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.