Search

The potash minerals on the Khorat Plateau consist of carnallite and sylvite. The origin of sylvite is still a matter of debate due to a lack of diagnostic evidence. In this study, we analyzed the petrography, trace element compositions, and B isotope compositions of various salt facies, including carnallites, sylvites, carnallite-associated halites, and sylvite-associated halites. The results show that the δ11B values of all of the salt samples range from 16.66 ‰ to 32.78 ‰, which are consistent with those of marine precipitates. Nevertheless, the sylvite-associated halites have higher B contents and elevated δ11B values, which differ greatly from those of the carnallite-associated halites but are similar to those of the potash facies. The variations in the B isotope compositions from the anhydrites to the carnallite-associated halites to the carnallites are consistent with the trend of marine salts from seawater evaporation. The Br contents of the sylvites and sylvite-associated halites are lower than those of their primary marine counterparts, suggesting a secondary origin. In contrast, the carnallites and carnallite-associated halites have Br contents within the range of the primary marine salts, denoting a primary origin. The trace element compositions, B isotopes, and petrography of the different salt facies suggest that the sylvites are secondary minerals and formed via the dissolution of primary carnallites. The carnallites were dissolved or leached by unsaturated waters, forming K and Mg enriched brine, from which the secondary sylvites and halites subsequently precipitated. This process usually occurred in the flanks of the salt domes where fractures were widely developed. We speculate that sylvites in those global giant Cretaceous potash deposits were most likely caused by extensive dissolution and recrystallization processes, and the initial and predominant potash mineral was carnallite. • The δ11B values of Khorat salt samples indicate a marine origin. • Sylvite in the Khorat Plateau were derived from dissolution of primary carnallites. • Extensive recrystallization occurred in global Cretaceous potash basins. [ABSTRACT FROM AUTHOR]

Asian aridification during the Cenozoic was strongly related to moisture flux modulated by sea incursion and the retreat fluctuations of the proto-Paratethys Sea via the Westerlies. However, the extents of the sea incursions are poorly constrained, and there is a lack of sedimentary records beyond the Central Asian basins. The Middle to Late Cretaceous Simao Basin and the Khorat Plateau in Southeast Asia contain thick continental sequences of red beds, evaporites, and aeolian deposits, but whether the depositional environment of the evaporites was marine or continental has been hotly debated. In this study, we compiled multiple sedimentology proxies, biomarkers, element geochemistry, and isotopic geochemistry of the fine-grained sediments of the Mohan section in the Simao Basin. Sedimentological analyses show that this section is composed of subtidal flat fine-grained siliciclastic and carbonate rocks and supratidal flat conglomerates and evaporites. The paleosalinity proxies show that the B concentration and B/Ga and Sr corrected /Ba ratios range from 30.3 to 81.7 ppm, 2.4 to 5.0, and 0.05 to 0.60, respectively, which indicate brackish conditions. The two typical increases in these proxies are consistent with the stratigraphic horizons of the diagnostic marine biomarkers 24-n-propyl- and 24-iso-propylcholestanes, which suggests seawater recharge. The carbonate δ13C and δ18O values (−4.84‰ to −1.75‰ and −8.25‰ to −3.69‰, respectively) and the bulk δ13C org values (−26.5‰ to −23.22‰) also indicate the influence of seawater. We propose that the seawater likely originated from the proto-Paratethys Sea during a marine incursion. We propose for the first time that the proto-Paratethys seawater incursions extended into the Simao Basin and the Khorat Plateau. These results provide a novel paleogeographic configuration for East Asia during the Middle to Late Cretaceous. The retreat of the proto-Paratethys Sea, the northeast Trade Winds, and the tectonic uplift dramatically decreased the moisture and intensified the extreme aridification in the Simao Basin and the Khorat Plateau. • The Simao-Khorat salt giant was recharged from seawater. • The sea incursions of the proto Para-Tethys Sea extend into Southeast Asia. • Moisture flux controlled by the sea fluctuation intensified the aridification. [ABSTRACT FROM AUTHOR]