Maha Sarakham Formation Research Articles

Shape design and safety evaluation of salt caverns for CO2 storage in northeast Thailand

Salt caverns are an adequate solution for the sequestration of CO2 (large capacity, safety and long-term operation). The large rock salt deposits of the Maha Sarakham Formation represent a very promising location. Designing salt caverns is still a complex issue. In this paper, the stability of typical cavern shapes (spherical, cylindrical, teardrop, bulb, and pear) was evaluated based on displacement, von Mises stress, safety factor, and volume change. The analysis aimed to find the optimal cavern shape for salt deposits at Ban Nong Plue, Borabue district, Maha Sarakham province, northeast Thailand. The Finite Element simulations investigating the cavern stability are carried out for a time span of 600 years (> 500 years is considered permanent storage). The bulb-shaped cavern yielded the best results, indicating that it constitutes the optimal shape in the given geological conditions. The stability of all analyzed caverns showed a dependence on the operating stage. Each stability factor exhibited large differences between the periods of cavern construction/brine discharge and pre-pressurization by CO2 injection. A lower cavern pressure negatively affected the evaluated factors. The results can be useful in planning future cavern fields for CO2 storage (and other gases) in salt deposits.

Geochemistry of evaporitic deposits from the Cenomanian (Upper Cretaceous) Maha Sarakham Formation in the Khorat Basin, northeastern Thailand

The Cenomanian (Upper Cretaceous) Maha Sarakham Formation of the Khorat Basin, northeastern Thailand consists of three evaporite units (Lower, Middle and Upper Salt) interbedded with clastic sediments and exhibits abundant deposits of potash. Although numerous studies have been carried out on the Khorat potash deposit, results are still equivocal with regard to the origin of the rock salt, whether it was marine or non-marine (hydrothermal and/or mixed fluids). The purpose of this study is to examine the origin of rock salt based on elemental compositions and boron isotope analyses in the southwestern part of the Khorat Basin. A stratigraphic correlation of five boreholes (K-201– 205) located in Bamnet Narong and Chaturat districts, Chaiyaphum province, Thailand, revealed a salt dome structure. The elemental composition and δ11B values in the longest borehole K-203 indicate a precipitation of halite and carnallite from seawater. Rare earth elements (REE) of claystone and siltstone from five boreholes (K-201– 205) are comparable to the REE of sandstone from the Simao Basin in China, which suggest a similar provenance. Stratigraphic comparisons and geochemical signatures are important for a better understanding of the recharge models of paleoseawater. In agreement with some previous studies, we conclude that the Cenomanian evaporites within the Khorat Basin are marine deposits.

Physical model simulation of surface subsidence under sub-critical condition

Physical modelling and numerical analyses are performed to assess the representativeness of some widely used profile functions developed to describe the subsidence configurations under sub-critical conditions. Synthetic gel is used to simulate the overburden. The surface profile measurements are used to calculate the maximum subsidence, slope and curvature angle. The model results agree well with those obtained from numerical analyses with discrepancies of less than 3%. Both suggest that the subsidence components increase with opening width-to-depth ratio. Under the same trough width, the trigonometric function slightly underpredicts the subsidence components, particularly for those induced by deep and small openings. The subsidence components predicted by the hyperbolic function are about 10% greater than those of the physical models. The exponential function underestimates the model measurements by about 5% for all cases. A set of empirical equations is derived to fit with the test results, which can be used to predict the subsidence components induced by the mine opening. Based on the scale law, the gel properties and the opening depths and widths can be correlated with those of the prototypes representing overburden above salt and potash mines in the Maha Sarakham formation.

Lithostratigraphy of the Late Cretaceous Khao Ya Puk Formation in Nakhon Thai Region, Thailand: Implication for Depositional Environment

The Khao Ya Puk Formation (>400 m thick) is well exposed in the Nakhon Thai Region, and is subdivided into three members. The Khao Ya Puk Formation overlies conformably on top of the anhydrite layer of the upper rock salt Maha Sarakham Formation. In turn, it is overlain unconformably by the Phu Khat Formation. The Khao Ya Puk Formation is interpreted to have been deposited by freshwater lake area in arid climate inland subkha in the Late Cretaceous. Lithostatigraphically, Khao Ya Puk Formation can be correlated with the Phu Tok Formation that has been deposited in the Khorat-Ubon basin and Udon-Sakon basin of the Khorat Plateau.

