permian</strong> rocks <strong>of</strong> the Phetchabun Fold Belt.<br />
More detailed studies on the structures in this<br />
region were carried out by Pothong (1986). The<br />
name ‘Saraburi Group’ in place <strong>of</strong> the name Ratburi
174 V. Chutakositkanon et al.<br />
Fig. 1 Index map. (a) Regional tectonic map <strong>of</strong> mainland SE Asia showing major tectonic blocks (Bunopas 1981, 1992). (b) Regional geologic map <strong>of</strong><br />
the Saraburi-Kaeng Khoi area, central Thailand (after Hinthong et al. 1985) showing major units <strong>of</strong> the Saraburi Group and the locality <strong>of</strong> the study area<br />
(inserted block). , Phu Phe Formation; , Nong Pong Formation; , Pang Asok Formation; , <strong>Khao</strong> Khad Formation; ,<br />
Sap Bon Formation; , terrace gravel; , Recent flood plain; , <strong>Khao</strong> Yai Volcanics; , Phra Ngam Diorite; , river and<br />
stream; , fault; , thrust fault; S, Saraburi city; K, Kaneg Khoi town. (c) Simplified geologic map <strong>of</strong> the <strong>Khao</strong> <strong>Pun</strong> <strong>Area</strong>, central Thailand.<br />
Group was first proposed by Bunopas (1981) and<br />
Bunopas et al. (1988) for the <strong>Permian</strong> succession<br />
which distributes in the west <strong>of</strong> the Khorat Plateau<br />
margin and the eastern part <strong>of</strong> the Central Plain<br />
(restricted to the Peninsula <strong>of</strong> Thailand). Recently,<br />
Dawson & Racey (1993) proposed the <strong>Permian</strong><br />
strata <strong>of</strong> central Thailand as a sequence <strong>of</strong> supratidal<br />
to outer platform facies comprising a Lowerupper<br />
Middle <strong>Permian</strong> transgressive/regressive<br />
carbonate platform succession. Recently, a detailed<br />
systematic study has been made on the <strong>Permian</strong><br />
lithostratigraphy around the <strong>Khao</strong> <strong>Pun</strong> study area<br />
at which the Kaeng Khoi Factory <strong>of</strong> the Siam<br />
Cement Public Company Ltd (SCC) is situated<br />
(V. Chutakositkanon, pers. data, 1996).<br />
The purposes <strong>of</strong> this study are, therefore,<br />
to present a detailed lithostratigraphy <strong>of</strong> the<br />
<strong>Permian</strong> rocks and to discuss the environment <strong>of</strong><br />
deposition related to tectonic setting <strong>of</strong> the area.<br />
Depending on their use, several classifications <strong>of</strong><br />
carbonate rocks are applied, following those <strong>of</strong><br />
Folk (1959, 1962), Dunham (1962), and Embry &<br />
Klovan (1971), slightly modified by the classification<br />
systems <strong>of</strong> Tucker (1981) and Adam et al.<br />
(1984). Approximately 30 spot locations were<br />
visited and revisited, and seven main drilled<br />
holes were logged and lithologically described.<br />
Then, 70 rock slabs and 70 thin sections were<br />
selectively prepared from more than 250 hand<br />
specimens collected from both field and drilled
<strong>Permian</strong> <strong>marine</strong> <strong>sequences</strong> in the Kao <strong>Pun</strong> 175<br />
cores. A chemical staining technique, described by<br />
Friedman (1959, 1977) and Friedman & Sternbach<br />
(1982), was used to aid identifying carbonate minerals.<br />
Major- and minor-oxide analyses were<br />
mainly performed by Krongkaew et al. (1991) and<br />
some by Chutakositkanon (1996).<br />
However, all samples analyzed by Krongkaew et<br />
al. (1991) were composite samples; therefore, in<br />
some cases the results do not represent the geologic<br />
units and are not similar to those performed<br />
by Chutakositkanon (1996), which indicate actual<br />
lithologic units. In this regard, we decided to use<br />
only the results from Chutakositkanon (1996),<br />
especially those for limestones in the Phu Phe<br />
Formation and for the mudrocks in the <strong>Khao</strong> <strong>Pun</strong><br />
Klang Member. However, in general, the results <strong>of</strong><br />
Krongkaew et al. (1991) are in agreement with our<br />
results.<br />
REGIONAL GEOLOGIC SETTING<br />
The Saraburi Group (Fig. 1b) in the current study<br />
area is in the southernmost part <strong>of</strong> the approximately<br />
north-trending continuous outcrops <strong>of</strong> the<br />
Upper Paleozoic carbonate/clastic unit, extending<br />
southwards at least 400 km from Lao PDR. The<br />
Saraburi Group, exposed as a chain <strong>of</strong> limestone<br />
hills, ridges, knobs and mounds, is observably<br />
bounded by the youngers, the <strong>Khao</strong> Yai Volcanics<br />
to the south, continental red-bed clastic rocks <strong>of</strong><br />
the Mesozoic Khorat Group to the east and northwest,<br />
and Quaternary alluvial deposits <strong>of</strong> the Chao<br />
Praya, the Lop Buri and the Pa Sak Rivers to the<br />
west. This grouping takes place along the westernmost<br />
part <strong>of</strong> the Indochina (Indosinian in<br />
places) microcontinent (Bunopas 1992).<br />
Several stratigraphers have tried to identify the<br />
complex stratigraphy here. Hinthong et al. (1985)<br />
subdivided the rocks, based upon stratification,<br />
fossils and structures, into six formations, from<br />
older to younger, as the Phu Phe, <strong>Khao</strong> Khwang,<br />
Nong Pong, Pang Asok, <strong>Khao</strong> Khad and Sap Bon<br />
Formations, covering the entire southern limit <strong>of</strong><br />
the Phetchabun–Saraburi trend.<br />
Structurally, Abele & Beeser (1963) believed that<br />
the regional rock sequence belonged to the southern<br />
flank <strong>of</strong> ‘a large gently westward plunging anticline’<br />
whose core lay somewhere further north. This<br />
gigantic structure is, however, mentioned as an<br />
anticlinorium by Hinthong (1981), N. Comviravong<br />
(unpubl. data, 1985) and Pothong (1986).<br />
The rocks in this region were intruded by<br />
plutonics <strong>of</strong> Permo-Triassic Phra Ngam Diorite<br />
(Hinthong 1981; Hinthong et al. 1985). Volcanics,<br />
dikes and sills <strong>of</strong> similar composition and age were<br />
