GEOLOGY OF THE PENGKALAN HULU-BETONG TRANSECT ...

A
Figure 16: Photographs showing the arenaceous facies of the Kubang Pasu Formation/Yaha
Formation (Ckp/yh) in Thailand.
A- Medium- to thick-bedded sandstone at Ban Phayao near Bang Lang Dam.
B- Quartzite at La Ong Rung Waterfall.
b) Argillaceous facies (Cyh2) of the Kubang Pasu/Yaha Formation (Ckp/yh)
Distribution
On the Thai side, the argillaceous facies of the Kubang Pasu/Yaha Formation (Ckp/yh)
is well exposed in the low relief terrain, near the eastern granite pluton in the middle part
of the Transect area. The sequence is exposed continuously at the NW-SE to NE-SW (as
a curvilinear) narrow belt in the Chantharat area. The succession is also exposed along the
road-cuts from Km 29 to Km 41 villages on the Yala-Betong road and extends to the
south of Bang Lang Dam.
Lithology
The argillaceous rocks in the lower part of the (Cyh2) of the Kubang Pasu/Yaha
Formation (Ckp/yh) are affected by local deformation and low-grade metamorphism (or
superimposed) in the shear and contact zones. The rocks are metamorphosed to thin- to
medium-banded phyllite, phyllitic schist, mica schist and quartz schist with minor
quartzite and calc-silicate (Figure 17). The thickness of this unit is unknown due to strong
folding.
In the Chantharat area (Thailand), the rocks are strongly folded and sheared, consisting
of thin-banded, dark grey to blueish grey quartz schist alternating with very thin-banded,
greenish grey phyllitic schist, and thin-banded, bluelish grey phyllite. Mica patches on
surfaces of these rocks are well developed and various directions of S1-S2 cleavages and
kink bands are very common. Some secondary pyrite and quartz gash fractures are found
in the rock sequence.
In the Sutan Tanun terrain, the succession consists of thin layers of blueish grey
phyllite and greenish grey schist. The rocks display strong S1 cleavage developed in the
N-S direction, generally dipping west. Kink bands and gash fractures are generally
recognized. Some medium banded quartzite and calc-silicate marble lenses are
occasionally found.
At Km 29 to Km 42 villages along the Betong-Yala road, the succession was affected
by a NE-SW shear zone and is pervasively metamorphosed by the granitic rocks. It is
B
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