Determination of time-dependent strengths of salt pillars based on strain energy principle

The objective of this study is to determine the time-dependent strengths of salt mine pillars in the Maha Sarakham formation, northeast of Thailand. Strain rate-controlled triaxial compression tests have been performed on salt specimens under confining pressures from 0 MPa to 12 MPa. The strain rates are from 10−7 s−1 to 10−4 s−1. The axial stresses and lateral strains are monitored through the strain-softening region. The results indicate that the strengths and elastic moduli increase exponentially with the strain rates. The power creep law parameters are calibrated with the test results, and hence allows constructing series of strain-time curves for the salt pillars under different depths and extraction ratios. The strain energy density principle is applied to develop a strength criterion for the salt pillars. Combining this criterion with the series of the strain-time curves the time-dependent strengths of the salt pillars for different extraction ratios can be predicted.

Potential impact of climate change on groundwater resources in the Central Huai Luang Basin, Northeast Thailand

The Central Huai Luang Basin is one of the important rice producing areas of Udon Thani Province in Northeastern Thailand. The basin is underlain by the rock salt layers of the Maha Sarakham Formation and is the source of saline groundwater and soil salinity. The regional and local groundwater flow systems are the major mechanisms responsible for spreading saline groundwater and saline soils in this basin. Climate change may have an impact on groundwater recharge, on water table depth and the consequences of waterlogging, and on the distribution of soil salinity in this basin. Six future climate conditions from the SEACAM and CanESM2 models were downscaled to investigate the potential impact of future climate conditions on groundwater quantity and quality in this basin. The potential impact was investigated by using a set of numerical models, namely HELP3 and SEAWAT, to estimate the groundwater recharge and flow and the salt transport of groundwater simulation, respectively. The results revealed that within next 30years (2045), the future average annual temperature is projected to increase by 3.1°C and 2.2°C under SEACAM and CanESM2 models, respectively, while the future precipitation is projected to decrease by 20.85% under SEACAM and increase by 18.35% under the CanESM2. Groundwater recharge is projected to increase under the CanESM2 model and to slightly decrease under the SEACAM model. Moreover, for all future climate conditions, the depths of the groundwater water table are projected to continuously increase. The results showed the impact of climate change on salinity distribution for both the deep and shallow groundwater systems. The salinity distribution areas are projected to increase by about 8.08% and 56.92% in the deep and shallow groundwater systems, respectively. The waterlogging areas are also projected to expand by about 63.65% from the baseline period.

Anomalously High Cretaceous Paleobrine Temperatures: Hothouse, Hydrothermal or Solar Heating?

Elevated surface paleobrine temperatures (average 85.6 °C) are reported here from Cretaceous marine halites in the Maha Sarakham Formation, Khorat Plateau, Thailand. Fluid inclusions in primary subaqueous “chevron” and “cumulate” halites associated with potash salts contain daughter crystals of sylvite (KCl) and carnallite (MgCl2·KCl·6H2O). Petrographic textures demonstrate that these fluid inclusions were trapped from the warm brines in which the halite crystallized. Later cooling produced supersaturated conditions leading to the precipitation of sylvite and carnallite daughter crystals within fluid inclusions. Dissolution temperatures of daughter crystals in fluid inclusions from the same halite bed vary over a large range (57.9 °C to 117.2 °C), suggesting that halite grew at different temperatures within and at the bottom of the water column. Consistency of daughter crystal dissolution temperatures within fluid inclusion bands and the absence of vapor bubbles at room temperature demonstrate that fluid inclusions have not stretched or leaked. Daughter crystal dissolution temperatures are reproducible to within 0.1 °C to 10.2 °C (average of 1.8 °C), and thus faithfully document paleobrine conditions. Microcrystalline hematite incorporated within halite crystals also indicate high paleobrine temperatures. We conclude that halite crystallized from warm brines rich in K-Mg-Na-Cl; sylvite and carnallite daughter crystals were nucleated during cooling of the warm brines sometime after deposition. Hothouse, hydrothermal, and solar-heating hypotheses are compared to explain the anomalously high surface paleobrine temperatures. Solar radiation stored in shallow density stratified brines is the most plausible explanation for the observed paleobrine temperatures and the progressively higher temperatures downward through the paleobrine column. The solar-heating hypothesis may also explain high paleobrine temperatures documented from fluid inclusions in other ancient halites.