also found in the <strong>Permian</strong> country rocks <strong>of</strong> the<br />
Saraburi Group. Hinthong (1981) and Hinthong et<br />
al. (1985) assigned the volcanics the name, Permo-<br />
Triassic <strong>Khao</strong> Yai Volcanics.<br />
Bryozoans, algae and fusulinids collected in<br />
carbonate rocks, as had been noted earlier by<br />
Pitakpivan (1965), Borax & Stewart (oral presentation<br />
at Economic Commission for Asia and the<br />
Far East Meeting in Bangkok, Thailand, ECAFE,<br />
1966), Tittirananda (1976), Dawson (1978a,b),<br />
Hinthong (1981), Ingavat-Helmcke (1993) and<br />
Dawson & Racey (1993), indicate the ages from<br />
Early to Late-<strong>Permian</strong>. These rocks were also<br />
noted to have deposited in a shallow shelf sea or<br />
platform environment.<br />
LITHOSTRATIGRAPHY<br />
The Upper Paleozoic sedimentary sequence in the<br />
<strong>Khao</strong> <strong>Pun</strong> area is composed mainly <strong>of</strong> limestones,<br />
subordinate mudstones and bedded cherts with<br />
minor andesitic hypabyssal rocks, such as dikes<br />
and sills. According to the works <strong>of</strong> Hinthong<br />
(1981) and Hinthong et al. (1985), the rock units in<br />
the present study area were grouped as the<br />
<strong>Khao</strong> Khad and Phu Phe Formations. However,<br />
the current field and subsurface stratigraphic<br />
investigations allowed us to re-group the <strong>Permian</strong><br />
sedimentary succession in the <strong>Khao</strong> <strong>Pun</strong> area<br />
from oldest to youngest, based essentially on the<br />
lithologic and physical appearances, stratifications<br />
and sedimentary structures, into three lithostratigraphic<br />
units, the Phu Phe, <strong>Khao</strong> Sung<br />
and <strong>Khao</strong> <strong>Pun</strong> Formations which are further<br />
subdivided into seven members as demonstrated<br />
(Fig. 2). In addition, 1796 composite samples<br />
(Krongkaew et al. 1991) and 14 systematically<br />
selected samples (Chutakositkanon 1996) from<br />
individual members were geochemically analyzed<br />
for major-oxide contents; the general results<br />
are shown (in the lithostratigraphic column <strong>of</strong><br />
Fig. 2).<br />
PHU PHE FORMATION<br />
The Phu Phe Formation (Hinthong 1981) in the<br />
study area consists <strong>of</strong> gray, thickly to very<br />
thickly bedded biomicrite and biomicrosparite or<br />
fossiliferous boundstone, packstone; some wackestone<br />
are distributed only in the southwesternmost<br />
portion. Regarding the chemical analysis
176 V. Chutakositkanon et al.<br />
Fig. 2 Lithostratigraphic column <strong>of</strong> the <strong>Khao</strong> <strong>Pun</strong> <strong>Area</strong> and average major-oxide results <strong>of</strong> individual units. Lithostratigraphic column uses the classical<br />
rock-section key shown in Selly (1985, Fig. 0.1). The grain size is drawn increasing to the left, following Wentworth grades. Numbers in parentheses<br />
are the total number <strong>of</strong> rock samples analyzed. *Analysis was performed by Krongkaew et al. (1991) and Chutakositkanon (1996); **Chutakositkanon<br />
(1996) only; ***no analysis performed.<br />
(Chutakositkanon 1996), the low SiO 2 , Al 2 O 3 , K 2 O<br />
and MgO contents (0.095, 0.02, < 0.01 and 1.34%,<br />
respectively) and high CaO content (53.6%) are<br />
considered to be limestones. Fusulinacean fossils<br />
can also be identified and the following species<br />
were reported by Hinthong (1981): Pseudoschwagerina<br />
cf. toriyamai Igo, Pseudoschwagerina<br />
(Zellia) turbida Kahler, Paraschwagerina sp. and<br />
others which indicate Early <strong>Permian</strong> Sakmarian.<br />
Notably, the bedding planes <strong>of</strong> the 85-meter-thick<br />
unit are steeply inclined to the northeast direction,<br />
directly opposite to the other units. This leads us<br />
to the interpretation that the Phu Phe Formation<br />
was possibly northward-upthrust on the youngest<br />
<strong>Khao</strong> <strong>Pun</strong> Formation. This Phu Phe Formation is<br />
also called the Rong Ngan Kaeng Khoi Member<br />
after the Kaeng Khoi Factory <strong>of</strong> the SCC, to be<br />
consistent with the members <strong>of</strong> the other units<br />
(Chutakositkanon 1996).<br />
KHAO SUNG FORMATION<br />
The <strong>Khao</strong> Sung Formation is entirely exposed in<br />
the northern part and its features are clearly dis-
<strong>Permian</strong> <strong>marine</strong> <strong>sequences</strong> in the Kao <strong>Pun</strong> 177<br />
played as a straight ridge in aerial photographs.<br />
The bedding planes, striking parallel to the NWtrending<br />
ridge, dip steeply in the southwesterly<br />
direction. Lithologically and structurally, this<br />
formation is distinguished from the others by the<br />
presence <strong>of</strong> cherts and the absence <strong>of</strong> hypabyssal<br />
rocks. However, the <strong>Khao</strong> Sung Formation is composed<br />
principally <strong>of</strong> three conformable members,<br />
namely, from older to younger, the <strong>Khao</strong> Sung<br />
Nua, <strong>Khao</strong> Sung Klang and <strong>Khao</strong> Sung Tai<br />
Members, respectively. The overall thickness <strong>of</strong><br />
this formation is ~ 830 m.<br />
<strong>Khao</strong> Sung Nua Member<br />
The <strong>Khao</strong> Sung Nua Member, the lowest unit<br />
<strong>of</strong> the <strong>Khao</strong> Sung Formation, is composed <strong>of</strong> a<br />
420-meter-thick uniform, southwestward-dipping<br />
succession <strong>of</strong> interbedding between thin- to<br />
medium-bedded black argillaceous limestones,<br />
black-bedded chert (Fig. 3a) and black shale.<br />
However, the argillaceous limestones are likely to<br />
contain a high percentage <strong>of</strong> organic materials and<br />
a very finely grained texture and are re-classified,<br />
herein, as micrite and biomicrite, or as mudstone<br />
and wackestone. Surprisingly, the limestones<br />
contain irregular-shaped microcrystalline quartz<br />
or chert with an average size <strong>of</strong> 5–7 mm. Individual<br />
chert layers, invariably interbedded with black<br />
