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ELSEVIER Sedimentary Geology 123 (1999) 31–62

Sedimentology of the Cretaceous Maha Sarakham evaporites in theKhorat Plateau of northeastern Thailand

Mohamed El Tabakh a,Ł, Cherdsak Utha-Aroon b,B. Charlotte Schreiber c

a School of Applied Geology, Curtin University, G.P.O. Box U 1987, Perth, WA 6001, Australiab Economic Geology Division, Department of Mineral Resources, Rama IV, Bangkok, Thailand

c Department of Geology, Appalachian State University, 118 Rankin, Science Building,Boone, NC 28608, USA

Received 14 May 1997; accepted 8 June 1998

Abstract

Evaporites of the Cretaceous to early Tertiary Maha Sarakham Formation on the Khorat Plateau of southeast Asia(Thailand and Laos) are composed of three depositional members that each include evaporitic successions, each overlainby non-marine clastic red beds, and are present in both the Khorat and the Sakon Nakhon sub-basins. These two basins arepresently separated by the northwest-trending Phu Phan anticline. The thickness of the formation averages 250 m but is upto 1.1 km thick in some areas. In both basins it thickens towards the basin centre suggesting differential basin subsidencepreceding or during sedimentation. The stratigraphy, lithological character and mineralogy of the evaporites and clasticsare identical in both basins suggesting that they were probably connected during deposition. Evaporites include thicksuccessions of halite, anhydrite and a considerable accumulation of potassic minerals (sylvite and carnallite) but containsome tachyhydrite, and minor amounts of borates. During the deposition of halite the basin was subjected to repeatedinflow of fresher marine water that resulted in the formation of anhydrite marker beds. Sedimentary facies and textures ofboth halite and anhydrite suggest deposition in a shallow saline-pan environment. Many halite beds, however, contain acurious ‘sieve-like’ fabric marked by skeletal anhydrite outlines of gypsum precursor crystals and are the product of earlydiagenetic replacement by halite of primary shallow-water gypsum. The δ34S isotopic values obtained from different typesof anhydrite interbedded with halite range from 14.3‰ to 17.0‰ (CDT), suggesting a marine origin for this sulphate.Bromine concentration in the halite of the Lower Member begins around 70 ppm and systematically increases upward to400 ppm below the potash-rich zone, also suggesting evaporation of largely marine waters. In the Middle Member theinitial concentration of bromine in halite is 200 ppm, rising to 450 ppm in the upper part of this member. The bromineconcentration in the Upper Member exhibits uniform upward increase and ranges from 200 to 300 ppm. The presence oftachyhydrite in association with the potassic salts was probably the result of: (1) the large volumes of halite replacement ofgypsum, on a bed by bed basis, releasing calcium back into the restricted waters of the basin; and (2) early hydrothermalinput of calcium chloride-rich waters. The borates associated with potash-rich beds likely resulted from erosion and influxof water from surrounding granitic terrains; however, hydrothermal influx is also possible. Interbedded with the evaporitesare non-marine red beds that are also evaporative, with displacive anhydrite nodules and beds and considerable amounts ofdisplacive halite. The δ34S isotopic values of this anhydrite have non-marine values, ranging from 6.4‰ to 10.9‰ (CDT).

Ł Corresponding author. E-mail: [email protected]

0037-0738/99/$ – see front matter c 1999 Elsevier Science B.V. All rights reserved.PII: S 0 0 3 7 - 0 7 3 8 ( 9 8 ) 0 0 0 8 3 - 9

32 M. El Tabakh et al. / Sedimentary Geology 123 (1999) 31–62

These data indicate that the Khorat and Sakhon Nakhon basins underwent periods of marine influx due to relative worldsea-level rise but were sporadically isolated from the world ocean. 1999 Elsevier Science B.V. All rights reserved.

Keywords: Khorat Plateau; Maha Sarakham; potash; evaporites

1. Introduction

A long recognised problem in evaporite deposi-tion is the origin and development of saline giantsfound in the rock record (Sloss, 1969; Hsu, 1972;Schreiber, 1988; Busson and Schreiber, 1997). TheMaha Sarakham Formation (Cretaceous through Ter-tiary) on the Khorat Plateau of northeastern Thailandis a saline giant that includes one of the largest saltdeposits in the world. The formation is composed ofthree depositional successions present in the north-ern Sakon Nakhon and the southern Khorat basins(Fig. 1). Lithologies of the formation are dominatedby halite with potassic minerals of sylvite (KCl)and carnallite (KCl MgCl2.6H2O), red beds and mi-nor anhydrite, tachyhydrite (CaCl2.MgCl2.12H2O)and borates [hilgardite, Ca2BCl (OH)2 and boracite,Mg3ClB7O13]. The origin of these potash mineralsis controversial because there are no specific internalfeatures that define depositional or diagenetic fea-tures and no modern potash-forming basin of marineorigin is active in recent time, hence we have no ob-servable analogue. Preserved petrographic texturesof evaporites of halite and gypsum and the para-genesis of the carnallite=sylvite assemblage in theMaha Sarakham Formation provide valuable infor-mation about its environment of deposition and earlydiagenesis.

Salt beds of the Maha Sarakham Formation werefirst discovered in groundwater wells in the KhoratPlateau (La Moreaux et al., 1959), and a preliminaryinvestigation (Gardner et al., 1967) was carried out.Later, sylvite was found near Vientiane, the capitalcity of Laos, and shortly thereafter, carnallite as wellas sylvite were found on the Thai side. Earlier worksby Hite (1971, 1974) and Hite and Japakasetr (1979)have outlined the general stratigraphy of the saltsequence and a broad overview of the depositionalelements of the Maha Sarakham Formation. Hiteand Japakasetr (1979) reported a sharp boundary be-tween the lower part of the evaporite section and theunderlying Khok Kruat Formation, suggesting a pos-

sible disconformity. They also present details of thebromine chemistry of the salt deposits and propose astratigraphic, environmental and structural evolutionof the salt beds. Utha-Aroon (1993) suggested non-marine depositional environment of the red-colouredclastics interbedded with the rock salt deposits basedon their sedimentary features.

Most giant evaporite deposits are associated withmarine shelf-carbonate sequences. The evaporites ofthe Maha Sarakham Formation, however, lie atop athick non-marine sequence of the Mesozoic KhoratGroup, are interbedded with non-marine red beds,apparently lack the more usual carbonates, and arefound in an inland basin on continental crust. Allof these features suggest a non-marine origin forthe Maha Sarakham salts. The objectives of thispaper are to combine details of the depositional fea-tures, petrography, and geochemistry of the MahaSarakham evaporites in order to more define theorigin of these evaporites and their diagenetic de-velopment and to document those features that arerelated to other similar saline deposits in the world.

2. Material and methods

This study is based on an examination of 235drilled cores obtained and archived by the D.M.R.(Department of Mineral Resources) of Thailand(Fig. 2). Individual cores are up to 1100 m long, butaverage about 300 m. Salts are well preserved andare sealed in double plastic covers which preventeddecay and dissolution of the salts. These depositswere studied by core logging and X-ray diffractionwas utilised to determine mineralogy. Bromine andpotassium concentrations in the evaporites were ob-tained by the D.M.R. The bromine concentrations inthe halite were determined by oxidation spectropho-tometry using samples of about 2 g of halite andthe analyses have 5% analytical error. A total of 100thin sections were obtained from different evaporitelithologies for petrographic examination. Scanning

M. El Tabakh et al. / Sedimentary Geology 123 (1999) 31–62 33

Fig. 1. Geological map of southeast Asia showing the general tectonic elements of the region and the location of the Khorat and theSakon Nakhon basins.

electron microscopy and back-scatter imaging wereused to define mineral textures and composition. Themineral analyses were performed at the centre ofMicroscopy and Microanalysis at the University ofWestern Australia in Perth, Australia. The δ34S of 30anhydrite samples were obtained from different coresand were analysed according to the methods of Holtand Engelkemeir (1970) at the CSIRO Laboratory inSydney, Australia.

