GEOLOGY OF THE PENGKALAN HULU-BETONG TRANSECT ...

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The regional structures are aligned in the N-S direction with local variations to NW-SE
and NE-SW. Apart from some local anticlines and synclines indicated on the geological
map, minor structures such as small scale tight folding and parallel folding are also
common. There is evidence indicating that the Lower Palaeozoic rocks have undergone
more than one series of deformation that had led to the development of rather complex
structures. The strata of the Kroh formation (SDkr) are tightly folded and are characterized
by steeply dipping limbs whereas strata of the Kubang Pasu Formation show broadly
folded strata. In general, the force that associated with the formation of fold in the study
area is from the E-W direction.
Faulting is widespread and present in all rock units. Normal faults are very common
but strike-slip and reverse faults also exist. All major faults within the Transect area are
N-S and NE-SW trending i.e. the Lebey fault.
2.2.2 Geologic setting of the Thai side
The oldest rocks, the Silurian-Devonian Betong Formation (SDbt), consist mainly of
very thin- to thin-bedded shale, siltstone, lithic sandstone with subordinate chert and
argillaceous limestone lenses in the middle to upper part. Thickness of the succession is
approximately 100 m. The succession is distributed in N-S trending low relief terrain
from Betong District to Piyamit III village in the southern part of the Thai side, and is
well exposed along the road-cuts from the Malaysia-Thailand border to Betong District.
Local metamorphism by Si Nakhon granite (Trgrsn) and Chantharat granite (Trgrch) to the
west resulted in the rocks being metamorphosed to hornfels or slate. Many fossil
assemblages, i.e. Ten taculites elegans, Tentaculites sp. and graptolite that occur in the
shale strata and some radiolarian assemblage discovered in the chert beds have been
determined; indicating the age of the rock sequence as Silurian-Devonian.
The Carboniferous clastic rocks of the Yaha Formation (Cyh1-2) conformably overlying
the Silurian-Devonian Betong Formation (SDbt), are largely exposed in the northwestern
and southeastern parts of the Transect area. Some of them occur as roof pendants in the
middle part of the area between two granitic belts. It is characterized by the presence of
very thin- to medium-bedded sandstone, siltstone, shale and minor chert in the lower part
and thick-bedded sandstone, argillite, and minor ribbon chert and limestone lenses in the
upper part. Total thickness of the Yaha Formation (Cyh1-2) exceeds 800 m. Fresh outcrops
are well exposed along the road-cuts near the Malaysia-Thailand border. In the central
part, the succession can be lithologically subdivided into two facies, the argillaceous
facies (Cyh1) and the arenaceous facies (Cyh2).
The argillaceous facies (Cyh1) is well exposed in areas of low relief in the middle-
eastern part of the Transect area, near the eastern granite pluton. It is usually affected by
the local deformation and low-grade metamorphism, which had taken place in the shear
and contact zones. The parent rocks were metamorphosed to thin- to medium-banded
phyllite, phyllitic schist, mica schist and quartz schist with minor quartzite and calcsilicate
schist. The thickness of this unit is inconclusive due to the strong folding of their
strata.
The arenaceous facies of the Yaha Formation (Cyh2) is well exposed in high relief
terrains. It is characterized by the presence of medium-bedded to thick-bedded sandstone,
quartzite and minor argillite with inconclusive thickness. Based on the presence of
radiolarians and conodonts in the siliceous rocks, the formation is considered to be
Carboniferous in age.