argillaceous limestones, range in thickness from a<br />
few centimeters to tens <strong>of</strong> centimeters or more,<br />
and both upper and lower surfaces are wavy. When<br />
viewed under the polarizing microscope, these<br />
bedded cherts are, in fact, porcellanite. The presence<br />
<strong>of</strong> shale in the member is still problematic<br />
because the member is not exposed clearly as an<br />
outcrop. The shale presence is interpreted from<br />
black, strongly weathered regoliths. Spherical<br />
radiolarians, the most observable fossils, are preserved<br />
very rarely in these siliceous sedimentary<br />
rocks.<br />
<strong>Khao</strong> Sung Klang Member<br />
The <strong>Khao</strong> Sung Klang Member, the middle unit<br />
<strong>of</strong> the <strong>Khao</strong> Sung Formation, is obviously distinguished<br />
because it is the only member in the formation<br />
that contains no cherts in succession. The<br />
member consists mainly <strong>of</strong> pinkish-gray, thickly<br />
to very thickly bedded fossiliferous or crinoidal<br />
floatstone, rudstone, packstone and grainstone<br />
or biointrasparite and intrabiosparite, with some<br />
packed (over 50% allochems) biointramicrite<br />
and packed intrabiomicrite. The southwestward<br />
dipping <strong>Khao</strong> Sung Klang Member, considered to<br />
be a large lens-shaped sedimentary body with a<br />
maximum thickness <strong>of</strong> 200 m, conformably overlies<br />
the <strong>Khao</strong> Sung Nua Member and underlies the<br />
<strong>Khao</strong> Sung Tai Member. The variable-sized fossils<br />
<strong>of</strong> crinoid stem fragments, calcareous algae and<br />
bryozoans can be observed easily. Due to its lens<br />
appearance, the geochemical data are not required<br />
for this study.<br />
<strong>Khao</strong> Sung Tai Member<br />
The youngest <strong>Khao</strong> Sung Tai Member <strong>of</strong> the <strong>Khao</strong><br />
Sung Formation, exposed entirely as cobble-sized<br />
loose blocks covering the southern foot <strong>of</strong> <strong>Khao</strong><br />
Sung, is outstandingly characterized by gray to<br />
dark gray laminated cherts (Fig. 3b) with some<br />
porcellanites and deep-red ironstones. According<br />
to its chemical data (Chutakositkanon 1996), the<br />
high SiO 2 content (approximately 89.9%) and small<br />
CaO concentration (approximately 3.90%) <strong>of</strong> this<br />
laminated siliceous sedimentary rock is regarded<br />
as a composition <strong>of</strong> chert to porcellanite following<br />
the classification <strong>of</strong> Krumbein & Sloss (1963). The<br />
section, carefully measured as ~ 210 m thick, overlies<br />
the <strong>Khao</strong> Sung Nua and <strong>Khao</strong> Sung Klang<br />
Members with abrupt contacts and underlies the<br />
<strong>Khao</strong> <strong>Pun</strong> Formation with a fault contact as a huge<br />
vertical cliff. Observed fossils are spherical radiolarians,<br />
sponge spicules and unidentified siliceous<br />
shell fragments, mostly aligned parallel to the lamination<br />
or banding (Fig. 3c). We discovered poorly<br />
preserved radiolarians, probably Follicucullus<br />
sp. (Fig. 4), suggesting an interval from middle<br />
Middle <strong>Permian</strong> to early Late <strong>Permian</strong>. The occurrence<br />
<strong>of</strong> the radiolarians places the age <strong>of</strong> the <strong>Khao</strong><br />
Sung Formation or the equivalent <strong>Khao</strong> Khad<br />
Formation <strong>of</strong> Hinthong et al. (1985) younger than<br />
that proposed by Hinthong et al. (1985).<br />
KHAO PUN FORMATION<br />
The youngest and thickest (1570 m thick) formation<br />
in the present study area is called the <strong>Khao</strong><br />
<strong>Pun</strong> Formation. The conformable sedimentary<br />
sequence, moderate-dipping in the southwesterly<br />
direction <strong>of</strong> the <strong>Khao</strong> <strong>Pun</strong> Formation, is set apart<br />
from the others by the presence <strong>of</strong> well-developed<br />
calcite veins and veinlets, andesitic dikes and sills<br />
<strong>of</strong> hypabyssal rocks, and the absence <strong>of</strong> chert<br />
layers. The <strong>Khao</strong> <strong>Pun</strong> Formation overlies the <strong>Khao</strong><br />
Sung Formation in the north and abnormally<br />
underlies the older Phu Phe Formation in the<br />
south with normal and thrust-fault contacts,
178 V. Chutakositkanon et al.<br />
Fig. 3 Samples from the <strong>Khao</strong> <strong>Pun</strong> area. (a) Natural exposure <strong>of</strong> intercalated argillaceous limestone (L, thicker beds) and chert (C, thinner beds) with<br />
both their upper and lower wavy surfaces in the <strong>Khao</strong> Sung Nua Member. (b) Rock slabs <strong>of</strong> laminated cherts from the <strong>Khao</strong> Sung Tai Member; bar =<br />
5 cm. (c) Photomicrograph <strong>of</strong> the laminated chert under crossed polars showing alignment and spherical outlines <strong>of</strong> radiolarians (R), bar = 0.5 mm. (d)<br />
Characteristics <strong>of</strong> well-bedded limestone strata <strong>of</strong> the <strong>Khao</strong> <strong>Pun</strong> Nua Member with extension cracks or gashes in the central part. (e) Fusulinid fragment<br />
(Verbeekina sp.) in a carbonate rock <strong>of</strong> the <strong>Khao</strong> <strong>Pun</strong> Nua Member, bar = 0.5 mm. (f) Road-cut exposure <strong>of</strong> the <strong>Khao</strong> <strong>Pun</strong> Klang Member showing the<br />
argillaceous limestone (L) intercalation in the slaty shale in the upper part <strong>of</strong> the member. (g) Normal graded-bedding and scoring in a core sample from<br />
the <strong>Khao</strong> <strong>Pun</strong> Klang Member proving the right-side-up sequence in the <strong>Khao</strong> <strong>Pun</strong> <strong>Area</strong>. Number indicates the depth (86.25 m) from surface <strong>of</strong> the drilled<br />
hole, bar = 5 cm. (h) Photomicrograph <strong>of</strong> andesite hypabyssal rock showing large biotite (B) and plagioclase (PL) phenocrysts with sericite alteration,<br />
bar = 0.5 mm.
