depositional environments of permian rocks of the...

ScienceAsia 33 (2007): 371-381

Depositional Environments of Permian Rocks of theKhao Khad Formation in Central Thailand

Sarawuth Thambunya,a* Visut Pisutha-Arnondb and Chaiyudh Khantaprabc

a TPI Polene Public Company Limited. 299 Moo 5, Mitraparp Road, Tabkwang, Kaeng Khoy, Saraburi 18260, Thailand.b Department of Geology, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand.c Higher Education Development Project. Building 5, Former Srinakharinwirot Pathumwan Campus, Henri

Dunant Road, Pathumwan, Bangkok 10330, Thailand.

* Corresponding author, E-mail: [email protected]

Received 6 Feb 2007Accepted 4 Jun 2007

ABSTRACT: The Khao Khad Formation of Saraburi Group in the central part of Thailand is the sequences oflimestone, dolomitic limestone and silty shale with nodular and banded cherts. The investigation of thesedimentary sequences has been undertaken on the basis of lithological characteristics and sedimentarystructures. Twenty nine measured sections were conducted in three study areas in the vicinity of Khao Khad,Khao Chan and areas along Pak Chong – Khao Yai route. Altogether 312 sampling locations and 539 rockspecimens were examined in this study. Fifteen rock units can be distinguished from the sedimentarysequence including: a) calcilutite with nodular chert, b) algal biolithite, c) crinoidal calcirudite, d) crinoidalcalcirudite with nodular chert, e) crinoidal calcirudite with banded dolomite, f) grade-bedded calcarenitewith banded chert, g) fusulinid-bearing fine calcirudite, h) coral biolithite, i) laminated dolomitic calcarenite,j) fusulinid and intraclasts-bearing calcirudite, k) fenestral and disturbed dolomitic calcilutite, l) fusulinidbearing calcarenite, m) intraclasts-bearing calcarenite, n) argillaceous limestone with silty shale, and o)cross-laminated calcarenite. The Khao Khad Formation was most likely deposited during a major transgressiveand regressive cycle of seawater during the Lower to Middle Permian time in the marine shelf conditionunder sub-environments of intertidal to subtidal zones near shore, subtidal zone of lagoon, shallow platform,barrier bar or shoal and foreslope of barrier bar.

KEYWORDS: sedimentology, lithostratigraphy, Permian, carbonate rocks, Khao Khad Formation.

INTRODUCTION

The Permian carbonate rocks in central Thailandare an economically important source of industrialrocks and minerals. They are the major raw materialscurrently being used for Portland cement and stoneindustries of the country. Besides, they were anticipatedand have been proved as a potential oil and gasreservoir1-2. A number of studies have addressed on theregional geology, structural geology, palaeontology,biostratigraphy and sedimentology of local areas3-11.However, there is still no comprehensive regional workdone on detailed lithology, depositional environmentsand diagenetic history. Hence, the aim of this study isto focus on a detailed investigation on primarysedimentary features of Permian carbonate sequencesnear the Saraburi town and on the western part ofNakhon Ratchasima in central Thailand in order todecipher the depositional environment (Fig. 1). Theselocalities were selected because they represent the wellexposed Permian carbonate rocks mapped as the KhaoKhad Formation in these areas.

Geological SettingThe Permian carbonate rocks are widespread in

Thailand. They are exposed as mountains with karstictopography in the northern and southern parts of thecountry including western part of the Khorat plateau.In central Thailand, the Permian carbonate rocks werenamed the ‘Saraburi Group’ by Bunopas12. Hinthong etal.4 later divided it into six formal formations, namely,Phu Phe Formation, Khao Khwang Formation, NongPong Formation, Pang Asok Formation, Khao KhadFormation and Sab Bon Formation, in ascending order(Fig. 1). The carbonate rock of the Saraburi Group inthis area is mainly the Khao Khad Formation, which isthe main target of this study. It is underlain by shale andslaty shale of the Pang Asok Formation and overlain bygray to brown tuffaceous sandstone and shale of theSab Bon Formation.

The area has undergone a complex geologicalhistory as a part of the western old Indochina landmass, which submerged as a shelf sea since MiddleCarboniferous period13. The thick sequences of mixedcarbonate-clastic sediments of the Saraburi Group weredeposited in a marine shelf environment under tropical

doi: 10.2306/scienceasia1513-1874.2007.33.371

372 ScienceAsia ScienceAsia ScienceAsia ScienceAsia ScienceAsia 33 (2007)33 (2007)33 (2007)33 (2007)33 (2007)

zone of the southern hemisphere. The paleolatitudedepositional environment during the Permian periodis nearly 10º south of the paleoequator11. Indosinianorogeny probably started since Late Permian wherethe convergence of the Shan-Thai and Indochina wereeventually collided in Late Triassic13. This caused theemergence of the shelf followed by strong folding in thearea, the Phetchabun Fold Belts. The post-collisionigneous activities took place mostly during Late Triassicto Early Jurassic4.

MATERIALS AND METHODS

Field work was carried out on three localities in thevicinity of Khao Khad, Khao Chan and areas along PakChong – Khao Yai route (Fig. 1). Twenty nine measuredsections were conducted to determine the lithologicalcharacteristics of sedimentary sequences. Rocksampling has been conducted from the sections for thedetailed petrographic study. Altogether there are 312sampling locations and 539 rock specimens.

