Page 95 - Sumatra Geology, Resources and Tectonic Evolution

82 CHAPTER 6

Origins of the volcanic units and their environments Rhyolite clasts in the Late Carboniferous?-Early Permian
of formation Pebbly Mudstone facies of the Bohorok Formation in Sibumasu
(East Sumatra Block) could be of any age, and plausibly were
Palaeozoic volcanism in Sumatra and the break-up of eroded from the same land area from which granite clasts in the
mudstone also originated. A trondhjemite clast from the compar-
Gondwanaland
able Singa Formation on Langkawi Island, west of Peninsula
Malaysia, has been dated at 1029 Ma (Hutchison 1989) suggesting
The andesite and basalt flows in the Lower Member of the a Proterozoic provenance. Volcanic rocks of the Condong Member
Kuantan Formation in the West Sumatra Block occur among
distal turbidites and debris flows indicative of deposition in a of the upper Mentulu Formation (Bohorok Formation equivalent)
and the Setiti plutons of the East Sumatra Plutonic-Volcanic Belt
deep-water environment, possibly in a forearc setting (Turner
1983). If these volcanics are contemporaneous with the sediments, (c. 298-276 Ma) have a similar Permian Asselian-Sakmarian
they are Vis6an (Lower Carboniferous) in age. Volcanic rocks of age, coinciding with the volcanic episode related to the break-
this age are unusual in SE Asia and Australia (Veevers & Tewari up of the Sibumasu/Gondwana margin. The East Sumatra
Plutonic-Volcanic Belt is of regional extent, being represented
1995). The Kuantan Volcanism may be related to seafloor spread-
ing in Palaeo-Tethys and be a precursor of the break-up volcanism by volcanics in the Bohorok Formation of North Sumatra
(Bennett et al. 1982c), and again by volcanic tufts which are
along the margin of the Gondwana Supercontinent. Volcanics widely distributed in the Mergui Series (comparable to the
from the Gondwana Break-up Sequence are known from the
Bohorok Formation) around Mergui and Tavoy (Chhibber 1934;
dating of drill samples from the West Australian margin Pascoe 1959) and in islands offshore Peninsular Myamar. The
(Veevers & Tewari 1995) and crop out in Timor where they are
stratigraphically well constrained (Charlton et al. 2002). These East Sumatra Plutonic-Volcanic Belt is related in time to the frag-
mentation of Sibumasu from Gondwana, but a great deal more
dated West Australian volcanics form a reference sequence for
comparison with the Sumatran Permian volcanics (Fig. 6.15). chemical and chronological data is required to amplify this
suggestion.

WESTERN SIBUMASU
Ma ~ STAGE AUSTRALIA SEA EAST SUMATRA BLOCK WEST SUMATRA BLOCK Gondwana Margin Events
boreholes ILEVEL TIMOR
_(2 II
-240~ ANISIAN ~v.ll ........ L~'el ris.._.e
~_ SCYTHIAN ~ Gondwana
' retreats to
GHANGHSINGIAN V g Volcanicity accompanies south
-250 WUCHAIPINGIAN V Sea-floor spreading in II
CApITANIAN V V Meso-Tethys (Phase 2)
WORDIAN I Calcareous
ROADIAN V V V V (-9 z Member
-260 z KUNGURIAN ! <=o_ (Tabir I Separation of
KLUET VvV - Formation) Sibumasu and Opening of
_ ARTINSKIAN VVVVV FORMATION ~u.. Volcanic Baoshan Blocks Meso-Tethys
-~O V V V , Member (Phase 1)
-270 rr con on V V (Palepat
iii Member ! Formation) tk
a. SAKMARIAN /,,,,,'
BOHOROK VV Mengkarang
1280 I & MENTULU Formation Rift faulting and
i
PENGABUHA FORMATIONS volcanicity Gondwana
ABSELIAN " Ice ', -- FORMATION 'Pebbly Glaciation of Sibumasu advances
-290-- i VolJne i Mudstones' and West Australian to north
i
Gondwana margin
GZELIAN "- '"
GANGSAL
FORMATION
-300 KASIMOVIAN i - i
MUSCOVIAN
-310 i i
ii
09 BASHKIRIAN
-320 D
O
cc , ,, 'Namurian' uplift of Gondwana
LU SERPUKOVIAN margin
I J_
-330
O
[]
I Limestone
-340 n- VISEAN Member
<
O
Lower
Kluet volcanism ?related to
-350 V V V Member sea-floor spreading in Palaeo-Tethys
......
TOURNAISIAN
-360
Opening of Palaeo-Tethys
Fig. 6.15. The Permian sequence in Timor after Charlton et al. (2002) showing volcanic horizons related to the break-up of the Gondwana margin and seafloor spreading
in the Meso-Tethys Ocean. Sibumasu is understood to have broken from Gondwana at the close of the Sakmarian (Metcalfe 1996) and the West Sumatra Block in the
Triassic.