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The boundary between tropical Permian faunas of the Inthanon Zone and Gondwana faunas in far NW Thailand has been long debated. Both the Late Devonian and the Permian Gondwanan platform margins lie a few kilometres west of the Mae Yuam/Mae Sariang Fault (MYMS FZ). In the Permian, the margin grades eastwards into hemipelagic radiolarites along the MYMS FZ and westwards into the Thitsiphin carbonate platform of Myanmar. The area west of the MYMS FZ is the Northern part of the West Thailand Region (NWTR). Quartz-rich limestones of Roadian age in the NWTR are succeeded by deep-water platform limestones in the NWTR and shallow-water carbonates in Myanmar and contain a distinctive fusulinid fauna including Monodiexodina which does not occur in palaeotropical terranes. A 300-m section of limestone 10 km west of the MYMS FZ contains Wordian microfauna and is placed in a deep shelf to slope environment. Carboniferous to Triassic continental margin, hemipelagic, non-hydrothermal, radiolarian cherts outcrop on either side of the Mae Yuam valley and were deposited on the upwelling margins of an ocean separating the Inthanon Zone and Sibumasu Terrane. The widely accepted allochthon model proposes that the Inthanon Zone Devonian-Triassic radiolarites were pelagic and deposited on a subducting ocean that supported seamounts with Visean to Permian tropical shallow-water carbonates lasting at least 90 my. We suggest an alternative hypothesis where the radiolarites of the Inthanon Zone were continental margin as shown by their geochemistry and deposited in deeper parts of small extensional basins with limited volcanism between long-lived, isolated carbonate platforms.

The Bayankhoshuu Ruins section in southern Mongolia is characterized by strongly thrusted and folded sequences. Overall, three sections ranging from Ordovician to Carboniferous rocks were studied. Facies analysis combined with stratigraphic data provide improved lithostratigraphic descriptions of Palaeozoic successions in the Mushgai region. The overall marine sedimentary sequence is punctuated by volcanic rocks–basaltic lava of Silurian and Middle Devonian age and volcaniclastic bentonite and tuff in the Middle to Late Devonian and Mississippian suggesting an island arc setting. The Minjin Member of the Botuulkhudag Formation (Middle Devonian to Late Devonian) is primarily composed of thick basaltic and subaerial volcanic rocks with minor silicified siltstone and chert inclusions. Thicker successions of limestone occur in the Ordovician/Silurian, Early Devonian, and the Mississippian. The macrofauna is scarce, except distinct limestone horizons where different fossil groups were recognized. Microfossils, such as radiolarians and conodonts, are scarce and generally poorly preserved. However, based on the re-study of collections from earlier publications and new conodont data, a more detailed biostratigraphic record of the Khoyormod, Botuulkhudag, and Arynshand formations of the Bayankhoshuu Ruins section can be developed. For instance, the Arynshand Formation likely ranges from the late Bispathodus ultimus conodont biozone to the Scaliognathus anchoralis–Doliognathus latus conodont biozone. A tectonic breccia occurs in the early Mississippian and is overlain by a red shale of remarkable thickness at the top of this formation which points to subaerial exposure in the early Mississippian (near the Tournaisian/Visean transition). Due to strong tectonic overprint and/or facies, some unconformities/hiatuses occur. Most strata are intensively folded and faulted, ranging from centimeter to meter scale. Overall, deposition likely occurred on either the Mandalovoo or Gurvansayhan Terrane.