3. Geology

The Khorat Plateau presently has a high escarp-ment of about 900 m above sea level along itswestern and southern edges whereas elevations of its

central area are only 100–300 m above sea level.Much of the sedimentary succession that makes upthe Khorat Plateau is a part of an extensive, largelynon-marine depositional system in the mainland ofsoutheast Asia. It covers an area of 170,000 km2 ofthe Esarn region in northeastern Thailand and centralLaos, and lies between latitudes 14º and 19ºN andbetween longitudes 101º and 106ºE (Fig. 1). Theplateau is located on the Indochina microplate andincludes the Sakon Nakhon Basin to the north andthe Khorat Basin to the south. These two basins areseparated by the Phu Phan anticline in northeast-ern Thailand. The plateau is a broad synclinoriumbounded to the west by the Shan Thai microplate,and to the north by the South China plate. The In-dochina microplate contains sediments ranging in


Fig. 2. Relative location of some representative cores of the Maha Sarakham Formation in the Khorat and Sakon Nakhon basins.

age from Late Cambrian to Recent (Sattayarak etal., 1991; Mouret, 1994). Early to Middle Triassiccollision between the Shan-Thai and the Indochinamicroplates along the Nan-Uttaradit suture (Sengor,1979, 1984; Bunopas and Vella, 1992; Drumm etal., 1993), was followed by Late Triassic tectonicrelaxation or extension which created half-grabenbasins, where thicknesses of up to 5 km of thenon-marine red beds of the Khorat Group were de-posited (Fig. 3). Fluvial and lacustrine facies filledthese basins with conglomerates, sandstones, andmudstones that range in age from latest Triassic tolate-Early Cretaceous (Racey et al., 1994; Mouret,1994). The source of the Khorat rocks in Thai-land is from erosion of the late Palaeozoic rocksexposed in the Nan-Uttaradit suture area in cen-tral north Thailand (Sengor, 1979; Hutchison, 1989)and possibly from eastern Laos and central Vietnam(Drumm et al., 1993). The Khorat deposits were in-truded by granites of Campanian and Cenomanian

age which resulted from thermal subsidence of theKhorat Plateau (Smith et al., 1996).

During the early Paleocene, compression fromthe northeast due to continental collision of the In-dochina microplate with southeast China plate andfrom backarc compression of the Shan Thai mi-croplate to the west, resulted in uplift and erosion ofabout 3000 m of the Khorat sediments and the forma-tion of the NW–SE-trending Phu Phan anticlinoriumin the central part of the Khorat Plateau (Cooper etal., 1989; Mouret, 1994; Bunopas and Vella, 1992).During the Cretaceous, the Indochina microplate waslocated near 20ºN latitude, suggesting arid climaticconditions (Achache et al., 1983).

The Maha Sarakham Formation was first namedby Gardner et al. (1967) and, based on palyno-morphs, it is of Albian–Cenomanian age (Sattayaraket al., 1991). The formation averages 250 m thickand is up to 1.1 km thick in the centre of the KhoratBasin. The variation in thickness appears to be due


Fig. 3. Mesozoic and the Cenozoic stratigraphy of the Khorat Basin in NE Thailand and summary of the geological history of theIndochina microplate during the Mesozoic and the Cenozoic.

to: (1) differential subsidence before and during de-position; (2) localised tectonic differentiation, suchas development of fault-controlled highs and lows;and (3) post-depositional dissolution, particularly ofthe Middle and Upper Units (due to very shallowburial). Lithological, stratigraphic and mineralogi-cal similarities of the formation in both the SakonNakhon and Khorat basins, suggest that a single gi-ant evaporite basin existed at least in the area of thepresent-day basins (Fig. 4).

4. Stratigraphy and depositional patterns

The Maha Sarakham Formation comprises threedistinctive depositional members (Lower, Middleand Upper) which are mainly composed of evapor-ites separated by red-coloured siliciclastics (Fig. 5).

All of the evaporative members contain beds com-posed of halite-replaced pseudomorphs of bottom-growth gypsum. While such beds are commonthroughout both basins, these replaced beds arethickest and most plentiful at the southwestern cor-ner of the basin. The following section of the paperis a petrographic description of all three membersand their component units. Because they all sharecommon diagenetic and geochemical histories, thedepositional and diagenetic background for all of thefacies will be reserved for the discussion section.

The original depositional morphology of the MaraSarakham has been complicated by deformation.Seismic data show that salt structures such as domes,anticlines, ridges and basins are found in the sub-surface of the Khorat plateau (see Figs. 16 and 17).Circular mounds and shapes of rounded landformsare related to shallow salt domes and some of these


Fig. 4. Idealised map showing the predominant rock types of the Khorat Plateau and the Maha Sarakham Formation. Large arrowindicates the postulated source of marine water inflow into the Khorat Plateau near Bamnet Narong area at the southwestern corner ofthe Khorat Plateau. KK marks the location of Khon Kaen City located in the central area of the Khorat Plateau.

salt structures are present as shallow as 50 m belowthe surface. These structures have apparently initi-ated from salt movement associated with differentialloading of clastics after the deposition of the thickLower Member. However, later stream channels haveeroded up to 200 m of the post-salt sedimentarycover, permitting further movement along axes ofsalt domes and ridges.

4.1. Lower Member

The Lower Member is well preserved throughoutboth basins. All of the component units are well dis-played and are readily traced from core to core. Defor-mational thickening (and thinning) has affected thismember more than the successive ones resulting in agreat difference in thickness from area to area. Thethickness of the formation ranges from 50 m in basinmarginal areas up to 1100 m in the basin centre areas.

4.1.1. Basal Anhydrite UnitThis unit is found at the base of the Maha

Sarakham Formation throughout both the Khorat and

the Sakon Nakhon basins with a consistent basin-widethickness of 1.1 m (Fig. 6). The unit does not appearto interfinger with or pass laterally into carbonates orclastics. The anhydrite exhibits laminar and nodularforms, and has a sharp and stylolitic basal contactwith the underlying sandstones of the Khok KruatFormation. Laminar and microcrystalline anhydriteis found just at the contact of salt and clastics. Thistype of anhydrite is made up of small and flattenedanhydrite nodules of up to 2 cm size, forming evenlaminations. The upper contact of the anhydrite unitwith the overlying halite is sharp and is marked bywhite anhydrite nodules found in thin layers. Gener-ally, the anhydrite comprises poorly defined layers ofhard, milky white to bluish-coloured nodules with adistorted and sheared mosaic fabric.

4.1.2. Halite L1 UnitThe Halite L1 Unit is the most fully preserved

and complete salt unit found in the Khorat Plateau.Thickness of this salt unit varies from 30 to 350m. This unit is widely distributed in both basins andconstitutes a single laterally continuous salt layer and


Fig. 5. Lithostratigraphy of a complete sedimentary section of the Maha Sarakham Formation.


Fig. 6. A composite core photograph showing the characteristic similarities seen in the Basal Anhydrite Member. The individual coresare obtained from different parts of the basin. This Basal Anhydrite Unit is traced for hundreds of kilometres and defines the base of theMaha Sarakham Formation. The unit averages 1 m in thickness. Length of individual core sample is 50 cm.

is present in all of the boreholes, but is somewhat de-formed. Individual halite beds are relatively thin andaverage 15 cm thick. Lithologies of this unit includehalite interbedded with minor anhydrite stringersand poorly preserved gypsum. In the lower section,the halite is almost pure (95%) and its crystals arenearly flattened, forming sheet-like halite crystals.The salt of this unit is dominated by sheared andrecrystallised smoky or grey halite with numerous

well-formed, white, chevron halite beds (Fig. 7A).The chevron halite commonly contains fine clasticmaterial ‘dusted’ along the chevron surfaces. Thetops of chevron halite layers are marked by irregulardissolution surfaces, outlined by fine clastic grains,and are overlain and infilled by thin-bedded, clearhalite making up bands of about 10 cm thickness. Insome cases halite-replaced, bottom-grown, gypsumlayers (up to 10 cm thick), overlie the clear halite


Fig. 7. Examples of halite from the Lower Member. (A) Core photograph showing sheared, massive and poorly bedded halite. Anindividual core segment is 1 m long. (B) Core photograph of halite with anhydrite nodules that define bedding. Core is 20 cm long.

layers. The depositional bedding of most of the haliteis indistinct, particularly when halite contains littleor no anhydrite. Where bedding is obvious, it isdefined by isolated nodules or irregular masses ofanhydrite stringers (Fig. 7B).

