Sao Khua Fm

At km post 35.2-41.5, Udon Thani-Nong Bua Lamphu Road; Imsamut (2003a) proposed reference sections at Khao Chamuk Khaek section, Lum Ngoen village of Nakhon Ratchasima province (821900E, 1614900N, 5338 II, L7018), at Phu Wiang Investigation Site 1 within Phu Wiang National Park, Khon Kaen province (204800E, 1846400N, 5442 III, L7018), at Phu khum Khao section of Sahat Sakhan district, Kalasin province (343300E, 1546000N, 5742 II, L7018). The best section of this formation is at Ban Kuan Bun along Highway 2239 in Tao Ngoi District, Sakhon Nakhon Province (Meesook, 2011). Parent unit: Khorat Gr.

Synonym: หมวดหินเสาขัว

[Figure: NE and SE Thailand, showing distribution of Cretaceous outcrops. (Meesook, 2011. Page 170). At least some of the Pre-Cretaceous sedimentary rocks on the map may be Jurassic and the position of the Jurassic-Cretaceous boundary is discussed in Meesook (2011).]

Siltstone, greyish-red to reddish-brown, mottled, calcareous and nonresistant; sandstone, red, yellowish-grey to yellowish-brown, medium- to fine-grained; minor conglomeratic sandstone. Imsamut (2003a) mentioned the occurrence of calcrete in the formation. It is interesting to note that the Sao Khua Formation in the Phu Phan Range is more sandy than in the western rim of the Khorat Plateau. Thickness: 404-720 m

[Figure: Sao Khua Formation in NE Thailand. (a) Thick channel sandstone at Phu Lon in Sakhon Nakhon Province; (b) thick channel sandstone at Pha Taem in Mukdahan Province; (c) fining- and thinning-upwards sequence at Pha Nam Yoi in Roi-et Province; and (d) thick sandstone with Trigonioides layers at Ban Huai Dua in Nongbua Lamphu Province (Meesook, 2011).]

Dinosaur fragments and turtle plates have been found at Tao Ngoi and Phu Phan Districts, Sakhon Nakhon Province and at Phu Khum Khao in Sahat Sakhan District, Kalasin Province. The Sao Khua Formation has yielded the richest and most diverse vertebrate faunas found in Thailand, which have been the subject of a rapidly growing literature (Buffetaut & Suteethorn 1998). The crocodilian Siamosuchus phuphokensis (Lauprasert et al. 2007) has been described as also have the theropod Siamotyranus isanensis, which is considered to be the oldest and most primitive known tyrannosaurid (Buffetaut et al. 1996), and the sauropod Phuwiangosaurus sirindhornae (Martin et al. 1994). Other vertebrate fossils include the first sinamiid fish from Southeast Asia (Cavin et al. 2007a, b), hybodont sharks (Cuny et al. 2005, 2006), turtles (Tong et al. 2006) and the first known fossil bird from the Mesozoic of Thailand (Buffetaut et al. 2005a). The Sirindhorn Museum exhibits many of the fossils found in the region and is located beside Highway 227 north of the city of Kalasin.

Invertebrate fossils, particularly bivalves, are also found in the Sao Khua Formation including Plicatounio, Unio, Koreanaia and Trigonioides (Meesook et al. 1995), Trigonioides (s.s.) trigonus, and ?Trigonioides cf. gaungsiensis (Sha Jingeng, pers. comm. 1997). Pieces of silicified wood in reddish-brown claystones are found in a few exposures. According to Hahn (1982), Racey et al. (1994, 1996) and Racey & Goodall (2009), abundant palynomorphs are present, suggesting a Berriasian–Barremian (Early Cretaceous) age for the formation (Meesook, 2011).

Lower Cretaceous (? Barremian-Aptian) by Racey and others (1994); originally assigned as Upper Jurassic (Ward and Bunnag, 1964); Berriasian–Barremian (Early Cretaceous) (Meesook, 2011). Spans ca. Barremian following Booth, J. & Sattayarak N., 2011, Chapter 9 Subsurface Carboniferous-Cretaceous geology of NE Thailand

Continental deposits. The environment of deposition is interpreted as low-energy meandering channels and extensive floodplain deposits with crevasse splays, deposited in warm to slightly cool semiarid conditions.