The first radiolarian fauna obtained from Permian carbonates in Thailand is of late Kungurian age and is present in the basal beds of the carbonate–mudstone–chert Phap Pha Formation, Ratburi Group. This succession contains several species of the radiolarian Pseudoalbaillella, and some sponge spicules. The radiolarian fauna consists of abundant Pseudoalbaillella aidensis and P. elegans together with P. fusiformis, P. longtanensis, P. m. rhombothoracata and P. sp. A. Other species include P. cf. aidensis, P. cf. elongata, P. cf. fusiformis, P. cf. ishigai, P. cf. lomentaria, P. cf. longicornis, P. cf. longtanensis, P. cf. ornata, P. cf. simplex, P. cf. m. scalprata, P. cf. m. postscalprata, P. cf. u–forma m. I, P. cf. u–forma m. II, and P. spp. The radiolarian assemblage suggests its correlation to the P. longtanensis Zone which, in turn, is correlated to the P. ishigai Zone of late Kungurian age. The occurrence of an abundant but generically low–diversity radiolarian fauna suggests restricted physical conditions and, with other evidence, suggests deposition along a cool deglaciating or deglaciated continental margin with an abundance of silica possibly provided by glacial meltwaters. The abundant chert in the Phap Pha Formation is part of the widespread Permian Chert Event.

Lower Palaeozoic rocks have been mapped in Kayin State in an area previously shown on published maps as either metamorphic or possibly Lower or Upper Palaeozoic rocks. Three new formations, with a total thickness of over 900 m, apparently overlain by an, at least, 100 m thick Upper Palaeozoic formation are mapped along the Salween River and along the road from Yinbaing, in Myanmar, to Tha Song Yang, in Thailand. The Lower Palaeozoic succession consists of the predominantly siliciclastic Kyaukpulu and Kushwe–e–we formations and an overlying, predominantly carbonate Meseik Ashe Formation which contains Middle Ordovician (Darriwilian) conodonts. The older two formations are probable correlates of the Ngwetaung and Lokeypin formations of the southern Shan State of Myanmar and the Lower Ordovician siliciclastics of western Thailand. The overlying, peritidal to shallow subtidal carbonates of the Meseik–Ashe Formation are correlates of the Wunbye and Sitha formations of Shan State, Myanmar. The thick–bedded, quartz arenites of the Nyaungwiang Formation are faulted against the Ordovician carbonates and are probable lithological correlates of the Carboniferous Taungnyo Formation. The folds in the Lower Palaeozoic rocks are overturned to the northeast and deformation was in one major phase between the Tournaisian and the Early Permian. The Lower Palaeozoic strata may probably be followed as a ridge for at least 100 km towards the NNW, close to the western border of the Sibuma Block which is separated by a postulated cryptic suture from the Irrawaddy Block to the west.

Limestone and chert sections from the Loei fold belt in Loei Province, northeastern Thailand, have yielded Givetian to Famennian conodonts and Late Devonian radiolarian faunas. The mid-Devonian siliciclastic section containing an abundant benthic assemblage is interbedded with volcaniclastics and is replaced laterally, in places, by pillow basalts. This siliciclastic section is conformably overlain by argillaceous limestones, divided into two microfacies. The lime mudstone (MF1) and spiculite wackestone (MF2) which contains Polygnathus linguiformis linguiformis, Belodella resima and Icriodus arkonensis, indicating the lower Givetian, deposited in a quiet subtidal to deeper water environment. This argillaceous limestone is succeeded by upper Givetian bioherm containing a diverse stromatoporoid–coral fauna. Microfacies of bioherm consists mainly of crinoidal wackestone/packstone (MF3) deposited in a flank and stromatoporoid–coral framestone/bafflestone/rudstone (MF4) forming a core of bioherm. Continuing relative sea-level rise led to drowning of the bioherms and the deposition of condensed section of non-oceanic, continental margin oozes containing the radiolarians Trilonche chiangdaoensis, Trilonche echinata, Trilonche elegans, Trilonche hindea, Trilonche minax and others and the Famennian conodonts Palmatolepis triangularis, Palmatolepis werneri, Palmatolepis minuta minuta and Palmatolepis wolskae. The transgressive Middle to Upper Devonian strata ranging from fossiliferous sandstones through argillaceous limestone and bioherm to radiolarian cherts are broadly similar to sections in South China and in the Rhenohercynian belt of Germany. Deposition in a continental margin basin, possibly on thinned continental crust, such as a back-arc basin, is more likely than previous models that suggested deposition in and around seamounts within a major ocean basin.

