Stratigraphy, sedimentology and tectonic setting of the Lampang Group, central north Thailand

The Lampang Group in central north Thailand is found to have been deposited as two adjacent sub-basins that contain Similar Ethological sequences which overlap in stratigraphy and age. The Lampang Group proposed here consists of seven formations, in ascending order the Phra That, Pha Kan, Hong Hoi, Doi Long, Pha Daeng, Kang Pla and Wang Chin Formations. The first five formations occur in the Lampang sub-basin in the west whereas the last three formations are in the Phrae sub-basin in the east, and only the Pha Daeng Formation is widespread over both sub-basins. The Lampang sub-basin formed during early Triassic to early late Triassic and contains Daonella, P aratrachyceras, Costatoria and Claraia while the Phrae sub-basin formed in late Triassic and contains Halobia . This classification contrast sharply with those of Piyasin (1972) and Chonglalcmani (1981), who considered lithologic sequences of these two sub-basins as belonging to the same stratigraphic level. The nomenclature of formations in this study also appears to compromise the contradictory names of the previous classifications. Discussion on these is provided. Both sub-basins contain a similar deepening-upward megasequence, starting with red beds upward to ramp carbonates and submarine fan sediments. The Lampang sub-basin also contains a shallowing-upward megasequence at the top, represented by a lithological change upward to platform carbonates and red beds. Time equivalence among the formations is common, since formations of the red beds and carbonates appear to be formed in relatively narrow areas while the mudstones of submarine fan occurred widely in the basins. The Phra That and Pha Daeng Formations consist mainly of red beds. A detailed sedimentologically study of the Pha Daeng Formation showed that it comprises mainly fining-upward sequences of gravity flow sediments deposited subaqueously under a shallow-water fan delta environment. The Pha Kan, Doi Long and Kang Pla are carbonate formations that were formed mainlyin shallow-marine ramp platform, drowned ramp and regressive platform environments. Rapid facies change of the carbonates is common. The Hong Hoi and Wang Chin Formations are mud-dominated and interpreted as having been formed as submarine fan environments with detached sand bodies, that may have been fed by multiple-point sources. Most sandstones display flat bases and no fan lobes. Volcaniclastic material dominates the majority of the Lampang sandstones. The compositional and chemical variations of the sandstones provide a record of shifting source areas within forearc basins. The source areas changed from active magmatic arcs in the Hong Hoi and Pha Daeng Formations to a combination of active magmatic arcs and recycled orogens during the sedimentation of the Wang Chin Formation. The change in source rocks corresponds well with stratigraphies of the new Lampang Group proposed here, and is interpreted as a result of interaction between the Shan-Thai and westward subducting Indochina terranes. The collision between these two terranes was possibly mild and occurred during late Triassic Period.