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During the Ordovician period, Western Thailand was part of the Sibumasu Block. Most studies on Ordovician conodonts in this region have focused on the Thong Pha Phum area, where the Triangulodus larapintinensis Range Zone of Floian and Aurilobodus leptosomatus Range Zone of Darriwilian have been documented. In this study, conodont samples were collected from two sections in the Ban Tha Kradan area of Western Thailand: from the Wat Mong Krathae (WMK) and Nautiloid Site Geosite (NSG) sections. In total 24 conodont samples were collected from the WMK section, all from the Tha Manao Formation characterized here by laminated and cross-bedded thin- to medium-bedded limestones. From the NSG section only three conodont samples were processed, one of them from the Tha Manao Formation and two from the revised Pa Kae Formation, which consists of thin- to medium-bedded laminated limestones and stylolitic limestones. Eleven Ordovician conodont species representing nine genera are documented and illustrated from the Ban Tha Kradan area for the first time. The species identified are Baltoniodus cf. alobatus (Bergström), Cornuodus longibasis (Lindström), Drepanoistodus pitjanti (Cooper), Drepanoistodus cf. pitjanti sensu (Zhen et al. 2021), Erraticodon patu (Cooper), Protopanderodus nogamii (Lee), Plectodina sp., Scabbardella altipes (Henningsmoen), Scolopodus striatus (Pander), Triangulodus larapintinensis (Crespin), and Triangulodus sp. In the WMK section, the Scolopodus striatus Assemblage Zone can be recognized in the Tha Manao Formation. Roughly, it corresponds to the eponymous assemblage zone in the northwestern peninsular Malaysia. Furthermore, the upper part of the Tha Manao Formation in the NSG section as identified earlier has been revised and assigned to the Pa Kae Formation, based on the lithostratigraphic features. Considering the occurrences of cephalopod Sinoceras chinense (Foord) and conodont Baltoniodus cf. alobatus, as well as on the recognition of the Guttenberg Carbon Isotope Excursion (GICE) in the lowermost part of the formation, the revised Pa Kae Formation in Western Thailand has a similar temporal range as that of the Pa Kae Formation in the southern peninsular Thailand. Thus, the Tha Manao Formation in Western Thailand is assigned to the upper Floian to Darriwilian, and the conformably overlying Pa Kae Formation to the Sandbianâlower Katian.

The three main regions of Indochina are defined as the Truong Son, Loei-Phetchabun, and Kontum terranes. The aim of this review is to integrate numerous petrological studies with sedimentary, palaeontological, and provenance studies in order to construct a preliminary tectonic model which shows the terranes docked in the earliest Carboniferous (Truong Son with Loei-Phetchabun) and in the Permian (Kontum). The Kontum Terrane is characterized by Proterozoic magmatism, mid-Ordovician to Early Devonian granites, and Permian charnockites. Major carbonate platforms developed in the Givetian to earliest Tournaisian on Truong Son and from the Visean to mid-Permian across Truong Son and Loei-Phetchabun terranes. The Truong Son has Silurian granites and a Late Ordovician to Silurian magmatic arc along its southern and western borders caused by subduction of oceanic lithosphere, the remnants of which are now partially preserved in the Loei and Tamky sutures. A region to the east of the Loei Suture in the Loei Foldbelt has a similar-age volcanic arc extending northwards into Laos and is included in Truong Son. A platform-margin coral-stromatoporoid reef developed on Truong Son in the Frasnian and was coincident with a gap in magmatism until the Pennsylvanian with granite magmatism became widespread until the Late Triassic. In the Loei-Phetchabun Terrane, Devonian siliciclastics, volcanics are overlain by Givetian limestones and patch reefs and in turn overlain by Late Devonian to earliest Tournaisian radiolarian cherts and siliciclastics. Tournaisian to early Visean siliciclastic turbidites were coincidental with initial uplift and deformation and were replaced by Visean terrestrial evaporites and coal and marine limestones. Arc magmatism along the Loei-Phetchabun Terrane indicates intermittent eastward subduction from mid-Devonian to mid-Triassic. Carboniferous-Triassic volcanism extends eastward along the northern margin of Cambodia and is probably an extension of Loei-Phetchabun. Ordovician within-plate basalts in Kontum and the transition from shallow marine Ordovician siliciclastics to Silurian deep-marine sediments and a volcanic arc in Truong Son suggest rifting of both terranes from Gondwana in the Late Ordovician. Initial docking of Truong Son with South China was probably in the late Silurian to Early Devonian and docking of Loei-Phetchabun and Truong Son was in the late Tournaisian to early Visean.

Furongian sandstones yield shallow-water trilobite faunas along the length of the Sibumasu Terrane in the four regions of NW Malaysia (Langkawi), southern Thailand (Satun), the Shan States of Myanmar and the Baoshan Block of western Yunnan. These are associated with felsic plutonics, volcanics or volcaniclastics in all four regions. Correlations of Langkawi Island and Satun Province shallow-water carbonates reveal a succession of dominantly molluscan, stromatoporoid-brachiopod faunas which, from conodont evidence, range from the mid-Tremadocian to the Darriwilian, and which have dominantly North China-Australian afinities. A major increase in sea-level in the late Darriwilian led to trilobite-brachiopod faunas with very strong South China afinities. Deep-water conditions of more than 150 m prevailed in the Katian with the deposition of red, stromatolitic limestones with cool/ deep-water trilobite and conodont faunas. This ‘Pa Kae facies’ re-occurs in limestones of Silurian and Devonian age in several areas of Sibumasu. Llandovery graptolitic shales occur along the length of the terrane. Silurian, probably outer shelf, limestones occur in NW Malaysia, the Shan States and Baoshan but have a limited distribution in Thailand. Most Katian to Devonian faunas in NW Malaysia, Thailand and Baoshan show evidence of deep-water deposition which continues into the Mississippian of NW Malaysia and into the Pennsylvanian of Thailand contrasting with the Baoshan Block’s extensive, Devonian to Mississippian shallow-water, coral-rich carbonates. A signiicant mid-Carboniferous to Pennsylvanian depositional gap in the Shan States and Baoshan may be due to regional uplift by the mantle plume that triggered Cisuralian rifting of Sibumasu from Gondwana. Palaeomagnetic results from Baoshan, along with biogeographic evidence and provenance data from Thailand show that Sibumasu was deinitely part of the Australian sector of Gondwana until the Cisuralian and did not experience major deformation until Late Triassic collision with Indochina.

Conodonts from radiolarian cherts in the Loei Terrane of Loei Province, Northeast Thailand, include Palmatolepis triangularis, P. minuta minuta, P. minuta wolskae and polygnathids and indicate a Famennian crepida Zone age. These cherts are about 20 my younger than Givetian reef limestones and are unlikely to have been deposited in a major ocean but rather in a deep marine basin close to a volcanic arc.