A Silurian radiolarian fauna based on 18 species is described from cherts from three sections in two quarries in the Sepon Mine area, central Laos. Each section contains distinctive assemblages butFutobari morishitai,F. solidus,Zadrappolus yoshikiensisandZ. tenuisare found in all three sections and are characteristic of the Ludlow–PridoliZ. yoshikiensisAssemblage. In addition, the Hoei Yang pit section containsZadrappolus hitoeganensis,Pseudospongoprunum sagittatum,Futobarisp. andSecuicollactasp. Section CQ containsPseudospongoprunum sagittatum,P. tazukawaensis,Praespongocoelia parva,P. sp.,Rotasphaerasp. cf.R. quadrata,Secuicollactasp. cf.cassa. Section QM also containsPseudospongoprunum parvispina,P.sp.,Zadrappolussp. andDevoniglansus(?)seponensisnew species Thassanapak is common. Evidence from Silurian and Devonian strata of the southern Truong Son Terrane indicates deepening towards the southern margin as defined by the Thakhek–Danang Shear Zone and exemplified by deep-marine Ludlow–Pridoli radiolarian cherts in the south, shallowing to fossiliferous, shallow-water sediments to widespread Old Red Sandstone type terrestrial facies 85 km further NE. The radiolarian cherts are in faulted contact with probable Llandovery limestone and Upper Ordovician – Llandovery graptolitic shales. The graptolitic shales are overlain with slight angular unconformity by the submarine fan, volcaniclastic-dominated, Namphuc Formation of Llandovery age. This fan received andesitic materials from the nearby Long Dai Volcanic Arc which extends eastwards to the Tamky Suture in Vietnam. This plutonic–volcanic arc was probably maintained by northwards subduction along the trend of the Permian–Recent Thakhek–Danang Shear Zone.

A reconnaissance survey of part of western Cambodia has found quarries in and near the town of Pailin containing chert, basalt, volcaniclastic rocks, gabbro and serpentinite. Abandoned quarries contain chert and basalt clasts in a volcaniclastic matrix and may constitute a mélange. A new, recently active, quarry contains these rock units in close and coherent contact but they lack the serpentinitic matrix of the Thung Kabin Mélange of eastern Thailand. The rock units are in close contact and may best be described collectively as a dismembered ophiolite. The mélange–ophiolite association constitutes a 3 km wide, 20 km long E–W belt separating a northern 200 km2 block of the mainly amphibolitic Pailin Crystalline Complex from a southern area of Triassic submarine fan siliciclastic rocks. The cherts yield the first documented radiolarian fauna from Cambodia and include a moderately well-preserved Asselian–Sakmarian age fauna consisting of Pseudoalbaillella sakmarensis, Pseudoalbaillella scalprata morphotype scalprata, Pseudoalbaillella sp. cf. P. simplex, Pseudoalbaillella u-forma morphotype II, Pseudoalbaillella sp. cf. P. elegans, Pseudoalbaillella sp. cf. P. lomentaria, Albaillella sp., Pseudoalbaillella spp., Trilonche? sp., Latentifistularia gen. et sp. indet. and Entactinaria gen. et sp. indet. The Pailin ophiolitic rocks, mélange and volcanic rocks occur within a generally E–W-trending belt, which suggests that the Sa Kaeo Suture does not extend southeastwards paralleling the Thai–Cambodian border, nor extend under the Cardamom Mountains but, rather, extends eastwards into Cambodia and possibly then turns southwards along the strike of the Pursat–Kampot Fold Belt.

