GEOLOGY OF THE PENGKALAN HULU-BETONG TRANSECT ...

Geology of the Pengkalan Hulu-Betong Transect area
along the Malaysia-Thailand border
by
The Malaysia-Thailand Working Group
EXECUTIVE SUMMARY
Detailed systematic geological mapping in the Pengkalan Hulu-Betong Transect area
on the scales 1:100,000 and 1:250,000 (using the 1:50,000 base maps) was carried out by
the Malaysian Working Group (Appendix 1) during parts of November 2002, January,
February, March and May 2003. On the Thai side, detailed geological mapping on similar
scales was carried out independently by the Thailand Working Group (Appendix 2)
during February 2003 and re-checked during April 2003. Joint field checks at selected
localities along the Transect area by geoscientists from both Malaysia and Thailand were
conducted on the Malaysian side (25 th to 28 th April 2003) and on the Thai side (29 th April
to 1 st May 2003). Problems in discontinuity among time rock unit boundaries between the
Palaeozoic rocks to Quaternary deposits, and the extension of granitic rocks (including
the mineral association), have been satisfactorily resolved.
The oldest rock unit, the Silurian-Devonian succession of the Kroh formation/Betong
Formation (SDkr/bt) consists mainly of very thin- to thin-bedded carbonaceous shale (often
siliceous), mudstone, siltstone, sandstone with subordinate chert and argillaceous
limestone lenses in the middle to upper part of the rock sequence. These rocks are
commonly metamorphosed to hornfels, calc-silicate hornfels, metasandstone and marble.
The succession is well exposed along the road-cuts in the middle and southwestern parts
of the Transect area. The thickness of the Silurian-Devonian rocks from representative
section exeeds 100 m. Fossil assemblages such as Spirograptus, Tentaculites elegans,
Tentaculites sp. and Styliolina were found in the shale strata indicate that the age of the
rock sequence is Silurian-Devonian.
The undifferentiated Carboniferous clastic rocks of the Kubang Pasu/Yaha Formation
(Ckp/yh) are largely exposed in the northwestern, middle and southeastern parts of the
Transect area. Some roof pendants of this rock unit are confined to the middle part of the
Rimba Telui/Si Nakhon granite (Trgrrt/sn) belt. The rock sequence of the Kubang
Pasu/Yaha Formation (Ckp/yh) with total thickness of more than 800 m is characterized by
the presence of very thin- to medium-bedded sandstone, siltstone, shale and chert at the
lower part; and thick- to very thick-bedded sandstone, argillite, and minor ribbon chert
and limestone lenses at the upper part of the formation. In the central-eastern part of the
Transect area, the succession can be lithologically subdivided into two facies i.e. the
argillaceous and arenaceous facies. The former is well exposed and forms a low relief in
the middle part of the area, near the Chantharat granite (Thailand). It is usually affected
by local deformations and low grade metamorphism that had taken place in the shear and
contact zones, and progrades to thin- to medium-banded phyllite, phyllitic schist, mica
schist and quartz schist with minor quartzite and calc-silicate. The thickness of this facies
is inconclusive due to strong folding of the strata. The arenaceous facies is well exposed
along several road-cuts on high relief terrains. It is characterized by the presence of
medium- to very thick-bedded sandstone, quartzite and minor argillite with inconclusive
v