<strong>Permian</strong> <strong>marine</strong> <strong>sequences</strong> in the Kao <strong>Pun</strong> 179<br />
Fig. 4 SEM photographs <strong>of</strong> radiolarians (Follicucullus sp.) from the<br />
<strong>Khao</strong> Sung Tai Member, bar = 100 mm.<br />
with either abrupt or intercalation contacts. The<br />
<strong>Khao</strong> <strong>Pun</strong> Nua Member consists predominantly<br />
<strong>of</strong> several fossiliferous beds. Essential fossils<br />
include crinoid stems, bryozoans and algae, with<br />
a minority <strong>of</strong> fusulinids (Fig. 3e) and small<br />
foraminifers. The following late Middle <strong>Permian</strong><br />
fusulinacean fossils have been identified from this<br />
member, namely Verbeekina verbeeki (Geinitz),<br />
Verbeekina sp., Neoschwagerina megaspherica<br />
Deprat, Afghanella cf. sumatrinaeformis Gubler<br />
and others (Hinthong 1981). Crinoid stems are<br />
the most abundant fossils and can be observed<br />
throughout the unit.<br />
respectively. Information obtained from both<br />
surface and subsurface investigation leads to the<br />
subdivision <strong>of</strong> the <strong>Khao</strong> <strong>Pun</strong> Formation into three<br />
major conformable members, from lower to upper,<br />
including the <strong>Khao</strong> <strong>Pun</strong> Nua, <strong>Khao</strong> <strong>Pun</strong> Klang and<br />
<strong>Khao</strong> <strong>Pun</strong> Tai Members.<br />
<strong>Khao</strong> <strong>Pun</strong> Nua Member<br />
The 560-meter-thick <strong>Khao</strong> <strong>Pun</strong> Nua Member is<br />
largely exposed in the northern portions <strong>of</strong> <strong>Khao</strong><br />
<strong>Pun</strong> and <strong>Khao</strong> Nong Kop as an E-trending strip<br />
passing the central part <strong>of</strong> the area. It consists<br />
generally <strong>of</strong> yellowish-gray or pinkish-gray to<br />
dark-gray, thickly to very thickly bedded (Fig. 3d)<br />
biointrasparite and intrabiosparite, with some<br />
packed biointramicrite and packed intrabiomicrite<br />
or fossiliferous or crinoidal floatstone, rudstone,<br />
packstone and grainstone. Dolomitic limestone,<br />
locally abundant, is exposed with a predominant<br />
weathering surface, the so-called ‘elephant skin’.<br />
Results from X-ray fluorescence analysis reveal<br />
that CaO averages 51.62% and MgO 1.18%. A<br />
value <strong>of</strong> MgO greater than 1% suggests that the<br />
mineral dolomite is likely to be present. However,<br />
most carbonate rocks in this member are classified<br />
as limestone, following the classification <strong>of</strong> calcitedolomite<br />
mixture (Pettijohn 1975) because the<br />
mineral dolomite is less than 5% <strong>of</strong> the total calcite–<br />
dolomite mixture. From subsurface investigation,<br />
an intercalation zone between the yellowish-gray<br />
limestone with reddish-brown calcareous shale,<br />
and the black argillaceous limestone with black<br />
shale can also be observed in this unit. The southwesterly<br />
dipping <strong>Khao</strong> <strong>Pun</strong> Nua Member overlies<br />
the <strong>Khao</strong> Sung Tai Member with a fault contact<br />
that formed as a large scarp and underlies the correspondingly<br />
dipping <strong>Khao</strong> <strong>Pun</strong> Klang Member<br />
<strong>Khao</strong> <strong>Pun</strong> Klang Member<br />
The thinnest, 180-meter-thick <strong>Khao</strong> <strong>Pun</strong> Klang<br />
Member mainly consists <strong>of</strong> high organic materials<br />
and a very finely grained sedimentary arrangement<br />
<strong>of</strong> mudstones as slaty shale to shale, with the<br />
argillaceous limestone intercalations near upper<br />
and lower contacts (Fig. 3f). Microscopically, the<br />
black color <strong>of</strong> mudstones in this unit is the result<br />
<strong>of</strong> finely disseminated organic matter and pyrite.<br />
The black argillaceous limestone is re-classified as<br />
micrite, quartz intramicrite and some as biomicrite<br />
or mudstone, wackestone and terrigenous quartz<br />
silt-sized wackestone. Geochemically, our mudstones<br />
contain averaged values <strong>of</strong> SiO 2 65.0%,<br />
Al 2 O 3 16.0%, Fe 2 O 3 5.02%, K 2 O 2.89%, MgO 1.86%,<br />
and Na 2 O 0.77%. Like the general shales, major<br />
minerals are clays and quartz; thus, the most<br />
abundant oxides are SiO 2 and Al 2 O 3 . The high percentage<br />
<strong>of</strong> Fe 2 O 3 might be due to the occurrence <strong>of</strong><br />
pyrite disseminated grains and framboids. The<br />
TiO 2 content (average 0.72%) in this mudstone is<br />
much higher than those <strong>of</strong> other sedimentary units<br />
(Chutakositkanon 1996, Table 5.2) and clearly<br />
distinguishes these rocks. However, averaged<br />
CaO content <strong>of</strong> SCC results (Krongkaew et al.<br />
1991) which is doubtfully higher than 16%, may<br />
be caused by the presence <strong>of</strong> calcite veins and<br />
veinlets in their samples. The southward to<br />
southwestward dipping, organic-rich, very finely<br />
grained clastic/carbonate rock unit overlies the<br />
fossiliferous carbonate rock <strong>of</strong> the <strong>Khao</strong> <strong>Pun</strong> Nua<br />
Member and underlies the youngest <strong>Khao</strong> <strong>Pun</strong> Tai<br />
Member with the abrupt or intercalated contacts.<br />
Subsurface study reveals that several sedimentary<br />
structures noticed in core-samples, such as normal<br />
graded-bedding (Fig. 3 g), cross-lamination, and<br />
scouring and flame structures, prove the normal or<br />
right-side-up sequence. Although we do not have<br />
enough chronological data for this member, it may
180 V. Chutakositkanon et al.<br />
be placed as late Middle <strong>Permian</strong>, based on the<br />
stratigraphical relationships between overlying<br />
and underlying members.<br />
<strong>Khao</strong> <strong>Pun</strong> Tai Member<br />
The 830-meter-thick <strong>Khao</strong> <strong>Pun</strong> Tai Member, the<br />
thickest and youngest unit, is clearly distinguished<br />
from the other members by its light gray color and<br />
aphanitic texture. The term ‘lithographic limestone’<br />
is used in field investigations for this dense,<br />
homogeneous and very fine-textured limestone.<br />
Although the limestone appears to be micrite in<br />
hand specimens, under the polarizing microscope<br />
these micrite crystals are observed to be microsparry<br />
calcite with average grain size <strong>of</strong> ~6 mm,<br />
following Ehlers & Blatt (1982), contrasting with<br />
typical micrite (1–4 mm) and typical microsparry<br />
calcite (5–15 mm). From this viewpoint, it is possible<br />
to classify the limestone in this unit as microsparite<br />
and dismicrosparite or mudstone, with rare wackestone.<br />
No dolomite has been recognized in the<br />
stained slab surfaces. This is consistent with the<br />
results from XRF analysis with average MgO contents<br />
at 0.53%. A very steep, southwesterly inclining<br />
unit overlies the conformable <strong>Khao</strong> <strong>Pun</strong> Klang<br />
Member with abrupt contact and abnormally<br />
underlying the older Rong Ngan Kaeng Khoi<br />
Member with thrust-fault contact.<br />
A highly weathered shale unit was observed to<br />
the north, near the study area. The observed 510-<br />
meter-thick mudstone unit predominantly consists<br />