All rock specimens were slab-cut, polished andsubjected to various detailed macroscopic andmicroscopic examinations. Standard microscopy wascarried out on 315 thin sections. The thin sections werestained with potassium ferricyanide and Alizarin RedS solutions to assist carbonate mineral identification aswell as to detect their texture and composition14. Therock classification and nomenclature of Grabau15 andFolk16 were selected for this study as field name andpetrographic name, respectively. The depositionaltextures of rocks in the study area are often obscuredby complex diagenetic history.

Lithostratigraphic sequence and depositionalenvironment

The sedimentary sequence of the Khao KhadFormation in the study area is composed of limestone,dolomitic limestone, argillaceous limestone and siltyshale with nodular and banded cherts. The detailedlithostratigraphic correlation among the threemeasured sections of the Khao Khad Formation givesfifteen different rock units (Fig. 2), namely from UnitsA to O consecutively in ascending order, and aredescribed as follows:

Fig 2. The lithostratigraphic correlation of the Khao KhadFormation from three measured sections, Khao Khad,Khao Chan and Pak Chong to Khao Yai areas.

Fig 1. Geologic map of central Thailand showing location ofstudy area (modified after Hinthong et al.4).

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Unit A: Calcilutite with nodular chertThe lower most rock unit of the Khao Khad

Formation is the calcilutite with nodular chert. It isexposed at both the Khao Khad and Khao Chan sectionswith the thickness of 251 to 294 m. This unit ischaracterized by medium light grey, thin- to medium-bedded calcilutite with dark grey chert nodules (Fig.3A). Generally, bioturbation features are poor. However,three subunits can be distinguished on the basis of thetype of bed and relative abundance of grain contents.The lower part of the unit is a parallel-bedded calcilutiterarely with skeleton grains. This is in contrast to themiddle part which is dominated by wavy- and non-parallel-bedded calcilutite containing abundant algalmats, brachiopod shell fragments and fusulinid tests ofmainly Robustoschwagerina sp. The upper part of the

unit is similar to the lower part and is a parallel-beddedcalcilutite with relatively rare shell fragments and sparsefusulinid tests. The rocks are classified as finelylaminated fossiliferous micrite, algal biomicrite, packedbiomicrite and packed biomicrudite based on Folk16

(Fig. 3B). Allochems are fusulinid tests, pellets,brachiopod and gastropod shell fragments, algalfragments and unidentified small micritized grains.Most of shells were completely micritized and somefusulinid tests and brachiopod shells are coated withmicrite. The microcrystalline calcite cement can berecognized around pellets and around contact pointarea of allochems. The chert nodules vary in size from5 to 10 cm and are composed mainly of microcrystallinequartz with some calcareous patches.

The micrite is commonly present in a low-energydepositional environment17 whereas the fecal pelletsare usually deposited in moderate- to low-energyconditions in a basinal or lagoon environment18.Micritized and micrite-coated grains are commonlyfound in the bio-active zone on the sea floor both in theintertidal and subtidal environments19. However, theformation of microcrystalline calcite cement indicatesthat the sediments might have occasionally beenexposed to marine vadose zone and frequently flushedby seawater or brine18. As such, the Unit A was mostlikely deposited in the subtidal zones of lagoon.

Unit B: Algal biolithiteThe algal biolithite of Unit B overlies conformably

on the Unit A and is found only at the Khao Khadsection with thickness of about 100 m. This unit isrepresented by light to medium grey, medium-beddedcalcarenite to calcirudite associated with algal biolithite.The rocks show a grain-supported fabric of differingtypes and sizes of grain components. Allochems arealgal encrustations, fusulinid tests, crinoid fragments,pellets, and shells of brachiopods, gastropods andcephalopods. The shell fragments are usually orientatedparallel or sub-parallel to bedding plane. The rocks areclassified as pelsparite, biopelsparite and biosparudite.All bioclasts were completely micritized and initiallycemented by microcrystalline calcite. The detailedpetrographic studies of algal stromatolite revealed thatthe pre-existing grains of pelsparite, biopelsparite andoncoilite were encrusted with many algae (Fig. 3C).There was some braking period of algal encrustationwhich was separated by radiaxial fibrous calcite cement.Many layers of the fibrous calcite cement are separatedby microcrystalline dolomite layers. Somemacrocrystalline dolomite rhombs occur in associationwith sparry calcite cement in the central area ofintergranular pores (Fig. 3D).

Even though the micritized grains and pellets couldoccur in the intertidal or subtidal zone of a shallow

Fig 3. A A A A A: An outcrop of the Unit A showing thin- to medium-bedded, parallel-bedded type, medium to dark greycalcilutite interbedded with dark grey nodular cherts.(UTM 706100E, 1634850N) BBBBB: Photomicrograph ofpacked biomicrite of the Unit A. (Stained, PPL, Sampleno. L3/22) C C C C C: Photomicrograph of laminated algal stro-matolite (1) accretion from the unidentified micritizedgrain (2) and dissolved grain (3) of the Unit B. (PPL,Sample no. 233/1) D D D D D: Photomicrograph depicting themacrocrystalline dolomite (1) associated with sparrycalcite cement (2) in the central area of intergranularpore spaces of the algal stromatolite (3) buildup of theUnit B. (Stained, PPL, Sample no. 233/1) EEEEE: A rock slabof the Unit C showing poorly sorted crinoidal fragments.(Sample no. L17/7) F F F F F: An exposure of very-thick-bed-ded calcirudite, 10 m thick between a series of thinly-grade-bedded calcarenite to calcilutite, in the middlepart of the Unit F. (UTM 731000E, 1621200N).


marine environment18, 19, the presence of algalstromatolite encrustation on those micritized grainsand pelsparite grains suggested that the sediments ofUnit B were rather deposited in low-energy intertidalenvironment (as seen in the modern analog at thePersian Gulf20). Furthermore, the occurrence ofmicrocrystalline calcite cement on fecal pellets andbioclasts indicates that the sediments had been exposedto the marine vadose zone whereas the formation ofmicrocrystalline dolomite interlayer further reinforcedthe deposition in upper intertidal environment21, 22, 23.It is therefore likely that the unit B was deposited in theintertidal.

