Page 37 - Sumatra Geology, Resources and Tectonic Evolution

Chapter 4
Pre-Tertiary stratigraphy

A. J. BARBER & M. J. CROW

In the early days of mineral exploration on behalf of the It has proved very difficult to establish with certainty the strati-
Netherlands East Indies Bureau of Mines and of petroleum graphic relationships between the various rock units which
exploration by the oil companies it was recognized that Pre- make up the exposed Pre-Tertiary basement of Sumatra. This is
Tertiary rocks were extensively exposed in the Barisan Mountains due to the generally fault-bounded contacts between rock units
in the western part of Sumatra (Fig. 1.4). These rocks are variably and the poor biostratigraphic control on their ages; over large
metamorphosed and were termed the 'Barisan-Schiefer' and the areas the rocks are apparently devoid of fossils. The varying
'Old-Slates Formation' (Veerbeek 1883) in Central Sumatra, and metamorphic grade of the basement units makes even lithological
the 'Crystalline Schists' in the Lampung area (Westerveld correlations difficult. As a result, formations have generally been
1941). Locally these rocks contain fossils, and it was recognized defined locally. When these local units have been extrapolated
that Carboniferous and Permian rocks occur within this Pre- over broader areas they are found to include a wide variety of
Tertiary basement. Some basement units were defined during the lithological types, so that correlation with the original units
mapping of Sumatra by the Netherlands Indies Geological becomes more and more uncertain.
Survey between 1927 and 1931, but the definition of units accord- The spate of new data on the geology of Sumatra generated
ing to modern stratigraphic principles began in the early 1970s, by the systematic geological survey of the whole island has stimu-
with the commencement of systematic mapping by the Indonesian lated attempts at regional synthesis, e.g. Cameron et al. (1980) and
Geological Survey in collaboration with the United States Pulunggono & Cameron (1984) in northern Sumatra and McCourt
Geological Survey, in the Padang area of West Sumatra et al. (1993) in southern Sumatra. These authors proposed a
(Kastowo & Leo 1973--Padang; Silitonga & Kastowo 1975-- stratigraphic scheme which distinguished a Carboniferous-
Solok; Rosidi et al. 1976--Painan and Muarasiberut). Permian Tapanuli Group, a Permo-Triassic Peusangan Group
Mapping and the definition of further units was continued in and a Jurassic-Cretaceous Woyla Group (Fig. 4.1 ). This terminol-
northern Sumatra by the Indonesian Directorate of Mineral ogy is used in the present account, although it is strictly applicable
Resources/British Geological Survey (DMR/BGS) between only to northern Sumatra where the units were defined.
1975 and 1980 as part of the Northern Sumatra Project and In this account the basement rocks of Sumatra are described
was extended into southern Sumatra in the 1980s and 1990s by from northern, central and southern Sumatra, as far as possible
the Indonesian Geological Research and Development Centre in terms of their stratigraphic age, although difficulties in
(GRDC), DMR amd BGS. The results of these surveys, which establishing these ages will be fully discussed. Five age units
established the distribution of the basement units, are published are recognized: Pre-Carboniferous basement, Carboniferous-
by GRDC as 1:250000 Geological Map Sheets coveting the ?Early Permian, Mid-Late Permian, Mid-Late Triassic and
whole of Sumatra and adjacent islands (Fig.l.5). The lithologies Jurassic -Mid-Cretaceous.
of each stratigraphic unit are briefly described in the keys to the
maps, and the units are described more fully in the accompanying
Explanatory Notes.
During these surveys the faunas from known fossil localities Pre-Carboniferous basement
were re-examined and new localities were found. Following the
survey the palaeontological evidence for the ages of stratigraphic Eubank & Makki (1981 ) record shales interbedded with quartzites
units in Sumatra has been reviewed by Fontaine & Gafoer (1989). from the boreholes, Pusaka-l, 85 km NE of Pekanbaru, and
It has now been established that fossiliferous rock units in the Pre- Rupat Island, in the Malacca Strait, which yielded palynomorphs
Tertiary basement of Sumatra range in age from Early Carbonifer- lu the Devonian-Carboniferous boundary, and used this
ous through to mid-Cretaceous. evidence to define an Upper Palaeozoic 'Quartzite Terrain' in
From the occurrence of tin granites in the eastern part of eastern Sumatra (Fig. 4.2). Some of these borehole records may
Sumatra, extending into the 'Tin Islands' of Bangka and Billiton, relate to quartz sandstones in the Triassic Kualu Formation and
it is supposed that the whole of Sumatra is underlain by a highly its correlative Tembeling Sandstone of Bangka (Ko 1986).
differentiated Pre-Carboniferous crystalline continental crust However, Eubank & Makki (1981) also obtained Rb-Sr ages of
with ages extending back into the Precambrian. Direct evidence 426 + 41.5 Ma (Silurian) and 335 + 43 Ma (Early Carboniferous)
for a Pre-Carboniferous basement has been obtained by isotopic from granites from boreholes put down into the basement beneath
dating of Silurian and Lower Carboniferous granitic rocks the Central Sumatra Basin. Turner (1983) reports gneissose
encountered in boreholes beneath the Tertiary Basins towards rocks included as xenoliths in dykes intruding Carboniferous
the northeastern side of the island (Eubank & Makki 1981). slates near Rao, Central Sumatra. These xenoliths were presum-
The oldest rocks identified by their fossil content were also ably derived from an underlying crystalline basement. A granitic
encountered in boreholes in eastern Sumatra. These rocks clast from pebbly mudstone encountered in a borehole, Cucut
contain palynomorphs from near the Devonian-Carboniferous No.l, gave an Rb-Sr age of 348 ___ 10 Ma, of Vis~an, Early
boundary (Eubank & Makki 1981). Older rocks, possibly Carboniferous age (Koning & Darmono 1984).
ranging down into the Devonian, were reported by Adinegoro & The occurrence of intrusive granites, possibly as old as Silurian,
Hartoyo (1974) from a borehole in the Malacca Strait, but no indicates that an older basement into which these granites were
details are given in their report and a Devonian age for sediments intruded underlies eastern Sumatra. This is highly probable, as
elsewhere in Sumatra has not been confirmed during subsequent Proterozoic and Lower Palaeozoic rocks occur in the Malaysian
drilling or by field studies, although rocks of this age, and older Langkawi Islands only some 300 km to the NE of Sumatra
ages back to the Proterozoic, occur in the Langkawi Island off along the strike (Jones 1961). Indeed, Hutchison (1994) has
NW Malaya, 300 km to the NE of Sumatra (Jones 1961). asserted that the buried Kluang Limestone south of Palembang,

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