<strong>of</strong> greenish-gray to yellowish-gray slaty shale or<br />
shale, rarely interbedded with argillaceous limestone.<br />
Its bedding planes were mostly inclined to<br />
the south and conformed with the other units<br />
in the present study area. Hinthong et al. (1985)<br />
mapped the rock unit in the north <strong>of</strong> the present<br />
study area as the <strong>Khao</strong> Khad Formation; however,<br />
we believe this unit could be re-mapped as the<br />
Pang Asok Formation <strong>of</strong> Hinthong et al. (1985)<br />
because <strong>of</strong> its similar lithology and overall thickness.<br />
We do not have sufficient paleontological data<br />
from this member. However, lithological features<br />
<strong>of</strong> this member may be correlated with those <strong>of</strong> the<br />
Facies 6 by Dawson & Racey (1993). Therefore, the<br />
age <strong>of</strong> this member is inferred as late Middle to<br />
early Late <strong>Permian</strong>.<br />
IGNEOUS ROCKS<br />
Igneous rocks in the present study area consist<br />
entirely <strong>of</strong> dark-green to greenish-gray hypabyssal<br />
rocks forming minor intrusions, such as dikes<br />
and sills. Geochemical results (Chutakositkanon<br />
1996) reveal that the rocks are <strong>of</strong> basaltic-andesitic<br />
to andesitic composition. The 40 Ar/ 39 Ar dating<br />
(Charusiri et al. 1999a) for hypabyssal dikes in this<br />
study area and nearby volcanics indicates the age<br />
is Jurassic, probably indicating the younger phase<br />
<strong>of</strong> more widely exposed Permo-Triassic plutonics<br />
(Phra Ngam Diorite) and volcanics (<strong>Khao</strong> Yai Volcanics).<br />
Petrographic investigation (Fig. 3 h) <strong>of</strong> the<br />
hypabyssal rocks reveals that the rocks consist<br />
mainly <strong>of</strong> relatively large-sized alkali feldspar,<br />
amphibole and brown biotite phenocrysts (up to<br />
2 cm) and a groundmass <strong>of</strong> plagioclase prism<br />
and quartz. Lath-shaped groundmass plagioclases<br />
exhibit a well-defined trachytic texture, possibly<br />
suggesting that the rocks formed at a shallow<br />
depth.<br />
GEOLOGIC STRUCTURES<br />
Generally, the bedding planes are very steep to<br />
moderate (40°–70°) and inclined towards SW to<br />
SSW directions with E to ESE trends. Subsurface<br />
studies indicate that several sedimentary structures<br />
noticed in core samples from the <strong>Khao</strong> <strong>Pun</strong><br />
Klang Member, such as normal-graded beddings,<br />
cross-laminations, scourings and flame structures,<br />
prove the normal or right-side-up sequence in the<br />
<strong>Khao</strong> <strong>Pun</strong> area (Chutakositkanon 1996), not the<br />
up-side-down sequence. These intermittently conformable<br />
sedimentary <strong>sequences</strong> may belong to<br />
the southern flank <strong>of</strong> ‘a large gently westward<br />
plunging antiformal anticline’ whose core lies<br />
somewhere further north <strong>of</strong> this study area, following<br />
the idea <strong>of</strong> Abele & Beeser (1963). However,<br />
we cannot prove the anticlinorium proposed by<br />
Hinthong (1981) and Pothong (1986).<br />
The NNW-trending thrust fault is observed in<br />
the southwestern part <strong>of</strong> the study area; whereas,<br />
the NE- to ENE-trending strike-slip faults are<br />
determined in the center, with the opposite sense<br />
<strong>of</strong> movement (Fig. 1c).<br />
DISCUSSION ON TECTONIC EVOLUTION<br />
AND PALEOENVIRONMENTS<br />
In tectonic terms, the <strong>Khao</strong> <strong>Pun</strong> area (Saraburi<br />
Province) is situated in the westernmost part <strong>of</strong><br />
the Indochina microcontinent (Bunopas 1981,<br />
1992; Pothong 1986; Bunopas & Vella 1992;<br />
Charusiri et al. 1999b).
<strong>Permian</strong> <strong>marine</strong> <strong>sequences</strong> in the Kao <strong>Pun</strong> 181<br />
After the Hercynian Orogeny (Middle Carboniferous),<br />
intermittent <strong>marine</strong> deposition took<br />
place in this region. According to Abele & Beeser<br />
(1963), Tittirananda (1976) and Wielchowsky &<br />
Young (1985), the sedimentation occurred in a<br />
shallow <strong>marine</strong> or shelf sea/platform environment<br />
in the <strong>Permian</strong> period. Recently, Dawson & Racey<br />
(1993) have proposed that the <strong>Permian</strong> limestone<br />
<strong>of</strong> Central Thailand is a supratidal sequence to<br />
outer platform bi<strong>of</strong>acies and consists <strong>of</strong> a Lower<br />
and an upper Middle Permain transgressive/<br />
regressive carbonate platform sequence. Paleoenvironmentally,<br />
their interpretation is appropriate<br />
for the more regional area than the limited<br />
<strong>Khao</strong> <strong>Pun</strong> area. Essentially, the depositional<br />
environment in this area must be analyzed by<br />
applying the <strong>marine</strong> carbonate depositional<br />
models <strong>of</strong> Irwin (1965), Tucker (1981) and Selley<br />
(1985).<br />
In this discussion, we explain the evolution <strong>of</strong><br />
the study area and areas nearby by dividing it into<br />
six stages. A depositional model for the development<br />
<strong>of</strong> the <strong>Khao</strong> <strong>Pun</strong> area during the Early to<br />
Late <strong>Permian</strong> time is shown (Fig. 5).<br />
THE FIRST STAGE<br />
In the first stage, we grouped rocks occurring in<br />
the <strong>Khao</strong> <strong>Pun</strong> area and areas nearby that were<br />
deposited in a relatively stable condition <strong>of</strong> a presumably<br />
and temporarily inactive margin during<br />
Early Middle <strong>Permian</strong>.<br />
The limestones <strong>of</strong> the Phu Phe and, presumably,<br />
the <strong>Khao</strong> Khwang Formations are dominantly<br />
gray-fossiliferous wackestone or biomicrite to<br />
biomicrosparite with fusulinids and bivalve shells<br />
indicating Early <strong>Permian</strong>. Although the Phu Phe<br />
Formation <strong>of</strong> the study area indicates the sheltered<br />
shallow or lagoonal environment, the occurrence<br />
<strong>of</strong> limestone-bedded chert alternation in<br />
the upper part <strong>of</strong> the <strong>Khao</strong> Khwang Formation, as<br />
defined by Hinthong (1981), probably indicates the<br />
deeper environment. Geochemical data (high<br />
values <strong>of</strong> CaO and LOI, and very low values <strong>of</strong><br />
SiO 2 , Na 2 O and K 2 O) as well as the occurrence <strong>of</strong><br />
micrite or microsparite matrix and recrystallized<br />
carbonate allochems <strong>of</strong> the Phu Phe Formation in<br />
the study area, indicate that no detrital or landderived<br />
minerals, such as quartz, feldspar or clay<br />
Fig. 5 Model <strong>of</strong> depositional environment <strong>of</strong> the <strong>Khao</strong> <strong>Pun</strong> <strong>Area</strong> during <strong>Permian</strong> Period. Stage 1, Phu Phe Formation; Early Stage 2, <strong>Khao</strong> Sung Nua<br />
Member; Late Stage 2, <strong>Khao</strong> Sung Klang Member; Stage 3, <strong>Khao</strong> Sung Tai Member; Stage 4, <strong>Khao</strong> <strong>Pun</strong> Nua Member; Stage 5, <strong>Khao</strong> <strong>Pun</strong> Klang Member;<br />
Stage 6, <strong>Khao</strong> <strong>Pun</strong> Tai Member. CCD, Carbonate compensation depth.
