Nong Dok Bua Fm

Small creak of Phu Chumpa, northwest of Nong Dok Bua village, Loei province by Charoenprawat and others (1984).

[Figure: Map of Thailand showing the distribution of the Devonian and closely associated Silurian and Carboniferous rocks. The belt of crystalline ‘basement’ rocks is mentioned in the text; they were previously thought to be Precambrian but radiometric dating shows them to be younger (Hansen & Wemmer 2011; Wongwanich & Boucot, 2011).]

Synonym: หมวดหินหนองดอกบัว

The Nong Dok Bua Formation consists of grey to dark grey thin-bedded chert interbedded with dark grey silicified shale, greenish grey quartzite and dark green tuffaceous sandstone and intercalated dark grey to black limestone in the north. The upper part of the sequence also changes to greenish grey, calcareous, pebbly greywacke and conglomerate in the north, especially in the Chiang Khan to Pak Chom area. In the area south of Pak Chom there are basalt, andesite and diabase dykes of possible Carboniferous age in the Nong Dok Bua Formation (Wongwanich and Boucot, 2011). Thickness: Not mentioned

[Figure: Black banded chert and intercalated black siliceous shale exposed in a quarry on the east side of Highway 2108 (17846.178 0 N, 101855.620 0 E) south of Pak Chom, from which Y. Kamata has obtained radiolaria indicating a late Devonian age (K. Ueno, pers. comm. 2009). The beds are folded in an apparently random fashion with axial planes ranging from near vertical to near horizontal (photograph courtesy of M. F. Ridd.) (Wongwanich and Boucot, 2011)

The beds in this area were formerly referred to the Carboniferous Wang Saphung Formation of Charoenprawat et al. (1976), the uppermost Devonian to Middle Carboniferous Nong Dok Bua Member, Wang Saphung Formation of Bunopas (1988) or the Upper Devonian to Lower Carboniferous locally siliceous shale unit with local intercalations of greywacke, chert and quartzite of Chairangsee et al. (1990). ]

Sashida et al. (1993) and Sashida & Igo (1999) described Late Devonian to Early Carboniferous radiolaria including Entactinia archocyrtium, Palaeoscenidium and Helenifore from black chert and siliceous shale at Phu Luang, Phu Sang and an abandoned quarry 2.5 km south of Phu Nong Ya Plong, previously referred to the Middle Devonian to Lower Carboniferous Nong Dok Bua Fm (Charoenprawat et al. 1976) or the Carboniferous Wang Saphung Fm (Bunopas 1988).

Later, Saesaengseerung et al. (2007a,b,c) described Late Devonian to Early Carboniferous radiolarian faunas, including Stigmosphaerostylus variospinax, Astroentactinia stellata, Astroentactinia multispinosus, Archocyrtium wonae and Archocyrtium riedeli from siliceous shale in a chert-clastic unit along the Mekong River near Wat Non Si Sa-at, Pak Chom city. Furthermore, highly contorted grey and black banded chert crops out in a quarry on the east side of Highway 2108 south of Pak Chom (Fig. 4.7) from which a late Devonian radiolarian fauna has been obtained by Y. Kamata (K Ueno, pers. comm. 2009) (Wongwanich and Boucot, 2011).

Lower to Middle Devonian chert interbedded with siliceous shale and tuff suggested to Bunopas (1988) deposition in a deeper or quieter marine environment on the western continental margin of the Indochina Block. This was followed by submarine volcanic activity on the ocean floor in the Middle Devonian, as evidenced by serpentinites, spilitic basalts and synsedimentary manganese ores in the Pak Chom area (Chairangsee et al. 1990) (Wongwanich and Boucot, 2011).

Correlation: Pak Chom chert Fm, Dok Du Fm (Department of Mineral Resources, 2007).