Abstract:
Marine Triassic strata in Thailand are distributed in three sedimentary basins: the Lampang Basin in the central north, the Mae Sariang - Kanchanaburi Basin in the northwest and west, and the Songkhla Basin in the south. Triassic strata in the Lampang Basin comprise the Lampang Group (Phra That, Doi Chang, Hong Hoi, and Doi Long Formations in ascending order), more than 3,000 m thick, and the overlying Pha Daeng Formation. Strata in the Mae Sariang - Kanchanaburi Basin consist of the Lower Mae Moei Group (Kamawkala Limestone, Tha Chang Tai and Um Yorn Formations, Unnamed redbed, and Thong Pha phum conglomerate) and the laterally equivalent Upper Ratburi Group. The newly proposed Mae Sariang Formation is included in the Lower Mae Moei Group. In the Songkhla Basin, marine sediments of confirmed Triassic age are represented by the Na Thawi formation. In this study 133 species and subspecies of bivalves and ammonoids are described. The bivalves, comprising 110 species and subspecies (31 new taxa and 10 unnamed), belong to 33 genera and subgenera, of which 3 are new (Siamopecten, Ngaoella, Retidaonella). Numerically dominant forms include thin-shelled Pectinacea, and heavier Pteriacea, Nuculanacea, Pachycardiidae and Myophoriidae. The ammonoids consist of 23 species and subspecies (2 new taxa, 7 unnamed) assigned to 18 genera and subgenera, mainly Trachyceratidae, Beyrichitidae, Haloritidae, Joannitidae, and Ophiceratidae. The proposed biostratigraphic divisions are based on stratigraphic sequences in the Lampang Basin. Twelve faunal assemblages can be distinguished: the Claraia - Ophiceras zone of the Upper Griesbachian; Hollandites - Leiophyllites, Hollandites - Balatonites, and Costatoria zones in the Anisian; Daonella indica zone (Ladinian); Paratrachyceras zone (Lower Karnian); Halobia styriaca, H. charlyana, H. parallela, and Trigonodus zones of the Middle-Upper Karnian; and the Halobia distincta and Indopecten zones in the Lower Norian. The faunal affinities of these assemblages and their correlation with well-known sections in other parts of the world are discussed. Three faunal associations of paleoecologic significance can be distinguished: the shallow water benthic bivalve facies, the slightly deeper cephalopod facies, and the deeper off-shore Halobia - Daonella facies. Thailand occupies part of the Burmese-Malayan Geosyncline. It belongs to two continental blocks, the Shan-Thai and the Indosinian, which have been joined together since Upper Triassic times. Triassic marine sediments were deposited on both margins of the Shan-Thai block. The paleogeographic history of the Lampang Basin may be divided into three stages: Scythian – Lower Karnian, Middle Karnian, and Upper Karnian - Lower Norian, followed by folding and uplift during the Middle Norian Indosinian Orogeny. The Mae Sariang - Kanchanaburi Basin appears to have remained rather stable, with continuous marine deposition up to the Jurassic, before folding in the Late Jurassic – Early Cretaceous Young Cimmerian Orogeny which ultimately consolidated, the whole country into a single stable landmass.