4.1.3. Anhydrite Marker Unit 1This unit comprises the thickest anhydrite bed in

all of the salt members and includes several layers

of laminated anhydrite and beds of well-developedgypsum pseudomorphs replaced by halite (Fig. 8).The unit is up to 3.45 m thick and separates theHalite L1 Unit, from the Halite L2 Unit and is de-fined by its unique thickness and wide distributionin the basin. Anhydrite layers are dense and com-monly exhibit even lamination and bedding. Somelayers have halite-replaced gypsum pseudomorphsin which outlines of the original gypsum crystals



are still in growth position (outlined by a rim ofanhydrite) with a vertical orientation, as originallydescribed by Hovorka (1992). Because of the an-hydrite rims surrounding halite fillings, these layersappear to have an unusual sieve-like texture.

4.1.4. Halite L2 UnitThe Halite L2 Unit varies from 10 to 200 m in

thickness. This unit is characterised by thick (up to1 m), white and clear salt beds with well definedbedding (Fig. 9). The crystals of halite in this unitare much less deformed than the lower section. Thehalite is dominated by chevron-type texture and iscomposed of white to creamy white halite that is richin fluid inclusions that cause the milky colouration.However, halite crystals in these layers are smallerthan those of the underlying Halite L1 Unit andin places exhibit a crystalline mosaic of anhedraltexture. The tops of almost all of the chevron halitelayers are truncated and marked by dissolution pits.The pits are filled by clear halite bands that average3 cm in thickness. These clear halite layers arecommonly overlain by the anhydrite stringers. Thinbeds of halite-replaced gypsum pseudomorphs andlaminated anhydrite are also present in this unit,making up to 8% of the rock volume and average5 cm in thickness (Fig. 9A). Toward the top of theL2 Unit thin, millimetric, anhydrite beds are present,interbedded with the halite (Fig. 9B).

4.1.5. Potash UnitThe distribution of potash deposits in the Khorat

Basin is known from boreholes and seismics. Thick-ness and distribution of the Potash Unit is highlyvariable, possibly due to some depositional differ-ences, but more importantly due to patchy dissolu-tion and deformation. Core examination (of over 100cores) shows that the potash deposits are extensive,seemingly laterally continuous, and in many areasare found at relatively shallow depths, at 100 m,

Fig. 8. Examples of Anhydrite Marker Unit (indicated by arrows). (A) Anhydrite layer is thick (about 80 cm) and includes ‘sieve-like’textures that exhibit pseudomorphs of halite after gypsum and thin laminar anhydrite layers. Individual core segment is about 0.8 m long.(B) A complete section of a marker bed of about 0.5 m thick, interbedded with halite and other minor anhydrite beds that define beddingwith halite. Individual core segment is about 100 cm long. (C) An example of a thick anhydrite marker bed which is 2.2 m thick and iscomposed of alternating laminar anhydrite and gypsum pseudomorphs. Individual core segment is 1 m long. The well defined gypsumforms suggest that the original gypsum crystals that nucleated at the bottom of the salt pan were replaced early on in the diagenetichistory of the deposits as synsedimentary diagenetic process.

in both basins. The potash minerals include sylvite(KCl) and carnallite (MgCl2ÐKClÐ6H2O) which arefound at the top of the Halite L2 Unit and are over-lain by banded and red-coloured halite beds. On amacroscopic scale, the strata of the potash sequenceinclude three zones: (1) a lower zone of massivehalite with traces of carnallite filling dissolution cav-ities in halite; (2) a middle zone of massive andpoorly bedded carnallite and halite; and (3) an up-per sylvinite zone (interbedded sylvite, sylvinite, andhalite) (Fig. 10).

Carnallite is by far the most widespread potashmineral in the Khorat Plateau and its first appearancetypically defines the base of the potash zone. Itis highly variable in thickness, ranging from 10to 80 m (average of 50 m). The lower part ofthe carnallite zone is composed of massive halitebeds having irregular dissolution surfaces markedby traces of carnallite, collapsed and recrystallisedhalite and cavities coated with microcrystalline andred-coloured carnallite. This zone grades upwardinto massive and poorly bedded carnallite with agranular to subrounded texture.

The middle part of the potash zone is dominatedby massive carnallite and halite. This zone has anaverage thickness of 20 m and is poorly bedded.Two types of halite are observed in this zone: (1)large, clear, recrystallised, halite crystals up to 4cm in size; and (2) granular and fine-grained halite.The first type is in the form of massive halite bedsand the latter type is found as carnallitite includingdisseminated halite crystals in carnallite. Anhydriteand other less soluble residues are scarce in thecarnallite zone. The carnallite is poorly bedded andin most cases is pale red or clear in colour and itscrystals are granular, of up to 1 cm in size.

In the uppermost part of the potash zone there isa sylvinite section (a mixture of halite and sylvite)that directly overlies the carnallite-rich zone. Theaverage thickness of this zone is 4 m. The sylvinite


Fig. 9. Typical sedimentary characteristics of bedded halite as seen in the upper section of the Maha Sarakham Formation. (A and B)Bedding of halite is defined by thin and irregular anhydrite stringers. Dark bands in halite beds include fine clastics within clear halite.The dark bands form during flushing of the halite salt pan by fresh water. Core segment is 1 m long.

zone begins at the top of the carnallite=halite whichgrades upward into sylvite or sylvite=halite. Eachsylvite-rich bed ranges from 1 to 15 cm in thickness.Macroscopically, sylvite layers are not homogeneousand several forms of sylvite are commonly foundboth intermixed and as thin but separate layers withfine clastics and thin halite beds (Fig. 11). Thesethin halite layers still exhibit well-defined bedding

and in many places display chevron halite textures(i.e. are primary). Some of the halite associated withsylvite, particularly in regions of structural deforma-tion, is coarsely crystalline and has a marked bluecolour. Blue-coloured halite is poorly crystalline andis found in zones of up to 10 m thick. Sylvite ispresent as: (a) individual crystals of up to 1 cmsize of euhedral forms, found in carnallite or halite

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Fig. 10. Sylvite lithologies. (A) A core photograph of massive, red-coloured, bedded sylvite and halite. Core sample is 12 cm long. (B) Core photograph showing bands of massivehalite and sylvite patches or clusters. Sylvite crystals show a characteristic ‘amoeboid’ texture. Individual sylvite crystal is up to 1 cm in size. Core sample is 12 cm long. (C) Acore photograph of massive sylvite. Sylvite crystals are showing interlocking fabrics. Sylvite crystals are coated with thin clay seams. Core is 15 cm long.

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Fig. 11. Microfabrics of the potash zone. (A) Large and euhedral sylvite crystal at the centre surrounded by carnallite (C). Scale bar is 1 mm. (B) Thin layer of sylvite (S) betweenhalite (H). Scale bar is 1 mm. (C) Large and euhedral sylvite crystals (S) surround subhedral halite (H). Scale bar is 1 mm. (D) Anhedral sylvite crystals (S) cementing euhedralhalite crystals. Scale bar is 1 mm.


(Fig. 11A); (b) massive lenses or layers that aver-age 3 cm in thickness or interlayered with halite(Fig. 11B); (c) small clusters and groups of crystalswithin carnallite or halite (Fig. 11C); (d) irregularpatches within massive halite beds (Fig. 11D); and(e) massive beds with an average thickness of 10 cm.Petrographically, sylvite crystals exhibit well crys-tallised subrounded shapes and contain finely dis-seminated haematite inclusions. Traces of anhydriteand thin beds of gypsum pseudomorphs composedof halite are present throughout the sylvite-rich zone.

