Page 46 - Sumatra Geology, Resources and Tectonic Evolution

PRE-TERTIARY STRATIGRAPHY 33

!
104~
~45' 105~ ~ Recent'Volcanoes
Late Cretaceous Granites
Menanga Formation
. ~"..." ~.~-~.~%,--~. ~'<-'~ Rive r ~--~'~_~ ~-,~ (mid-Cretaceous)
mpung ~ , ~
" <z;~ ~ ~ Gunungkasih Complex_
- 5o15 ,
(Palaeozoic)
%
"\\ ~o /~'~. -"--~, BANDARLAMPUNG

\ %,~% ~ KOTAAGUNG
~s

atk
Fig. 4.8. The distribution of the Pre-
Tertiary units of the Bandar Lampung area,
southern Sumatra after GRDC geological
map sheets of Kotaagung and
Tanjungkarang (Amin et al. 1994b; Andi
- 5~ ' \ 5~45 '-
Mangga et al. 1994a). The Gunungkasih
Complex is correlated with the Palaeozoic
Tapanuli Group and the Menanga
"~'~. Strike-slip Faults Formation with the Jurassic-Cretaceous
"~ Thrust Faults 0 ................... . ................... 50km Woyla Group of northern Sumatra (see
.
.
below). In areas left blank the older rocks
104~ ' 104~ ' 105~ ' are covered by Tertiary and Quaternary
I ........................ l ................................ l ....................... I sediments and volcanics.

Vis~an age has also been established for the Limestone Member of interdigitates with, and passes into the Kluet Formation; they
the Kuantan Formation (Fontaine & Gafoer 1989; Metcalfe 1983; regarded the latter as the lateral equivalent of the Bohorok
Vachard 1989a, b). The record by Turner (1983) of plant remains Formation, representing a more distal turbidite facies. Similar
in the Nior member of the Kuantan Formation is compatible with relationships are described from Central Sumatra between the for-
this age attribution. Turbiditic sandstones and pelites, similar to mations in the Tigapuluh Group (Fig. 4.6). Cameron et al. (1980)
those of the Kluet and Bohorok formations, occur interbedded also observed a systematic reduction in the size and proportion of
with limestones characteristic of the Alas Formation, suggesting clasts towards the SW in the pebbly mudstones and in conglomer-
to the surveyors that the Alas is part of the same sedimentary ates throughout the Bohorok and Kluet formations. The inference
sequence as the other units (Cameron et al. 1980). They therefore from these observations is that the sedimentary provenance of
considered that the Bohorok, Alas and Kluet/Kuantan formations the Tapanuli/Tigapuluh Group lay to the NE of Sumatra
are lateral facies variants of a coherent sedimentary assemblage. and that deposition occurred on a continental margin extending
Clasts in the pebbly mudstones of the Bohorok, and conglomer- out into an ocean lying to the SW, in present day coordinates.
ates in the Bohorok, Kluet and Kuantan formations and also in As reported above, Cameron et al. (1980) suggested that the
the Tigapuluh Group of Central Sumatra, include the same range Kluet and the Bohorok formations were related facies of
of lithologies. Analysis of the composition of the clasts shows the same age. The erroneous identification of a fossil coral
that all these units were derived from a low-grade metamorphic from the Alas Formation led Cameron et al. (1980) to suppose
terrane composed of slates, phyllites, calc-silicate schists, that the Alas Formation was of Early Permian age and was there-
marbles and quartzites which were intruded by granitic rocks. A fore preserved in a syncline, overlying the older Kluet and
K/Ar age of 1029 Ma from a trondjemite clast from pebbly Bohorok formations. Cameron et al. (1980) proposed a strati-
mudstones in the Langkawi Islands (Hutchison 1989, p. 16) indi- graphic scheme for the Tapanuli Group of northern Sumatra
cates that the source area included rocks of Proterozoic age. Some based on an analogy with stratigraphic relationships seen near
argillaceous clasts show evidence from slaty cleavage and crenu- Phuket in Peninsular Thailand (Garson et al. 1975) (Fig. 4.2). At
lation cleavages that they had already undergone multiple defor- Phuket, pebbly mudstones of the Phuket Group, similar to those
mation. Locally the metamorphic grade in the source region was of the Bohorok Formation of Sumatra, are underlain and inter-
higher, indicated by clasts of mica schist and granitic gneiss. bedded with a thick and extensive series of turbiditic sediments.
The granitic gneisses may have been formed by synkinematic Fossils in the turbidites include the trilobite Cyrtosymbole (wari-
deformation of granites intruded into an active shear zones. Rare bole) perlisensis Kobayashi and Hamada (Mitchell et al. 1970)
chert clasts, may indicate the presence of oceanic rocks incorpor- of Late Devonian to Early Carboniferous age. The same fossil
ated in a collisional suture and rhyolite clasts indicate acid volcan- occurs near the base of the pebbly mudstones and sandstones
ism. In fact, the palaeogeology of the area from which the forming the Sings Group, in Langkawi, a group of islands offshore
sediments of the Tapanuli and Tigapuluh groups were derived Peninsular Malaysia (Jones et al. 1966) (Fig. 4.2). In Phuket, the
resembles very closely the present-day geology of northern pebbly mudstones are overlain by thin-bedded sandstones contain-
Sumatra. ing a fauna of bryozoa and brachiopods and then by a 'Bryozoan
Cameron et al. (1980) report that, within the Bohorok Bed' considered to be of Early Permian age (Mitchell et al. 1970;
Formation, pebbly mudstones die out in a southwesterly direction. Garson et al. 1975). Cameron et al. (1980) drew an analogy
With the loss of pebbly mudstones the Bohorok Formation between the Pangururan Bryozoan Bed of northern Sumatra and