JT Sandstone

The Khorat Group The sandstones of the Khorat Group are one of the truly remarkable rock formations of Southeast Asia in terms of aerial extent, thickness, and lithological uniformity and consistency. The red beds of the Khorat Group, or its their equivalents, cover many square kilometres (extending well outside the Mekong Basin) and attain thickness in excess of 6 kilometres. Until recently, these exclusively terrestrial sediments were thought to be molase, or the erosion products, of the denudation of the mountain ranges created during the Early Triassic Indonsinian Orogeny and were therefore assumed to span the entire Jurassic and Cretaceous systems. However, recently new fossil evidence, including spores and pollens and vertebrates such as dinosaurs, have shown that the Khorat Group at its base is unlikely to be any older than the very latest Late Jurassic and that the Group was deposited almost entirely during the Early Cretaceous. This revision their age means that the source of the sediments of Khorat Group is unlikely to be related to the Indonsian Orgeny as the time gap between this event and the oldest Khorat Group strata is simply too long. Their most likely provenance is now believed to be in China. Whatever their provenance, it is impossible to overlook the effect these hard and indurated sandstones have had on the landscape of the Mekong Basin. Their resilience to erosion means that the more massive sandstones commonly form ridges, the rims of breached anticlines and isolated outliers that from table mountains standing above softer, and more easily eroded, strata. The most impressive landform is probably the cuesta, (the Khorat Escarpment (A ridge with a gentle slope on one side and a cliff on the other)) that completely encircles the Khorat Plateau almost without break. This ridge, which defines limits of the plateau, also marks the watershed of some of the basin’s most important tributary catchments, including those of the Mun and Songkhram rivers. Such is the resilience of the Khorat sandstones that escarpment is only breached by the Mekong itself and the Nam Ngum in the Vientiane Basin. Outliers of the Khorat Group beyond the Khorat Plateau also contribute significantly to the landscape of the basin. The plateau which forms the basin for the Nam Theun 2 impoundment is formed of sandstones of the Khorat Group, so while outlier is developed as a structural syncline it is the escarpment formed by these sandstones that give the plateau its appearance of being a ‘lost world’. In addition to the characteristic sandstones of the Khorat Group, the upper part of the succession comprises evaporate deposits including rock salt, potash, anhydrites, and gypsum (Mahara Sarakham Formation). The evaporates were deposited in isolated inland lakes (playa lakes) that developed during late Cretaceous in between structural lineaments such as the Phu Phan Uplift. As the ion rich waters in these lakes evaporated salts such as sodium chloride (halite) and potassium chloride were precipitated forming beds that may be many metres thick. In areas where these deposits are very thick, such as the Vientiane Basin, they may form salt pillows and domes. In places these diapiric structures may penetrate to the surface where they are exploited in open cast mines. Elsewhere, the salts are dissolved by underground water flows contributing largely to the highly salt contaminated soils that are an increasing problem in the Khorat Plateau of NE Thailand. Cuesta where Mekong traverses the Khorat group: A ridge with a gentle slope on one side and a cliff on the other.

Sand and siltstones from Boloven Plateau, 13 cm and 8 cm wide

Below: Contact layer between Khorat sandstone (Bottom) and basalt (left) from Boloven, Tad Lou, Lao PDR. Sandstone with rippels (8 cm).