Unit C: Crinoidal calciruditeOverlying conformably on the Unit B is the crinoidal

calcirudite of Unit C. This unit is well exposed at boththe Khao Khad and Khao Chan sections with thicknessof 32 to 116 m. This unit is light to medium grey, medium-to thick-bedded crinoidal calcirudite. The rocks containmore than 50 % of large (0.5 mm to 2 cm) crinoid stems(Fig. 3E). The beds are of parallel-bedded type withvery thin bands of grayish black to reddish silty shalebetween each crinoidal bed. The rocks are classified asunsorted crinoidal biosparite and biosparudite.Allochems are mainly crinoidal and bryozoanfragments. They are cemented together by coarse sparrycalcite. The fine-grained matrix is rarely recognized asmicrite and pseudosparite with sparse dolomite rhombsin some parts. Most crinoidal fragments were over-grown by syntaxial calcite cement.

The Unit C was probably deposited in current- orwave-agitated shallow subtidal environment of barrierbar or shoal. This assumption is inferred by the abundantoccurrence of cleanly-washed and large-sized crinoidfragments analogous to those found in the BeticMountains, southern Spain24. The rather thick crinoidalcalcirudite unit certainly requires the deposition undersubsiding condition. The presence of thin bands of siltyshale between crinoidal limestone beds howeversuggests a brief interrupt of the carbonate depositionby the influx of fine-grained siliciclastic sediments.

Unit D: Crinoidal calcirudite with nodular chertThe crinoidal calcirudite with nodular chert of Unit

D overlies conformably on the Unit C at both the KhaoKhad and Khao Chan sections with thickness of 57 to117 m. This unit is characterized by grey to dark grey,medium- to thick-bedded crinoidal calcirudite withscattered dark grey elongated chert nodules. The bedsare of parallel-bedded type and slightly dolomitic in theupper part of the unit. Allochems are crinoid stems,foraminifera, intraclasts and bryozoans, respectivelyin decreasing order of abundance. It is noted that therelicts of crinoid and foraminifera are also observed in

chert nodules. The rocks are classified as sortedbiosparite and biosparudite. However, microcrystallinedolomite is locally present along the boundaries ofcarbonate grains as dark brown rhombs. The crinoidfragments show syntaxial overgrowth cement. Thechert nodules vary in size from 5 to 30 cm and aremainly composed of microcrystalline quartz locallydisseminated with calcite rhombs.

There seems to be a certain degree of similarity inlithology between the Unit C and the Unit D, except thatthe Unit D contains chert nodules and some foraminiferaand intraclasts. Intraclasts are the former semi-consolidated sediments being ripped off in the high-energy intertidal zone near shore or barrier bar orshoal. They commonly re-accumulated in subtidal zoneor on the back side of the bar together with other graincomponents as have been found in the Bahama Bank25.As such the depositional environment of the Unit D wasprobably a slowly subsiding back barrier.

Unit E: Crinoidal calcirudite with banded dolomiteThe crinoidal calcirudite with banded dolomite of

Unit E overlies conformably on the Unit D at both KhaoKhad and Khao Chan sections with the thickness of 92to 213 m. This unit is represented by light grey, medium-to thick-bedded, coarse calcarenite to fine calcirudite.The beds are of parallel-bedded type with thin beddeddolomite alternating with dolomitic limestone beds.Allochems of this unit are similar to those of theunderlying Unit D. The rocks are quite similar to thoseof the Unit D but suffer higher degree of dolomitization.The Unit E comprises sorted biosparite, biosparudite,poorly-washed biosparite and finely to coarselycrystalline dolomite. Within the dolomitic limestone,dolomite rhombs are sparsely embedded in the blockycalcite cement, while in the dolomite bed the rockexclusively consists of finely to coarsely crystallinedolomite replacing both allochems and cements.

Resemble to the Unit D, the Unit E was probablydeposited in a high-energy zone of back barrier bar inwhich the dolomitization could have been morefavorable26.

Unit F: Grade-bedded calcarenite with banded chertThe grade-bedded calcarenite with banded chert

of Unit F is only exposed at the Khao Chan section. Itoverlies conformably on the Unit E with the thicknessof 181 m. This unit is a medium to dark grey, thin- tovery-thick-bedded, calcilutite to calcarenite. The bedsare of parallel-bedded type and show graded beddingwith dark grey chert bands. The reddish brown fissilesilty shale is often thinly-laminated to thinly-beddedbetween each limestones bed. In the middle part of theunit, there are four distinct calcirudite beds of 9-13 mthick (Fig. 3F). These four beds contain remarkably


large grain components (1 to 50 cm) of angular boulders,cobbles, pebbles and sands of intraclasts withsubordinate calcareous skeletons, mostly crinoidossicles, foraminiferal tests and other skeletonsfragments. The rocks are classified as poorly-washedintrasparudite, poorly-washed intrasparite, packedintramicrite and fossiliferous micrite. The banded cherts(occasionally associated with packed intramicrite andfossiliferous micrite) consist mainly of microcrystallinequartz, subordinate chalcedonic and megacrystallinequartz, and sparse remnants of both calcareous anddolomite patches.