Associated with sylvite and halite are accessoryborate minerals of hilgardite [Ca2BCl (OH)2] andboracite (Mg3ClB7O13). These minerals occur in twoforms: as white thin layers up to 1 cm thick insylvinite beds or as dispersed massive and irregularnodules and grains from 1 mm up to 5 cm thick. Eu-hedral tachyhydrite (CaCl2.MgCl2.12H2O) crystalsare commonly found in these potash-rich deposits.The identification of tachyhydrite was confirmed byX-ray microanalysis with scanning electron micro-scope (SEM). Tachyhydrite crystals are small andaverage 1 mm in size and are commonly found withhalite and carnallite (Fig. 12), but is not detectedwith sylvite.

4.1.6. Clastics L UnitThe thickness of this unit is 10–60 m. The Clas-

tics L1 Unit is a mudstone of reddish to brown colourthat directly overlies the Lower Member (above thehalite and potash-rich units). The mudstone lay-ers are mainly composed of illite, quartz, haematitewith minor K-feldspar. These layers are massiveand are interbedded with rare siltstone layers. Dis-placive halite hoppers of up to 3 cm size occur inthese red mudstones, and chaotic mudstone texturescomposed of clastic matrix with irregular masses ofhalite skeletal crystals are common. Randomly ori-ented fractures filled with halite spar of up to 1 mthick are found in this unit. A few root structuresand weakly developed soil profiles have been noted;however, the disrupted and chaotic textures are thedominant fabric.

4.2. Middle Member

The Middle Member ranges from 40 to 130 me-tres in thickness, is composed of two halite units

separated by an anhydrite marker bed and containsmost of the same depositional and diagenetic fea-tures as the Lower Member. There is little or nopotassic salt in this member.

4.2.1. Halite M1 UnitThe thickness of this halite unit is 20–60 m. This

unit directly overlies the Clastics L Unit, and iscomposed of well-bedded halite layers that average10 cm in thickness. The halite beds are interbeddedwith thin beds of anhydrite and layers of gypsumpseudomorphs that are composed of halite with typ-ical anhydrite rims. Some disseminated sylvite andcarnallite crystals are found in rare intervals of thissequence. Halite beds in this unit typically exhibitcoarse chevron structures and have a characteristicdark honey-colour, due to finely disseminated ironoxide.

4.2.2. Anhydrite Marker MThis marker bed is extensive and contains abun-

dant well-formed gypsum pseudomorphs. The unit isup to 0.5 m thick, and is similar to the unit found inthe Lower Member. It consists of beds composed ofone or more thin, laminated anhydrite layers of upto 10 cm thick immediately overlain by more mas-sive beds composed of gypsum pseudomorphs. Thegypsum is replaced by halite and the original gyp-sum crystals still show original vertical orientationas defined by anhydrite rims.

4.2.3. Halite M2 UnitThe thickness of this halite unit is 10–70 m thick

and is separated from the Lower Halite M1 Unitby the Anhydrite Marker Bed. This unit is largelycomposed of bedded dark honey-coloured rock salt,interbedded with dark smoky-coloured halite beds.Thin anhydrite layers of up to 5 cm thick are present.These layers are composed of anhydrite nodules,laminated anhydrite, and halite-replaced, gypsumpseudomorphs with anhydrite rims.

4.2.4. Clastics M UnitThe thickness of this unit is 20–70 m. Clastics

consist of massive red to purple claystone and siltymudstone. The bedding in this unit is well defined,with laminations and root traces and beds up to50 cm thick. Internally the clastic beds are highly


Fig. 12. Microfabrics of halite and tachyhydrite. (A) Thin-section photomicrograph of euhedral tachyhydrite crystals (T) found in blockyhalite. Note that the tachyhydrite crystals are well formed. (B) SEM photomicrograph of euhedral tachyhydrite (T) in halite (H). Scalebar is 0.1 mm.

fractured and are poorly consolidated. At the baseof the unit, near the contact of halite and clastics,characteristic grey anhydrite layers of up to 1 m thick

are present. Displacive cubes of halite crystals andnodules of anhydrite of up to 4 cm size are founddispersed in the fine clastic matrix.


4.3. Upper Member

The entire sedimentary section of the MahaSarakham Formation was sampled by 235 cores butonly about 12 cores contained the Upper Member, asit is poorly preserved in the rock record largely dueto its shallow burial in a wet climate. Based on whathalite is now preserved in this unit, it seems that atleast half of the salt of this unit has been leached.Where preserved the total thickness of this memberranges from 3 to 17 m.

4.3.1. Halite U1 UnitThe thickness of this unit ranges from 1 to 15

m. This salt unit is the least preserved halite unitin the Khorat Plateau because of later dissolutionand possible non-deposition in some parts of thebasins. However, salt beds when observed are theleast deformed in the sequence. This unit is mainlycomposed of layers of chevron halite that average 5cm in thickness. Halite is dark smoky grey to orangein colour with minor anhydrite stringers and somelayers of gypsum pseudomorphs.

4.3.2. Anhydrite Marker Unit UThe thickness of this anhydrite unit where present

is up to 0.5 m. This unit is composed of thin beds oflaminated anhydrite and layers of gypsum pseudo-morphs that are composed of halite with an anhydriteoutline.

4.3.3. Halite U2 UnitThe thickness of this unit is 2 m. This unit is

rarely sampled by the studied cores due to later dis-solution. Where present it contains minor anhydritestringers with thin carbonaceous bands that are in-terbedded with the halite layers. Halite of this unit iswell bedded and is dark smoky to orange in colourwith beds up to 10 cm thick.

4.3.4. Clastics U UnitThis unit represents the uppermost sedimentary

layer of the Maha Sarakham Formation. The thicknessof this unit is highly variable, up to 680 m. The clas-tics of this unit are composed of pale reddish-brownsilty claystones and sandstones. Bedding is well de-fined with even laminations, and sets of cross-bedsare commonly observed. Some layers contain well-

defined root traces. Sands are mainly composed ofdetrital quartz grains of up to 2 mm in diameter butminor amounts of feldspar are present. The detritalgrains are rounded to subrounded. Composite sets ofcross-beds of silty and sandy mudstones interbeddedwith massive mudstone layers occur in this unit. Thecontact of this unit with the clastics of the overlyingPhu Thok Formation is not clearly defined; however,generally the clastics of the Phu Thok are coarser.

4.4. Sedimentology synthesis

4.4.1. Primary depositional faciesSulphates. Evaporated marine water, upon reaching asalinity range of 140–300 ppt, results in the formationof crystalline gypsum at the bottom of the salt wa-ter body. Schreiber and Kinsman (1975) and Geisler-Cussey (1982) describe the gypsum morphologies ofdeposits forming in shallow environments and notea number of different gypsum crystal forms, but themost common primary crystal forms are twinned gyp-sum crystals, commonly present in regular, coalescentbeds. Such beds make up much of the Upper Miocenegypsum present all around the Mediterranean. Com-monly when such gypsum deposits become buried todepths greater than 1 km they gradually lose their wa-ter of crystallisation and become massive, featurelessbeds of anhydrite (Jowett et al., 1993). The character-istic gypsum crystal morphology is usually destroyedwith the 40% volume loss and is preserved only underunusual circumstances (Schreiber et al., 1982). Pseu-domorphs of such shallow-water gypsum beds (see‘diagenesis’ below) are found throughout the MahaSarakham Formation and are particularly common inalmost every bed near the southwestern corner of theKhorat Basin.

Several other morphologies of calcium sulphateevaporites are found in the Maha Sarakham Formation(Fig. 13). Beds of laminar and microcrystalline anhy-drite are commonly found within the M and U clasticunits. Isolated nodules of anhydrite are also foundwithin these clastic units. Well bedded and crystallineanhydrite that exhibits a nodular-mosaic (‘chicken-wire’) texture is found in clastics at the western areaof the Khorat Basin, in the Bamnet Narong area. Thesebeds are found in sections of up to 15 m thick. Thistype of anhydrite is white and is found in thick beds ofnodular anhydrite that a exhibit mosaic texture. Shear-


Fig. 13. Examples of different calcium sulphate lithologies in the Maha Sarakham Formation. (A) Pseudomorphs of gypsum crystals noware replaced by anhydrite. The original ingrowth morphologies suggest that these forms are still in their crystallisation position. Sampleis 15 cm long. (B) Beds of anhydrite that exhibit twin gypsum forms. Sample is 20 cm long. (C) Small and poorly bedded anhydritenodules in composite mosaic forms. Core is 20 cm long. (D) A core photograph showing numerous anhydrite nodules.

man (1978) and Butler et al. (1983) have demonstratedthat such nodules and layers form displacively withinan already existing subaerial matrix adjacent to a ma-rine water source, as early diagenetic features. Veigas(1997) and Ortı (1997) show that many of the samevery early displacive morphologies may form on thefloors of and adjacent to sporadically desiccated playalakes. Because of the morphological similarities be-tween nodular calcium sulphates, in many cases, theonly way to differentiate between marine and con-tinental-sourced sulphates is by examination of thesulphur isotope values.

