Bulletin of the Geological Society of Malaysia, Volume 20, August 1986, pp. 249 – 267
Geology Department, Chuklalongkorn University, Bangkok 10500, Thailand
Abstract: Analyses of 65 drill-hole cores which penetrated to depths ranging from 60-465 metres within the Bamnet Narong area of approximately 170 square kilometres reveal three cycles of evaporitic sequences in the Maha Sarakham Formation of the Khorat Group. Mineralogical, petrographical, geochemical and geophysical data have been employed to define the lithostratigraphy, sedimentary facies, as well as for the reconstruction of the depositional environment and post-depositional changes.
The lower evaporitic cycle is characterized, from bottom to top, by ferruginous sandstone, calcareous sandstone, an anhydrite zone, a halite zone, and a potash zone. This sequence is almost exclusively equivalent to lower-, middle-, and some parts of higher- orders of theoretical marine evaporitic facies. The middle evaporitic and the upper evaporitic cycles are represented by the alternation of halite and anhydrite zones. 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 clastic rocks of reasonable thicknesses.
Many lines of evidence indicate that the depositional basin was a shallow epeiric sea with a gentle sloping depositional surface. During the initial marine transgression, the sediment substrate was mainly of terrigenous clastics. The condition of the depositional environment changed from an open marine to a restricted marine under the influence of threshold depths of the basin floor elsewhere outside the study area. Subsequently, the brine concentration mechanism as well as the palaeosalinity has been attributed to a marine transgression and regression into the depositional basin under an arid to semi-arid palaeoclimate. The evaporitic sub-facies also indicate that the palaeosalinity varied within the range of penesaline, saline, to supersaline conditions according to the “bar-basin” theory.
With respect to post-depositional changes, it is recognized that in the area where the evaporitic facies have been structurally deformed to gentle anticlines, the characteristics of depositional facies have remarkably altered. In the light of groundwater activities, carnallite in the potash zone has been chemically transformed into sylvite through the process of incongruent alteration. The formation of anhydrite caps at the crests of anticlines has been explained as a residual accumulation after the leaching of halite/anhydrite layers by tile groundwater.