research article lithostratigraphy of permian marine

Research ArticleLithostratigraphy of Permian marine sequences, Khao Pun Area,

central Thailand: Paleoenvironments and tectonic history

VICHAI CHUTAKOSITKANON,1 PUNYA CHARUSIRI1 AND KATSUO SASHIDA2*1Department of Geology, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand and 2Institute

of Geoscience, University of Tsukuba, Ibaraki 305-8571, Japan (email: [email protected])

Abstract Geologic mapping and subsurface lithostratigraphic investigations were carriedout in the Khao Pun area (4 km2), central Thailand. More than 250 hand specimens, 70 rockslabs, and 70 thin sections were studied in conjunction with geochemical data in order toelucidate paleoenvironments and tectonic setting of the Permian marine sedimentarysequences. This sedimentary succession (2485 m thick) was re-accessed and re-groupedinto three lithostratigraphic units, namely, in ascending order, the Phu Phe, Khao Sungand Khao Pun Formations. The Lower to lower Upper Permian sedimentary facies indi-cated the transgressive/regressive succession of shelf sea/platform environment to pelagicor abyssal environment below the carbonate compensation depth. The sedimentologicaland paleontological aspects, together with petrochemical and lithological points of view,reveal that the oldest unit might indicate an Early Permian sheltered shallow or lagoonalenvironment. Then the depositional basin became deeper, as suggested by the prolongedoccurrence of bedded chert-limestone intercalation with the local exposure of shallowercarbonate build-up. Following this, the depositional environment changed to pelagic depo-sition, as indicated by laminated radiolarian (e.g. Follicucullus sp.) cherts. This crypticevidence might indicate the abyssal environment during middle Middle to early LatePermian; whereas, previous studies advocated shelf-facies environments. Following this,the depositional condition might be a major regression on the microcontinent close toIndochina, from the minor transgressive/regressive cycles that developed within a skele-tal barrier, and through the lagoon with limited circulational and anaerobic conditions, onto the tidal flat to the sheltered lagoon without effective land-derived sediments.

Key words: central Thailand, marine sequences, paleoenvironment, pelagic, Permian, Radiolaria, shelf, tectonics.

stone and the other Permian limestones through-out the country as the Ratburi Limestone. Borax & Stewart (1966) worked on the Paleozoic strati-graphic correlation of northeastern Thailand. Tittirananda’s (1976) PhD dissertation was on thestratigraphy and paleontology of the limestonesalong Highway no. 21. Hinthong et al. (1985) systematically compiled the geology of the entire1:250 000-scale topographic map sheet of ND 47–8 (Changwat Phra Nakhon Si Ayutthaya). Wielchowsky & Young (1985) studied lithofacies inthe Permian rocks of the Phetchabun Fold Belt.More detailed studies on the structures in thisregion were carried out by Pothong (1986). Thename ‘Saraburi Group’ in place of the name Ratburi

INTRODUCTION

The Khao Pun area, 4 km2 in size, is located in theKaeng Khoi District, Saraburi Province, centralThailand (Fig. 1). Physiographically, it consists of limestone mountains trending northwest-southeast, covered by a dense forest.

The Upper Paleozoic carbonate/clastic unit inthis region has been visited by several groups ofgeoscientists. One of the first studies was per-formed by Brown et al. (1951) who named this lime-

*Correspondence.

Accepted for publication 20 December 1999.© 2000 Blackwell Science Asia Pty Ltd.

The Island Arc (2000) 9, 173–187

174 V. Chutakositkanon et al.

Fig. 1 Index map. (a) Regional tectonic map of mainland SE Asia showing major tectonic blocks (Bunopas 1981, 1992). (b) Regional geologic map ofthe Saraburi-Kaeng Khoi area, central Thailand (after Hinthong et al. 1985) showing major units of the Saraburi Group and the locality of the study area(inserted block). , Phu Phe Formation; , Nong Pong Formation; , Pang Asok Formation; , Khao Khad Formation; ,Sap Bon Formation; , terrace gravel; , Recent flood plain; , Khao Yai Volcanics; , Phra Ngam Diorite; , river andstream; , fault; , thrust fault; S, Saraburi city; K, Kaneg Khoi town. (c) Simplified geologic map of the Khao Pun Area, central Thailand.

Group was first proposed by Bunopas (1981) andBunopas et al. (1988) for the Permian successionwhich distributes in the west of the Khorat Plateaumargin and the eastern part of the Central Plain(restricted to the Peninsula of Thailand). Recently,Dawson & Racey (1993) proposed the Permianstrata of central Thailand as a sequence of suprati-dal to outer platform facies comprising a Lower-upper Middle Permian transgressive/regressivecarbonate platform succession. Recently, a detailedsystematic study has been made on the Permianlithostratigraphy around the Khao Pun study areaat which the Kaeng Khoi Factory of the SiamCement Public Company Ltd (SCC) is situated (V. Chutakositkanon, pers. data, 1996).

The purposes of this study are, therefore, to present a detailed lithostratigraphy of thePermian rocks and to discuss the environment ofdeposition related to tectonic setting of the area.Depending on their use, several classifications ofcarbonate rocks are applied, following those ofFolk (1959, 1962), Dunham (1962), and Embry &Klovan (1971), slightly modified by the classifica-tion systems of Tucker (1981) and Adam et al.(1984). Approximately 30 spot locations werevisited and revisited, and seven main drilledholes were logged and lithologically described.Then, 70 rock slabs and 70 thin sections wereselectively prepared from more than 250 handspecimens collected from both field and drilled

cores. A chemical staining technique, described byFriedman (1959, 1977) and Friedman & Sternbach(1982), was used to aid identifying carbonate min-erals. Major- and minor-oxide analyses weremainly performed by Krongkaew et al. (1991) andsome by Chutakositkanon (1996).

