ScienceAsia 31 (2005): 183-191 |doi: 10.2306/scienceasia1513-1874.2005.31.183

Middle Miocene Molluscan Assemblages in Mae Moh Basin, Lampang Province, Northern Thailand

Wickanet Songtham^a, Hiroaki Ugai^b, Suvapak Imsamut^a, Somkiat Maranate^b, Wattana Tansathien^a, Assanee Meesook^a and Wirote Saengsrichan^a

ABSTRACT: Molluscan beds in the Mae Moh basin have been observed and studied. Each individual molluscan bed has its own unique assemblage and occurs in a stratigraphic succession which allows for close stratigraphic correlations to be made within the Middle Miocene Mae Moh Group. Each molluscan species occurs in a particular facies showing close relationship between each taxon and its habitat. ?Paludina occurs in claystone and ligneous claystone indicating that its habitat was a lake with no vegetation or only sparse vegetation growing around it. Specimens of the Family Planorbidae must have preferred living in swamp containing dense vegetation since they occur in coal beds and ligneous claystone. Melanoides sp. cf. M. tuberculata occurs in claystone, suggesting that their habitats were in lake with little or no vegetation growing in it, similar to the living Melanoides tuberculata, which is a burrower into the lake sediments. The specimens of the Genus Bellamya have probably lived in the same conditions as Melanoides sp. cf. M. tuberculata did but not burrow. The twelve metre-thick Bellamya Bed was formed by a dynamic sedimentary process rather than by snail dying and being deposited in situ. The snails were regarded as being somehow transported for a short distance. This thick shell bed has been probably interpreted as the result of water level changes in the Mae Moh Lake under a desiccation regime. Viviparous snails such as Margarya occurred in a restricted local swamp of a fluvial system at a time when the Huai Luang Formation was deposited. Faunal changes in the molluscan assemblages have been occurred in direct response to the changes of their depositional environment.

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a Bureau of Geological Survey, Department of Mineral Resources, Bangkok, Thailand
b Goshoura Cretaceous Museum, Goshoura-cho, Kumamoto Prefecture, Japan

Received 9 Jul 2004, Accepted 14 Feb 2005