GEOLOGY OF THE PENGKALAN HULU-BETONG TRANSECT ...

The Permian Ratburi Group (Prb), underlain by the Carboniferous rocks, is well
distributed in the northeastern part of the Transect area. Some patches of the unit are
locally distributed in small hills in the western part of the Betong town. The succession
can be lithologically subdivided into two formations: the lower clastic, Sri Paen formation
(Psp) and the upper carbonate, Tham Krachaeng formation (Ptk).
The Sri Paen formation (Psp) is 500 m thick comprising predominantly thin- to
medium-bedded argillaceous limestone, shale, siltstone and chert with minor sandstone in
the upper to middle part. Some successions are locally deformed and metamorphosed to
phyllitic shale, schist and calc-silicate.
The Tham Krachaeng formation (Ptk) is mostly recrystallized and consists of thickbedded
to massive recrystallized limestone to marble with chert nodule patches. The
thickness of the formation is inconclusive.
Based on the occurrences of fossil assemblages, i.e. bivalves and brachiopods in
siltstone strata, radiolarian assemblage in siliceous beds and brachiopod fragments, and
fusulinids in limestone layers, the age of the Tham Krachaeng formation (Ptk) is assigned
as Middle Permian and the Sri Paen formation (Psp) rock is Early Permian.
Unconsolidated sediments were deposited in non-marine environments and can be
subdivided into two formations in ascending order: the Ai Yoi Boe Chang gravel beds
(Qtay) and Recent fluvial deposit (Qa). The Pleistocene gravel beds of Ai Yoe Boe Chang
sediments are well exposed as small hills around Betong District. The other localities are
situated southeastwardly in the low land area near the border. They are composed of
gravel, sand, silt, and laterite with abundant iron concretions. The size of the pebbles in
the gravel beds is decreasing westwards, towards the Betong town. The thickness of this
rock unit is interpreted to be more than 20 m. The Holocene fluvial deposit is well
deposited as channel fills in the river i.e. Khlong Ka Pae and Pattani rivers. It is mainly
made up of sand, silt and clay.
Triassic granitic rocks intruded the Palaeozoic succession in the Transect area. The Si
Nakhon granite (Trgrsn) is located near the Malaysia-Thailand border. It is characterised
by medium- to coarse-grained, porphyritic biotite granite with inequigranular
(heterogranular) to good megacrystic texture and K-feldspar megacrysts in various shapes
and sizes. It also includes pegmatite, minor phases of aplite or leucocratic granite, barite
and base metal sulphide minerals. Pa Ret Tu granite (Trgrpr) is exposed in the central part
on the Thai side of the Transect area. It is mainly composed of fine- to medium-grained
(with local two-phase variants texture), equigranular to sparsely porphyritic, tourmalinebiotite-muscovite
granite. Biotite is the most abundant mafic mineral. Tin mineralization
is commonly associated with this granitoid. In the eastern part near the Hala Forest
Reserve, Chantharat granite (Trgrch) is mainly composed of medium- to coarse-grained,
porphyritic biotite granite including aplite, pegmatite and minor phases of chlorite and
tourmaline biotite granite.
N-S and NE-SW trending quartz dykes usually intruded the country rocks, especially
the Carboniferous rocks in the southern and the middle parts of the area. Contact
metamorphism and associated late stage mineralizations were observed in the country
rocks.
Structurally, strata of the Palaeozoic succession are strongly inclined close folds
(isoclinal folds), deformed into repeated sequence, especially near the igneous pluton.
The normal and strike-slip faults are conspicuous with three main N-S, NW-SE and NE-
SW strike directions especially in the western part of the Bang Lang Dam site.
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