However, all samples analyzed by Krongkaew etal. (1991) were composite samples; therefore, insome cases the results do not represent the geo-logic units and are not similar to those performedby Chutakositkanon (1996), which indicate actuallithologic units. In this regard, we decided to useonly the results from Chutakositkanon (1996),especially those for limestones in the Phu Phe Formation and for the mudrocks in the Khao PunKlang Member. However, in general, the results ofKrongkaew et al. (1991) are in agreement with ourresults.

REGIONAL GEOLOGIC SETTING

The Saraburi Group (Fig. 1b) in the current studyarea is in the southernmost part of the approxi-mately north-trending continuous outcrops of theUpper Paleozoic carbonate/clastic unit, extendingsouthwards at least 400 km from Lao PDR. TheSaraburi Group, exposed as a chain of limestonehills, ridges, knobs and mounds, is observablybounded by the youngers, the Khao Yai Volcanicsto the south, continental red-bed clastic rocks ofthe Mesozoic Khorat Group to the east and north-west, and Quaternary alluvial deposits of the ChaoPraya, the Lop Buri and the Pa Sak Rivers to thewest. This grouping takes place along the west-ernmost part of the Indochina (Indosinian inplaces) microcontinent (Bunopas 1992).

Several stratigraphers have tried to identify thecomplex stratigraphy here. Hinthong et al. (1985)subdivided the rocks, based upon stratification,fossils and structures, into six formations, fromolder to younger, as the Phu Phe, Khao Khwang,Nong Pong, Pang Asok, Khao Khad and Sap BonFormations, covering the entire southern limit ofthe Phetchabun–Saraburi trend.

Structurally, Abele & Beeser (1963) believed thatthe regional rock sequence belonged to the south-ern flank of ‘a large gently westward plunging anti-cline’ whose core lay somewhere further north. Thisgigantic structure is, however, mentioned as ananticlinorium by Hinthong (1981), N. Comviravong(unpubl. data, 1985) and Pothong (1986).

The rocks in this region were intruded by plutonics of Permo-Triassic Phra Ngam Diorite

(Hinthong 1981; Hinthong et al. 1985). Volcanics,dikes and sills of similar composition and age werealso found in the Permian country rocks of theSaraburi Group. Hinthong (1981) and Hinthong etal. (1985) assigned the volcanics the name, Permo-Triassic Khao Yai Volcanics.

Bryozoans, algae and fusulinids collected in carbonate rocks, as had been noted earlier byPitakpivan (1965), Borax & Stewart (oral presenta-tion at Economic Commission for Asia and the Far East Meeting in Bangkok, Thailand, ECAFE,1966), Tittirananda (1976), Dawson (1978a,b),Hinthong (1981), Ingavat-Helmcke (1993) andDawson & Racey (1993), indicate the ages fromEarly to Late-Permian. These rocks were alsonoted to have deposited in a shallow shelf sea orplatform environment.

LITHOSTRATIGRAPHY

The Upper Paleozoic sedimentary sequence in theKhao Pun area is composed mainly of limestones,subordinate mudstones and bedded cherts withminor andesitic hypabyssal rocks, such as dikesand sills. According to the works of Hinthong(1981) and Hinthong et al. (1985), the rock units inthe present study area were grouped as the Khao Khad and Phu Phe Formations. However,the current field and subsurface stratigraphicinvestigations allowed us to re-group the Permiansedimentary succession in the Khao Pun area from oldest to youngest, based essentially on thelithologic and physical appearances, stratificationsand sedimentary structures, into three litho-stratigraphic units, the Phu Phe, Khao Sung and Khao Pun Formations which are further subdivided into seven members as demonstrated(Fig. 2). In addition, 1796 composite samples (Krongkaew et al. 1991) and 14 systematicallyselected samples (Chutakositkanon 1996) fromindividual members were geochemically analyzedfor major-oxide contents; the general results are shown (in the lithostratigraphic column of Fig. 2).

PHU PHE FORMATION

The Phu Phe Formation (Hinthong 1981) in thestudy area consists of gray, thickly to very thickly bedded biomicrite and biomicrosparite orfossiliferous boundstone, packstone; some wacke-stone are distributed only in the southwestern-most portion. Regarding the chemical analysis

Permian marine sequences in the Kao Pun 175


(Chutakositkanon 1996), the low SiO2, Al2O3, K2Oand MgO contents (0.095, 0.02, < 0.01 and 1.34%,respectively) and high CaO content (53.6%) areconsidered to be limestones. Fusulinacean fossilscan also be identified and the following specieswere reported by Hinthong (1981): Pseudoschwa-gerina cf. toriyamai Igo, Pseudoschwagerina(Zellia) turbida Kahler, Paraschwagerina sp. andothers which indicate Early Permian Sakmarian.Notably, the bedding planes of the 85-meter-thickunit are steeply inclined to the northeast direction,directly opposite to the other units. This leads us

to the interpretation that the Phu Phe Formationwas possibly northward-upthrust on the youngestKhao Pun Formation. This Phu Phe Formation isalso called the Rong Ngan Kaeng Khoi Memberafter the Kaeng Khoi Factory of the SCC, to beconsistent with the members of the other units(Chutakositkanon 1996).