182 V. Chutakositkanon et al.<br />
minerals, are involved in this carbonate diagenesis,<br />
possibly due to the very low relief landmass or,<br />
in other words, the landmass providing terrigeneous<br />
sediments was located far apart at this<br />
stage.<br />
Towards the late stage, the deposition recognized<br />
by the interbedded limestone-shale <strong>of</strong> the<br />
Nong Pong Formation (Hinthong 1981) and the<br />
shale-dominated sequence <strong>of</strong> the Pang Asok Formation<br />
(Hinthong 1981) still occurs in restricted<br />
<strong>marine</strong> areas or a lagoon, but it was supported by<br />
very finely grained clastic sediments and organic<br />
matter during Middle <strong>Permian</strong>.<br />
THE SECOND STAGE<br />
This stage, characterized by the development <strong>of</strong><br />
the interbedded chert-argillaceous limestone lith<strong>of</strong>acies<br />
<strong>of</strong> the <strong>Khao</strong> Sung Nua Member, had the<br />
depositional condition during Middle <strong>Permian</strong> <strong>of</strong><br />
an open-sea or outer-shelf environment with sedimentation<br />
below wavebase and away from bottom<br />
currents. The fine-grained size indicates deposition<br />
in quite a low-energy <strong>marine</strong> environment,<br />
while the rarity <strong>of</strong> fauna and the absence <strong>of</strong><br />
benthonic algae imply that the floor was below the<br />
limits <strong>of</strong> the photic zone. The lithological and paleontological<br />
backgrounds indicate that the depositional<br />
basin may have been deeper than that <strong>of</strong><br />
the first stage, as evidenced by the occurrence <strong>of</strong><br />
bedded chert and the preservation <strong>of</strong> spherical<br />
radiolarians. Chert associated with micrite limestone<br />
indicates that deposition occurred at depths<br />
<strong>of</strong> perhaps 200–1000 m (Boggs 1995).<br />
At the end <strong>of</strong> this stage, the sedimentary basin<br />
was shallower and locally evinced by the carbonate<br />
build up <strong>of</strong> the <strong>Khao</strong> Sung Klang Member,<br />
which is indicative <strong>of</strong> the shallow high-energy<br />
<strong>marine</strong> environment <strong>of</strong> migrating shoals and<br />
banks. The observed fossils, such as both sessile<br />
benthos and encrusting colonial organisms (including<br />
crinoids, calcareous algae, bryozoans and rare<br />
corals), <strong>of</strong> this carbonate are ecologically zoned<br />
with respect to the barrier or reef mounds.<br />
THE THIRD STAGE<br />
Lithologically and paleontologically, the third<br />
stage is distinguished by deep <strong>marine</strong> deposits <strong>of</strong><br />
laminated radiolarian chert with some porcellanite<br />
and ironstone. Radiolarians, siliceous pelagic<br />
organisms, have been thought traditionally to<br />
accumulate in sediments on the deep ocean floor<br />
and to be originally amorphous opaline silica<br />
(Tucker 1981). The radiolarians in the <strong>Khao</strong> Sung<br />
Tai Member were converted to opal-CT as indicated<br />
by X-ray diffraction method (XRD) analysis<br />
(Chutakositkanon 1996), also called disordered<br />
cristobalite, alpha-cristobalite or lussatite (Jones<br />
& Segnit 1971), showing low first-order interference<br />
colors in crossed polars. Opaline silica is<br />
metastable and then decreases in abundance back<br />
through time and is absent from Paleozoic cherts<br />
(Tucker 1981). Although the data are limited, we<br />
suggest that this stage occurred during middle<br />
Middle to early Late <strong>Permian</strong>, based on the radiolarian<br />
species, Follicucullus sp.<br />
By analogy with modern siliceous oozes (Tucker<br />
1981; Selley 1985), radiolarian-rich cherts <strong>of</strong> the<br />
third stage are interpreted as deep water in an<br />
abyssal area where depths exceed the carbonate<br />
compensation depth (CCD), ~ 4.5 km (Garrison &<br />
Fischer 1969; Tucker 1981). With regard to the<br />
presence <strong>of</strong> radiolarian chert in the third stage, the<br />
present study may be the first that evince the open<br />
ocean and pelagic or abyssal environment below<br />
the CCD, far from continental influence in middle<br />
Middle to early Late <strong>Permian</strong> in the southern<br />
end <strong>of</strong> the Phetchabun–Saraburi trend, because<br />
several authors believe that the <strong>Permian</strong> limestone<br />
in this region is merely a sequence <strong>of</strong> supratidal<br />
to outer platform <strong>of</strong> shelf deposits. According to<br />
Helmcke & Kraikhong (1982), Helmcke & Lindenberg<br />
(1983), Winkel et al. (1983), Helmcke et al.<br />
(1985) and Wielchowsky & Young (1985), the existence<br />
<strong>of</strong> deep-<strong>marine</strong> depositional environments<br />
was also reported in Phethcabun, further to the<br />
north <strong>of</strong> the study area during Early <strong>Permian</strong> to<br />
late Middle <strong>Permian</strong>. Their studies give us a paleo-<br />
Phetchabun-Saraburi Tethys trend during the<br />
<strong>Permian</strong>, with slight facies changes from north to<br />
south, ranging from Early <strong>Permian</strong> to late Middle<br />
<strong>Permian</strong> in the northern part, to middle Middle to<br />
early Late <strong>Permian</strong> in its southern end.<br />
In addition to the paleomagnetic result <strong>of</strong><br />
Bunopas & Vella (1983, 1992) for the <strong>Permian</strong><br />
rocks <strong>of</strong> Shan-Thai Terrane, the mean paleolatitude<br />
for the <strong>Permian</strong> is between 0° S and 10° S.<br />
Unfortunately, there are no paleomagnetic data<br />
from the Paleozoic rocks <strong>of</strong> Indochina in their<br />
studies. Paleontologically radiolarian oozes at<br />
present occur in the equatorial region <strong>of</strong> the Pacific<br />
and Indian Oceans where the depth surpasses<br />
the CCD (Tucker 1981; Selley 1985). This might<br />
be the evidence which indicates that the paleoposition<br />
<strong>of</strong> the Indochina microcontinent during the<br />
third stage in the <strong>Permian</strong> moved to low latitudes,<br />
presumably in the paleoequatorial zone at that
<strong>Permian</strong> <strong>marine</strong> <strong>sequences</strong> in the Kao <strong>Pun</strong> 183<br />
time, which overlapped the present equatorial<br />
zone.<br />
THE FOURTH STAGE<br />
In the fourth stage, the depositional environment<br />
was thought to be a barrier during late Middle<br />
<strong>Permian</strong>, following the complex carbonate shelf<br />
model <strong>of</strong> Selley (1985). Generally, this stage is recognized<br />
in the <strong>Khao</strong> <strong>Pun</strong> Nua Member. The presence<br />
<strong>of</strong> skeletal floatstone, rudstone, grainstone<br />
and wackestone, is paleoecologically interpreted<br />
to represent non-reef bioherm or mound-like<br />
accumulation. The recognized fossils (both sessile<br />
benthos and encrusting colonial organisms, including<br />
crinoids, calcareous algae, bryozoans, and rare<br />