More than thirty species of radiolarians are described from maroon chert and siliceous shales from the Den Chai and Santisuk sections in the east of the Sukhothai fold belt. The fauna obtained from Den Chai comprises Muelleritortis cochleata cochleata, M. cochleata tumidospina, M. expansa, Triassocampe deweveri, T. coronata, T. scalaris, Annulotriassocampe companilis, A. multisegmantatus, A. sulovensis, Pseudostylosphaera coccostyla coccostyla, P. imperspicua, Canoptum inornatus, C. levis, Corum kraineri, Spongoserrula rarauana, Orbiculiforma karnica and others. The assemblage may be correlated with those reported in the Fang-Chiang Dao and Lumphun areas in the north of Thailand and in the Changning-Menglian belt of western Yunnan, South China and suggests a late Ladinian (Middle Triassic) age. The radiolarian fauna from the Santisuk Section comprises Muelleritortis cochleata cochleata, M. sp., Striatotrissocampe nodosoannulata, Triassocampe. deweveri, T. sp., Pseudogodia? sp., Pseudostylosphaera coccostyla coccostyla, Archaeocenosphera sp., Annulotriassocampe multisegmantis, Pseudostylosphaera sp., and others which indicates a late Ladinian age. Middle Triassic radiolarian chert and siliceous shale from Den Chai and Santisuk are interpreted as having been deposited in an extensional continental margin within the Sukhothai fold belt and not in the Devonian–Permian back-arc basin of the Nan suture. These deep marine sequences were deposited in a subsiding continental margin in the Lampang-Phrae Basin. We consider these sequences to be part of the Hong Hoi Formation (Anisian–Ladinian) of the Lampang Group which, in turn, may be compared to the Choushui Formation and other associated Middle Triassic formations in the Simao Basin of South China.

More than 42 species of well-preserved Upper Devonian radiolarians have been obtained from cherts and siliceous shales in four sections (Chom Noi, Phu Kham Phe (P), Phu Kham Phe (C) and Pha Samyod) from the NE Thailand sector of the Loei fold belt, Indochina terrane. They include Ceratoikiscum sp. cf. planistellare Foreman, Helenifore laticlavium Nazarov & Ormiston, Astroentactinia sp. cf. paronae (Hinde), Stigmosphaerostylus herculea (Foreman), S. pusilla (Hinde), S. cf. vulgaris (Won), Trilonche davidi (Hinde), T. echinata (Hinde), T. elegans Hinde, T. hindea (Hinde), T. palimbola (Foreman), T. vetusta Hinde, Polyentactinia polygonia Foreman, Spongentactinella sp. cf. windjanensis Nazarov, Archocyrtium sp.?, Palaeoscenidium sp., Triaenosphaera sp., and others. Radiolarian faunas from these sections indicate a range from lower Frasnian to Famennian. Analyses of rare earth, trace and major elements suggest that the Upper Devonian chert and siliceous successions from Loei were deposited in a continental margin environment near to a volcanic arc. These successions contain distinctive weak negative Ce anomalies with strong positive Eu anomalies which differ from Upper Devonian cherts reported from northern Thailand, the Truong Son fold belt (Laos) and from South China. Negative Ce anomalies and low to moderate Lan/Cen ratios imply proximity between the Chom Noi and Phu Kham Phe (C), which were deposited in more distal parts of the depositional basin than the Pha Samyod and Phu Kham Phe (P) samples. Most of the Pha Samyod and Phu Kham Phe (P) samples exhibit high continental-derived trace element levels including Cr, Zr, Hf, Rb, Th and in part V, which supports close proximity to terrigenous sources. Paleozoic stratigraphic sequences of the Loei fold belt and geochemical results suggest that the Upper Devonian deep marine sequences in the Loei fold belt were deposited within a rifted, continental-margin basin, possibly a back-arc basin and not in a large oceanic basin as has been previously suggested.