KHAO SUNG FORMATION

The Khao Sung Formation is entirely exposed inthe northern part and its features are clearly dis-

Fig. 2 Lithostratigraphic column of the Khao Pun Area and average major-oxide results of individual units. Lithostratigraphic column uses the classi-cal rock-section key shown in Selly (1985, Fig. 0.1). The grain size is drawn increasing to the left, following Wentworth grades. Numbers in parenthesesare the total number of rock samples analyzed. *Analysis was performed by Krongkaew et al. (1991) and Chutakositkanon (1996); **Chutakositkanon(1996) only; ***no analysis performed.

played as a straight ridge in aerial photographs.The bedding planes, striking parallel to the NW-trending ridge, dip steeply in the southwesterlydirection. Lithologically and structurally, this formation is distinguished from the others by thepresence of cherts and the absence of hypabyssalrocks. However, the Khao Sung Formation is com-posed principally of three conformable members,namely, from older to younger, the Khao SungNua, Khao Sung Klang and Khao Sung TaiMembers, respectively. The overall thickness ofthis formation is ~ 830 m.

Khao Sung Nua Member

The Khao Sung Nua Member, the lowest unit of the Khao Sung Formation, is composed of a 420-meter-thick uniform, southwestward-dippingsuccession of interbedding between thin- tomedium-bedded black argillaceous limestones,black-bedded chert (Fig. 3a) and black shale.However, the argillaceous limestones are likely tocontain a high percentage of organic materials anda very finely grained texture and are re-classified,herein, as micrite and biomicrite, or as mudstoneand wackestone. Surprisingly, the limestonescontain irregular-shaped microcrystalline quartzor chert with an average size of 5–7 mm. Individ-ual chert layers, invariably interbedded with blackargillaceous limestones, range in thickness from afew centimeters to tens of centimeters or more,and both upper and lower surfaces are wavy. Whenviewed under the polarizing microscope, thesebedded cherts are, in fact, porcellanite. The pres-ence of shale in the member is still problematicbecause the member is not exposed clearly as anoutcrop. The shale presence is interpreted fromblack, strongly weathered regoliths. Sphericalradiolarians, the most observable fossils, are pre-served very rarely in these siliceous sedimentaryrocks.

Khao Sung Klang Member

The Khao Sung Klang Member, the middle unit of the Khao Sung Formation, is obviously distin-guished because it is the only member in the for-mation that contains no cherts in succession. Themember consists mainly of pinkish-gray, thickly to very thickly bedded fossiliferous or crinoidalfloatstone, rudstone, packstone and grainstone or biointrasparite and intrabiosparite, with somepacked (over 50% allochems) biointramicrite and packed intrabiomicrite. The southwestward

dipping Khao Sung Klang Member, considered tobe a large lens-shaped sedimentary body with amaximum thickness of 200 m, conformably overliesthe Khao Sung Nua Member and underlies theKhao Sung Tai Member. The variable-sized fossilsof crinoid stem fragments, calcareous algae andbryozoans can be observed easily. Due to its lensappearance, the geochemical data are not requiredfor this study.

Khao Sung Tai Member

The youngest Khao Sung Tai Member of the KhaoSung Formation, exposed entirely as cobble-sizedloose blocks covering the southern foot of KhaoSung, is outstandingly characterized by gray todark gray laminated cherts (Fig. 3b) with someporcellanites and deep-red ironstones. Accordingto its chemical data (Chutakositkanon 1996), thehigh SiO2 content (approximately 89.9%) and smallCaO concentration (approximately 3.90%) of thislaminated siliceous sedimentary rock is regardedas a composition of chert to porcellanite followingthe classification of Krumbein & Sloss (1963). Thesection, carefully measured as ~ 210 m thick, over-lies the Khao Sung Nua and Khao Sung KlangMembers with abrupt contacts and underlies theKhao Pun Formation with a fault contact as a hugevertical cliff. Observed fossils are spherical radio-larians, sponge spicules and unidentified siliceousshell fragments, mostly aligned parallel to the lam-ination or banding (Fig. 3c). We discovered poorlypreserved radiolarians, probably Follicucullussp. (Fig. 4), suggesting an interval from middleMiddle Permian to early Late Permian. The occur-rence of the radiolarians places the age of the KhaoSung Formation or the equivalent Khao Khad Formation of Hinthong et al. (1985) younger thanthat proposed by Hinthong et al. (1985).

KHAO PUN FORMATION

The youngest and thickest (1570 m thick) forma-tion in the present study area is called the KhaoPun Formation. The conformable sedimentarysequence, moderate-dipping in the southwesterlydirection of the Khao Pun Formation, is set apartfrom the others by the presence of well-developedcalcite veins and veinlets, andesitic dikes and sillsof hypabyssal rocks, and the absence of chertlayers. The Khao Pun Formation overlies the KhaoSung Formation in the north and abnormallyunderlies the older Phu Phe Formation in thesouth with normal and thrust-fault contacts,