fusulinids and small foraminifers) are skeletal or<br />
fragmental debris. Although true corals in this<br />
member are relatively rare, similarly to the<br />
<strong>Permian</strong> reefs <strong>of</strong> West Texas (Selley 1985), these<br />
fossils still indicate that the depositional environment<br />
<strong>of</strong> the fourth stage must be barrier reef or<br />
skeletal mound (migrating shoals and banks).<br />
With regard to the subsurface lithostratigraphy<br />
<strong>of</strong> the <strong>Khao</strong> <strong>Pun</strong> Nua Member, the latest <strong>of</strong> the<br />
fourth stage is characterized by the transition or<br />
intercalation <strong>of</strong> reddish-brown calcareous shale<br />
and argillaceous limestone/black shale at the upper<br />
part <strong>of</strong> the <strong>Khao</strong> <strong>Pun</strong> Nua Member. Its character<br />
presumably indicates transgression/regression.<br />
During transgression, the carbonate and finegrained<br />
clastic sedimentation on the barrier was<br />
accompanied by anaerobic-starved conditions. This<br />
transgression alternated with a regressive drop<br />
in sea level, and the barrier was temporarily and<br />
intermittently subaerially exposed and cemented<br />
by the oxidizing condition.<br />
THE FIFTH STAGE<br />
The fifth stage is marked by the sequence <strong>of</strong> the<br />
late Middle <strong>Permian</strong> mudstone <strong>of</strong> the <strong>Khao</strong> <strong>Pun</strong><br />
Klang Member. This black shale is devoid <strong>of</strong> fossils<br />
and, with the disseminated pyrite grains and<br />
framboids, may indicate an oxygen deficiency. The<br />
organic matter may be preserved at depth, but<br />
the surface sediments could still support benthic<br />
epifauna; where there are anoxic conditions on the<br />
seafloor, there is invariably plentiful H 2 S in the<br />
water and benthic organisms are absent. Both<br />
lithological and paleontological lines <strong>of</strong> evidence<br />
for the <strong>Khao</strong> <strong>Pun</strong> Klang sequence strongly point<br />
to the depositional environment <strong>of</strong> limited circulation<br />
and anaerobic conditions in the restricted<br />
<strong>marine</strong> or lagoon with sedimentation below effective<br />
wave base, cut <strong>of</strong>f from the open sea by the<br />
bars <strong>of</strong> skeletal limestones <strong>of</strong> the <strong>Khao</strong> <strong>Pun</strong> Nua<br />
Member.<br />
THE SIXTH STAGE<br />
Towards the sixth stage during late Middle to<br />
early Late <strong>Permian</strong> time, the depositional environment<br />
was rather quiet and conditions were<br />
stable. The last stage is distinguished by the homogeneous<br />
and light-colored limestone <strong>of</strong> the <strong>Khao</strong><br />
<strong>Pun</strong> Tai Member and the shale-dominated succession<br />
<strong>of</strong> the Sap Bon Formation (Hinthong 1981).<br />
The <strong>Khao</strong> <strong>Pun</strong> Tai Member is composed entirely<br />
<strong>of</strong> fine-grained microsparry calcite (average 6 mm)<br />
with a minority <strong>of</strong> sparry calcite cement in fenestral<br />
pores that parallel the bedding plane. From<br />
this viewpoint, the <strong>Khao</strong> <strong>Pun</strong> Tai Member is likely<br />
to deposit from the tidal flat to the sheltered<br />
lagoon where there are no land-derived sediments<br />
with oxidizing conditions on the passive margin.<br />
When comparing the paleoenvironments <strong>of</strong> the<br />
fourth to sixth stages with the carbonate shelf<br />
model <strong>of</strong> Selley (1985), the major regression from<br />
the barrier <strong>of</strong> the fourth stage to the fifth stage is<br />
characterized by movement from a lagoon with<br />
limited circulation and anaerobic conditions on the<br />
tidal flat to the sheltered lagoon <strong>of</strong> the sixth stage.<br />
Although, with little evidence and less clarity<br />
in the northern extension in southern Yunan<br />
(Charusiri et al. 1999b), it must be emphasized that<br />
the <strong>Permian</strong> <strong>sequences</strong> <strong>of</strong> central Thailand are<br />
similar to those <strong>of</strong> Loei (Charusiri et al. 1998) and<br />
Pichit-Phetchabun (T. Nuchanong, pers. comm.,<br />
1998). Charusiri et al. (1998) considered that the<br />
Loei <strong>Permian</strong> <strong>sequences</strong> are atoll-like carbonate<br />
buildups associated with bedded cherts and a<br />
scarcity <strong>of</strong> <strong>marine</strong> clastics deposited onto the<br />
Carboniferous Paleotethyan basaltic ocean-floor<br />
(Intasopa 1993) in Loei, and with tuffs in Pichit<br />
(T. Nuchanong, pers. comm., 1990) intervening in<br />
Shan-Thai and Indochina. This stable (or passive)<br />
ocean floor subsequently collided with the<br />
westernmost part <strong>of</strong> Indochina where its Upper<br />
Paleozoic strata are characterized by the silicic<br />
shelf-dominated <strong>sequences</strong> <strong>of</strong> continental margin<br />
(Wielchowsky & Young 1985). All the <strong>Khao</strong> <strong>Pun</strong><br />
carbonates are, therefore, tectonically regarded<br />
as unlikely to represent the carbonate-shelf sediments<br />
deposited immediately at the continental<br />
margin, as previously thought. In addition, based<br />
on the airborne geophysical interpretation by<br />
Tulyatid & Charusiri (1999), carbonates <strong>of</strong> the
184 V. Chutakositkanon et al.<br />
Saraburi Group are surrounded by a high magnetic<br />
anomaly. Tulyatid & Charusiri (1999) interpreted<br />
this to indicate the occurrence <strong>of</strong> the<br />
paleo-ocean floor prior to the <strong>Permian</strong> times.<br />
Therefore, we consider that the <strong>Permian</strong> carbonates<br />
may have been deposited on the ocean<br />
floor.<br />
After the <strong>Permian</strong>, or during the Permo-Triassic<br />
periods <strong>of</strong> <strong>marine</strong> sedimentation, the place grew<br />
the venue <strong>of</strong> the folding/faulting. Because the<br />
Indosinian Orogeny was generally sited in this<br />
region, it was thought to be the one dominant,<br />
significant structural feature. The Indosinian<br />
Orogeny that took place between Triassic to Early<br />
Jurassic (Workman 1975; Charusiri 1989) brought<br />
about the plate interaction and collision between<br />
Indochina to the east and the Shan-Thai to the<br />
west and, hence, finished the <strong>marine</strong> deposition in<br />
now-mainland Thailand (Bunopas & Vella 1983,<br />
1992). The thrusting nature in the area may indicate<br />
the compressional tectonics. In accordance<br />
with the paleomagnetic (Bunopas 1981) and dating<br />
(Charusiri 1989) data, the Indosinian Orogeny and<br />
the collision <strong>of</strong> both microcontinents developed in<br />
the Early Mesozoic era. The <strong>Permian</strong> carbonate<br />
rocks in the study area were crosscut by the<br />
andesitic dyke; the hornblende from this dyke<br />
yielded the<br />
40 Ar/ 39 Ar date at Early Jurassic<br />
(Charusiri et al. 1999a). In addition to the north <strong>of</strong><br />
