Page 53 - Sumatra Geology, Resources and Tectonic Evolution

40 CHAPTER 4

This conclusion can be extended throughout eastern Sumatra an abundant necktonic-planktonic fauna, was deposited in a
where the Tapanuli Group, the Malarco or Malang Formation on basin of restricted circulation with anaerobic bottom conditions.
Kundur Island, the Persing Complex of Singkep Island and He suggests that the chert beds may have resulted from the
the Pemali Group of northern Bangka were all deformed prior to dissolution of volcanic glass in ash falls from volcanic activity
the mid-Triassic. However, it cannot be extended to central at some distance from the site of deposition, as no beds of ash
Sumatra. Although the Kuantan Formation in central Sumatra or pyroclastic deposits have been recognized in Malaya.
shows the same slaty cleavage with multiple deformation as the However, volcaniclastic sediments and tuffs are recorded in the
Kluet Formation in the same area, the Permian Barisan, the Cubadak and Tuhur formations of west central Sumatra (Rock
Triassic Tuhur and the Jurassic Rawas and Asai formations also et al. 1983; Turner 1983).
show slaty cleavage and multiple deformation. Evidently in In Malaya and in Bangka Island the increase in grain size and
central Sumatra the major deformation event occurred after the frequency of the sandstone units towards the east, suggest that
deposition of the Jurassic sediments. the source area for the Semanggol sediments lay in this direction.
Late Upper Permian and the earliest Lower Triassic deposits However, there are also indications in current directions within the
have not yet been recognized anywhere in Sumatra (Fig. 4.11). sandstones for derivation of sediments from local sources within
However, Mid-Late Triassic rocks are extensively developed in the basin. The pebbles in the Conglomerate Member are composed
the northern part of Sumatra, from Aceh to West Sumatra and in mainly of vein quartz, quartzite and dark-coloured chert, which
the islands of Bangka and Billiton. The period between Late could have been derived from Palaeozoic rocks in the central
Permian and Middle Triassic was a period of regression and part of the Malay Peninsula, which was evidently being uplifted
erosion, as reworked mid-Late Permian fusulinids are found in latest Triassic times. The Conglomerate Member may pass
abundantly in clasts in the mid-Late Triassic sediments of the upwards into the Tembeling Formation of presumed Jurassic age
Tuhur and Limau Manis formations (Silitonga & Kastowo 1975; (Burton 1973), which corresponds in age with the Tabir, Asai,
Turner 1983). Therefore, the concept that the scattered outcrops Peneta and Rawas formations of central Sumatra (Rosidi et al.
of Permo-Triassic formations throughout Sumatra constitute a 1976; Kusnama et al. 1993b; Suwarna et al. 1994) to be described
stratigraphic 'Group' is not valid. In future studies it would be sen- later.
sible to divide these formations into Permian and Triassic groups. Mid-Late Triassic sediments in the western Malay Peninsula
and northern Sumatra represent deposition on a broad continental
Triassic Correlation with West Peninsular Malaysia. A close corre- shelf which was undergoing extension, with the formation of
lation can be made between the Triassic rocks of northern localized deep rift basins in which black shales and chert were
Sumatra and those of Peninsular Malaysia. The Mid-Late deposited and into which, from time to time, turbidity cun'ents
Triassic age of part of the limestones of the Situtup Formation carried coarse clastic sediments. Carbonate was deposited on shal-
has been established by foraminifers (Cameron et al. 1983); the lower parts of the shelf to form the massive limestone units in both
age of the Kaloi Formation, part of the Batumilmil Formation, northern Sumatra and western Malaya. In the basin, sandstone
the Sibaganding Limestone Member of the Kualu Formation by units increase in thickness upwards through the sequence and
conodonts, and the Kualu Formation, the Cubadak and Limau are replaced in Malaya by conglomerates, indicating uplift of
Manis formations by ammonites and the presence of abundant the eastern source area. According to Metcalfe (2000) this uplift
Halobia. This whole assemblage of Triassic rocks in northern resulted from the collision between the Sibumasu (Sumatra) and
Sumatra can be correlated directly with the Upper Triassic Indochina blocks (East Malaya) which was taking place at this
Semanggol and Kodiang Limestone formations which crop out time. In his recent publications Metcalfe (2000) interprets the
in Kedah and Perak in NW Malaya, some 200-250 km to the tectonic environment in which the Semanggol Formation was
east across the Malacca Strait (Metcalfe 2000). deposited as a foredeep basin, related to the collision.
The Semanggol Formation of Malaya has been divided into
three members: a lower Chert Member, a Rhythmite Member
and an upper Conglomerate Member (Burton 1973). The Chert
Member, as its name implies, contains chert beds interbedded Woyla Group (Jurassic-Cretaceous)
with shales and sandstones, the sandstones commonly showing
disharmonic folding as convolutions and slumps. The Chert Woyla Group in Aceh
Member may be correlated directly with the Pangunjungan
Member of the Kualu Formation of northern Sumatra. The The Woyla Group was defined in Aceh, northern Sumatra, where
Rhythmite Member, interpreted as a turbidite sequence with the rocks are extensively exposed, but Jurassic-Cretaceous units
graded bedding, cross lamination slump folds and sole marks correlated with the Woyla Group have been identified in the
in the sandstones, and its fauna of thin-shelled bivalves, may be Barisan Mountains throughout western Sumatra (Fig. 4.12).
correlated with the thin-bedded sandstones, siltstones and mud- In Aceh, areas of outcrop of the Woyla Group are shown on
stones of the type section of the Kualu Formation in the Sungai the GRDC Banda Aceh, Calang, Tapaktuan and Takengon
Kualu. The Conglomerate Member of the Semanggol Formation 1:250 000 Quadrangle Sheets (Bennett et al. 1981a, b; Cameron
has not been recognized in northern Sumatra, although sandstone et al. 1982b, 1983). The Woyla River, from which the Woyla
units become more common in the upper part of the Kualu Group was named, is on the Takengon Sheet (Fig. 4.13).
Formation. The Conglomerate Member may be represented by The descriptions given below, except where specified, are taken
the conglomeratic sandstones of the Papan Formation on Kudur largely from the reports which accompany these maps. An
Island to the south of Singapore and the Tempilang Sandstone account of the lithological units which make up the Woyla
of Bangka Island (Cameron et al. 1982c; Ko 1986). Group and a detailed discussion of their interpretation is given
The massive Kodiang Limestone in northern Kedah, Malaya, by Barber (2000).
has been identified as of Mid-Late Triassic age from the presence During the DMR/BGS survey 13 lithostratigraphic units were
of conodonts (Ishii & Nogami 1966), and may be correlated distinguished in the Woyla Group in Aceh, as well as a unit of
directly with the massive limestone units in northern Sumatra 'undifferentiated Woyla'. Many of the mapping units distin-
described as Situtup, Kaloi, Batumilmil formations and the guished in the Woyla Group of Aceh during the DMR/BGS
Sibaganding Limestone Member of the Kualu formation, which survey are made up of the same rock types, but in varying
have all yielded Mid-Late Triassic conodonts (Metcalfe 1989a). proportions. It is clear that they represent geographical, rather
Burton (1973) suggested that the lower part of the Semanggol than genuine lithostratigraphical units. A different name was
Formation, with black carbonaceous shales and mudstones and given to each distinguishable unit on each map sheet. The outcrops