A sequence of thin and grade-bedded calcareniteto calcilutite is favor in the distal part of turbidity grainflows occurring in the foreslope of barrier bar27. Asdescribed in the final stage of evolution of the Niagaranbuildup of Indiana23, the association of current sweptplatform morphology and an overproduction ofcrinoids created ideal conditions for the generation ofslumping and turbidity currents which was a majormechanism for the radial distribution down the buildupflanks. The occurrences of the four thick calciruditebeds in this unit therefore suggest that the depositioncould have taken place under exceptionally strongenergy condition of presumably turbidite channel inthe fore-slope of the barrier bar.

Unit G: Fusulinid-bearing fine calciruditeThe fusulinid-bearing fine calcirudite of Unit G is

only exposed at the Khao Khad section with the thicknessof 184 m. It overlies on the Unit E with fault contact.This unit is characterized by medium to dark grey,medium- to very-thick-bedded, grain-supportedcalcarenite to fine calcirudite. The beds are of parallel-bedded type. The chert nodules and dolomite patches(10 to 30 cm in size) are found scattering throughoutthe unit. Allochems are mainly fusulinid tests (1 to 5 mmin size) and the matrix is a mixture of pellets and smallshell fragments (Fig. 4A). The rocks are poorly washedbiosparite, unsorted biopelsparite and biosparudite.These rocks have been silicified and dolomitized incertain areas. Some pellets and contact point of grainswere earlier cemented by thin-layer coating of light-colored microcrystalline calcite (Fig. 4B). The remainingintergranular and intragranular pore spaces were filledby fine sparry calcite cement. It appears that the outersurface of shell fragments had been coated with micritebefore the fragments were broken. Some elongatedshell fragments lie parallel or sub-parallel to the beddingplane. The nodular chert is mainly microcrystallinequartz with some scattered dolomite rhombs (10 – 300µm in size) and calcareous patches (60 – 100 µm insize). Some parts of biosparite also contain scattereddolomite rhombs.

The Unit G contains various rock types indicating

both high and low-energy depositional environments.The grain-supported calcarenite to fine calcirudite wererather deposited in high-energy environment ofintertidal zone. The packed biopelmicrite and poorlywashed biosparite were probably formed in somewhatlower-energy condition in which micrite componenthas not been completely removed whereas theunsorted biopelsparite, biosparite and biosparuditeindicate slightly higher energy to winnow all of themicrite. It is therefore likely that the Unit G wasdeposited in an intertidal to subtidal zone of lagoonsimilar to those found in the Middle Cambrian DaegiFormation28.

Fig 4. A A A A A: A fine calcirudite slab of the Unit G showing grain-supported fabrics of mainly fusulinid tests. (Sample no.89/1) BBBBB: Photomicrograph of unsorted biopelsparite ofthe Unit G showing grain-supported fabrics. Grains wereinitially cemented by microcrystalline calcite (1), thenby isopachous fibrous (2) and finally by sparry calcite(3). (PPL, Sample no. 87/3) C C C C C: A coral biolithite slab ofthe Unit H showing small coral colony in growth posi-tion (light grey) on hard ground calcilutite. (Sampleno. 225/1) D D D D D: A rock slab of pinkish grey, moderatelysorted, fusulinid and intraclast-bearing fine calciruditeof the Unit J. (Sample no. L1/1) E E E E E: A coquinite slab ofthe Unit K showing flattened broken shell fragments(white) embedded in the dark grey calcilutite matrix.(Sample no. 203/1) FFFFF: A medium grey calcilutite slab ofthe Unit K showing abundant burrows of Chondritesichnosp. (white steaks) filled with microcrystalline do-lomite. (Sample no. 210/1).


Many burrow phases of Diplocraterions ichnosp.,Planolites ichnosp. and Skolithos ichnosp. are filled withmicrite, finely crystalline dolomite and sparry calcitecement. The dolomite beds are essentially mediumcrystalline dolomite replacing the pre-existingcalcareous parts.

The laminated dolomitic calcarenite was probablydeposited in the intertidal zone of lagoon. Thisinterpretation is inferred by the abundant occurrenceof burrows filled with micrite and finely crystallinedolomite. The presence of Diplocraterions ichnosp. is agood indicator of tidal flat30 while the Planolites ichnosp.and Skolithos ichnosp. are known as feeding anddwelling burrows. They are also common in theintertidal zone of lagoon30.

Unit J: Fusulinid and intraclasts-bearing finecalcirudite

Overlying conformably on the Unit I is the fusulinidand intraclasts-bearing fine calcirudite of Unit J. It iswell exposed in all sections with thickness of 122 to 322m. This unit is light to pinkish grey, medium- to thick-bedded, moderately sorted calcarenite to finecalcirudite (Fig. 4D). The beds are of parallel-beddedtype. The fine calcirudite shows both grain-supportedand matrix-supported fabric with some nodular chertsand dolomitic patches. The rocks are fusulinid-bearingbiosparite, intrasparite, biopelsparite, packedbiomicrite and intrasparudite. Allochems are fusulinids,crinoid stems, intraclasts, bryozoans, pellets andunidentified skeletal fragments. Grains are completelymicritized and the internal pores are occluded by sparrycalcite cement. Some grains and pellets showmicrocrystalline calcite cement near the grain contact.The nodular chert is mainly microcrystalline quartzwith minor chalcedonic quartz and macrocrystallinequartz.

