Sedimentology and depositional history of the Pha Nok Khao Platform in the Loei-Phetchabun Fold Belt, Northeast Thailand

[Truncated abstract] The Pennsylvanian to Permian Pha Nok Khao (PNK) platform crops out in the northern Loei-Phetchabun Foldbelt, northeastern Thailand, and represents an outcrop analogue for coeval carbonate gas reservoirs beneath the nearby Khorat Plateau. This study focuses on geological mapping, sedimentology and depositional history of the major carbonate-dominated unit of this platform, known regionally as the Pha Nok Khao Formation. The study area is located in a major syncline with associated smaller scale folds and several major fault sets related to the Indosinian Orogeny and subsequent Cenozoic to Recent Himalayan Orogeny. The Pha Nok Khao Formation-equivalent (PNKF-e) and the underlying Wang Saphung Formation have been divided into 27 carbonate and six siliciclastic facies, and grouped into 11 facies associations. These associations represent five major carbonate platform depositional environments; toe-of-slope to basin, slope, platform margin, inner platform and restricted inner platform. Lateral distribution of platform margin facies associations indicates that they developed along the present-day northern and western parts of the study area with the inner platform located towards the southeast. Microbial-algal mounds and algal boundstones are locally present, and together with microbialite debris from local buildups, are also key evidence for interpreting windward margins along the present-day northern and western sides of the platform. Slope and basin associations are recorded outboard of the carbonate platform to the east, north and west. PNKF-e is thickest to the southeast (~1200 m) but is truncated in the north due to uplift and subsequent erosion of Middle Permian strata (~850 m thick).