Page 52 - Sumatra Geology, Resources and Tectonic Evolution

PRE-TERTIARY STRATIGRAPHY 39

TETHYAN

STAGES STAGES

RHAETIAN
NORIAN

CARNIAN
LADINIAN
ANISIAN

SCYTHIAN
TATARIAN DORASHAMIAN
DZULFIAN
KAZANIAN MIDIAN
UFIMIAN MURGHABIAN
KUNGURIA KUBEGANDIAN
BOLORIAN
ARTINSKIA
YAHTASHIAN
SAKMARI3
SAKMARIAN
ASSELIAN ASSELIAN

Fig. 4.11. Palaeontological evidencc for the ages of Permo-Triassic stratigraphic units in Sumatra (data from Fontaine & Gafoer 1989).

outcrop extensively in the Barisan Mountains southwards from Carboniferous-Permian Tapanuli Group, which is invariably
Muarasipongi and are also found in Bangka and Billiton. affected by greenschist metamorphism, with the development of
Lower Permian formations in southern Sumatra include the slates and phyllites, and the Permo-Triassic Peusangan Group,
andesitic, basaltic and rhyolitic volcanics of the Palepat Formation which is relatively undeformed and unmetamorphosed, except
and the lower part of the Silungkang Formation. These volcanics where it occurs in metamorphic aureoles (Cameron et al. 1980;
are frequently interbedded with limestones and clastic sediments, Pulunggono & Cameron 1984). It was therefore proposed that
and the limestones in particular, frequently contain large fusulinid the major phase of deformation occurred between the deposition
foraminifera and other fossils which have allowed precise age of these two units. In order to establish the age of deformation
determinations. Early Permian, Asselian to Kungurian ages, and metamorphism affecting the older unit, it is essential to deter-
have been established for the Barisan and Palepat formations, mine the ages of the units in the Tapanuli and Peusangan groups
and also for the Mengkarang Formation with its 'Jambi Flora' more precisely. The age of the Pangururan Bryozoan Bed is
(Fontaine & Gafoer 1989). Cameron et al. (1980) interpreted critical in this respect. The Bryozoan Bed is interbedded with tur-
these Lower Permian volcanics and the associated rocks as pro- biditic rocks identified as part of the Kluet (Bohorok?) Formation
ducts of a Permian volcanic arc with its volcaniclastic sedimentary and is deformed with a slaty cleavage in exactly the same way as
apron and carbonate reefs. the surrounding rocks (Aldiss et al. 1983). Deformation of the
Pulunggono & Cameron (1984) extended this interpretation into Kluet/Kuantan, Alas and Bohorok formations therefore occurred
northern Sumatra on the basis of the occurrence of volcanic rocks after the deposition of this unit. As has been reported above, the
in the Situtup Formation and volcanics of the Toweren Member fragmentary fauna obtained from the Pangururan Bryozoan Bed
of the Tawar Formation. However, no fossils have yet been indicates a Late Carboniferous to Early Permian age, although
found in the Tawar Formation so that its age is unknown, and fusu- the palaeontologists from the British Museum who made the deter-
linids in the Situtup Formation have not been dated more precisely minations favoured the later age. If this age determination is
than mid-Permian. As noted above, it is possible that the accepted, the major deformation of the Tapanuli Group occurred
epidotized basaltic rocks of the Situtup Formation and the after the deposition of the Bryozoan Bed, while the mid-Permian
Toweren Member of the Tawar Formation, should more properly Situtup Limestone and Batumilmil Limestone formations of the
be classified with the Jurassic-Cretaceous Woyla Group, cropping Peusangan Group are undeformed. The main deformation in north-
out in the same area, which includes similar lithologies. On the ern Sumatra therefore occurred in the late Early Permian or Early
basis of the available evidence the case for the extension of the Middle Permian as Cameron et al. (1980) proposed. Certainly the
Early Permian volcanic arc into northern Sumatra is unproven. main deformation of the Tapanuli Group in northern Sumatra
Geochemical studies and isotopic dating of the volcanic rocks occurred before the Triassic, as the Sibaganding Member of the
are required to resolve this problem. Mid-Late Triassic Kualu Formation, cropping out along the
shores of Lake Toba near the outcrop of the Bryozoan Bed,
Ages of deformation and metamorphism. During the Northern shows open folding, but the associated argillaceous units do not
Sumatra Survey a distinction was made between the show a penetrative slaty cleavage.