GEOLOGY OF THE PENGKALAN HULU-BETONG TRANSECT ...

metaconglomerate occurs in the upper reaches of the Sungai Weng valley, at the central
western part of the Transect area on the Malaysian side.
In Thailand, the Kubang Pasu/Yaha Formation (Ckp/yh) is well distributed in the northwestern,
eastern and south-eastern parts of the Transect area, including roof pendants
lying on the Si Nakhon and Chantharat granites in the central part of the Transect area. In
the middle part of the Transect area, the formation can be subdivided into two facies: the
argillaceous facies (Cyh1) of the Kubang Pasu/Yaha Formation (Ckp/yh) or the lower part,
and the arenaceous facies (Cyh2) of the Kubang Pasu/Yaha Formation (Ckp/yh) or the upper
part of the complete sequence.
Lithology
In Malaysia, the Kubang Pasu/Yaha Formation (Ckp/yh) comprises essentially thick
interbeds of sandstones, argillites and minor siliceous rock. The thickness of individual
bed ranges from 20 cm to 1 m, forming a complete rock sequence of 300 m to 350 m
thick. This rock sequence consists of (from bottom to top) thick-bedded sandstones,
greywacke-subgreywacke intercalated with thick- to very thick-bedded grey argillite
followed by thick-bedded to massive, well-sorted, graded and cross bedded quartz arenite
overlain by thin beds of chert or siliceous shale. Medium-grained subgreywacke
(generally grey to white in colour) is the most common arenaceous rock with lesser
quartzite (quartz arenite), feldspathic sandstone (arkose) and grit. The siliceous rock
consists of thin bedded chert and siliceous shale. The chert (ribbon chert) exposures are
limited within the vicinity of Sungai Teliang valley, in the north-western part of the
Transect area and in some parts of the Malaysia-Thailand border security road. It contains
minute and rounded structures made up of microcrystalline quartz and chalcedony that
might represent trace of replaced radiolarian fossils. Primary sedimentary structures such
as graded bedding, cross lamination, fucoidal marking and slump structures are common
(Figure 12).
The arenaceous facies of the Kubang Pasu Formation is made up of moderate- to thickbedded
sandstone, quartzite and minor argillite. In the Gunung Kenderong and Kerunai
areas (in the central south of the Transect area), the sandstone succession is
metamorphosed and composed predominantly of medium-grained, pale-coloured
protoquartzite containing 5% – 15% mica with minor tourmaline, feldspar and
carbonaceous material. Orthoquartzite and subgreywacke are also present. The grains are
poorly sorted and more often grades into pebbly metasandstone and metaconglomerate or
into metasiltstone. The rock unit is commonly thick-bedded to massive, with occasionally
observed large-scale cross bedding. Thin interbeds of phyllitic shale occasionally occur
within the metasandstone. In the Belum area (in the southeast of the Transect area), the
sandstone comprises protoquartzite, orthoquartzite, metagreywacke and some
conglomerates.
The argillite consists of shale, mudstone and a variety of poorly sorted argilloarenaceous
sequence varying from muddy silstone through wacke and subgreywacke, of
dark grey to predominantly light grey, buff and pink to brick red. Shale is generally grey
to dark grey in colour. The shale beds are commonly 0.3 m to 0.6 m thick, ocassionaly
laminated with lamination parallel to the bedding plane. Siltstone occurs commonly as
interbeds and intercalations (generally less than a few centimetres thick) within the shale
sequences.
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