Effects of carnallite content on mechanical properties of Maha Sarakham rock salt

Mechanical characterization tests are performed to determine the effects of carnallite contents on the strength, elasticity, and time-dependent parameters of rock salt specimens obtained from the Lower Member of the Maha Sarakham formation. The specimens are prepared with carnallite content (C %) varying from 0% (pure halite) to 100% (pure carnallite). The compressive and tensile strengths and elastic moduli of the specimens exponentially decrease with increasing C %. Specimens with higher C % tend to dilate more than those with lower C %, as evidenced by the increasing of the Poisson’s ratio. The strength reduction due to the carnallite content decreases as the confining pressures increase. The elastic, visco-elastic, and visco-plastic parameters of the creep test specimens are defined as a function of C %. They rapidly decrease with increasing C %. Pure halite tends to behave as the Burgers material while pure carnallite behaves as the Maxwell material. The different creep rates and deformation mechanisms between halite and carnallite explain the occurrences and structures of potash ore deposit at the shelves and flanks of the salt domes and anticlines in the salt basin.

Isotope Dating of the Potash and Rock Salt Deposit at Bamnet Narong, NE-Thailand

Bamnet Narong is located in northeastern Thailand (Chaiyaphum Province). It is the largest salt mine in the country and has been mined for decades. The landscape in this part of Thailand is characterised by a low plateau, which is called the Khorat Plateau. The plateau is divided into two basins by the Phu Phan Range, the Sakhon Nakhon Basin in the north and the Khorat Basin in the south. The analysed potashes and rock salts are deposited in the Maha Sarakham Formation, which represents the salt-bearing strata of the Khorat Basin. The stratigraphic age of this deposit has been debated since the late 1960’s. The assigned ages range from Mid-Cretaceous to Late Cretaceous and up to the Eocene. In this study different isotopic dating systems (Rb-Sr, Sr-Sr, K-Ar and K-Ca) were applied. The stratigraphic age for the time of deposition was confirmed to be Mid Cretaceous (Cenomanian). Furthermore, the homogeneity of the carnallites was investigated in order to trace a possible redistribution of rubidium.

First SHRIMP U–Pb zircon ages of the potash-bearing Mengyejing Formation, Simao Basin, southwestern Yunnan, China

High temperatures and arid climates were extensive throughout the Tethyan domain during the mid-Cretaceous, and thus greatest volumes of evaporites were deposited and widely distributed. The Mengyejing Formation, which contains the only pre-Quaternary potash salt deposit in China, is thought to be genetically related to the Late Cretaceous Maha Sarakham Formation in the Khorat Basin. The age of the Mengyejing Formation has been a subject of debate and controversy and was previously interpreted as Paleocene in age mainly based on a few palynology and ostracoda fossils. In this paper, we report U–Pb sensitive high-resolution ion microprobe (SHRIMP) ages between 100 Ma to 110 Ma for the first time from the tuff beds interbedded with gypsum units and siltstones from the upper part of the Mengyejing Formation in the Simao Basin, southwestern Yunnan. This age indicates that the age of the Mengyejing Formation is likely Albian to Cenomanian providing precise geochronological constraint for the age of the Mengyejing Formation in the Simao Basin, which is consistent with the age of the Maha Sarakham Formation. Combined with marine origin of δ34S values for gypsum units in the Mengyejing Formation, the ages indicate that marine incursions from the Neo-Tethyan Ocean in the mid-Cretaceous were caused by global high sea level. However, the stratigraphic succession of the Mengyejing Formation cannot be well correlated with those of the Maha Sarakham Formation caused by marine incursion and tectonic inversion in the mid-Cretaceous.

Provenance and paleogeography of the Late Cretaceous Mengyejing Formation, Simao Basin, southeastern Tibetan Plateau: Whole-rock geochemistry, U–Pb geochronology, and Hf isotopic constraints

