New species of algae Actinastrum
lagerheim and Closterium nitzsch ex Ralfs from Middle Miocene Sediments of Chiang Muan basin, Phayao, Thailand, with tropical pollen composition

Wickanet Songtham,^a,b,* Benjavun Ratanasthien^a and Dallas C Mildenhall^c

ABSTRACT: Two forms of algae were described including Actinastrum bansaense n. sp. and Closterium thailandicum n. sp. They occurred abundantly in some sedimentary horizons of Middle Miocene age from Chiang Muan basin. Abundant occurrence of the two algal species with some rare forms of algae Botryococcus and Striatriletes susannae aquatic fern spores reveals that the Chiang Muan basin formation occurred in a freshwater depositional environment. Abundant spores of Crassoretitriletes vanraadshoovenii (Lygodium microphyllum-type) were also recovered with common Dipterocarpaceae, Lagerstroemia, Ilexpollenites, Myrtaceidites, and Combretaceae with rare forms of Florschuetzia, Homonoia, Calophyllum, Striatriletes susannae, Botryococcus and Mimosaceae. There are abundant Laevigatosporites haardtii fern spores in some horizons with various forms of as yet unidentified tricolporate and tricolpate pollen. Three acme palynological zones were established and used for describing variation in paleoenvironment through time of sedimentation. The sporomorphs representing tropical palynofloras derived from tropical monsoon forests, accumulated mainly in lacustrine depositional environments. Origins and distributions of the families Dipterocarpaceae and Myrtaceae are discussed and criticised and new criteria elucidating their paleophytogeographic histories in relation to northern Thailand are proposed.

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a Department of Geological Sciences, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand.
B Bureau of Geological Survey, Department of Mineral Resources, Rama VI Road, Ratchathevee, Bangkok 10400, Thailand.
C Institute of Geological and Nuclear Sciences, P.O. Box 30-368, Lower Hutt, New Zealand.

* Corresponding author, E-mail: wickanet@dmr.go.th

Received 23 May 2003, Accepted 24 Dec 2003