Fig. 3 Samples from the Khao Pun area. (a) Natural exposure of intercalated argillaceous limestone (L, thicker beds) and chert (C, thinner beds) withboth their upper and lower wavy surfaces in the Khao Sung Nua Member. (b) Rock slabs of laminated cherts from the Khao Sung Tai Member; bar =5 cm. (c) Photomicrograph of the laminated chert under crossed polars showing alignment and spherical outlines of radiolarians (R), bar = 0.5 mm. (d)Characteristics of well-bedded limestone strata of the Khao Pun Nua Member with extension cracks or gashes in the central part. (e) Fusulinid fragment(Verbeekina sp.) in a carbonate rock of the Khao Pun Nua Member, bar = 0.5 mm. (f) Road-cut exposure of the Khao Pun Klang Member showing theargillaceous limestone (L) intercalation in the slaty shale in the upper part of the member. (g) Normal graded-bedding and scoring in a core sample fromthe Khao Pun Klang Member proving the right-side-up sequence in the Khao Pun Area. Number indicates the depth (86.25 m) from surface of the drilledhole, bar = 5 cm. (h) Photomicrograph of andesite hypabyssal rock showing large biotite (B) and plagioclase (PL) phenocrysts with sericite alteration, bar = 0.5 mm.

respectively. Information obtained from bothsurface and subsurface investigation leads to thesubdivision of the Khao Pun Formation into threemajor conformable members, from lower to upper,including the Khao Pun Nua, Khao Pun Klang andKhao Pun Tai Members.

Khao Pun Nua Member

The 560-meter-thick Khao Pun Nua Member islargely exposed in the northern portions of KhaoPun and Khao Nong Kop as an E-trending strippassing the central part of the area. It consists generally of yellowish-gray or pinkish-gray todark-gray, thickly to very thickly bedded (Fig. 3d)biointrasparite and intrabiosparite, with somepacked biointramicrite and packed intrabiomicriteor fossiliferous or crinoidal floatstone, rudstone,packstone and grainstone. Dolomitic limestone,locally abundant, is exposed with a predominantweathering surface, the so-called ‘elephant skin’.Results from X-ray fluorescence analysis revealthat CaO averages 51.62% and MgO 1.18%. A value of MgO greater than 1% suggests that themineral dolomite is likely to be present. However,most carbonate rocks in this member are classifiedas limestone, following the classification of calcite-dolomite mixture (Pettijohn 1975) because themineral dolomite is less than 5% of the total calcite–dolomite mixture. From subsurface investigation,an intercalation zone between the yellowish-graylimestone with reddish-brown calcareous shale,and the black argillaceous limestone with blackshale can also be observed in this unit. The south-westerly dipping Khao Pun Nua Member overliesthe Khao Sung Tai Member with a fault contactthat formed as a large scarp and underlies the cor-respondingly dipping Khao Pun Klang Member

with either abrupt or intercalation contacts. TheKhao Pun Nua Member consists predominantly of several fossiliferous beds. Essential fossilsinclude crinoid stems, bryozoans and algae, with a minority of fusulinids (Fig. 3e) and smallforaminifers. The following late Middle Permianfusulinacean fossils have been identified from thismember, namely Verbeekina verbeeki (Geinitz),Verbeekina sp., Neoschwagerina megasphericaDeprat, Afghanella cf. sumatrinaeformis Gublerand others (Hinthong 1981). Crinoid stems are the most abundant fossils and can be observedthroughout the unit.

Khao Pun Klang Member

The thinnest, 180-meter-thick Khao Pun KlangMember mainly consists of high organic materialsand a very finely grained sedimentary arrange-ment of mudstones as slaty shale to shale, with theargillaceous limestone intercalations near upperand lower contacts (Fig. 3f). Microscopically, theblack color of mudstones in this unit is the resultof finely disseminated organic matter and pyrite.The black argillaceous limestone is re-classified asmicrite, quartz intramicrite and some as biomicriteor mudstone, wackestone and terrigenous quartzsilt-sized wackestone. Geochemically, our mud-stones contain averaged values of SiO2 65.0%,Al2O3 16.0%, Fe2O3 5.02%, K2O 2.89%, MgO 1.86%,and Na2O 0.77%. Like the general shales, majorminerals are clays and quartz; thus, the mostabundant oxides are SiO2 and Al2O3. The high per-centage of Fe2O3 might be due to the occurrence ofpyrite disseminated grains and framboids. TheTiO2 content (average 0.72%) in this mudstone ismuch higher than those of other sedimentary units(Chutakositkanon 1996, Table 5.2) and clearly distinguishes these rocks. However, averaged CaO content of SCC results (Krongkaew et al.1991) which is doubtfully higher than 16%, may be caused by the presence of calcite veins and veinlets in their samples. The southward to southwestward dipping, organic-rich, very finelygrained clastic/carbonate rock unit overlies thefossiliferous carbonate rock of the Khao Pun NuaMember and underlies the youngest Khao Pun TaiMember with the abrupt or intercalated contacts.Subsurface study reveals that several sedimentarystructures noticed in core-samples, such as normalgraded-bedding (Fig. 3 g), cross-lamination, andscouring and flame structures, prove the normal orright-side-up sequence. Although we do not haveenough chronological data for this member, it may


Fig. 4 SEM photographs of radiolarians (Follicucullus sp.) from theKhao Sung Tai Member, bar = 100 mm.


be placed as late Middle Permian, based on thestratigraphical relationships between overlyingand underlying members.