Halite. Halite beds having chevron structures werefirst described by Shearman (1970). Shearman

(1970) pointed out that these structures develop dur-ing rapid growth of halite crystals at the bottomof shallow salt pans, and that their milky colouris caused by fluid inclusions incorporated on thegrowth faces of the salt during rapid crystal growth(Shearman, 1978). Truncation and erosion of uppersurfaces of these beds (including solution pitting)results from short-term subaerial exposure, and=orinflux of new, less concentrated water (below satura-tion). Infill of such solution pits and surface irregu-larities by clear, banded halite suggests flooding bynew water and renewed concentration of this ensuingbatch of water covering the surface. Lowenstein andHardie (1985) show the types of halite formed indeeper water and contrasts them to the shallow-water


forms. The Maha Sarakham Formation contains re-peated beds of shallow chevron halite beds and fewexamples of deeper-water halite; therefore its originis largely from very shallow waters.

4.4.2. Diagenetic overprintsHovorka (1992), noted that early replacement of

gypsum crystal beds by halite (and anhydrite) maytake place and the resultant pseudomorphs clearly re-tain the characteristic forms of the original shallow-water gypsum. Replacement of the bottom nucleatedgypsum actually occurs in the floors of salt panswhen hot brines (33–37ºC or greater), that are un-dersaturated in gypsum and oversaturated in halite,sink into the previously accumulated gypsum sub-strate (Schreiber and Walker, 1992). Schreiber andWalker point out that it is the hot, halite-saturatedbrines that heat the now cooler underlying gypsum,dissolve it, and as the sinking brines begin to cool,replace most of the gypsum with halite. The anhy-drite outlines of the original gypsum crystals remainpreserved, floating in the replacive halite (Hovorka,1992).

The Basal Anhydrite Member found at the baseof the Maha Sarakham Formation is defined as adissolution residue that resulted from flushing ofthe salt by compacted basin waters. This processresulted in accumulation of interbedded anhydritewith the salt as a dissolution residue at the base ofthe salt. Lithologic, petrographic and geochemicaldata supporting this interpretation are given in ElTabakh et al. (1998).

Tectonic effects. In many halite beds of the Lowerand Middle Members, the halite crystals appear to bestrongly deformed, even where the beds themselvesstill reflect primary depositional features. Shearedand flattened crystals are common. Salt deforma-tion on structure has been recognised for a longtime (Clabaugh, 1962), but experimental studies ofhalite recrystallisation, resulting in foliated fabrics,did not take place until much later. Investigationof salt domes as hydrocarbon storage facilities re-sulted in studies of experimentally induced crys-tal changes developed under fairly low strain rates(Larsen, 1985). As a consequence of these investi-gations it is clear that pervasive recrystallisation cantake place even where the beds are only modestly

deformed. The original geometry of particularly thesylvite deposits is not defined in the Maha SarakhamFormation. This is because later extensive dissolu-tion and alteration by groundwater and deformationof salt occurring during development of salt domes,ridges and depressions, have drastically changed thegeometry of the potash deposits and also the rocksalt. The surface extension and vertical changes ofthickness of the sylvite and carnallite associationsgreatly change from one core to another. Exten-sion and vertical changes of thickness of the potashgreatly change from one core to another, even on thescale of hundreds of metres between cores.

Other features. Blue-coloured halite is commonlyfound overlying deformed zones of sylvite (andsylvinite) in the Maha Sarakham. Sonnenfeld (1995)has pointed out that zones of blue-coloured halitecommonly develop along paths of circulating brinesin association with deposits of sylvinites. He fur-ther suggests that the colour is not due to impuritieswithin the salt but is probably due to recrystallisationof the associated halite and the loss of bromine atomsfrom the crystal lattices, leaving metallic sodium be-hind. This type of halite has probably resulted fromfluid migration resulting from stress during domingof the salts and is not a primary feature.

The effects of dissolution of the Maha Sarakhamsalts are obvious in the observed thinning of saltunits and the absence of units in several cases, par-ticularly around the basin edge. Dissolution of saltleads to lack of preservation of the Middle and Uppersalts in many cores. It also resulted in accumulationof anhydrite residue from dissolution of salt in somebeds. This residue is found between the overlyingclastics and the underlying salt unit (Fig. 14A). Typ-ical anhydrite-dominated thin residual layers tend tocap underlying salt beds. These anhydrite residuestend to follow modern hydrological and topographicpatterns. The contacts between the residue and theoriginal halite beds show characteristic angular dis-continuities (Fig. 14B).

5. Geochemistry of the evaporites

The concentration of bromine in halite and iso-topic composition of δ34S in anhydrite are conven-


Fig. 14. Dissolution features of Maha Sarakham salt. (A) Core photograph showing horizontal, stacked, light anhydrite bands overlyinginclined halite beds with thin layers of anhydrite stringers. (B) A core photograph of anhydrite layers of about 2.5 m thick found betweenhalite and clastics. The anhydrite bands apparently are a residue from dissolution of salt layers along tops and flanks of salt domes.

tionally used to help define the origin of the brinesfrom which the evaporative sediments have formed.These studies cannot stand alone but must be com-pared to the petrology of the sediments from whichthey are taken.

5.1. The Br composition in the salts

The bromine content of halite may be used as ageneral test for the origin of the water from whichthe halite was formed. This is a useful measurementbecause in nature bromine may substitute, in part,for chlorine in the halite crystal lattice. The amount


of bromine substitution for chlorine depends on theconcentration of bromine in the original brine. Be-cause the bromide ions have some difficulty in fittinginto the chloride positions in the crystal lattice ofthe halite, the percentage of bromine entering intothe crystal growth is lower than its percentage inthe solution. Thus more and more bromine remainsin the parent solution as salt is precipitated. Theconcentration of bromine in halite is therefore usedto determine brine evolution during halite precipita-tion (Holser, 1979). The first halite precipitates fromevaporation of seawater will contain 65–75 ppm Brand will progressively increase during further halitedeposition rising to values of 320 to 400 ppm atthe point where potash minerals should precipitatefrom evaporated seawater (Holser, 1966, 1979). Inthe Lower Member, the initial bromine content atthe base of halite averages 70 ppm (Fig. 15). Thebromine content of this unit shows a slow but con-tinuous increase from the bottom (70–90 ppm), tothe middle (200 to 250 ppm), to the top where itrapidly increases to 450 ppm (just below the potashlayer). In some cores, Br concentration in the LowerHalite Member is constant from the base to the topand averages 75 ppm. In the Middle Member theinitial concentration of bromine is 200 ppm at thebottom, and increases rapidly to 400 ppm in the mid-dle, reaching up to 450 ppm in the upper part of thismember. The bromine concentration in the UpperMember exhibits uniform an upward increase andranges from 180 up to 400 ppm. However, bromineconcentration may retreat to about 80 ppm in themiddle of the Upper Salt and then increase rapidly.

5.2. The Ž34S isotope values in anhydrite

The evolution and variation of the sulphur iso-topes in marine evaporites has been documented bya world-wide analysis of evaporites of different ages(Claypool et al., 1980). In order to determine the δ34Sisotopic values of anhydrite in the Maha SarakhamFormation 30 anhydrite samples were taken andanalysed from the different types of anhydrite (Ta-ble 1). These values are used to trace the originof aqueous sulphate from which evaporites precip-itated. The δ34S isotopic composition in anhydritetaken from the halite beds ranges from 14.8‰ to17.7‰ (CDT). The δ34S isotopic composition in an-

hydrite nodules taken from the thick clastic unitsshows significantly lower isotopic values, rangingfrom 6.4‰ to 10.9‰ (CDT).