A new locality of Frasnian radiolarian chert and siliceous shale is reported in the Vilabouly area, within the Truong Son fold belt of central Laos, in the eastern part of the Indochina Terrane. Samples from this locality, together with previously described radiolarian–bearing siliceous shale from Ban Phonxai, 100 km further north, have been geochemically analysed. Most of the Ce anomalies from Vilabouly and Ban Phonxai exhibit negative normalized values suggesting deposition in an open–marine basin and in a transition zone between open–marine and continental margin environments, respectively. NASC and chondrite-normalized REE and La/Ce plots and trace element contents support this interpretation. Normalized La/Yb values indicate removal of LREEs relative to HREEs from the water column, suggesting deposition far from terrigenous sources for the Vilabouly samples. The Eu/Eu* values are not pronounced which, with the high values of Al2O3/(Al2O3 + Fe2O3), indicate minimal hydrothermal influence. Both sections are characterized by a distinctive Frasnian radiolarian and tentaculitid assemblage. The new radiolarian fauna from Vilabouly includes 23 species including Ceratoikiscum sp. cf. C. bujugum, Archocyrtium sp., Astroentactinia cf. stellata, Helioentactinia aster, H. perjucunda, Spongentactinella corynacantha, S. veles, Spongentactinia concinna, S. exquisita, Stigmosphaerostylus profundisulcus, Trilonche davidi, T. echinata, T. elegans, T. hindea, T. palimbola, T. vetusta, Palaeoscenidium cladophorum and others. The tentaculitids, Homoctenus ultimus and Costulatostyliolina vesca, have been previously described from South China and from Ban Phonxai. The similarity of the radiolarian fauna to those from South China, northern Thailand and Western Australia suggests free interchange of species. Based on geochemistry, stratigraphic sequences and structural geology, the radiolarian chert/siliceous shales are interpreted as highstand deposits in pull-apart basins. This was possibly caused by transtension along the NW–SE–trending strike–slip fault of the Thakhek–Danang Shear Zone during the Late Devonian. Copyright © 2015 John Wiley & Sons, Ltd.

A Middle Permian carbonate sequence at Khao Somphot in the south of the Khao Khwang Platform has been measured and analyzed. This sequence is characterized by prolific fossil content and large, bizarre alatoconchid bivalves. Nine major microfacies types are differentiated and consist of: algal-foram facies, fusuline facies, alatoconchid facies, lime mudstone/wackestone facies, laminated bindstone facies, fine-grained cortoid grainstone facies, coral biostrome facies, crinoidal packstone facies and carbonate breccia/conglomerate facies. Laminated bindstone, lime mudstone with fenestral fabric and algal foram facies represent loferites deposited in a restricted intertidal zone of an inner ramp. Fusuline grainstone and cortoid grainstone facies indicate sand shoals of an inner ramp. The fusuline wackestone/packstone and alatoconchid facies were deposited in a subtidal, below fair-weather wave base environment in the mid-ramp. The crinoidal facies, which overlies collapse breccia, possibly accumulated during transgression in the deeper part of the mid-ramp. Storm deposits are prevalent throughout as thin accumulates and as alatoconchid floatstone/rudstone (coquinite) layers and are common in mid-ramp setting. The Khao Khwang Platform probably evolved from a rimmed platform in the Early Permian to a ramp in the Middle Permian. The general trend of δ 13C composition from both brachiopod shell and rock matrix from the lower part of the study section (Wordian) is significantly high. Moreover, the δ 13C signature from this interval is up to 8 VPDB‰, which suggests high productivity on the tropical Tethyan shelf of the Indochina Terrane. An abrupt negative shift in δ 13C in the late Wordian and late Capitanian indicating significant changes in paleoenvironment, productivity and the carbon cycle is probably contemporaneous with the Kamura event recorded from sediments of the mid-Panthalassa Ocean in Japan and elsewhere related to global cooling and a sea-level lowstand.

