Khlong Min Fm

The type section of the Khlong Min Formation is at Khlong Min, a north-flowing stream NE of Khao Sam Chom and c. 12 km NNW of Bang Khan; reference section at Laem Pleo of Bo Muang village, Khlong Thom district, Krabi province (Teerarungsigul and others, 1999; Raksaskulwong, 2002). Parent unit: Thung Yai Gr (Trang Group).

Synonym: หมวดหินคลองมีน, Khao Lak Fm (in Chumphon Basin)

[Figure: Geological map of the Thung Yai Gr in Khlong Thom-Bang Khan area, Peninsular Thailand (Meesook, 2011)]

Mudstone interbedded with fossiliferous limestone in the lower part; change upward to siltstone and fossiliferous limestone, to sandstone, and fossiliferous limestone in the upper part. The Khlong Min Formation in the Khlong Thom–Bang Khan area (Teerarungsigul et al., 1999) consists of four lithofacies(in ascending order: (I) mudstone intercalated with fossiliferous limestone, containing abundant fossils including eustheria, ostracods (Darwinulla sp.), sporopollen (Classopollis sp.) and hybodont shark remains, (II) siltstone and limestone, (III) calcareous sandstone, and (IV) limestone and calcareous sandstone. Thickness: 58-116 m.

[Figure 1: Non-marine facies of the Jurassic Khlong Min Formation on the shore at Laem Pleo, Krabi Province. Grey claystone with lignite streaks passes up into conglomeratic sandstone with fragments of bone and a large piece of partly ferruginous fossilized wood, which in turn is overlain by well-bedded siltstone and sandstone showing some soft-sediment deformation (photograph courtesy of M. F. Ridd). (Meesook & Saengsrichan, 2011)]

[Figure 2: Generalized lithological column of the Thung Yai Gr in southern Peninsular Thailand (modified after Teerarungsigul et al. 1999). The upper three formations are Cretaceous while the lowest, the Khlong Min Fm, is Jurassic. Arrows indicate current directions determined from cross-bedding. (Meesook, 2011)]

"Brackish- water bivalve assemblages, for example, Protocardia sp., Myrene sp., Actinostreon sp. and Praemytilus sp. are abundant. In some beds, lignitic shale and siltstone contain ostracods, gastropods, conchostracans and palynomorphs (Classopollis sp. and Dicheiopollis sp.) and wood fragments. In the upper part of this sequence the rocks grade up to non-marine lacustrine deposits with calcrete horizons containing non-marine vertebrates including hybodont sharks, Lepidotes-like actinopterigians, lung fishes, temnospondyl amphibians, mesosuchian crocodilians and cryptodiran turtles, an assemblage indicative of a Jurassic age." (Meesook & Saengsrichan, 2011)

Seems that Jurassic basins begin in ca. earliest Toarcian (comparison Fig. 7.3 of Peninsular basins in Jurassic chapter of Geology of Thailand); and gradual upward transition into Lam Thap Fm of earliest Cretaceous.

The fossil assemblages reflect a lagoonal environment with brief marine incursions and a gradual change to fluviatile and lacustrine conditions.

[Figure: Characteristics of the Jurassic Khlong Min Formation in the Khlong Thom–Bang Khan area, Krabi and Nakhon Si Thammarat Provinces, showing the environments of deposition interpreted by Teerarungsigul et al. (1999) (redrawn by Meesook & Saengsrichan, 2011).]