The Unit J, similar to that of the Unit G, comprisesvarious rock types indicating both high and low-energydepositional environments. There seems to be somechanges between low and high-energy conditions. Thepresence of intraclast indicates that the sediments wereripped off in the intertidal zone and re-deposited in thesubtidal zone as mentioned earlier. As such, the Unit Jwas probably deposited under the intertidal to subtidalzones of lagoon or inner shelf behind the barrier barsor shoal.

Unit K: Fenestral and disturbed dolomiticcalcilutite

The fenestral and disturbed dolomitic calcilutite ofUnit K overlies conformably on the Unit J and is onlyexposed at the Khao Khad section with thickness ofabout 120 m. This unit is medium to dark grey, thin- tothick-bedded calcilutite to calcarenite with coquinite,

Unit H: Coral biolithiteThe coral biolithite of Unit H overlies on Unit G with

fault contact and is exposed at both the Khao Khad andPak Chong to Khao Yai sections with thickness of 123to 211 m. This unit is characterized by medium to darkgrey, thin- to very-thick-bedded calcilutite to calciruditeassociated with coral biolithite. In hand-specimen, therock contains many small elongate fusulinid tests (0.5to 1.5 mm in size) embedded in fine-grained micritematrix. The coral biolithite is the colonies of coral ingrowth positions (10 to 50 cm in size) and usuallyassociated with poorly sorted rocks with diversebioclasts (Fig. 4C). The dark grey calcilutite usuallyassociates with coralline colonies by filling in theirintergranular pores. Chert nodules are also common inthis unit and mostly form elongate shape (10 to 20 cmlong). The rocks are classified as biopelmicrite, packedbiomicrite, packed biomicrudite, packed intramicriteand intraclast-bearing biomicrudite. Allochems arelarge shell fragments of mainly brachiopods andgastropods, large fusulinid tests, fecal pellets, abundantsmall intraclasts of micrite and coral fragments packedwith small unidentified fragments. The packedintramicrite also contains many dolomite rhombs havingmicro-stylolitic grain contact. The nodular chert isessentially microcrystalline quartz.

The presence of the coral colonies in growthpositions together with diverse bioclasts and intraclasts,as mentioned above, shows that the Unit H was acarbonate buildup, caused by active growth of colonialsedentary organisms. The accumulation of carbonateprobably occurred in the intertidal to subtidal zone ofshallow platform, analogous to those found in theDuranguesado carbonate platform in the northernSpain29.

Unit I: Laminated dolomitic calcareniteThe laminated dolomitic calcarenite of Unit I

overlies conformably on the Unit H and is exposed atboth the Khao Khad and Pak Chong to Khao Yai sectionswith thickness of 86 to 180 m. This unit is representedby medium to dark grey, thin- to medium-beddedcalcilutite interbedded with thinly-bedded dolomiticlimestone and dolomite beds. The dolomitic limestoneand dolomite appear in the fields as bands of white tolight grey ‘elephant skin’ texture on the weatheredsurface that marks the difference from the other unit.This is quite in contrast to medium to dark greycalcilutite. The cross lamination is also present in thedolomitic limestone. The rocks are classified as packedbiomicrite, disturbed micrite containing microcrystallineand macrocrystalline dolomite. The packed biomicritecomprises diverse small bioclasts (10 to 500 µm in size)embedded in micrite matrix. The disturbed micrite ischaracterized by abundant burrows in micrite matrix.


algal encrusted coquinite and many dolomitic limestonebeds. This unit contains some large shell fragments inthe calcilutite matrix (Fig. 4E), and shows bird’s eyestructures, fenestral features and burrows of Chondritesichnosp. (Fig. 4F), Planolites ichnosp., Skolithos ichnosp.and Trichichnus ichnosp. The rocks are packedbiomicrite and dismicrite. Small shell fragments (10 to600 mm in size) are abundant and packed together inmicrite matrix. The bio-disturbed micrite such asburrow is partially filled with microcrystalline andmesocrystalline dolomite and sparry calcite cement(Fig. 5A).

The fenestral and disturbed dolomitic calcilutite ofthe Unit K was probably deposited in the intertidal

zone of lagoon environment. The abundantoccurrences of bio-disturbed micrite and fenestralfeatures are usually found in the intertidal to subtidalzones30, 31.

Unit L: Fusulinid-bearing calcareniteThe fusulinid-bearing calcarenite of Unit L overlies

conformably the Unit K and is exposed only at the KhaoKhad section with the thickness of about 150 m. Thisunit is characterized by medium to brownish grey, thin-bedded calcarenite. Allochems are mainly of fusulinidtests (1 to 2 mm in size) and some brachiopod shellfragments (Fig. 5B). Most of this unit has been silicifiedand weathered into friable siliceous matter. The rocksare classified as packed biomicrite. The micritizedfusulinids were transformed into microsparite. As suchthe internal pores were filled with coarse sparry calcitecement. The silicified packed biomicrite containsabundant chalcedonic spherulite.

As has been repeatedly mentioned micrite usuallyoccurs in low-energy depositional environments17 andarenaceous fusulinids are also abundant in the lagoonenvironment. It is therefore likely that the Unit L wasdeposited in the subtidal zone of lagoon similar tothose found in the Lower Member of the KhuffFormation in Saudi Arabia31.

