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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.

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.

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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 include Hindeodus gulloides, Pseudohindeodus oertlii, Mesogondolella siciliensis and Sweetognathus subsymmetricus which 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 (<500 m) carbonate turbidites in Thailand and in very deep deposits in Oman and Sicily, but generally not in shallow-water tropical limestones in Thailand, Oman and Sicily. The chert sequence yields a radiolarian fauna consisting of 11 confidently assigned species of which Albaillella asymmetrica, A. sinuata, Tormentum delicatum and Latentifistula patagilaterata suggest a latest Kungurian – earliest Roadian age, deposited at a palaeodepth of c. 500 m. Ostracodes consist of 16 genera and 23 species which belong to Shivaella, Paraberounella, Carinaknightina, Paraparchites, Shemonaella, Bairdia, Cryptobairdia, Bairdiacypris?, Spinocypris, Pseudobythocypris, Baschkirina, Microcheilinella, Basslerella, Polycope and Cyathus, of which Shivaella 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 caperata and C. elliptica show a palaeobiogeographic link to south China.