Boundary and evolution of rock salt in the Maha Sarakham formation in the northeast. Thailand(Thai)

Abstract

Analysis of geological and geophysical data in the areas of the Khorat and the Sakon Nakhon basins, Northeast Thailand has provided a new evidence of subsurface geology and evolution of the rock salt and claystone in the Maha Sarakham Formation. Results from the analysis of 59 industry seismic lines, borehole data, geochronology, and semi 3D geophysical surveys show the following. (1) Rock salt underlying below the ground surface in the two basins cover areas of 45,944 km² or about 28% of the Northeast total area. (2) The rock salt in the Maha Sarakham Formation is found in 16 provincial administrations. Each province has different area underlain by the rock salt. This can be ranked from 6 largest percentage as follows: Nongkai (86%), Maha Sarakham (85%), Nakon Phanum (79%), Roi Ed (66%), Sakon Nakhon (58%), Yasothorn (53%). Nong Bua Lumpoo, Mukdaharn, and Loei provinces do not have subsurface rock salt deposit. (3) Integrated seismic and resistivity surveys can be used to map 3D of the salt dome. (4) During Abian-Cenomanian (91-113 million years) the Maha Sarakham and the Phutok formations were deposited in an inland salt lake. The bottom of the Maha Sarakham Formation overlain unconformity on the Khorat Group whereas at the top of it formed a depositional contact with the Phutok Formation. The Phutok Formation was overlain unconformity by the Tertiary rock or the Quaternary soils. Thus evolution of the Maha Sarakham Formation can be described as follows. The inland salt lake was first created by a collision of the Western Burma plate and Chan Thai plate during Cretaceous time. Thereafter, there was seawater invasion into the lake and later the formation of evaporates and clay were deposited. There were three cycles of invasion resulting in three cycle of rock salt layers and claystone. The evaporite deposits were ended because the lake had been filled with wind blown silt and sand particles. At the beginning of the Paleocene to early Eocene (60-65 million year), the Khorat and Sakon Nakhon basins and the Phu Phan fold belt were created by folding mechanism in conse-quence of the collision of the Indian Plate and the Eurasian Plate. The evaporites in which covered the Phu Phan areas were transported into the Khorat and the Sakon Nakhon basins, because of plasticity properties of the rock salt. These interpretations suggest that the Northeastern Thailand stratigraphic units and geologic map need revision in order to prevent misleading geologic map where the Maha Sarakham Formation and the Phu Tok Formation were outlined.