Unit M: Intraclast-bearing calcareniteThe intraclast-bearing calcarenite of Unit M overlies

conformably the Unit J and is exposed only at the KhaoChan section with the thickness of about 159 m. Thisunit is characterized by dark grey and thin- to thick-bedded calcarenite (partly calcilutite) alternating withreddish brown silty shale. Some parts of this unit werepatchily dolomitized. The elongate nodular cherts ofless than 20 cm in size are scattered throughout thisunit. In addition, the nodular cherts are also associatedwith some dolomitic limestone (Fig. 5C). The rocks arepacked intramicrite and biomicrite. Allochems areintraclasts, minor broken crinoid ossicles, andforaminiferal tests. There are some detrital quartz anddolomite grains (10 to 100 µm in size) deposited withcalcareous grains. They are packed together andcemented by ferroan sparry calcite. The nodular chertis microcrystalline quartz with abundant calcareousremnants and microcrystalline dolomite. Some partsof nodular chert contain carbonate vadose silts.

The detrital quartz, dolomite grains, and intraclastsoccur preferably in restricted zone of inner shelf,especially in the intertidal and subtidal zones near theshoreline. Similarly, calcarenite is also common in theintertidal zone of moderately high-energy. On thecontrary, the occurrences of micrite and thinly-laminated silty shale indicate low-energy environmentwhile the presence of vadose silt suggests the sediments

Fig 5. A A A A A: Photomicrograph of disturbed micrite of the Unit K.The burrows of Chondrites ichnosp. were earlier filled bymicrocrystalline dolomite (1) and later cemented bysparry calcite (2). (Stained, PPL, Sample no. 210/1) B B B B B: Acalcarenite slab of the Unit L showing matrix-supportedfabric of fusulinid test. (Sample no. 201/1) C C C C C: An expo-sure of medium-bedded, dark grey calcarenite of theUnit M showing elongate chert nodules and dolomiticlimestone patches embedded in limestone beds. (UTM731100E, 1620500N) D D D D D: An outcrop of thin-bedded,dark grey mixed calcarenite and argillaceous limestoneof the Unit N. (UTM 732000E, 1620000N ) EEEEE: A rockslab of calcirudite of the uppermost part of the Unit Nshowing abundant intraclasts packed with foraminiferaltests and crinoidal fragments. (Sample no. L24/6) F F F F F:Photomicrograph of rounded packed biomicrite of theUnit N showing micrite coated grains with roundedrims. Grains were completely replaced by sparry cal-cite. (PPL, Sample no. 119/1).


were exposed and affected by meteoric water. Thus, itis likely that the Unit M was deposited in intertidal andsubtidal zones near shore.

Unit N: Argillaceous limestone with silty shaleThe argillaceous limestone with silty shale of Unit

N is exposed at both the Khao Khad and Khao Chansections. It overlies conformably on the Unit M with thethickness of 239 to 435 m. This unit is represented bya sequence of medium to dark grey, thin-beddedcalcilutite and calcarenite in the lower part, graduallypassing upward into clastic associations of thin-beddedsilty shale, silty sandstone and porcelanite with verythick-bedded calcirudite in the uppermost part. Thecalcilutite and calcarenite usually contain abundantargillaceous matter (Fig. 5D). The uppermost calciruditebeds are 1 to 2 m thick and comprise essentiallycompacted intraclasts (1 to 10 cm in size, Fig. 5E). Thecumulative thickness ratio of carbonates andsiliciclastics of this unit is approximately 4:1. The rocksare poorly-washed biosparite, packed intramicrudite,packed biomicrite and micrite. Allochems are abundantskeletal fragments of crinoid and foraminifera withsome algal and coral fragments, unidentified grains,intraclast and detrital quartz. Some grains are roundedto sub-rounded and appear as micrite-coated orcompletely micritized grains (hence unable to identify,Fig. 5F). The elongate grains are usually orientedsubparallel to the bedding plane. In part, the carbonatehas been silicified which appears as microcrystallinequartz, as well as minor chalcedonic quartz andmegacrystalline quartz. Also in the fine-grained chertmatrix there are some of both non-ferroan and ferroancalcite remnants. There are also some ferroanmicrocrystalline dolomites disseminated in the fine-grained chert matrix. The porcelanite containsabundant sponge spicules of 10 to 100 µm in size mixedwith silicified argillaceous matter.

There seems to be a certain degree of graduallypassing upward sequence from Unit M to Unit Nbecause of the increasing of the proportion of intraclastsand siliciclastic sediments in the sequence. As previouslybeen documented, the sequence of thinly-beddedlimestone, dolomitic limestone, argillaceous limestoneand shale were found to deposit in shallow to deepramp in the Dumugol Formation32. The packedintraclastic calcirudites or packed intramicrudite werefound in the Chanda limestone, Penganda Group, ofIndia33 and the Kelley-Snyder Field in West Texas34.These sediments have been interpreted as debris flowsrepresenting a re-deposition of semi-consolidatedsediments35. Therefore, the depositional environmentof this unit could be high-energy intertidal and subtidalzones.

Unit O: Cross-laminated calcareniteThe cross-laminated calcarenite of Unit O is the

upper most unit of the Khao Khad Formation. It overlieson the Unit N with fault contact and is exposed only atthe Khao Chan section with the thickness of about 365m. This unit is characterized by medium to dark grey,thin- to thick-bedded calcarenite rarely with small chertnodules. Cross-lamination can be recognized in thelower part of the unit. The rocks are poorly-washedbiosparite, packed biomicrite and sparse biomicrite.Allochems are bioclasts of foraminiferal tests, crinoids,unidentified skeletal fragments and bryozoans togetherwith intraclasts and detrital quartz. The chert noduleis made up essentially of microcrystalline quartz withsome calcareous patches.

The formation of calcarenite from diverse graincomponents by itself suggests that the rock representsa slightly high-energy zone of shallow marine. However,the presence of micrite in poorly-washed biospariteand packed biomicrite seems to indicate a low-energyenvironment. In addition, the cross-lamination usuallyformed in the intertidal zone of restricted inner shelf.As such this unit was probably deposited in the intertidaland subtidal zones of inner shelf.