Khao Pun Tai Member

The 830-meter-thick Khao Pun Tai Member, thethickest and youngest unit, is clearly distinguishedfrom the other members by its light gray color andaphanitic texture. The term ‘lithographic lime-stone’ is used in field investigations for this dense,homogeneous and very fine-textured limestone.Although the limestone appears to be micrite inhand specimens, under the polarizing microscopethese micrite crystals are observed to be micro-sparry calcite with average grain size of ~6 mm, following Ehlers & Blatt (1982), contrasting withtypical micrite (1–4 mm) and typical microsparrycalcite (5–15 mm). From this viewpoint, it is possibleto classify the limestone in this unit as microspariteand dismicrosparite or mudstone, with rare wacke-stone. No dolomite has been recognized in thestained slab surfaces. This is consistent with theresults from XRF analysis with average MgO con-tents at 0.53%. A very steep, southwesterly inclin-ing unit overlies the conformable Khao Pun KlangMember with abrupt contact and abnormallyunderlying the older Rong Ngan Kaeng KhoiMember with thrust-fault contact.

A highly weathered shale unit was observed tothe north, near the study area. The observed 510-meter-thick mudstone unit predominantly consistsof greenish-gray to yellowish-gray slaty shale orshale, rarely interbedded with argillaceous lime-stone. Its bedding planes were mostly inclined tothe south and conformed with the other units in the present study area. Hinthong et al. (1985)mapped the rock unit in the north of the presentstudy area as the Khao Khad Formation; however,we believe this unit could be re-mapped as thePang Asok Formation of Hinthong et al. (1985)because of its similar lithology and overall thick-ness. We do not have sufficient paleontological datafrom this member. However, lithological featuresof this member may be correlated with those of theFacies 6 by Dawson & Racey (1993). Therefore, theage of this member is inferred as late Middle toearly Late Permian.

IGNEOUS ROCKS

Igneous rocks in the present study area consistentirely of dark-green to greenish-gray hypabyssal

rocks forming minor intrusions, such as dikes and sills. Geochemical results (Chutakositkanon1996) reveal that the rocks are of basaltic-andesitic to andesitic composition. The 40Ar/39Ar dating(Charusiri et al. 1999a) for hypabyssal dikes in thisstudy area and nearby volcanics indicates the ageis Jurassic, probably indicating the younger phaseof more widely exposed Permo-Triassic plutonics(Phra Ngam Diorite) and volcanics (Khao Yai Vol-canics). Petrographic investigation (Fig. 3 h) of thehypabyssal rocks reveals that the rocks consistmainly of relatively large-sized alkali feldspar,amphibole and brown biotite phenocrysts (up to 2 cm) and a groundmass of plagioclase prism and quartz. Lath-shaped groundmass plagioclasesexhibit a well-defined trachytic texture, possiblysuggesting that the rocks formed at a shallowdepth.

GEOLOGIC STRUCTURES

Generally, the bedding planes are very steep tomoderate (40°–70°) and inclined towards SW toSSW directions with E to ESE trends. Subsurfacestudies indicate that several sedimentary struc-tures noticed in core samples from the Khao PunKlang Member, such as normal-graded beddings,cross-laminations, scourings and flame structures,prove the normal or right-side-up sequence in theKhao Pun area (Chutakositkanon 1996), not the up-side-down sequence. These intermittently con-formable sedimentary sequences may belong tothe southern flank of ‘a large gently westwardplunging antiformal anticline’ whose core liessomewhere further north of this study area, fol-lowing the idea of Abele & Beeser (1963). However,we cannot prove the anticlinorium proposed byHinthong (1981) and Pothong (1986).

The NNW-trending thrust fault is observed inthe southwestern part of the study area; whereas,the NE- to ENE-trending strike-slip faults aredetermined in the center, with the opposite senseof movement (Fig. 1c).

DISCUSSION ON TECTONIC EVOLUTIONAND PALEOENVIRONMENTS

In tectonic terms, the Khao Pun area (SaraburiProvince) is situated in the westernmost part ofthe Indochina microcontinent (Bunopas 1981,1992; Pothong 1986; Bunopas & Vella 1992;Charusiri et al. 1999b).

After the Hercynian Orogeny (Middle Car-boniferous), intermittent marine deposition tookplace in this region. According to Abele & Beeser(1963), Tittirananda (1976) and Wielchowsky &Young (1985), the sedimentation occurred in ashallow marine or shelf sea/platform environmentin the Permian period. Recently, Dawson & Racey(1993) have proposed that the Permian limestoneof Central Thailand is a supratidal sequence toouter platform biofacies and consists of a Lowerand an upper Middle Permain transgressive/regressive carbonate platform sequence. Paleoen-vironmentally, their interpretation is appropriatefor the more regional area than the limited Khao Pun area. Essentially, the depositional environment in this area must be analyzed byapplying the marine carbonate depositionalmodels of Irwin (1965), Tucker (1981) and Selley(1985).

In this discussion, we explain the evolution ofthe study area and areas nearby by dividing it intosix stages. A depositional model for the develop-ment of the Khao Pun area during the Early toLate Permian time is shown (Fig. 5).

THE FIRST STAGE

In the first stage, we grouped rocks occurring inthe Khao Pun area and areas nearby that weredeposited in a relatively stable condition of a pre-sumably and temporarily inactive margin duringEarly Middle Permian.

