Page 80 - Sumatra Geology, Resources and Tectonic Evolution

PRE-TERTIARY VOLCANIC ROCKS 67

Table 6.3. Palaeozoic volcanic units in the East Sumatra Block
Formation Unit with volcanies Age Description Reference
Probably Asselian-Artinskian Tufts beneath Tertiary sediments Eubank & Makki ( 1981)
Permian Stages
Mentulu Condong Member 200-250 m of metatuff, tuffaceous claystone Simandjuntak et al. (1991) &
and grey to brown, hard and porphyritic Suwarna et al. ( 1991)
andesitic to basaltic tuff
Bohorok ?Asselian-Artinskian Crystal tufts and other tuffaceous rocks Bennett et al. (1981c)
Permian Stages
Pebbly Mudstone Facies Rhyolite clasts (unknown age) Cameron et al. (1980)

Permian age. In their reconstruction of the geology of the Pre- Block, and form a discontinuous belt, much disrupted by
Tertiary basement Eubank & Makki (1981) show an area of strike-slip movements along the Sumatra Fault Zone, parallel to
tufts encountered in boreholes to the NW of Pakanburu that may the west coast of Sumatra (Fig. 6.1). In Table 6.4 these volcanic
be related to a volcanic centre, and are similar to those of the rocks are described from north to south, and their relationships
Condong Member. to the local stratigraphy are illustrated in Figure 6.4.
In the Langsa Quadrangle Bennett et al. (1981 c) describe 'some Katili (1969, 1973) described these plutonic-volcanic rocks
crystal tufts and other tuffaceous rocks' belonging to an unnamed as a continental margin arc, on the basis of lithology, but the
volcanic unit within the Bohorok Formation. Cameron et al. non-genetic term 'belt' is used here. Two extensive but poorly
(1980) recorded rhyolite clasts within the Pebbly Mudstone exposed formations are distinguished to the south of the equator.
Facies of the Bohorok Formation in Northern Sumatra, indicating The Silungkang Formation, named by Klomp6 et al. (1961),
the presence of rhyolitic volcanics in the source region from which lies to the SE of Lake Singkarak, consists of Volcanic
which the pebbles were derived. These rhyolites could be of any and Calcareous Members. The petrology was described by Katili
age prior to the Permian. (1969) and the geological setting by Silitonga & Kastowo
(1975). The other unit, the Palepat Formation (Rosidi et al.
1976), was previously known as the Air Kuning Beds (Zwierzijcki
1935). The main outcrop lies to the SW of Muarabungo. Earlier
West Sumatra Permian Plutonic-Volcanic Belt this formation was mapped by Tobler (1922) as the 'Oudere
(Early-Mid-Permian volcanism) diabaasformatie' (Palaeodyas or Lower Permian age), overlain
by the 'Porfierformatie' (Neodyas or Upper Permian age).
Lower-Middle Permian volcanics and sediments and several Tobler (1917, 1922) shows outcrops of the Porfierformatie west
associated granitic plutons crop out within the West Sumatra of the main Palepat Formation outcrop in the vicinity of the

SIBOLGA SILUNGKANG FORMATION PALEPAT FORMATION KUANTAN FORMATION
System Stage Ua Aspden et al. (1982b) Katili (1969); Fontaine & Gafoer (1989) Fontaine & Gafoer (1989) 1 .Silitonga & Kastowo (1975)
Silitonga & Kastowo (1975) 2. As used in present account
i UJ Chansingian
IWuchi piogian
~. Capitanian
. ~ 3_,j.,,..L~ Guguk Bulat 2.
W Wordian
Shale Bukit Pendopo -%~,4E~-.~--. Pawan & Tanjungpuah
Formations
~' v V[
s
Phyllite & shale
uji~ Roadian Basalt Tabir Formation ~'~ ............... Member
n 256.1 Silungkang Ngaol Formation {:::: : ]
Kungurian
l--- 259.7 Sibolga Granite ~ Limestone lvv vv v vvl
Artinskian Volcanic Member iv v v v v v v I Palepat Volcanics
1268.8 264+_6
,~ Sakmarian
U.l i281.5 Shale ii!iiiii~ii:iiii 1 Mengkarang Formation
Asselian
!290
Gzhelian
Kusimovian
o Muscovian
Bashkirian
8 Serpukovian
~ ~ J ~ ~JLimestoneMember
Visean
< Lower Member
O Tournaisian

Fig. 6.4. Stratigraphy of units within the West Sumatra Permian Plutonic-Volcanic Belt.