Reconstruction of Depositional ModelBased on the analysis of those 15 rock units, it can

be summarized that the carbonate and siliciclasticsequences of the Khao Khad Formation were depositedin the marine shelf condition under sub-environmentsof intertidal to subtidal zones near shore, subtidal zoneof lagoon, shallow platform, barrier bar or shoal andforeslope of outer shelf. By careful examination of thevertical succession of lithostratigraphic units andassociated environments in the measured sections,there appears to be a major transgressive and regressivecycle of seawater during the Lower to Middle Permiantime.

The Transgressive SequencesThe transgressive sequences probably occurred

during the deposition of Unit A to H in which the sub-environments might have varied from intertidal andsubtidal zones of lagoon to barrier bars (Fig. 6A). Thesequence started with the Unit A which was the subtidalzone of lagoon at both Khao Khad and Khao Chanareas (might be absent or not exposed in the Pak Chongto Khao Yai area). The Khao Chan area seemed to havehigher amount of micrite deposition than that at theKhao Khad area. The Unit B, which is locally present inthe Khao Khad area, was algal stromatolites in theintertidal zone. It probably represented a minorregression or progradation. The Unit C to E were barrierbar or shoal at the Khao Khad and Khao Chan areascontained various proportion of grain components of


Table 1. Summary of lithological characteristics and depositional environments from lower to upper units.

UnitUnitUnitUnitUnit ThicknessThicknessThicknessThicknessThickness Lithological CharacteristicsLithological CharacteristicsLithological CharacteristicsLithological CharacteristicsLithological Characteristics DepositionalDepositionalDepositionalDepositionalDepositional(meters)(meters)(meters)(meters)(meters) environmentenvironmentenvironmentenvironmentenvironment

O 365 Medium to dark grey, thin- to thick-bedded, cross-lamination calcarenite with rare small Intertidalchert nodules. The rocks are poorly-washed biosparite, packed biomicrite, and sparse biomicrite andwith some microcrystalline quartz. Allochems are foraminiferal tests, crinoid stems, bryozoan subtidaland intraclasts with some detrital quartz. It overlies on the Unit N with fault contact. zone of inner shelf

N 239-435 Sequences of medium to dark grey, thin-bedded calcilutite to calcarenite in the lower part Highand gradually passing upward into clastic associations of thin-bedded silty shale, silty sand energyand porcelanite with very thick-bedded calcirudite in the uppermost part. The rocks are intertidalpoorly-washed biosparite, packed intramicrite, packed biomicrite and micrite. Allochems are andcrinoidal and algal fragments, fusulinid tests and intraclasts with some coral and detrital quartz. subtidalThe unit rests conformably on the Unit M. zones

M 159 Dark grey, thin- to thick-bedded calcarenite, partly calcilutite with dark grey chert nodules Subtidaland silt-shale between limestone beds. The rocks are packed intramicrite and biomicrite. andAllochems are intraclasts, crinoid ossicles and fusulinid tests with some detrital quartz. intertidal zonesThe unit rests conformably on the Unit J. near shore

L 150 Medium to brownish grey, thin-bedded calcarenite. The rock is packed biomicrite with grains Subtidalof fusulinid tests and brachiopod fragments. The silicified argillaceous limestone contains zoneabundant chalcedonic spherulite. The unit rests conformably on the Unit K. of lagoon

K 120 Medium to dark grey, thin- to thick-bedded calcilutite to calcarenite with coquinite, algal Intertidalencrusted coquinite and dolomitic limestone beds. The rocks are packed biomicrite and zonedismicrite with abundant burrows and fenestral features. Allochems are shell fragments. ofThe unit rests conformably on the Unit J. lagoon

J 122-322 Light to pinkish grey, medium- to thick-bedded, calcarenite to fine calcirudite with nodular Intertidalcherts. The rocks are fusulinid bearing biosparite, intrasparite, biopelsparite, packed to subtidalbiomicrite and intrasparudite with microcrystalline, macrocrystalline and chalcedonic quartz. zoneAllochems are fusulinid tests, crinoidal and bryozoan fragments, intraclast and pellets. of lagoonThe unit rests conformably on the Unit I.

I 86-180 Medium to dark grey, thin- to medium-bedded calcilutite interbedded with thin-bedded Intertidaldolomitic limestone and dolomite beds. The rocks are packed biomicrite, disturbed zone ofmicrite with some burrows. Allochem is rare. The unit rests conformably on the Unit H. lagoon

H 123-211 Medium to dark grey, thin- to very-thick-bedded calcilutite to calcirudite and coral biolithite Shallowwith nodular cherts. The rocks are classified as biopelmicrite, packed biomicrite, packed platformbiomicrudite, packed intramicrite and intraclasts-bearing biosparudite. Allochems are buildupbrachiopods and gastropods shells, fusulinid tests, fecal pellets and intraclasts with coralfragments. It overlies on the Unit G with fault contact.

G 184 Medium to dark grey, medium- to very-thick-bedded, parallel-bedded type, calcarenite to fine Intertidalcalcirudite with nodular cherts and dolomite patches. The rocks are poorly washed biosparite, tounsorted biopelsparite and biosparudite with microcrystalline quartz and macrocrystalline subtidaldolomite. Allochems are mainly large fusulinid tests, pellets and small shell fragments. It zone ofoverlies on the Unit E with fault contact. lagoon.