The limestones of the Phu Phe and, presumably,the Khao Khwang Formations are dominantlygray-fossiliferous wackestone or biomicrite to biomicrosparite with fusulinids and bivalve shellsindicating Early Permian. Although the Phu PheFormation of the study area indicates the shel-tered shallow or lagoonal environment, the occur-rence of limestone-bedded chert alternation in the upper part of the Khao Khwang Formation, asdefined by Hinthong (1981), probably indicates thedeeper environment. Geochemical data (highvalues of CaO and LOI, and very low values ofSiO2, Na2O and K2O) as well as the occurrence ofmicrite or microsparite matrix and recrystallizedcarbonate allochems of the Phu Phe Formation inthe study area, indicate that no detrital or land-derived minerals, such as quartz, feldspar or clay


Fig. 5 Model of depositional environment of the Khao Pun Area during Permian Period. Stage 1, Phu Phe Formation; Early Stage 2, Khao Sung NuaMember; Late Stage 2, Khao Sung Klang Member; Stage 3, Khao Sung Tai Member; Stage 4, Khao Pun Nua Member; Stage 5, Khao Pun Klang Member;Stage 6, Khao Pun Tai Member. CCD, Carbonate compensation depth.


minerals, are involved in this carbonate diagene-sis, possibly due to the very low relief landmass or,in other words, the landmass providing terrige-neous sediments was located far apart at thisstage.

Towards the late stage, the deposition recog-nized by the interbedded limestone-shale of theNong Pong Formation (Hinthong 1981) and theshale-dominated sequence of the Pang Asok For-mation (Hinthong 1981) still occurs in restrictedmarine areas or a lagoon, but it was supported byvery finely grained clastic sediments and organicmatter during Middle Permian.

THE SECOND STAGE

This stage, characterized by the development ofthe interbedded chert-argillaceous limestone litho-facies of the Khao Sung Nua Member, had thedepositional condition during Middle Permian ofan open-sea or outer-shelf environment with sedi-mentation below wavebase and away from bottomcurrents. The fine-grained size indicates deposi-tion in quite a low-energy marine environment,while the rarity of fauna and the absence of benthonic algae imply that the floor was below thelimits of the photic zone. The lithological and pale-ontological backgrounds indicate that the deposi-tional basin may have been deeper than that of the first stage, as evidenced by the occurrence ofbedded chert and the preservation of sphericalradiolarians. Chert associated with micrite lime-stone indicates that deposition occurred at depthsof perhaps 200–1000 m (Boggs 1995).

At the end of this stage, the sedimentary basinwas shallower and locally evinced by the carbon-ate build up of the Khao Sung Klang Member,which is indicative of the shallow high-energymarine environment of migrating shoals andbanks. The observed fossils, such as both sessilebenthos and encrusting colonial organisms (includ-ing crinoids, calcareous algae, bryozoans and rarecorals), of this carbonate are ecologically zonedwith respect to the barrier or reef mounds.

THE THIRD STAGE

Lithologically and paleontologically, the thirdstage is distinguished by deep marine deposits oflaminated radiolarian chert with some porcellaniteand ironstone. Radiolarians, siliceous pelagicorganisms, have been thought traditionally toaccumulate in sediments on the deep ocean floorand to be originally amorphous opaline silica

(Tucker 1981). The radiolarians in the Khao SungTai Member were converted to opal-CT as indi-cated by X-ray diffraction method (XRD) analysis(Chutakositkanon 1996), also called disorderedcristobalite, alpha-cristobalite or lussatite (Jones& Segnit 1971), showing low first-order interfer-ence colors in crossed polars. Opaline silica ismetastable and then decreases in abundance backthrough time and is absent from Paleozoic cherts(Tucker 1981). Although the data are limited, wesuggest that this stage occurred during middleMiddle to early Late Permian, based on the radio-larian species, Follicucullus sp.

By analogy with modern siliceous oozes (Tucker1981; Selley 1985), radiolarian-rich cherts of thethird stage are interpreted as deep water in anabyssal area where depths exceed the carbonatecompensation depth (CCD), ~ 4.5 km (Garrison &Fischer 1969; Tucker 1981). With regard to thepresence of radiolarian chert in the third stage, thepresent study may be the first that evince the openocean and pelagic or abyssal environment belowthe CCD, far from continental influence in middleMiddle to early Late Permian in the southern end of the Phetchabun–Saraburi trend, becauseseveral authors believe that the Permian limestonein this region is merely a sequence of supratidal to outer platform of shelf deposits. According toHelmcke & Kraikhong (1982), Helmcke & Linden-berg (1983), Winkel et al. (1983), Helmcke et al.(1985) and Wielchowsky & Young (1985), the exis-tence of deep-marine depositional environmentswas also reported in Phethcabun, further to thenorth of the study area during Early Permian tolate Middle Permian. Their studies give us a paleo-Phetchabun-Saraburi Tethys trend during thePermian, with slight facies changes from north tosouth, ranging from Early Permian to late MiddlePermian in the northern part, to middle Middle toearly Late Permian in its southern end.

In addition to the paleomagnetic result ofBunopas & Vella (1983, 1992) for the Permianrocks of Shan-Thai Terrane, the mean paleolati-tude for the Permian is between 0° S and 10° S.Unfortunately, there are no paleomagnetic datafrom the Paleozoic rocks of Indochina in theirstudies. Paleontologically radiolarian oozes atpresent occur in the equatorial region of the Pacificand Indian Oceans where the depth surpasses the CCD (Tucker 1981; Selley 1985). This might be the evidence which indicates that the paleopo-sition of the Indochina microcontinent during thethird stage in the Permian moved to low latitudes,presumably in the paleoequatorial zone at that

time, which overlapped the present equatorialzone.

