GEOLOGY OF THE PENGKALAN HULU-BETONG TRANSECT ...

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Microscopic study of a calc-silicate rocks sample collected from Sungai Bahoi
(05 o 25′ 24″ N, 101 o 01′ 14″ E) shows that the calc-silicate is epidote-diopsidevesuvianite
hornfels.
The calcareous facies
This unit is composed of lenticular bodies of limestone (often metamorphosed to
marble) and other calcareous rocks. The limestone is light to dark grey in colour, thick
bedded to massive, carrying a large quantity of non-carbonate impurities. It is frequently
platy and bedded owing to the post-depositional segregation of argillaceous impurities
from the carbonate. Coarse-grained crystalline marble occurs in the vicinity of the granite
margin especially in the Belukar Semang area where it is in contact with the calc-silicate
member. The calcareous facies generally shows a very restricted vertical-lateral
distribution in which the development of features of karst topography was insufficient.
However, minor karst topography can be seen in the Tanah Hitam, Belukar Semang and
Kampung Pong areas.
On the Thai side, the Silurian-Devonian Kroh formation/Betong Formation (SDkr/bt)
consists mainly of very thin- to thin-bedded shale, siltstone, lithic sandstone with
subordinate chert and argillaceous limestone lenses. Local metamorphism by Si Nakhon
(Trgrsn) granite resulted in the rocks becoming slate or phyllite. The lower part of the
formation displays good outcrops along the road-cut from the Malaysia-Thailand border
to Betong District and from Km 4 village to Bo Nam Ron village. The rocks consist of
interbedded layers of very thin- to thin-bedded, laminated, grey to dark grey mudstone
and silicified shale to very fine-grained sandstone.
The middle part is well exposed on the road-cuts along the border to Betong District
i.e. in Ku Nung Chanong and Ka Pae villages. Other localities are situated at the rock
aggregate quarry in Bo Nam Ron village, Km 4 to Km 8, and Km 19 villages along the
Yala-Betong road. The rocks consist of very thin- to thin-bedded, laminated, grey to dark
grey mudstone interbedded with silicified shale to very fine-grained sandstone and thinbedded,
laminated, friable, porous, and light pink shale with Tentaculites sp., trilobite,
and graptolite. The lenses of thin- to medium-bedded, argillaceous limestone and dark
grey shale (stylolite) are intercalated in the upper part of the sequence from 4 – 20 m in
thickness.
The upper part is represented by good outcrops in the road-cuts of the Suan Som-Ai
Yoe Boe Chang villages, near the border and the eastern part of Betong District. Other
localities are also exposed at the road-cuts in Ban Piyamit III village. The succession
consists of interbedded layers of very thin- to thin-bedded, laminated, dark grey to black
mudstone, silicified shale, very fine-grained sandstone and thin-bedded to ribbon, brown
to grey chert with rare radiolarians and locally, limestone lenses. The thin-bedded,
laminated, friable, porous, light pink shale with Tentaculites is partly intercalated in the
upper part of the sequence from 2 – 5 m in thickness.
The rock displays moderately developed S1 cleavage in the N-S direction with general
dip towards east, and kink bands are generally recognized. The total thickness of the
Silurian-Devonian rocks from the representative section exceeds 100 m.