The Late Cretaceous Mengyejing Formation, which contains the only pre-Quaternary potash salt deposit in the Simao Basin, southeastern Tibet, is thought to be genetically related to the Maha Sarakham Formation in the Khorat Basin. The provenance and paleogeography of these two basins have been under debate, although little diagnostic evidence has been previously published. A combined analysis of whole rock geochemistry, zircon U–Pb chronology, and Hf isotopic compositions was performed to characterize the provenance of the Mengyejing Formation. These formation's sandstones are characterized by moderate chemical index of alteration (CIA) values. These values, together with plots of the Th/U–Th ratios, suggest that certain samples have undergone moderate weathering and sedimentary recycling. The major and trace elements (La/Th–Hf, Th/Sc–Zr/Sc, Eu/Eu*–Th/Sc, TiO2–Fe2O3T+MgO, Al2O3/SiO2–Fe2O3T+MgO, K2O/Na2O–SiO2) indicate that the sedimentary sources were felsic rocks from an active continental margin or continental arc with a minor amount of recycled sediment from a passive continental margin. The Mengyejing Formation contains detrital zircons primarily with U–Pb ages of 2.45–2.57Ga, 1.8–1.9Ga, 740–880Ma, 410–470Ma, and 215–300Ma. The results reveal that the pre-Devonian zircons are derived from the recycled sediments of the Yangtze block originating in the Qinling Orogenic Belt, and they share this provenance with the coeval sediments in the Khorat Basin. The magmatic rocks of the Ailaoshan and Lincang areas are responsible for supplying the Devonian to Triassic detrital zircons. These provenance data combined with published paleocurrent results suggest that the Simao Basin was situated on the western margin of the Khorat Basin during the Late Cretaceous. The basins were connected when marine incursion occurred. We propose that pre-Devonian materials from the southwestern Sichuan Basin first supplied detritus to the Simao Basin and subsequently to the Khorat Basin.

Intrinsic variability of the mechanical properties of Maha Sarakham salt

Abstract Laboratory mechanical tests were carried out to study the intrinsic variability of rock salt specimens obtained from the Middle and Lower members of the Maha Sarakham formation in the Khorat basin. Prior to the mechanical tests, the types and amount of inclusions were identified by visual examination and after testing by X-ray diffraction and dissolution methods. The uniaxial compressive strength of the specimens linearly increases from 27 MPa to about 40 MPa as the anhydrite content (by weight) increases from 0% (pure halite) to 100% (pure anhydrite). The combined stiffness of the salt and the anhydrite causes an increase of specimen elasticity from 22 GPa (pure salt) to as high as 36 GPa (pure anhydrite). Tensile strengths increase with increasing anhydrite content, particularly when the content is above 60% by weight. Below this limit the anhydrite has an insignificant impact on the specimen tensile strength. The tensile strength of salt crystals can be as high as 2 MPa, whereas that of the inter-crystalline boundaries is 1 MPa. The visco-plasticity increases exponentially with crystal size, as dislocation glide mechanisms become predominant for the specimens comprising large crystals. Salt specimens with fine crystals deform by dislocation climb mechanisms, and hence reduce the specimen9s visco-plasticity. The true understanding of the impact of the inclusions on the mechanical properties of the salt is important for the design and stability analysis of underground mines and storage caverns.

Palynology and stratigraphy of the Mesozoic Khorat Group red bed sequences from Thailand

Abstract New palynological results are presented and integrated with previous data for the Mesozoic Khorat Group continental red bed succession of NE Thailand. The Khorat Group is traditionally considered to comprise, from bottom to top, the Nam Phong, Phu Kradung, Phra Wihan, Sao Khua, Phu Phan and Khok Kruat Formations. The group is unconformably overlain by the continental evaporitic Maha Sarakham Formation, which has been palynologically dated as mid-Albian–Cenomanian, therefore giving a minimum age for the underlying Khorat Group. Traditionally the Nam Phong Formation is considered to be Late Triassic, the Phu Kradung, Phra Wihan and Sao Khua Formations are assigned to the Jurassic, and the Phu Phan and Khok Kruat Formations to the Early Cretaceous. This age dating has mainly been based on plant macrofossil and vertebrate studies. Palynology supports the Late Triassic age for the Lower Nam Phong and underlying Huai Hin Lat Formations but suggests an Early Cretaceous (Berriasian–Barremian) age for the Phu Kradung, Sao Khua and Phra Wihan Formations, and an Aptian age for the Khok Kruat Formation. The Phu Phan has yet to yield useful age-diagnostic palynomorphs but must also be Early Cretaceous based on the age of the under- and overlying formations. The key palynomorph present for ascribing an Early Cretaceous age is Dicheiropollis etruscus , a taxon that is restricted to this age interval and has been found in the Phu Kradung (both at outcrop and in subsurface exploration wells), Phra Wihan and Sao Khua Formations. The lithostratigraphy and age of the Nam Phong Formation is, at present, still problematic. From subsurface seismic data it is clear that the Nam Phong Formation comprises two distinct units separated by an unconformity, referred to herein as the Lower and Upper Nam Phong Formations. The age of the Lower Nam Phong Formation based on palynology is Late Triassic (Rhaetian) whereas the age of the Upper Nam Phong Formation is poorly constrained but can be no older than Pliensbachian based on data from the Phu Horm-1 well. The absence of the Early Cretaceous marker D. etruscus may be taken as indirect evidence for a Jurassic age for the Upper Nam Phong Formation in the Phu Horm-1 well. This revised age dating suggests the presence of a significant depositional hiatus within the Nam Phong Formation, and consequently the Lower Nam Phong Formation should be removed from the Khorat Group. Overall the palynomorph assemblages of the Khorat Group are dominated by the gymnosperm pollen Corollina (synonymous with Classopollis ) and Dicheiropollis , both of which belong to the Cheirolepidaceae and would indicate deposition in a warm, dominantly seasonally dry subtropical climate.

