Origin of dolomites in the Cretaceous Maha Sarakham evaporites of the Khorat Plateau, northeast Thailand

Abstract

The Khorat Plateau of northeast Thailand and Laos was an area of widespread deposition of evaporites and siliciclastics (Maha Sarakham Formation) during the Cretaceous and three types of dolomites are associated with this formation: (1) limpid dolomite, (2) coarse, subhedral and euhedral dolomite, and (3) saddle dolomite. Limpid dolomite is present as isolated crystals in recrystallized, clear halite beds while the other two types are only present as authigenic clusters and individual large crystals within the widespread Basal Anhydrite Unit (BAU) (1.1 m), which defines the lowest member of the formation. The δ 18O PDB isotopic values and the 87Sr/ 86Sr ratios of the limpid dolomite are within the range of Cretaceous seawater. Petrography and carbon and oxygen isotopes suggest that limpid dolomite was formed under shallow burial during times of freshening in a mostly marine-fed aggrading halite salina. Dolomite formation was possibly influenced by bacterial metabolism. The coarse crystalline and saddle dolomite types float within a matrix of coarse, recrystallized anhydrite in the Basal Anhydrite. The δ 18O isotopic values of the coarse, subhedral and euhedral dolomite types are more negative than the limpid dolomite, and the 87Sr/ 86Sr ratios are more radiogenic than typical marine dolomite of the Cretaceous. Saddle dolomite has even more negative δ 18O isotopic values than the coarse euhedral dolomite. The 87Sr/ 86Sr ratios of the saddle dolomite are also radiogenic. The isotopic and petrographic data indicate that the dolomites in the Basal Anhydrite Unit are late and authigenic but do not appear to replace a precursor carbonate. Rather, they appear to have precipitated within the deep burial environment along a 1–2-m-thick subsurface mixing interface between warm upwelling basinal brines and dense highly saline brines created by dissolving the underside of the overlying impervious halite bed. The upwelling basinal waters presumably rose through the underlying siliciclastics of the Khorat Group and then moved laterally along the base of the Maha Sarakham salt. The result of this inferred hydrologic process apparently is an accumulation of burial dolomite and recrystallized residual anhydrite in a petrographically complex unit that ultimately appear to be a solution residue, the Basal Anhydrite Unit.