Conodonts are rare in the Permian carbonates of Indochina but abundant conodonts and ostracodes have been obtained from turbiditic limestones of the Permian E-Lert Formation along with radiolarians from overlying cherts, all deposited on the margins of the interplatform Nam Duk Basin. Conodonts are typically Tethyan and are very similar to faunas from Sicily and south China. They includeHindeodus gulloides,Pseudohindeodus oertlii,Mesogondolella siciliensisandSweetognathus subsymmetricuswhich indicate a probable late Kungurian – Roadian age range although a Wordian age cannot be excluded.M. siciliensis, which has a high blade and small cusp supposedly typical of warm-water conodonts, is found in deep (Albaillella asymmetrica,A. sinuata,Tormentum delicatumandLatentifistula patagilateratasuggest a latest Kungurian – earliest Roadian age, deposited at a palaeodepth ofc.500 m. Ostracodes consist of 16 genera and 23 species which belong toShivaella,Paraberounella,Carinaknightina,Paraparchites,Shemonaella,Bairdia,Cryptobairdia,Bairdiacypris?,Spinocypris,Pseudobythocypris,Baschkirina,Microcheilinella,Basslerella,PolycopeandCyathus, of whichShivaella elertensis sp. nov.Chitnarin is newly described. The ostracodes are a palaeoecologically mixed assemblage comprising external platform and deeper-water forms, suggesting that the limestone turbidites were deposited on the proximal part of the slope.Cyathus caperataandC. ellipticashow a palaeobiogeographic link to south China.

A 4. m thick section of silicified shales from Ban Phonxai in central Laos contains the radiolarians Trilonche davidi , T. hindea , T. minax , T. palimbola , Stigmosphaerostylus spp. and Ceratoikiscum ? sp. and the tentaculitids Homoctenus ultimus and Costulatostylionina vesca which together indicate a Frasnian age. This pelagic, deep shelf fauna from the Indochina Terrane is similar to that from South China and shows that tentaculitids may be common and biostratigraphically useful in radiolarian cherts and shales in South East Asia. This discovery probably necessitates mapping of the Late Devonian Phon Tiou Fm in this area which is currently mapped as the Early Carboniferous Boulapha Formation.

Geochemical analysis reveals that Middle Triassic radiolarian cherts from northern Thailand, including Chiang Dao, Lamphun and Den Chai, are of biogenic origin. These cherts present slightly high SiO2 content which was possibly modified by diagenetic alteration and migration processes as indicated by negative correlation between SiO2 and most of the other major elements. The relatively high content of Cr, Zr, Hf, Rb and Th and high positive correlation of these elements with Al and Ti from the majority of cherts suggest a close relation to terrigenous component. The Ce anomaly (Ce/Ce*) with geometric means ranging from 0.85 to 0.93 is compatible with that of continental margin composition (0.67–1.52) from Murray et al. (1990) which is also consistent with low Eu anomalies (Eu/Eu*, 0.91–0.94). Moreover, the slightly low ratios of La and Ce NASC normalized (Lan/Cen, 0.91–0.94) and the low LREE and HREE ratios in most of our samples (Lan/Ybn, 0.62–0.85) are in agreement with the continental margin. The result from Lan/Cen vs. Al2O3/(Al2O3+Fe2O3) discrimination diagrams also supports the continental margin (residual basin, s. str.) interpretation. These geochemical results are compatible with geological evidence, which suggest that during the Middle Triassic, radiolarian cherts were deposited within a deeper part of a residual basin in which an accommodation space was possibly controlled by faults under extensional regime subsequent to Late Variscan (Permian) orogeny. Paleogeographically, the main Paleotethys which closed during Late Triassic should be located further to the west of these study localities. This scenario is in agreement with the current view of the Tethys in this part of the world.

Middle Triassic radiolarians were obtained from a bedded chert sequence in Chiang Dao, northern Thailand. Sixty-four species were identified into two assemblages, which indicate two stratigraphic levels including lower Illyrian (lowermost upper Anisian) and upper Illyrian (uppermost Anisian). The occurrence of upper Anisian radiolarians in our study indicates the former existence of a relatively deep marine environment in northern Thailand with a seaway between eastern and western Palaeotethys. The stratigraphic units containing bedded chert in this area were formerly dated on the basis of fossil evidence and petrological similarities. However, the strata in the study area are possibly not part of a continuous stratigraphic sequence, but consist of tectonic slices. The fossil age from any slice indicates only the geological age of the given slice, and adjacent slices are often of different ages. Parts of the sequence formerly mapped as Carboniferous and Permian are Middle Triassic in the study area. The sequence covers the Carboniferous to Middle Triassic and can be correlated with the eastern zone of the Changning-Menglian Belt in southwestern Yunnan, China.