5.3. Discussion of geochemical studies

Geochemical analysis of bromine in the halite ofthe Maha Sarakham Formation demonstrates the fol-lowing: (1) concentration of bromine in the LowerMember ranges from 70 ppm to 400 ppm (base totop), that more or less conforms with the evaporativeevolution of marine water as pointed out by Hite andJapakasetr (1979); and (2) bromine systematicallyincreases in each of the three members and can betraced from one core to another. Bromine concen-tration and upward systematic increase in all halitemembers across both basins demonstrates excellentlateral continuity, and that both basins were con-nected during deposition. Bromine data for both theMiddle and Upper Halite Members suggest recyclingand dissolution=reprecipitation of the salt. This in-terpretation is also supported by Hite and Japakasetr(1979). Data presented here and those of Hite andJapakasetr (1979) show Br curves for the Middle andUpper Halite Members that suggest synsedimentaryrecycling and dissolution=reprecipitation of the salt.The concentration of bromine in the Maha SarakhamFormation indicates the following: (1) Br is initiallyformed from marine water as in the base of theLower Salt Member; (2) Br is recycled as in Middleand Upper Salt Members; and (3) there are internalvariations in the concentration of Br in the UpperSalt Member.

Later chemical reworking or dissolution of halitemay deplete the original salt in bromine as invokedby Hite and Japakasetr (1979) for the pattern of Brcurves in the Maha Sarakham Formation. However,early chemical fractionation in a stratified water bodyis preferred because it may cause enrichment of theremaining brine in bromine. This same mechanism issuggested by Cendon et al., 1998 for a synsedimen-tary origin of sylvite deposits of Subiza in Navarra,Spain.

The Mesozoic era was a time of world-wide for-mation of evaporative basins leading to significantisotopic variations in marine evaporites, but thesulphur isotope values of anhydrite samples takenfrom the Maha Sarakham salts are in agreement


Fig. 15. Br profiles of different salt units of the Maha Sarakham Formation. The bromine concentration increases upward in most of thehalite units suggesting its early sedimentary recycling.


Table 1The isotopic composition of sulphur in different anhydrite forms in the Maha Sarakham Formation

Unit Core δ34S Depth Description(m)

Upper Clastics k-95 8.6 264 Anhydrite nodule in clasticsUpper Clastics k-36 10.9 184 Anhydrite nodule in clasticsUpper Halite k-49 15.4 63 Anhydrite residue in haliteMiddle Halite RS 2.7 15.9 254 Anhydrite nodule in haliteMiddle Halite k-11 16.2 154 Anhydrite nodule in haliteLower Clastics k-62 6.4 410 Anhydrite nodule in clasticsLower Clastics k-62 8.6 212 Anhydrite nodule in clasticsTop L. Halite k-31 14.3 318 Anhydrite cap=residueTop L. Halite k-91 15.1 108 Anhydrite residue=capTop L. Halite RS 2.22 15.9 93 Anhydrite residue=capLower Halite k-14 15.4 78 Anhydrite nodule in haliteLower Halite kk-03 15.5 265 Laminar anhydrite in haliteLower Halite k-49 16.0 81 Laminar anhydrite in haliteLower Halite k-3 16.0 63 Anhydrite nodule in haliteLower Halite k-16 15.8 34 Anhydrite marker in haliteLower Halite RS 2.7 17.0 211 Anhydrite marker in haliteLower Halite RS 2.1 16.9 192 Anhydrite residue in haliteLower Halite k-47 16.0 96 Anhydrite marker in haliteLower Halite k-64 16.2 266 Anhydride in haliteLower Halite k-11 16.1 490 Anhydrite residue in haliteLower Halite k-40 15.9 500 Anhydrite nodule in haliteLower Halite k-104 14.7 240 Anhydrite nodule in haliteLower Halite k-56 16.9 204 Anhydrite in a marker bedLower Halite k-8 14.8 157 Anhydrite nodule in haliteLower Halite k-19 15.1 224 Anhydrite nodule in haliteLower Halite RS 2.12 17.7 139 Anhydrite bedBasal Anhydrite k-29 16.0 245 Anhydrite bedBasal Anhydrite k-16 15.1 359 Anhydrite bedBasal Anhydrite k-19 15.4 224 Anhydrite bedBasal Anhydrite k-56 15.0 365 Anhydrite bed

Data show marine values of anhydrite taken from halite and from anhydrite residues in halite. However, anhydrite nodules in red-colouredclastics are of low sulphur isotopic values, suggesting non-marine origin of these evaporites.

with world-wide Cretaceous marine evaporite valueswhich range from 14‰ to 17‰ (CDT). The thickclastic units, that ended each of the three marineevaporite phases, exhibit sedimentary features char-acteristic of non-marine fluvial to lacustrine environ-ments. The sulphates present in these clearly non-marine units do not fit the word-wide marine curves(6.4‰ to 10.9‰ (CDT)) and are appropriately theproduct of continental or mixed-water precipitation.

6. Sedimentology and stratigraphic correlation

Several factors have contributed to the forma-tion and preservation of the salt units of the Maha

Sarakham Formation such as geographic isolation,arid climate, and a strong supply of solutes. Litholo-gies typical of evaporative basin margins such asevaporitic carbonates, dolomites, and reefs are notknown from the Khorat Plateau. This is possibly dueto (1) limited sampling, (2) non-deposition, or (3)subsequent erosion of these lithologies. The primaryfabrics in halite beds are composed of inclusion-richchevron halite, halite-replaced gypsum (verticallyoriented) and equant halite (recrystallised). Thesetextures suggest deposition in a shallow saline-pan environment (Shearman, 1970; Lowenstein andHardie, 1985; Hovorka, 1992). These beds are cross-cut by abundant dissolution surfaces filled with acoarsely crystalline cement of clear halite. Bottom-


grown gypsum crusts and anhydrite together withnumerous dissolution surfaces suggest influx of di-lute waters of possibly marine origin in salinas ofshallow brine depths (Warren, 1982). The dilutionwas, however, largely marine, based on isotopic val-ues of the sulphates; however, continental and evenhydrothermal waters may have entered the basin.

Stratigraphic correlation of different units of theMaha Sarakham Formation is based on lithologicaland depositional features seen in cores and thereis little evidence of fauna and=or flora. A typicalcross-section of the Maha Sarakham Formation inthe Khorat Basin is shown in Fig. 16. The thicknessof the formation increases toward the central basinarea. In all cases, halite and sandstone units increasein thickness and reach their maximum thickness inthe central basin area. Siliciclastics in the basin areaare composed of alternating cross-bedded siltstonesand massive mudstones of fluvial origin suggestingfluvial deposition in the basin province.

Lithologies and stratigraphy of the MahaSarakham are relatively uniform over vast areas ofboth basins, suggesting stability of deposition asseen in seismic lines (Fig. 17). The continuity ofthe evaporites was disrupted by later salt domingand dissolution, causing relative thinning of differentunits at different parts of the basin. The stratigraphyand continuity of the salt strata which typify manyof the saline giants of the world was confirmed bycoring along and near the seismic lines.

Gypsum layers and anhydrite partings lack re-working and suggest that no major phases of halitedissolution took place during freshening events (nonotable synsedimentary collapse structures). This isalso supported by a lack of significant insolubleresidue between halite layers. Only thin dustingsof clastic grains are present anywhere in the manyhalite beds.

Depositional conditions suitable for sulphate andhalite formation and preservation were disrupted bythree periods, marked by deposition of siliciclasticsand subaerial exposure evidenced in the three ma-jor mudstone interbeds within halite and associatedhalite=mudstone mixtures. Sedimentary structures inmudstones such as mudcracks, chaotic textures, rootstructures and burrows suggest a subaerial setting.However, haloturbation caused by early diageneticdisplacive growth of halite in mudstone beds suggests

sporadic brine flooding and then drying of subaerialexposed areas. Input of clastics into the basins impliesthat the source areas were exposed to similarly spo-radic variations in weathering, erosion and rainfall.