the study area, the andesite volcanics in the Lam<br />
Narai area, Lopburi, were dated by Charusiri<br />
et al. (1999a), using the same method, to be Early<br />
Jurassic. Petrochemical investigation (Charusiri<br />
et al. 1999a) reveals that these volcanics have<br />
magmatic arc-affinity. Therefore, with regard to<br />
the dating results and petrogenetic investigation,<br />
we infer that the occurrence <strong>of</strong> these volcanics are<br />
indicative <strong>of</strong> the recurrence <strong>of</strong> E-dipping oceanic<br />
subduction beneath the Indochina block (Fig. 6).<br />
Additional evidence <strong>of</strong> such tectonic setting is supported<br />
by K–Ar dating results <strong>of</strong> volcanic rocks in<br />
Lao PDR, further to the north, as noted by Stokes<br />
et al. (1996). If such a scenario is correct, there<br />
must be a paleo-ocean floor in-between the Shan-<br />
Thai block to the west and the Indochina block to<br />
the east, the so-called ‘Nakhon Thai block’, as mentioned<br />
by Charusiri et al. (1999b). Consequently,<br />
the oceanic slab may have existed along the suture<br />
zone <strong>of</strong> the Indochina and the Nakhon Thai block,<br />
and may have rejuvenated during Early Jurassic<br />
period.<br />
From Jurassic to Cretaceous (Charusiri 1989) or<br />
Early Tertiary period, a shear tectonism took<br />
place with the occurrence <strong>of</strong> sinistral and dextral<br />
Fig. 6 Simplified tectonic model for the development <strong>of</strong> the <strong>Khao</strong> <strong>Pun</strong><br />
<strong>Area</strong>, (a) Carboniferous-<strong>Permian</strong>; (b) Triassic-Early Jurassic. 1, <strong>Permian</strong><br />
atoll-like carbonate–buildup in association with bedded cherts deposited<br />
onto the Carboniferous Paleotethyan basaltic ocean-floor in Loei<br />
(Charusiri et al. 1998); 2, Age <strong>of</strong> volcanics and plutonics <strong>of</strong> the <strong>Khao</strong> Yai<br />
Volcanics and Phra Ngam Diorite (Hinthong 1981; Hinthong et al. 1985);<br />
3, The 40 Ar/ 39 Ar dating (Charusiri et al. 1999a) for hypabyssal dikes in the<br />
<strong>Khao</strong> <strong>Pun</strong> <strong>Area</strong> and nearby volcanics.<br />
<strong>of</strong>fset faults. Charusiri (1989) noted that the sinistral<br />
<strong>of</strong>fset along the Mae Ping Fault occurred as a<br />
result <strong>of</strong> the collision (compression tectonics)<br />
between the Shan-Thai and Western Burma microcontinents<br />
at the end <strong>of</strong> the Cretaceous period.<br />
The NE–SW structural trend in this area could<br />
have resulted from the Mae Ping Fault drag. The<br />
alternative definitions for the NW structural<br />
trending here are not plausible, because there are<br />
insufficient data collected in this small area.<br />
After the Mesozoic era, the last orogenic episode<br />
took place, that is, the 45–50 Ma Himalayan<br />
Orogeny (India–Asia collision). The significant<br />
deformation associated with the development <strong>of</strong><br />
the tensional tectonic regime resulted in the<br />
normal faulting in the study area.<br />
CONCLUSIONS<br />
The results obtained from new data <strong>of</strong> lithostratigraphy,<br />
lithology, sedimentology, paleontology<br />
and geochemistry, are used to interpret the paleoenvironments<br />
and associated tectonic settings <strong>of</strong>
<strong>Permian</strong> <strong>marine</strong> <strong>sequences</strong> in the Kao <strong>Pun</strong> 185<br />
the <strong>Khao</strong> <strong>Pun</strong> area. During the <strong>Permian</strong> period,<br />
the evolution <strong>of</strong> the <strong>marine</strong> sedimentation which<br />
took place in the western margin <strong>of</strong> the Indochina<br />
microcontinent can be divided into six stages.<br />
Regarding the character <strong>of</strong> the oldest formation,<br />
the <strong>Khao</strong> <strong>Pun</strong> area was under the relatively and<br />
temporarily stable conditions <strong>of</strong> an inactive margin<br />
with sheltered shallow or lagoonal environment in<br />
the first stage. The second stage, distinguished<br />
by chert and argillaceous limestone interbeds,<br />
indicates the deposition in a low-energy <strong>marine</strong><br />
environment such as an open sea or outer shelf<br />
with sedimentation below wavebase and away<br />
from bottom currents on the Indochina microcontinental<br />
margin. Towards the end <strong>of</strong> this stage, the<br />
depositional basin was locally shown by the carbonate<br />
build up and signified the shallow highenergy<br />
<strong>marine</strong> environment <strong>of</strong> the barrier. By<br />
analogy with modern siliceous oozes, radiolarian<br />
chert in the member <strong>of</strong> the third stage is interpreted<br />
to represent an abyssal area where depths<br />
exceed the CCD. This might have guided the paleoposition<br />
<strong>of</strong> the microcontinent during the third<br />
stage and was moved to low latitudes, possibly in<br />
the paleoequatorial zone.<br />
In the fourth stage, the depositional environment<br />
was generally thought to be a barrier or<br />
skeletal mound with transgressive/regressive<br />
changes by lithological and paleontological characters.<br />
The fifth stage is recorded by the black mudstone<br />
with sporadically disseminated pyrite grains<br />
and framboids in texture. Both lithological and<br />
paleontological lines <strong>of</strong> evidence for the sequence<br />
in this stage disclose the depositional environment<br />
<strong>of</strong> limited circulation and anoxic conditions in the<br />
restricted <strong>marine</strong> environment <strong>of</strong> the microcontinental<br />
margin. During the last stage, the paleoenvironment<br />
<strong>of</strong> the Indochina microcontinent was<br />
in the quiet and stable condition <strong>of</strong> a temporarily<br />
inactive margin, and was distinguished by very<br />
light-colored, homogeneous and very fine-grained<br />
limestone. Its depositional environment might be<br />
interpreted as tidal flat to sheltered lagoon uninfluenced<br />
by land-derived sediments.<br />
ACKNOWLEDGEMENTS<br />
Grateful and sincere acknowledgements are<br />
extended to the Thailand Research Fund and Siam<br />
Cement Public Company Ltd for the kind financial<br />
support and geochemical analysis, with special<br />
thanks to the Kaeng Khoi Factory, Saraburi<br />
Province, for support in labor and convenience<br />
during the field investigations. Assistance in the<br />
field by Mr Suvapak Imsamut (Geologic Survey<br />
Division, Department <strong>of</strong> Mineral Resources) is<br />
greatly acknowledged. The authors express their<br />
gratitude to Mr Sone Bhongaraya (graduate<br />
student in Geology, Chulalongkorn University),<br />
who made time available to help us. Furthermore,<br />
thanks are extended to the Department <strong>of</strong><br />
Geology, Faculty <strong>of</strong> Science, Chulalongkorn University<br />
for some instrumental support in the<br />
course <strong>of</strong> the study.<br />
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