Origin of dolomites in the Cretaceous Maha Sarakham evaporites of the Khorat Plateau, northeast Thailand

The Khorat Plateau of northeast Thailand and Laos was an area of widespread deposition of evaporites and siliciclastics (Maha Sarakham Formation) during the Cretaceous and three types of dolomites are associated with this formation: (1) limpid dolomite, (2) coarse, subhedral and euhedral dolomite, and (3) saddle dolomite. Limpid dolomite is present as isolated crystals in recrystallized, clear halite beds while the other two types are only present as authigenic clusters and individual large crystals within the widespread Basal Anhydrite Unit (BAU) (1.1 m), which defines the lowest member of the formation. The δ 18O PDB isotopic values and the 87Sr/ 86Sr ratios of the limpid dolomite are within the range of Cretaceous seawater. Petrography and carbon and oxygen isotopes suggest that limpid dolomite was formed under shallow burial during times of freshening in a mostly marine-fed aggrading halite salina. Dolomite formation was possibly influenced by bacterial metabolism. The coarse crystalline and saddle dolomite types float within a matrix of coarse, recrystallized anhydrite in the Basal Anhydrite. The δ 18O isotopic values of the coarse, subhedral and euhedral dolomite types are more negative than the limpid dolomite, and the 87Sr/ 86Sr ratios are more radiogenic than typical marine dolomite of the Cretaceous. Saddle dolomite has even more negative δ 18O isotopic values than the coarse euhedral dolomite. The 87Sr/ 86Sr ratios of the saddle dolomite are also radiogenic. The isotopic and petrographic data indicate that the dolomites in the Basal Anhydrite Unit are late and authigenic but do not appear to replace a precursor carbonate. Rather, they appear to have precipitated within the deep burial environment along a 1–2-m-thick subsurface mixing interface between warm upwelling basinal brines and dense highly saline brines created by dissolving the underside of the overlying impervious halite bed. The upwelling basinal waters presumably rose through the underlying siliciclastics of the Khorat Group and then moved laterally along the base of the Maha Sarakham salt. The result of this inferred hydrologic process apparently is an accumulation of burial dolomite and recrystallized residual anhydrite in a petrographically complex unit that ultimately appear to be a solution residue, the Basal Anhydrite Unit.