7. Discussion

7.1. Depositional model

Several depositional processes are interpretedfrom the study of the Maha Sarakham Formation:(1) marine-derived flooding onto the Khorat Plateau;(2) evaporation of seawater associated with aridityand hydrological restriction; (3) synsedimentary dis-solution of salt beds; (4) basin-wide and repeatedinflux of marine water (freshening) that is evidencedin the deposition of thick sulphate marker beds whichare now seen as largely replaced by halite. The thick-ness of this evaporite accumulation, particularly thepotassic salts suggests extreme aridity that favouredprecipitation and preservation of halite and potashminerals (Kinsman, 1976).

Despite the fact that this seems to be a marine-sourced deposit based on bromine data and sul-phate isotope studies, the evaporites studied here donot have a regional setting along a typical passivecontinental depositional border. Based on availablelithological data, we suggest that the location of themarine entrance into the Khorat Basin was in theBamnet Narong area which is located in the south-western part of the Khorat Plateau and depositionof the Maha Sarakham saline giant took place in alarge inland and isolated basin which was separatedfrom the Cretaceous ocean by a barrier located inthe southwestern corner of the Khorat Plateau. Thisis because large amounts of calcium sulphate evap-orites comprising anhydrite and secondary gypsumare found interbedded with red siliciclastics in thesoutheastern corner of the plateau.

The deposition of these salts coincides with aworld-wide high sea level in the Late Cretaceousand spill of marine-sourced water into what is nowthe Khorat Plateau (Haq et al., 1987). Other exam-ples of saline giants in the world suggest severalmodels of deposition of evaporites which include:(1) shallowing of water in a deep basin, as in theMediterranean (Hsu, 1972); (2) drops in sea level be-

M.E

lTabakh

etal./Sedim

entaryG

eology123

(1999)31–62

55

Fig. 16. A west–east cross-section taken across the Bamnet Narong area at the southwestern corner of the Khorat Basin showing progressive increase in thickness of salt toward the centreof the basin. Anhydrite and gypsum are abundant in several cores at the western part of the section, near the basin’s western margin.

56M

.El

Tabakhet

al./Sedimentary

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123(1999)

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Fig. 17. Subsurface structures of the salt domes in the Maha Sarakham Formation. (a) A shallow seismic line showing a salt dome. The salt members are relatively thinner ontop of the dome and are progressively thicker away from the dome due to dissolution by groundwater. Line is obtained from Khon Kaen area and the relative location of the lineis given in Fig. 2. (b) A deep seismic line showing several salt domes at the subsurface of the Khorat Plateau. Notice the continuous pattern of deposition of the salt members aswell as the strata of the Khorat non-marine clastics. Line is obtained from the near central area of the Khorat Basin and the relative location of the line is given in Fig. 2.


low the marginal topographic highs as in the ParadoxBasin (Williams-Stroud, 1994); (3) seawater spillinginto opening rifts from surrounding oceans as inthe early Mesozoic basins of the Atlantic passivemargins (Burke, 1975); (4) barred marginal marineto ephemeral salt pan (Williams, 1991); and (5) re-striction and drawdown of a marine-sourced basinlocated entirely on continental crust as in the Michi-gan Basin (Cercone, 1988). Deposition of evaporitesof the Khorat Plateau seem to have been controlledby repeated drops of sea level below the marginaltopographic highs located in the southwest corner ofthe plateau in a depositional area that was locatedentirely on continental crust.

7.2. Potash mineralisation

The origin of potash minerals is controversial be-cause no modern potash-forming basins of marineorigin are active in recent time. Modern potash de-posits typically form from non-marine-sourced wa-ters such as Lake Qaidam Basin, China (Lowensteinet al., 1989; Casas et al., 1991), Chatt El Djerid,southern Tunisia (Bryant et al., 1994), and DanakilDepression, Ethiopia (Holwerda and Hutchinson,1968). The chemistry and mineralogy of halite andanhydrite of the Maha Sarakham Formation suggesta marine origin of these evaporites. However, themineralogy of the potash sequence suggests a pos-sible non-marine influence on mineralisation or atleast recycled salts that resulted from dissolution oforiginally marine salts by non-marine water. A pri-mary origin of sylvite formed by accumulation ofcrystals precipitated at the brine–air interface wassuggested by Lowenstein and Spencer (1990) for thesylvite deposits of the Rhine graben, Permian Saladoand Devonian Prairie formations.

Textures of the potash minerals in the MahaSarakham Formation suggest primary and=or earlydiagenetic origin including: (1) presence of carnalliteat the base of the potash zone as fillings of disso-lution cavities in halite beds; (2) lateral continuityand well-defined layering of sylvite and presence ofinterbedded primary chevron halite with the sylvitebeds; and (3) fillings of sylvite in early dissolutioncavities of halite layers.

During deposition, once carnallite was formed asa near-surface deposit, it may dissolve when fresh

or nearly concentrated brine enters the basin. Subse-quently sylvite will form as a primary deposit duringthese reactions. In the Khorat Plateau a substantialamount of carnallite is still preserved and sylvite ex-hibits primary textures suggesting that if carnallite tosylvite conversion occurred, it was minor. Evidenceof freshening in the Khorat Plateau during sylviteformation includes: (1) presence of thin clastic bedscomposed of fine detritus and clays associated withsylvite; (2) presence of traces of anhydrite and mm-size gypsum pseudomorphs, replaced by halite; (3)presence of both chevron and clear halite beds in-terbedded with sylvite beds; and (4) presence ofiron oxides in both carnallite and sylvite which wereleached from red-coloured clastics, derived by in-coming non-marine waters into the basin.

New brine would fill pore spaces in carnallite andhalite beds and original pore-filling brine would seepaway. In the modern Qaidam Basin in China, car-nallite and halite sediments exhibit porosity and per-meability which allowed less saline and fresh waterto enter the carnallite sediments (Casas et al., 1991).Due to dilution, carnallite would decompose intosylvite and MgCl2 dissolved in solution (Braitsch,1971; Richter-Bernburg, 1972; Ortiz and Mur, 1984).Water released from the structure of carnallite wouldthen seep down and react further with carnallite andconvert it into sylvite. Thickness of sylvite variesfrom one core to the another that mainly depends onthe original permeability of carnallite and chemistryof original potash-forming brine and the new brinesthat resulted from a carnallite–sylvite transforma-tion. Brines rich in magnesium chloride end-memberwould have been produced in the potash deposits asresidual brine and would escape by seepage throughthe deposit or become diluted by overflow brine(Spencer, 1983; Sonnenfeld, 1984).

The potash deposits of the Maha Sarakham For-mation lack MgSO4-bearing salts. Final evaporationof modern seawater fails to form MgSO4-bearingsalts that should form before K and Mg-chloride-bearing salts, e.g. polyhalite, kainite, kieserite. Sev-eral reasons can be given for the absence of thesesalts: (a) unusual marine waters, (b) modification oforiginal mineralogy, (c) mixing of marine C non-marine parent waters, (d) non-marine origin, and(e) partial removal of sulphate by sulphate-reduc-ing bacteria (Braitsch, 1971). The Stassfurt evaporite


minerals of the Permian Zechstein are the only evap-orite suits which appear to follow the predicted evap-oration of modern seawater at 25ºC (Harvie et al.,1982; Lowenstein and Spencer, 1990). The formationof MgSO4-deficient potash evaporites is possible innon-marine basins which contain CaCl2-rich brinessuch as in the Chatt el Djerid depression, southernTunisia, Qaidam Basin in China, and the Danakil De-pression, Ethiopia. In these areas, marine-like halite,gypsum and anhydrite are commonly found associ-ated with potash deposits. Magnesium sulphate-poorevaporites occur in cratonic sag basins where risingdeep-seated hydrothermal brines are not expected.However, the hydraulic head, produced during thedesiccation of deep evaporite basins, may cause deepand hot groundwaters to migrate upward and enterthe basin (Kendall, 1989).

