Palaeoenvironmental implications of geochemistry and radiolarians from Upper Devonian chert/shale sequences of the Truong Son fold belt, Laos

Abstract

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.

Llandovery (lower Silurian) graptolites from the Sepon Mine, Truong Son Terrane, central Laos and their palaeogeographical significance

“…Those specimens identified to species level were assigned to Homoctenus ultimus, which is widely geographically distributed, and Costulatostylionina vesca, previously recorded only from the South China Terrane. The same two tentaculitid species were recorded from Vilabouly, 150 km SE of Ban Phonxai, Laos (Fig.3) by Udchachon et al (2017a).…”

“…The radiolarians that have been described recently from central Laos and north-east Thailand (Thassanapak et al, 2012, in press;Udchachon et al, 2017aUdchachon et al, , 2017b provide useful biostratigraphical constraints on the strata from which they have been extracted, but the taxa recorded appear to have been geographically widespread and thus do not assist with palaeogeographical reconstructions in terms of palaeolatitude or faunal provincialism. They do, however, provide extremely important evidence for regional palaeoenvironmental interpretations which have a bearing on the questions of the unity or proximity of the Indochina terranes and South China in the mid Palaeozoic.…”

“…Udchachon et al (2017a) recorded Frasnian radiolarian cherts from Vilabouly area of central Laos (Truong Son Terrane) deposited in "relatively deep marine environments" and in their palaeogeographical reconstruction of the region show the radiolarian-bearing sediments being deposited between an emergent part of Indochina and another small landmass termed Phu Hoat with a marine strait approximately 250 km wide separating Phu Hoat from South China.Udchachon et al (2017a, p. 155) noted that that during the Frasnian Indochina was "separated from the South China Block and from Gondwana by oceanic lithosphere. "3.5 Graptolites from the Truong Son TerraneGraptolitic strata are exposed in many sections within the Truong Son Terrane, most notably in central Vietnam from which Nguyen Van Phuc (2002) recorded many Llandovery graptolite biozones, with generally more sporadic records of Wenlock to Devonian biozones.In the same paper are details also of graptolitic sections from northern Vietnam (South ChinaTerrane).…”

Litho-, bio-, and chemostratigraphy of the Middle Triassic carbonate succession in the North-Central Coast Region of Vietnam

“…The territory of Vietnam spans the South China and Indochina blocks. The boundary between them is considered to be the Red River Fault Zone (Tapponnier et al 1990;Osanai et al 2008) or the Song Ma Suture (Lepvrier et al 2004(Lepvrier et al , 2008Metcalfe 2011Metcalfe , 2012Udchachon et al 2017), both of which strike NW-SE. The Indochina Block occupies the majority of Vietnam, with the South China Block being limited to the country's northern corner.…”

“…δ 18 O values in carbonates reflect the temperature of precipitation and the δ 18 O composition of the water from which they precipitated (e.g., Tucker and Wright 1990). Based on the equation relating the temperature, and the δ 18 O values of carbonates and the water (Friedman and O'Neil 1977), and on conversion between the VPDB and VSMOW scales (Coplen et al 1983), we can calculate the δ 18 O values of calcite precipitated in oxygen isotope equilibrium with ambient seawater (equilibrium calcite).…”