THE FOURTH STAGE

In the fourth stage, the depositional environmentwas thought to be a barrier during late MiddlePermian, following the complex carbonate shelfmodel of Selley (1985). Generally, this stage is rec-ognized in the Khao Pun Nua Member. The pres-ence of skeletal floatstone, rudstone, grainstoneand wackestone, is paleoecologically interpreted to represent non-reef bioherm or mound-like accumulation. The recognized fossils (both sessilebenthos and encrusting colonial organisms, includ-ing crinoids, calcareous algae, bryozoans, and rarefusulinids and small foraminifers) are skeletal orfragmental debris. Although true corals in thismember are relatively rare, similarly to thePermian reefs of West Texas (Selley 1985), thesefossils still indicate that the depositional environ-ment of the fourth stage must be barrier reef orskeletal mound (migrating shoals and banks).

With regard to the subsurface lithostratigraphyof the Khao Pun Nua Member, the latest of thefourth stage is characterized by the transition orintercalation of reddish-brown calcareous shaleand argillaceous limestone/black shale at the upperpart of the Khao Pun Nua Member. Its characterpresumably indicates transgression/regression.During transgression, the carbonate and fine-grained clastic sedimentation on the barrier wasaccompanied by anaerobic-starved conditions. Thistransgression alternated with a regressive drop in sea level, and the barrier was temporarily andintermittently subaerially exposed and cementedby the oxidizing condition.

THE FIFTH STAGE

The fifth stage is marked by the sequence of thelate Middle Permian mudstone of the Khao PunKlang Member. This black shale is devoid of fossilsand, with the disseminated pyrite grains and framboids, may indicate an oxygen deficiency. Theorganic matter may be preserved at depth, but the surface sediments could still support benthicepifauna; where there are anoxic conditions on theseafloor, there is invariably plentiful H2S in thewater and benthic organisms are absent. Bothlithological and paleontological lines of evidencefor the Khao Pun Klang sequence strongly pointto the depositional environment of limited circula-tion and anaerobic conditions in the restricted

marine or lagoon with sedimentation below effec-tive wave base, cut off from the open sea by thebars of skeletal limestones of the Khao Pun NuaMember.

THE SIXTH STAGE

Towards the sixth stage during late Middle toearly Late Permian time, the depositional envi-ronment was rather quiet and conditions werestable. The last stage is distinguished by the homo-geneous and light-colored limestone of the KhaoPun Tai Member and the shale-dominated succes-sion of the Sap Bon Formation (Hinthong 1981).The Khao Pun Tai Member is composed entirelyof fine-grained microsparry calcite (average 6 mm)with a minority of sparry calcite cement in fenes-tral pores that parallel the bedding plane. Fromthis viewpoint, the Khao Pun Tai Member is likelyto deposit from the tidal flat to the shelteredlagoon where there are no land-derived sedimentswith oxidizing conditions on the passive margin.When comparing the paleoenvironments of thefourth to sixth stages with the carbonate shelfmodel of Selley (1985), the major regression fromthe barrier of the fourth stage to the fifth stage ischaracterized by movement from a lagoon withlimited circulation and anaerobic conditions on thetidal flat to the sheltered lagoon of the sixth stage.

Although, with little evidence and less clarity in the northern extension in southern Yunan(Charusiri et al. 1999b), it must be emphasized thatthe Permian sequences of central Thailand aresimilar to those of Loei (Charusiri et al. 1998) andPichit-Phetchabun (T. Nuchanong, pers. comm.,1998). Charusiri et al. (1998) considered that theLoei Permian sequences are atoll-like carbonatebuildups associated with bedded cherts and ascarcity of marine clastics deposited onto the Carboniferous Paleotethyan basaltic ocean-floor(Intasopa 1993) in Loei, and with tuffs in Pichit (T. Nuchanong, pers. comm., 1990) intervening inShan-Thai and Indochina. This stable (or passive)ocean floor subsequently collided with the westernmost part of Indochina where its UpperPaleozoic strata are characterized by the silicicshelf-dominated sequences of continental margin(Wielchowsky & Young 1985). All the Khao Puncarbonates are, therefore, tectonically regarded as unlikely to represent the carbonate-shelf sedi-ments deposited immediately at the continentalmargin, as previously thought. In addition, basedon the airborne geophysical interpretation byTulyatid & Charusiri (1999), carbonates of the



Saraburi Group are surrounded by a high mag-netic anomaly. Tulyatid & Charusiri (1999) inter-preted this to indicate the occurrence of thepaleo-ocean floor prior to the Permian times.Therefore, we consider that the Permian car-bonates may have been deposited on the oceanfloor.

