Paleokarst in Phong Nha - Ke Bang - Hin Nam No and its Geomorphological and Geological Values

Abstract: Paleokarst, a karst landform developed in the geologic past and often buried by younger sedimentary rocks, has not received much attention in Vietnam. Field surveys and thin-section analyses have confirmed the presence of paleokarsts in the two adjacent national parks Phong Nha - Ke Bang (Vietnam) and Hin Nam No (Laos). They are mainly found in two types of buried karst surfaces (karren, polje) and filled karst fissures and caves, developed on the Middle Devonian (Givetian), Upper Devonian (Famennian), Lower Carboniferious and Carboniferous - Permian limestones. The karstification for the first time over whole unified territory of Phong Nha - Ke Bang - Hin Nam No (PNKB HNN) started in the Triassic and ended in the Middle Jurassic due to the unconformable coverage of Upper Jurassic - Cretaceous sediments. The paleokarst discoveries are additional evidences to shed light on the karst evolution from Triassic to Jurassic in the area. In addition, it also suggests a new direction in karst research in Vietnam, especially in dating karst landforms. Paleokarst evidences date Triassic to Jura for karstic landforms in PNKB HNN, much older than previous studies. Paleokarst with many significant sites found in PNKB HNN, deserves to be a new Outstanding Universal Value (OUV) for PNKB HNN.