GEOLOGY OF THE PENGKALAN HULU-BETONG TRANSECT ...

Small amounts of interbedded sedimentary rocks are also included in the tuff and
possible slumping of included shale and greywacke are also recorded. Tuffaceous rocks
containing lenticular bodies of dark grey impure limestone as observed in the Gerik area
is also considered as part of the Gerik formation (Pgk). Minor crystal tuff only exposed
around the Kuala Rui area in the southeastern part of the Transect area was also observed.
It is greenish in colour, medium- to coarse-grained with euhedral quartz and pinkish to
white prismatic K-feldspar phenocrysts. The hand specimen of this rock may be
mistakenly identified as altered granite under the naked eye, but in the thin section
(microscopic study), it does not show any granite characteristics. In general, the crystal
tuff is composed of grains and crystal fragments of quartz, K-feldspar, perthitic feldspar,
and plagioclase feldspar, up to 5 mm in size, set in a fine-grained or cryptocrystalline
matrix of quartz, mica, and chlorite.
Age and correlation
Jones (1970) suggested that the age of Grik tuff (now Gerik formation) might be Late
Ordovician. Burton (1986) interpreted the age of the Grik tuff as Middle Ordovician.
However, Middle Ordovician to Early Silurian age was thought to be more probable for
this pyroclastic rock. During recent field check, fossils of Anisopyge and Phillipsia
(trilobite) and Chonetid (brachipod) were found in the calcareous shale interbedded with
tuff and dark grey impure limestone at Kampung Batu 2, Gerik (05 0 27′ 36″ N,
101 0 08′ 00″ E). The presence of Anisopyge sp. and Phillipsia sp. (trilobite) strongly
indicates Middle Permian age of the clastic sediments (Mohd Shafeea Leman, Pers.
Comm.). Thus, this finding suggests the age of the Gerik formation (Pgk) is not older than
Carboniferous, which disqualifies the interpretation on the age of the Grik tuff as Middle
Ordovician-Early Silurian by Jones (1970) and Burton (1986). Furthermore, their
interpretations were solely based on lithological correlation (without any fossil evidence).
Fragments of brachiopod spines were also observed in the thin section of the limestone
from the abandoned Bukit Jagor quarry (05 0 25′ 31″ N, 101 0 09′ 02″ E).
1mm
I
A
II
1cm
Figure 28: Photographs of fossils found in the Gerik formation (Pgk), Malaysia.
A- Trilobite belongs to Anisopyge sp. (I) and Phillipsia sp. (II) along
the road-cut at Kampung Batu 2, Gerik.
B- Brachiopod found near the border road.
B
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