GEOLOGY OF GUNUNG JERAI, KEDAH
S.S. ALMASHOR
UNIVERSITI KEBANGSAAN MALAYSIA
1974
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ABSTRACT --
The present study aims at improving the current geological understanding of the Jerai formation and estab- lishing the nature of the rock which has been termed as
"quartz porphyry". The Jerai formation covers about three- quarters of the area of Gunung Jerai, Kedah. The other quarter of Gunung Jerai is occupied by a granite stock
which is characteristically transectea with pegmatite dykes.
The eastern 5, fzerEV7r!y of Gunung Jerai is underlain ’ by shale Formation. The shale is not described in this thesis; b%yxern likely that it conformably
overlies the Jerai formation.
The Jerai formation consists of mappable rock units without complex interbedding between them as otherwise thought before. The formation is m.ade up of a thick sequ- ence of phyllite overlain by another thick sequence of quartzite which is intercalated with a few tuff beds, Minor occurances of talc-silicate rocks are found in the phyllite and quartzite members. The phyllite member is not less than 1240 metres thick, while the quartzite
member (including the tuff beds) measures about 1050 metres thick. The tuff member consists of two dominant tuff beds
(75 and 95 metres thick each) and a felr thin ones,
The previously-called "quartz prophyry" turned out to be this tuff. Its identification T?as substantiated by
a detailed field examination and the study of its thin section and chemistry. The tuff is found to be chemically similar to the tuff in the Grik area located about 100 km to the southeast of Gunung Jerai. By lithostratigraphic correlation with the Grik area, the Jerai formation is found to be probably of Ordovician age.
On the basis of the plate tectonic theory, it is envjsaged that during: the Ea.sly Ordovician the Gunung Jerai area received pelitic sediments from the Dorthwest; and during the Late Ordovician, the arep was supplied continous- ly with sands an3 ucc~sivunlly with volcaniclastic materials, from the southeast.
Intrusion of the granite stock during the Jd3aFBsiw CretaC?!ous seems to have arched UD the Jerai formation
into ti du:ne-shape& mourltsiu, which was subsequently reshaped by differential weathering and erosion to the present
topography. The Granite stock caused an approximately 20-metre wide contect-metamorphic aureole. The rest of the Jerai formation had been rcgionall:~ metamorphosed into a hornblonde-hornfcls facies. A structural analysis
of the area shows that it had been subjected tc a stress directed in ?120W-S2OE. 1
,- .~~~ _
The southern portion of the are? contains econcmically- rich quantities of alluvial caasiterite irnd residual iron ore a The cassiterite is believed to have been released by weathering; from pegmatite dykes which transect the country rock, while the iron ore is the product of residual
concentration and nlteration of primary mapnetite veins found in the tuCf.