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

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 overlain by non-marine clastic red beds, and are present in both the Khorat and the Sakon Nakhon sub-basins. These two basins are presently separated by the northwest-trending Phu Phan anticline. The thickness of the formation averages 250 m but is up to 1.1 km thick in some areas. In both basins it thickens towards the basin centre suggesting differential basin subsidence preceding or during sedimentation. The stratigraphy, lithological character and mineralogy of the evaporites and clastics are identical in both basins suggesting that they were probably connected during deposition. Evaporites include thick successions of halite, anhydrite and a considerable accumulation of potassic minerals (sylvite and carnallite) but contain some tachyhydrite, and minor amounts of borates. During the deposition of halite the basin was subjected to repeated inflow of fresher marine water that resulted in the formation of anhydrite marker beds. Sedimentary facies and textures of both halite and anhydrite suggest deposition in a shallow saline-pan environment. Many halite beds, however, contain a curious `sieve-like' fabric marked by skeletal anhydrite outlines of gypsum precursor crystals and are the product of early diagenetic replacement by halite of primary shallow-water gypsum. The δ 34 S isotopic values obtained from different types of 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 to 400 ppm below the potash-rich zone, also suggesting evaporation of largely marine waters. In the Middle Member the initial concentration of bromine in halite is 200 ppm, rising to 450 ppm in the upper part of this member. The bromine concentration in the Upper Member exhibits uniform upward increase and ranges from 200 to 300 ppm. The presence of tachyhydrite in association with the potassic salts was probably the result of: (1) the large volumes of halite replacement of gypsum, on a bed by bed basis, releasing calcium back into the restricted waters of the basin; and (2) early hydrothermal input of calcium chloride-rich waters. The borates associated with potash-rich beds likely resulted from erosion and influx of water from surrounding granitic terrains; however, hydrothermal influx is also possible. Interbedded with the evaporites are non-marine red beds that are also evaporative, with displacive anhydrite nodules and beds and considerable amounts of displacive halite. The δ 34 S isotopic values of this anhydrite have non-marine values, ranging from 6.4‰ to 10.9‰ (CDT). These data indicate that the Khorat and Sakhon Nakhon basins underwent periods of marine influx due to relative world sea-level rise but were sporadically isolated from the world ocean.

Diagenetic origin of Basal Anhydrite in the Cretaceous Maha Sarakham salt: Khorat Plateau, NE Thailand

Development of a diagenetic anhydrite bed at the base of the Cretaceous Maha Sarakham Saline Formation (the `Basal Anhydrite' member) of the Khorat Plateau in north‐eastern Thailand took place due to leaching and/or pressure dissolution of salt at the contact between an underlying active sandstone aquifer system and an overlying massive halite‐dominated evaporite sequence. Basal evaporites composed of halite with intercalated anhydrite of the latter sequence are undergoing dissolution as a result of subsurface flushing, with anhydrite produced as the insoluble residue. The result is a 1·1 m thick interval of nodular anhydrite displaying unique, basin‐wide continuity. Observed textures, petrographic features and chemical data from the anhydrite and associated authigenic minerals support the origin of the Basal Anhydrite Member as an accumulation residue from the dissolution of the Maha Sarakham salts. Petrographically, the anhydrite in this unit is made up of crystals that are blocky and recrystallized, sheared, generally elongated and broken, and is bounded at the bottom by organic‐rich stylolite surfaces. Authigenic and euhedral dolomite and calcite crystals are associated with the anhydrite. Traces of pyrite, galena and chalcopyrite are present along the stylolite surfaces suggesting supply of fresh water from the underlying sandstone at highly reducing conditions of burial. The δ34S of sulphate in the Basal Anhydrite averages 15 ‰ (CDT) and falls within the isotopic composition of the anhydrite in the Cretaceous Maha Sarakham Formation proper and the Cretaceous values of marine evaporites. Measured δ18O in dolomite range from −4·37 to −14·26‰ (PDB) suggesting a re‐equilibration of dolomite with basinal water depleted in 18O and possible recrystallization of dolomite under relatively elevated temperatures. The δ13C, however, varies from +1·57 to −2·53‰ (PDB) suggesting a contribution of carbon from oxidation of organic matter. This basal anhydrite bed, similar to basinwide beds found at the bottom of many giant evaporite sequences, has always been considered to be depositional. Here, at the base of the Maha Sarakham Formation, we demonstrate that the anhydrite is diagenetic in origin and was formed by accumulation of original anhydrite by dissolution of interbedded halite from waters circulating though the underlying aquifer: it represents an `upside‐down' caprock.

Clay-mineral assemblages from some levels of K-118 drill core of Maha Sarakham evaporites, northeastern Thailand

Clay-mineral assemblages in Middle Clastic, Middle Salt, Lower Clastic, Potash Zone, and Lower Salt, totalling 13 samples from K-118 drill core, in the Maha Sarakham Formation, Khorat Basin, northeastern Thailand were studied. The clay-size particles were separated from the water-soluble salt by water leaching. Then the samples were leached again in the EDTA solution and separated into clay-size particles by using the timing sedimentation. The EDTA-clay residues were divided and analyzed by using the XRD and XRF method. The XRD peaks show that the major-clay minerals are chlorite, illite, and mixed-layer corrensite including traces of rectorite? and paragonite? The other clay-size particles are quartz and potassium feldspar. The XRF results indicate Mg-rich values and moderate MgAl atom ratio values in those clay minerals. The variable Fe, Na, and K contents in the clay-mineral assemblages can explain the environment of deposition compared to the positions of the samples from the core. Hypothetically, mineralogy and the chemistry of the residual assemblages strongly indicate that severe alteration and Mg-enrichment of normal clay detritus occurred in the evaporite environment through brine-sediment interaction. The various Mg-enrichment varies along the various members reflecting whether sedimentation is near or far from the hypersaline brine.

