On the origin of evaporite deposits in the Maha Sarakham formation in Bamnet Narong area, Changwat Chaiyaphum

The present investigation aims at utilizing the existing 65 drill-hole information coupled with detailed laboratory data to synthesize the sub-surface geology of the Maha Sarakham Formation in Bamnet Narong Area, Changwat Chaiyaphum form the ground surface down to the depth range of 60-465 meters. Consequently, the depositional model of evaporite formation as well as the post-depositional changes have been reconstructed and presented. The sub-surface lithostratigraphy of the complete sequences of the Maha Sarakham Formation of the Khorat Group in the study area can be subdivided into five members, namely, Basal Salt, Lowere Clastics, Middle Salt, Middle Clastis, and Upper Salt. It is noted that almost complete evaporitic sequences are present only in the Basal Salt Member. With regard to the sub-surface lithostratigraphy of the incomplete sequences of the Maha Sarakham Formation in the salt anticline areas, it consists mainly of Basal Salt Member with incomplete evaporitic sequences, and the uppermost Clastics Members. Besides, the complete sequences represent the depositional sequences with only slightly recognizable post-depositional changes. In contrast, the incomplete sequences have undergone post-depositional changes up to a certain degree. Therefore, the sedimentary facies of the evaporite-bearing Maha Sarakham Formation can be categorized into two types : depositional facies and disturbed facies, respectively. For the depositional facies of the evaporite-bearing Maha Sarakham Formation, three sedimentary cycles have been recognized and defined in terms of evaporitic facies. Due to the fact that the evaporitic facies of the three sedimentary cycles are essentially the primary precipitates of marine evaporites in almost all parts, the reconstruction of depositional environment and paleosalinity can, therefore, be made on the basis of successions of evaporitic sub-facies present. The nature and characteristics of the depositional basin is a shallow epeiric sea with gentle sloping depositional surface. Originally, the sediment substrate of the shallow epeiric sear during the initial marine transgression period was mainly terrigenous clastic type. Then the condition of the depositional environment had changed from the open marine to the restricted marine under the influences of threshold depths of the basin elsewhere outside the study area. Subsequently, the brine concentration mechanism as well as the paleosalinity have been essentially reconstructed from the successive evaporitic sub-facies concerned. Besides, the influences of marine transgression and regression in the depositional basin have been accordingly proposed. Two aspects of post-depositional changes, notably, structural deformation, and chemical alterations have been recognized. Some parts of the evaporitic facies had been structurally deformed. The mechanism which caused this deformation of salts might be either the differential loading of the overlying sediments and the high plasticity of the evaporitic sequences or regional tectonic disturbances or the combination of both. Considering the post-depositional chemical alterations, the groundwater has been mainly responsible for the mechanism of transforming of carnallite to sylvite through the process of incongruent alteration. Besides, the formation of the cap anhydrite in the anticline areas was resulted of a residual accumulation after leaching of halite/anhydrite layers by the groundwater. Finally, the reconstruction of the dynamic depositional models and the post-depositional changes has been synthesized and proposed to explain the marine evaporite deposits in the Maha Sarkham Formation of Bamnet Narong Area, Changwat Chaiyaphum.