Discovery of Early-Late Triassic microfaunas from the bedded-chert and ‘Jurassic base-conglomerate’ at Mae-Sot, NW Thailand : Preliminary report for the Shan-Thai Terrane and end-Triassic orogeny

The Middle and Late Triassic radiolarian faunas were extracted from a vitric tuff rich laminated bedded-chert in the Mae-Sot and Umphang areas of NW Thailand. The radiolarian faunas from the bedded-chert succession are characterized by Triassocampe postdeweveri (Early Ladinian), Pachus multinodosus (Early Norian), Sarla natividadensis (Middle Norian) and Canoptum rhaeticum (Norian-Rhaetian), individually. The Triassic chert-sequence is overlain by the ‘Jurassic base-conglomerate’, an ill-sorted breccia in a reddish-silt matrix. The limestone and chert clasts in the conglomerate yield Early - Late Triassic conodonts and Middle - Late Triassic radiolarians, respectively. Chert clasts in the conglomerate yield among others Norian - Rhaetian radiolarians like Pachus multinodosus and Canoptum rhaeticum, while Early - Late Triassic conodonts as Platyvillosus costatus, Cratognathodus cf. cuspidatus, Metapolygnathus polygnathiformis, M. carpathicus, Ancyrogondolella quadrata and A. spatulata are found in limestone clasts. The silici-pelagic origin of the clasts suggests the presence of an ocean before the end Triassic orogeny along the Mae Sariang Zone that amalgamated the parts of the Shan-Thai block. This first finding of Late Triassic (Norian - Rhaetian) radiolarians from bedded-cherts, next to the Middle Triassic and older radiolarian faunas, adds another element to the reconstruction of the sequence now comprised in the Mae Sariang Zone, W of the Nan-Uttaradit Suture. The occurrence of Triassic limestone, as that of the Chaiburi Formation in the Phatthalung area of the Mae Sariang Zone or the Kodiang Limestone in the ‘Western zones’, may elucidate the question about the provenance of the Triassic conodont-bearing limestone clasts in the Jurassic base-conglomerate that seals the Mae-Sariang Zone. The newly dated Triassic sequence is further sealed by the continental-shelf deposits of the Toarcian - early Bajocian Hua Fai Group.