After the Permian, or during the Permo-Triassicperiods of marine sedimentation, the place grewthe venue of the folding/faulting. Because theIndosinian Orogeny was generally sited in thisregion, it was thought to be the one dominant, significant structural feature. The IndosinianOrogeny that took place between Triassic to EarlyJurassic (Workman 1975; Charusiri 1989) broughtabout the plate interaction and collision betweenIndochina to the east and the Shan-Thai to thewest and, hence, finished the marine deposition innow-mainland Thailand (Bunopas & Vella 1983,1992). The thrusting nature in the area may indi-cate the compressional tectonics. In accordancewith the paleomagnetic (Bunopas 1981) and dating(Charusiri 1989) data, the Indosinian Orogeny andthe collision of both microcontinents developed inthe Early Mesozoic era. The Permian carbonaterocks in the study area were crosscut by theandesitic dyke; the hornblende from this dykeyielded the 40Ar/39Ar date at Early Jurassic(Charusiri et al. 1999a). In addition to the north ofthe study area, the andesite volcanics in the LamNarai area, Lopburi, were dated by Charusiri et al. (1999a), using the same method, to be EarlyJurassic. Petrochemical investigation (Charusiri et al. 1999a) reveals that these volcanics have magmatic arc-affinity. Therefore, with regard tothe dating results and petrogenetic investigation,we infer that the occurrence of these volcanics areindicative of the recurrence of E-dipping oceanicsubduction beneath the Indochina block (Fig. 6).Additional evidence of such tectonic setting is sup-ported by K–Ar dating results of volcanic rocks inLao PDR, further to the north, as noted by Stokeset al. (1996). If such a scenario is correct, theremust be a paleo-ocean floor in-between the Shan-Thai block to the west and the Indochina block tothe east, the so-called ‘Nakhon Thai block’, as men-tioned by Charusiri et al. (1999b). Consequently,the oceanic slab may have existed along the suturezone of the Indochina and the Nakhon Thai block,and may have rejuvenated during Early Jurassicperiod.

From Jurassic to Cretaceous (Charusiri 1989) orEarly Tertiary period, a shear tectonism tookplace with the occurrence of sinistral and dextral

offset faults. Charusiri (1989) noted that the sinis-tral offset along the Mae Ping Fault occurred as aresult of the collision (compression tectonics)between the Shan-Thai and Western Burma micro-continents at the end of the Cretaceous period.The NE–SW structural trend in this area couldhave resulted from the Mae Ping Fault drag. Thealternative definitions for the NW structuraltrending here are not plausible, because there areinsufficient data collected in this small area.

After the Mesozoic era, the last orogenic episodetook place, that is, the 45–50 Ma HimalayanOrogeny (India–Asia collision). The significantdeformation associated with the development ofthe tensional tectonic regime resulted in thenormal faulting in the study area.

CONCLUSIONS

The results obtained from new data of lithos-tratigraphy, lithology, sedimentology, paleontologyand geochemistry, are used to interpret the pale-oenvironments and associated tectonic settings of

Fig. 6 Simplified tectonic model for the development of the Khao PunArea, (a) Carboniferous-Permian; (b) Triassic-Early Jurassic. 1, Permianatoll-like carbonate–buildup in association with bedded cherts depositedonto the Carboniferous Paleotethyan basaltic ocean-floor in Loei(Charusiri et al. 1998); 2, Age of volcanics and plutonics of the Khao YaiVolcanics and Phra Ngam Diorite (Hinthong 1981; Hinthong et al. 1985);3, The 40Ar/39Ar dating (Charusiri et al. 1999a) for hypabyssal dikes in theKhao Pun Area and nearby volcanics.

the Khao Pun area. During the Permian period,the evolution of the marine sedimentation whichtook place in the western margin of the Indochinamicrocontinent can be divided into six stages.

Regarding the character of the oldest formation,the Khao Pun area was under the relatively andtemporarily stable conditions of an inactive marginwith sheltered shallow or lagoonal environment inthe first stage. The second stage, distinguished by chert and argillaceous limestone interbeds,indicates the deposition in a low-energy marineenvironment such as an open sea or outer shelfwith sedimentation below wavebase and awayfrom bottom currents on the Indochina microcon-tinental margin. Towards the end of this stage, thedepositional basin was locally shown by the car-bonate build up and signified the shallow high-energy marine environment of the barrier. Byanalogy with modern siliceous oozes, radiolarianchert in the member of the third stage is inter-preted to represent an abyssal area where depthsexceed the CCD. This might have guided the pale-oposition of the microcontinent during the thirdstage and was moved to low latitudes, possibly inthe paleoequatorial zone.

In the fourth stage, the depositional environ-ment was generally thought to be a barrier orskeletal mound with transgressive/regressivechanges by lithological and paleontological charac-ters. The fifth stage is recorded by the black mud-stone with sporadically disseminated pyrite grainsand framboids in texture. Both lithological andpaleontological lines of evidence for the sequencein this stage disclose the depositional environmentof limited circulation and anoxic conditions in therestricted marine environment of the microconti-nental margin. During the last stage, the paleoen-vironment of the Indochina microcontinent was in the quiet and stable condition of a temporarilyinactive margin, and was distinguished by verylight-colored, homogeneous and very fine-grainedlimestone. Its depositional environment might beinterpreted as tidal flat to sheltered lagoon unin-fluenced by land-derived sediments.

ACKNOWLEDGEMENTS

Grateful and sincere acknowledgements areextended to the Thailand Research Fund and SiamCement Public Company Ltd for the kind financialsupport and geochemical analysis, with specialthanks to the Kaeng Khoi Factory, SaraburiProvince, for support in labor and convenience

during the field investigations. Assistance in thefield by Mr Suvapak Imsamut (Geologic SurveyDivision, Department of Mineral Resources) isgreatly acknowledged. The authors express theirgratitude to Mr Sone Bhongaraya (graduatestudent in Geology, Chulalongkorn University),who made time available to help us. Furthermore,thanks are extended to the Department ofGeology, Faculty of Science, Chulalongkorn Uni-versity for some instrumental support in thecourse of the study.

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