GEOLOGY OF THE PENGKALAN HULU-BETONG TRANSECT ...

A B
Figure 33: Exposures of the Nenering beds/Ai Yoe Boe Chang Gravel beds (Qnn/ay) at
Ban Ai Boe Chang, Thailand.
A- Side view of the gravel bed along the security road.
B- Close-up view of the gravel bed along the security road.
The gravel beds in the northern edge of the Betong basin are spectacularly exposed as
road-cut outcrops in Ko Mo 7 village at more than 25 m thick (Figure 34). The
succession, unconformably overlying the Carboniferous Kubang Pasu/Yaha Formation
(Ckp/yh2), is mainly composed of three to four layers of gravel beds (1 – 5 m in thickness)
interbedded with coarse-grained sand beds (2 – 3 m in thickness). The clast of each
individual gravel beds varied in size from gravel to boulder (average 5 – 10 cm in
diameter). The clasts are subrounded to rounded shape. Large boulders of limestone,
sandstone, quartzite, reddish brown sandstone and conglomerate also occur.
In the northwestern edge of the Betong basin, the gravel beds, as thick as 30 m, are
well exposed at a quarry 2 – 3 km north of the Betong town, near the Betong-Yala road.
The succession, unconformably overlying the Silurian-Devonian Kroh formation/Betong
Formation (SDkr/bt), is mainly composed of thick-bedded (3 – 5 m thick) gravel beds
interbedded with medium- to thick-bedded, and medium- to coarse-grained sand beds.
The clasts of individual gravel bed consist of various sizes (2 – 10 cm in diameter,
averagely 3 – 5 cm) of quartzitic and arkosic sandstone, chert, phyllite and schist. A
southward paleocurrent is recognised by a southwardly decrease in clast size of the gravel
beds.
In the eastern edge of the Betong basin, i.e. Ya Rom, Ro Mong and Ba Yo Ta Pae
villages, the gravel beds are exposed at a quarry and along road-cuts. The succession is
mainly composed of medium- to thick-bedded (1 – 2 m thick) gravel beds interbedded
with thick-bedded medium- to coarse-grained sand layers. Clasts of the gravel beds are
made up of gravel to pebble sizes (0.5 – 2 cm in diameter) of quartzitic sandstone and
chert. A westwardly decrease in clast size of the gravel beds can also be observed.
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