F 181 Medium to dark grey, thin- to very-thick-bedded, parallel-bedded type, graded calcarenite to Foreslopecalcilutite with dark grey banded chert and thinly-laminated to thinly-bedded, reddish brown, offissile silty shale between each limestone bed. The rocks are poorly-washed intrasparudite, barrierpoorly-washed intrasparite, packed intramicrite and fossiliferous micrite. The banded barcherts consist of microcrystalline, megacrystalline and chalcedonic quartz. Allochems consistof angular boulders, cobbles, pebbles and sands of intraclasts, crinoid stems and fusulinid tests.The unit rests conformably on the Unit E.

E 92-213 Light grey, medium- to thick-bedded, parallel-bedded type, coarse calcarenite to fine calcirudite Backwith dolomitic limestone beds. The rocks are sorted biosparite, biosparudite, poorly-washed barrierbiosparite with finely to coarsely crystalline dolomite. Allochems are crinoid stems, fusulinid bartests, intraclasts and bryozoans. The unit rests conformably on the Unit D.

D 57-117 Grey to dark grey, medium- to thick-bedded, parallel-bedded type, crinoidal calcirudite with Backdark grey chert nodules. The rocks are sorted biosparite and biosparudite with some barriermicrocrystalline quartz and microcrystalline dolomite. Allochems are crinoid stems, fusulinid bartests, intraclasts and bryozoans. The unit rests conformably on the Unit C.

C 32-116 Light to medium grey, medium- to thick-bedded, parallel-bedded type, crinoidal calcirudite. BarrierThe rocks are unsorted crinoidal biosparite and biosparudite. Allochems are crinoidal and bar orbryozoan fragments. The unit rests conformably on the Unit B. shoal

B 100 Light to medium grey, medium-bedded calcarenite to calcirudite and algal biolithite. The Intertidalrocks are pelsparite, biopelsparite and biosparudite. Allochems are algal and crinoid stems, zone ofpellets, fusulinid tests, and shells of brachiopods, gastropods and cephalopods. It overlies shallowconformably on the Unit A. marine

A 251-294 Medium light grey, thin- to medium-bedded, parallel- to wavy-bedded types, calcilutite with Subtidaldark grey chert nodules. The rocks are classified as finely laminated fossiliferous micrite, algal zonesbiomicrite, packed biomicrite and packed biomicrudite with some microcrystalline quartz. ofAllochems are fusulinid tests, pellets, brachiopod and gastropod shells and algal fragments. lagoon


crinoid fragments, intraclasts, some fusulinid tests,bryozoans and shell fragments. The Unit F was theforeslope of barrier bar as indicated by the repetitivesequence of graded calcarenite to calcilutite withabundant intraclasts. This unit was only exposed at theKhao Chan area. The Unit G was deposited in intertidalto subtidal zones of lagoon at the Khao Khad area andcontained abundant fusulinid tests. The Unit H wasdeposited in the shallow platform or bioconstructedbuildup zone, approximately 100 m wide, at the KhaoKhad and Pak Chong to Khao Yai areas. This environmentmight be initiated during low stand of shallow outershelf which was quite different from crinoidal barrierbar of the Unit C. In this shallow platform area therewere abundant buildups of corals or patch reef, largebrachiopods and gastropods without crinoids. Thebuildups were probably acting as the barrier separatedthe lagoon from open marine conditions.

The Regressive SequencesThe regressive sequences probably occurred during

the deposition of the Unit I to O. The depositionalenvironments change from intertidal and subtidal zonesof back barrier bar to intertidal zone of near shore (Fig.6B). The regression probably started from the Unit Iand J which deposited in the intertidal to subtidal zoneof lagoon probably related to subsequent exposure ofshallow platform buildup. After that sea level fell down

then unit K was deposited in the intertidal zone oflagoon with abundant bioturbation and fenestralfeatures forming by processes such as upward escapeof gas bubbles. The Unit L was deposited in the areaslightly deeper in subtidal zone of lagoon. During thedeposition of Unit M and N, there was the increasinginflux of terrigeneous sediments into the intertidal andsubtidal zones near shore. The siliciclastic sedimentswere deposited together with fine-grained intraclastsand minor skeleton fragments of crinoids andfusulinids. Subsequently, the supply of terrigeneoussediments was decreasing and the Unit O was depositedin the intertidal to subtidal zones of inner shelf withslightly high-energy condition.

CONCLUSION

The depositional model of the Khao KhadFormation is represented by the lagoon of inner shelfwith barrier bar or shallow platform in outer shelf andforeslope. The barrier bar and shallow platformenvironments protected the inner shelf from the openmarine wave energy and promoted diverse bio-communities on the back side. The situation may varyfrom low to slightly high-energy conditions probablyrelated to the variation of paleogeography of the seafloor. The low-energy condition was dominated by thedeposition of abundant micrite with large allochemssuch as fusulinid tests and brachiopod and gastropodshells. The high-energy condition was dominated bythe deposition of crinoid fragments and intraclasts.The barrier bar in the outer shelf was changed fromshallow-water carbonate platform at the Khao Khadand Pak Chong to Khao Yai areas into crinoidal barrierbar at the Khao Khad area. There might have been somechannels cross-cutting the barrier bar by which theturbidity current could transport the carbonatesediments into the foreslope. The overalllithostratigraphic sequence indicates that the KhaoKhad Formation was deposited during the transgressionand regression of the Lower to Middle Permian sea.

ACKNOWLEDGEMENTS

Thanks to the Department of Geology,Chulalongkorn University, for the permission to uselaboratory facilities.

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