GEOLOGY OF THE PENGKALAN HULU-BETONG TRANSECT ...

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a) The Sri Paen formation (Psp)
Distribution
The Sri Paen formation (Psp) is only exposed in Thailand. It is well distributed along a
narrow NNE-SSW direction, high relief terrain in the central and northern parts of the
area, and bounded by porphyritic biotite granite to the west and Carboniferous rocks to
the east and south (see geology map). In the central part, the sequence is continuously
exposed along the Betong-Yala road-cut outcrops from Ta Phayao-Ban To-Khok Chang
village and extends northwardly to the eastern part of Than To District. The other
succession is also exposed along road-cuts from Bo Hin-Ba Ta village and Cho Si Po
Dam site along the high relief terrain in the northern part i.e. Kala Po-Bu Su village. The
good reference sections of the unit are exposed along road-cut outcrops from the Ta
Phayao-Ban To village, Than To-Sri Paen village and Na Kaset village located in the
central part of the area.
Lithology
The succession of the Sri Paen formation (Psp) predominantly consists of thin- to
medium-bedded, argillaceous limestone, shale, siltstone, and chert with minor sandstone
in the upper to middle part. The rocks in some successions are locally deformed and
metamorphosed to be phyllitic shale, schist and calc-silicate.
The lower part of the formation is shown by the good road-cut outcrops along the road
from Ta Payao to Ban To village, and from Kala Po to Si Paen village. The rocks, 250 –
280 m thick, display obvious thickening upward sequence (Figure 18) as follows:
i) The lower portion is represented by the evenly thin- to very thin-bedded,
brownish grey to brown shale, mudstone and siltstone (80%) intercalated with
the stringers of very fine-grained sandstone and chert strata (Figure 19). Some
fragments of bivalves are revealed in shale strata. The sequence gradually
thickens upwardly interbedded with thin- to medium-bedded, grey to dark grey
phyllitic shale, laminated, dark grey argillaceous limestone, and sandy limestone
with the stringering of sandstone.
ii) The upper portion (Figure 20) consists of moderate- to thick-bedded, pale brown,
well sorted, medium- to coarse-grained, calcareous quartzitic and lithic
sandstones interbedded with thin- to thick-bedded, grey to dark grey and
yellowish brown shale and siltstone with greenish grey to dark grey argillaceous
and sandy limestone lenses (30 – 60 cm in general). The limestone lenses and
siltstone show the thickening upward sequence and contain some brachiopod
fragments.