The Middle to Late Cretaceous marine incursion of the Proto-Paratethys Sea and Asian aridification: A case study from the Simao-Khorat salt giant, Southeast Asia

Abstract

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 Srcorrected/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 δ13Corg 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.