Usually in an evaporative deposit originating frommarine water, there is little calcium remaining in wa-ter concentrated above 325 ppt (start of halite precip-itation). In this basin, because of early halite replace-ment of primary gypsum, CaCl2-rich brines enter thewaters of the basin and modify the chemistry of thesurface waters and of the associated groundwater.Minerals associated with potash rocks can then beused to infer the paragenesis of the potash sequence.In the Maha Sarakham Formation tachyhydrite isabundant in the potash zone and although it is verysoluble it is present in considerable quantity in thedeposits. Here, tachyhydrite is commonly found as-sociated with carnallite as alternating bands or as amixture with carnallite. Core examination indicatesthat tachyhydrite is more concentrated in the centralpart of both basins. These areas were the most sub-sident parts of the basins, and subsidence allowedbrines to flow into low areas. Tachyhydrite dissolvesin contact with air because it is highly deliquescentand its presence in the Maha Sarakham evaporitessuggests that these evaporites were deposited sub-aqueously or as a subsurface product of diagenesisby migrating groundwater.

Based on geochemical data obtained from evap-oration of seawater of different pathways and com-positions, the mineral tachyhydrite is conclusivelyproven not to form from the final evaporation of puremarine-sourced waters (Hardie, 1990). CaCl2-richwaters were described in association with potash de-posits and may form in geothermal areas (Holwerda

and Hutchinson, 1968). The study by Hardie (1990)suggested that hydrothermal input into the potashprecipitating environments must have occurred to ac-count for the formation of tachyhydrite. In the Kho-rat Plateau, there is some evidence of hydrothermalactivities and thermal event during the Cretaceouswhich have possibly supplied CaCl2-rich waters intothe basin (Smith et al., 1996). Dating by K–Ar ofgranite intrusions in the area gave ages of Cenoma-nian and Aptian that are coincident with the ther-mal subsidence of the Khorat Plateau. The timingand mechanism of the input of these hydrothermalsources is still not fully understood. However, theseobservations suggest an origin and=or contributionof waters from non-marine or thermal sources intothe depositional area.

Borate minerals are common in the potash-richbeds of the Maha Sarakham Formation and mayresult from non-marine input into the basin such asin the Michigan Basin (Nurmi and Friedman, 1977),the middle Oligocene lacustrine sediments whichare interlayered with volcanic sediments and flows(Kyle, 1991), and in the Sierra Nevada lakes of thelong Valley Caldera (Felmy and Wear, 1986).

7.3. Comparison to other potash basins

The depositional characteristics of the MahaSarakham saline deposits are comparable to otherCretaceous saline deposits, including deposits of theEarly Cretaceous Congo–Gabon basins (deRuiter,1979), and deposits of the Sergipe in Brazil (Ward-law and Nicholls, 1972). These characteristics are:(1) including large volumes of tachyhydrite; (2)stratigraphic position directly overlying non-marinerocks with minor amounts of anhydrite and a lackof carbonates; (3) interbedded with non-marine redbeds; (4) presence of borate minerals; and (5) asso-ciation of subsequent igneous intrusions adjacent tothe deposits.

The Congo–Gabon basins in West Africa andthe Sergipe Basin in east South America wereformed during rifting and were possibly intercon-nected with non-marine rift lakes and hot hydrother-mal CaCl2-rich waters prior to continental rifting.Such rift setting is similar to modern-day lakes Ma-gadi and Natron and they contain sodium carbonatebrines (Eugster and Hardie, 1978). Deep and hot cal-


cium-rich brines of hydrothermal origin commonlyflow through deep-seated faults such as at the bottomof the Red Sea (Craig, 1969). In modern settingswhere calcium magnesium-rich brines exists, tachy-hydrite does not deposit even under dry conditions.Tachyhydrite would easily form if brine is extraor-dinarily hot, and under geothermal conditions whichwould then combine the source of heat and thechemistry of water. Boron-rich inflow, reflected indeposition of borate minerals, suggests either conti-nental (from granitic terranes) or geothermal inputinto the Khorat Plateau.

In summary, the Maha Sarakham saline giant wasdeposited in an extensive largely land-locked, butpartly marine, evaporite basin in southeast Asia dur-ing the Cretaceous. This basin received influx fromthe world ocean through an inlet at the present south-western corner of what is now the Khorat Plateau.Marine inflow was apparently choked off or ceasedduring the formation of the red beds that lie be-tween the three salt phases. The basin experienceda substantial input of marine water during three ma-jor episodes that most likely represent the productof short-term sea-level rises. The apparent excessamounts of calcium, present in the deposits of tachy-hydrite, probably originated from the replacementof gypsum by halite. Potash mineralisation resultedfrom extreme evaporation of seawater under veryarid conditions; however, some non-marine inputinto the saline environment took place, based on thepresence of borates.

8. Summary and conclusions

The Khorat Plateau area of southeast Asia un-derwent periods of marine influx due to relativesea-level rise but was sporadically isolated fromthe world ocean. These evaporites mark a majormarine deposition over the Khorat Plateau in north-east Thailand, following a long depositional historyof non-marine sedimentation of the Khorat Group.Evaporites of the Maha Sarakham Formation weredeposited as a single evaporite giant during the Cre-taceous to early Tertiary in the Khorat and the SakonNakhon sub-basins. These two sub-basins are sepa-rated by the northwest-trending Phu Phan anticlinewhich was formed during the Tertiary collision of

southeast Asia and south China. The stratigraphyand depositional features of the formation are sim-ilar throughout the plateau area. The formation iscomposed of three depositional members which in-clude three evaporitic successions, separated by non-marine clastic red beds that include some anhydritenodules and beds. The formation thickens towardsthe basin centre in both basins suggesting basin sub-sidence during sedimentation.

Evaporites include halite, anhydrite and a consid-erable accumulation of potassic minerals (sylvite andcarnallite). Some tachyhydrite and minor amounts ofborates are also present in the potash section sug-gesting input of non-marine or hydrothermal watersinto the Khorat Plateau during deposition of evapor-ites. Sedimentary facies and textures of both haliteand anhydrite suggest deposition in a shallow saline-pan environment. Interbedded with the halite bedsare anhydrite outlines of gypsum precursor crystalsthat form and which are the product of early diage-netic replacement by halite of primary shallow-watergypsum in saline ponds under relatively elevatedtemperatures caused by solar heating of the salinepan.

The sulphur isotopic values of anhydrite interbed-ded with halite range from 14.3‰ to 17.0‰ (CDT),suggesting a marine origin for this sulphate. Bromineconcentration in all halite member begins around 70–200 ppm and systematically increases upward to 400ppm towards the top of the halite, also suggesting amarine water source for the Maha Sarakham salts.Tachyhydrite found with the potassic salts resultedfrom releasing of calcium into the restricted watersof the basin due to the replacement of gypsum byhalite replacement of gypsum, and early hydrother-mal input of calcium chloride-rich waters into thedepositional area. Erosion and influx of water fromsurrounding Triassic-age granitic terrains is the pos-sible source for borates associated with potash-richbeds; however, hydrothermal influx is also possi-ble. The non-marine red beds interbedded with theevaporites are fluvial or alluvial deposits and includedisplacive anhydrite nodules and beds and displacivehalite in cubic forms. The low δ34S isotopic valuesof this anhydrite suggesting non-marine sources forsulphate.


Acknowledgements

We are grateful to the Deputy Director General ofthe DMR in Bangkok (Department of Mineral Re-sources) of Thailand Mr. Prayong Augsuwathana forpermission to study and sample the Maha Sarakhamcores and to Mr. Thawat Japakasetr of the DMRfor his encouragement and for permitting the study.Thanks to Mr. Nares Sattayarak of the Petroleum Ex-ploration Group in the DMR for sharing ideas withus about the geology of the Khorat Plateau area andfor his encouragement. The DMR has kindly fundedsome of the needed field expenses in northeasternThailand. We thank the personnel of the Ground Wa-ter Division of the DMR in Khon Kaen for their sup-port and for allowing us to use their core facilities.Gratitude is extended to Bruce Sellwood and Sed-imentary Geology reviewers M.M. Blanc-Valleronand Peter Sonnenfeld for their valuable suggestions.This research was funded by the ARC-2445 grant(Australian Research Council) supporting M.E. as anARC Postdoctoral Research Fellow.

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