Continental origin of the Maha Sarakham evaporites, northeastern Thailand

Abstract

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.