Phu Kradung Fm

The Phu Kradung Formation has its type section on the 1316 m high mountain of that name in Loei Province. Parent unit: Khorat Gr.

Synonym: หมวดหินภูกระดึง

[Figure: The three sedimentary basins in which the marine Jurassic occurs, and their relationships to the main faults which might subsequently have displaced them (MYF, Mae Yuan Fault; MPF, Mae Ping Fault; TPF, Three Pagodas Fault; KMF, Khlong Marui Fault). The main outcrop areas of non-marine Jurassic rocks are also shown. In some areas, particularly on Peninsular Thailand, the Jurassic succession contains both marine and non-marine facies (From Meesook A. & Saengsrichan W., 2011. Redrawn from Meesook 1994; Charusiri et al. 2002).]

The rocks are predominantly maroon siltstone and claystone, although sandstone and conglomerate are also present. The sandstone units can be up to 10 m thick and are whitish grey in places. At the top of the claystone intervals, calcrete nodules and caliches are found. Locally the rocks show fining- and thinning-upwards sequences, from channel conglomerate and channel sandstone beds up through crevasse-splay facies to flood plain sandstone beds at the top (Meesook and Saengsrichan, 2011). Thickness: The formation is c. 1000 m thick at its type section (Ward & Bunnag 1964) and ranges from 800 to 1200 m in other areas.

[Figure: Composite stratigraphic column of the Mesozoic rocks of the Khorat Plateau, NE Thailand (Meesook and Saengsrichan, 2011).]

The Phu Kradung Formation in the Khorat Plateau contains both invertebrate and vertebrate faunas and floras, for example, freshwater sharks (Cuny et al. 2007), turtle plates, temnospondyl amphibian remains, crocodilian teeth, theropod and sauropod dinosaur teeth (Buffetaut & Ingavat 1984a; Buffetaut et al. 1997), bivalves and palynomorphs. The bivalves Unio thailandica Hayami, Unio sp. and ?Neomiodon khoratensis Hayami have been collected from Khon Kaen and Chaiyaphum Provinces, and the freshwater crocodile Sunosuchus thailandicus was found along the Nongbua Lamphu–Udon Thani road (Buffetaut & Ingavat 1980, 1984b). Buffetaut et al. (2006) have reviewed the dinosaurs and compared them with those from the Late Jurassic Upper Shaximiao Fm of Sichuan and the Shishugou Fm of Xinjiang, China. Abundant palynomorphs along the Nongbua Lamphu–Udon Thani road include Ballosporites sp., Calamospora sp., Classopollis striatus Madler, Ballosporites hians Madler, ?Ovalipollis sp. and cf. Cyclotriletes subgranulatus Madler (Racey et al. 1994, 1996).

At the type section of the Phu Kradung Formation, Classopollis sp., Cyathidites sp. and Ballosporites hians Madler are also found. Racey & Goodall (2009) attach particular importance to the presence of Dicheiropollis etruscus at outcrop and in at least one borehole (Phu Horm 1) which they consider to indicate a Berriasian to Barremian (i.e. Early Cretaceous) age for all but the lower part of the Phu Kradung Formation (Meesook and Saengsrichan, 2011).

Lower Jurassic; Lower Jurassic to Lower Cretaceous (Racey and others, 1994); Middle to Upper Jurassic (Department of Mineral Resources, 2007). Spans latest Jurassic-early Berriasian following Booth, J. & Sattayarak N., 2011, Chapter 9 Subsurface Carboniferous-Cretaceous geology of NE Thailand.

The sedimentological aspects and vertebrate fossils of the Phu Kradung Formation indicate that it was deposited by meandering rivers, with a high-energy regime along the paleochannels, followed by deposition in a floodplain and lacustrine environment. The presence of eroded calcrete paleosol in the topmost part of the floodplain deposits suggests that the rocks were deposited in a semiarid paleoclimate (Meesook 2000), although less arid than the Sao Khua Fm higher in the Khorat Group judging from the thicker paleosol horizons of the latter. A change to a braided and occasional meandering river environment with a slightly more humid climate marks the boundary with the overlying Phra Wihan Fm (Meesook and Saengsrichan, 2011).