Page 47 - Sumatra Geology, Resources and Tectonic Evolution

34 CHAPTER 4

the Early Permian Bryozoan Bed of Phuket. In Thailand the submarine mass wasting on a continental slope (e.g. Mitchell
Phuket Group is overlain by the Mid-Late Permian Ratburi et al. 1970).
Limestone, which Cameron et al. (1980) correlated with the In Peninsular Thailand, NW Malaysia and Baoshan in SW
Alas Formation of Sumatra. Now that the age of the Alas China (Wang et al. 2001) pebbly mudstones are interbedded
Formation is firmly established as Early Carboniferous, the latter with sediments containing Early Permian fossils. In Australia
correlation is no longer valid. the occurrence of glacial deposits indicates that glaciation com-
The present situation is, that although it is possible that Tapanuli menced in the Namurian, reached its peak in the Stephanian and
Group and its correlatives, the Kuantan Formation and Tigapuluh Sakmarian and had ceased by the Artinskian (Quilty 1984). it is
Group of Central Sumatra extend down into the Devonian, the therefore possible that the Bohorok Formation with the diamictites
only age diagnostic fossils so far identified in Sumatra are of ranges in age from the Late Carboniferous to the Early Permian.
Lower Carboniferous, Vis6an age. No Toumaisian or Upper
Carboniferous rocks have so far been recognized. The only rock Palaeogeography. Cameron et al. (1980) suggest that the Tapanuli
unit which could possibly be of Late Carboniferous age is the Group represents a continental margin sequence deposited on a
Pangururan Bryozoan Bed from Lake Toba (Fig. 4.4). As already rifted passive margin. The reduction in clast sizes in the mudstones
reported above, fossils collected from this locality have been ident- and conglomerates of the Bohorok and Kluet formations, with a
ified as of Late Carboniferous to Early Permian age, with the decrease in the frequency and grain size of sandstone units in a
balance of opinion in favour of the later age (Aldiss et al. 1983). southwesterly direction, suggest that in Carboniferous times an
This age determination confirms the correlation with the Early open ocean lay in this direction. In this model turbiditic sandstones
Permian Bryozoan Bed of Phuket proposed by Cameron et al. and shales were deposited in rift basins, while limestones of the
(1980). The Pangururan Bryozoan Bed is interbedded with, and is Alas and Kuantan formations formed carbonate banks on horst
deformed, to the same extent as the associated sandstones and blocks of uplifted basement, perhaps represented by the high
slates of the Kluet (Bohorok?) Formation, which must also therefore grade metamorphic rocks associated with the Alas Formation in
be partly of Early Permian age. No unconformities have so far been the field.
recognized within the Tapanuli Group so that it is probable that the Following Cameron et al. 1980, Fontaine & Gafoer (1989)
group also includes rocks of Upper Carboniferous age. interpreted the Carboniferous rocks in the northern part of
As has been reported above interbedded quartzites and shales Sumatra as a series of contemporaneous sedimentary facies
were encountered beneath Tertiary sediments in boreholes to formed on a continental margin (Fig. 4.9). They suggest that the
the NE of Pekanbaru, in the Malacca Strait and in the Persing Kubang Pasu and Kenny Hill formations in the western part of
Complex of Singkep Island. These occurrences were used by the Malay Peninsula, and quartzites and quartz sandstones
Eubank & Makki (1981) to define a 'Quartzite Terrain' encountered in oil company boreholes along the Malacca Straits
(Fig. 4.2). Palynomorphs from the shales indicated an age near represent littoral and shelf facies sands in the east. The pebbly
the Devonian-Carboniferous boundary. Similar rock units
composed of quartz-rich sandstones with shales and mudstones
described as the Kubang Pasu and Kenny Hill formations occur
on the eastern side of the Malacca Strait (Fig. 4.2). The Kubang
Pasu Formation outcrops in eastern Perlis and NW Kedah
where it is dated by Devonian trilobite pygidia at the base and
Carboniferous goniatites and brachiopods higher in the sequence,
and passes upwards conformably into the Lower Permian Chuping
Limestone Formation. The Kenny Hill Formation which outcrops
near Kuala Lumpur contains only trace fossils and poorly
preserved body fossils which do not provide a reliable indication
of age. However, it is considered to be of Carboniferous age
because it is younger than the adjacent Silurian Kuala Lumpur
Limestone Formation, but is cut by Mesozoic granites and ore
bodies (Stauffer, in Gobbett & Hutchison 1973). These quartz-
rich units appear to have been derived from the east and are
considered to be stratigraphically equivalent to the Bohorok,
Kluet and Alas formations.
Pebbly mudstones. As noted above, pebbly mudstones similar to
those of the Bohorok Formation occur in the Langkawi Islands
and in Perlis in Peninsular Malaysia and at Phuket in Peninsular
Thailand. Similar deposits occur in the Mergui Series of the
Shah States of Myanmar and in the Salt Ranges of Pakistan.
Wherever they occur, there has been much discussion concerning
the origin of these pebbly mudstones.
Stauffer & Lee (1986), as part of their studies of the Singa
Formation in the Langkawi islands, described 'dropstone' struc-
tures beneath clasts in laminated mudstones, which they attribute
to the deposition of pebbles and boulders carried by floating ice.
They conclude that the pebbly mudstones were deposited in a
0 250 500km
glacio-marine environment. Similar detailed sedimentological I I I
studies of the pebbly mudstones and their associated deposits
are required in Sumatra. Following the studies of Stauffer & Lee
(1986) a glacial origin for pebbly mudstones throughout the
region has generally been accepted, although dissenting opinion Fig. 4.9. Carboniferous palaeogeography of Sumatra and the Malay Peninsula
has interpreted the pebbly mudstones, fi-om their association (from Fontaine & Gafoer 1989). The description of the facies and the
with turbidite deposits, as the product of debris flows, due to palaeogeographic interpretation are given in the text.