Palaeoenvironment and palaeogeography of Middle and Upper Devonian strata from the Loei fold belt, Indochina terrane (northeast Thailand)

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.