Triassic carbonate rocks of Pha Kan and Doi Long Formations of Lampang Group consist mostly of carbonate successions deposited in the Sukhothai Zone, northern Thailand. These formations are mixed carbonate and siliciclastic sediments and widely exposed in the Lampang area. These deposits are important for the reconstruction of depositional environments and regional correlations of the Triassic carbonate rocks with respect to the volcanic arc of the Sukhothai Zone during the Triassic. The goal of this study is to analyze the lithology and microfacies of the carbonate rocks of the Pha Kan and Doi Long Formations and reevaluate the existing depositional models and carbonate settings. Both formations have diverse skeletal fossils, including foraminifers, sponges, calcimicrobes, calcareous algae, bivalves, echinoderm spines and fragments, ostracods, gastropods, and other fossils. Based on field observations and microfacies analysis, twelve major facies, corresponding to specific depositional environments within a carbonate setting, have been established. The inferred depositional environment is a carbonate platform with a well-developed lagoon, small-scale reefs, and bioclastic and oolitic shoals, as parts of a carbonate ramp. The microfacies and sedimentological study show that this carbonate system was influenced by the nearby volcanic arc. Consequently, our study shows that the Lampang Group in northern Thailand can be correlated with the Lincang Massif of the western Yunnan Province, PRC and the northwestern Lao PDR.

The Nam Pat Group of the Nam Pat Basin, Uttaradit Province, Thailand is situated in the Nan Suture Zone. Two contrasting tectonic framework models of the Nan Suture have previously been proposed: as a main Paleo-Tethyan suture and as a remnant of a closed back-arc basin. The goal of this study is to analyze the tectonic framework of the Nan Suture and reevaluate the existing tectonic models. This research infers the tectonic setting of the basin based on basin-filled lithofacies distributions, provenance, and paleocurrents. Three facies groups are identified. Facies A, B, and C are polymictic conglomerate, interbedded sandstone and mudstone, and mudstone with thin-bedded sandstone, respectively. Generally, the strata strike NE-SW, steeply dip NW, and fine toward the SE. Petrographic results of sandstone samples show that the majority of lithic fragments are volcanic rocks with lesser amount of metamorphic rocks and sedimentary rocks. The modal compositions of sandstones are analyzed on QFL, QmFLt, QmPK, LmLvLs, and QpLvmLsm ternary diagrams. The plots indicate that the sandstones of the Nam Pat Group have high proportions of volcanic-arc detritus. Paleocurrents, determined from the orientation of clast imbrications and cross-stratification, are toward the SE. The results suggest that large quantities of sediment traveled southeastward from a nearby volcanic arc into the basin. The Nam Pat Group stratigraphically overlies the Pak Pat Volcanics. The chemical features of the Pak Pat Volcanics, located to the west of the basin, show that they are andesitic volcanic rocks formed as a magmatic arc. Thus, the nearby Pak Pat Volcanics are the main sediment source of the Nam Pat Group, and the basin is best interpreted as back-arc basin rather than as a forearc basin.