Continental origin of the Maha Sarakham evaporites, northeastern Thailand

Recent sedimentologic analysis of the potash-bearing Maha Sarakham Formation (Upper Cretaceous) shows that it is not of marine origin as previously thought. The main reason is the lack of evidence suggesting transgression onto the underlying redbeds of the Khok Kruat Formation, which are desert stream deposits. Similar lack of evidence also holds for the salt/red clastic contacts within the evaporites. Sedimentary features preserved in cores also point towatd the nonmarine origin. Algal lamination, palmate and swallow-tail fabrics in anhydrite (formerly gypsum) suggest shallow gypsum pan environment, while truncation and dissolution of the primary halite crystals suggest ephemeral salt pan conditions. The clastic units alternating in the formation contain features which characterize saline and dry mudflat environments, for instance, desiccation cracks, caliche, and displacive anhydrite nodules. In the potash zone, neither carnallite nor sylvite shows structures that may indicate sedimentation in a standing brine body or growing upward from the pan floor. Moreover, it is discernible at places that carnallite and sylvite were formed interstitially in the salt pan halite host, which implies their diagenetic origin. Considering the information gained so far, it is apparent that there was no restricted sea in northeastern Thailand during Late Cretaceous time. The Maha Sarakham Formation was in fact deposited in arid desert, probably deep in the continent. The new interpretation raises several quest0ons to the consensus on the formation, i.e. basin history, stratigraphic subdivision of the Khorat Group, and relationship among the formations within. It may also affect the economic aspects of the formation, i.e. continuity of the potash horizons and occurrence and migration of subsurface brines.

The evaporite deposits in Bamnet Narong area, Northeastern Thailand

Analyses of 65 drill- hole cores which penetrated to depths ran ging from 60-465 metres with in th e Bamnet Narong area of approxi mately 170 square kilometres reveal three cycles of evaporiti c sequences in th e Maha Sarakham Formation of th e K.horat Group. Mineralogical , petrographical , geochemical and geophysical data have been employed to define the lithostratigraph y, sedimentary facies, as well as for the reconstruction of the depositi onal environment and post-depositional changes. The lower evaporiti c cycle is characterized, from bottom to top, by ferr uginous sandsto ne, calcareous sandstone, an anhydrite zone, a halite zo ne, and a potash zone. Thi s sequence is almost excl usively equivalent to lower- , middle-, and so me parts of hi gher- orders of th eoretical marine evaporiti c facies. The middle evaporitic and the upper evaporiti c cycles are re presented by the alternation of halite and anhydrite zo nes. These two cycles can be interpreted as lower- and middle­ orders of theoretical evaporitic facies. It was also noted that all evaporitic cycles were interbedded with fine-grained clas tic rocks of reasonable thicknesses. Many lines of evidence indicate that the deposi ti onal bas in was a shallow epeiri c sea with a gentl e sloping depositional surface. During th e ini tial marine tran sg ression, the sediment substrate was mai nl y of terrigenous clastic s . The cond ition of th e depositional environment changed from an open marin e to a restricted marine under the influence of threshold depths of th e basin fl oor e lsewhere outside th e stud y area . Subsequently, th e brin e concentrati on mechanism as we ll as tl1e palaeosalinity has been attributed to a marine transgression and regress ion into th e depositi onal basin under an ari d to semi-ari d pal aeoclimate. The evaporitic sub-facies also indicate th at the palaeosalinity varied within th e range of penesaline, saline, to supersaline conditions accordin g to the bar-basin theory. With respect to po st-deposition al changes, it is recognized th at in the area where the evaporiti c facies have been structurall y deformed to gentle anticlines, the characteris tics of depositi onal fac ies have remarkably altered. In th e light of groundwater activities, carnallite in the potash zone has been chemically transformed into sylvite through th e process of incongruent alteration. The formation of an hydrite caps at the crests of anticlines has been explained as a residual accumulation after the leachin g of hal ite/anhydrit e layers by til e groundwater.