Page 57 - Sumatra Geology, Resources and Tectonic Evolution

44 CHAPTER 4

00oe \ Recent Volcanoes
I

, 0\ Langsat Volcanics I~
Palaeogene granites
-,- 9 -..
- . . . - . - . -
Late Cretaceous granites
Vlanunggal
Batholith 'Sorik WOYLA GROUP
Batang Natal 9 Merapi
River Section, ~ Limestones
QKOTANOPAN I~ ~' ~ M~langes

NATAL i ii!ii Muarasoma Formation
Belok Gadang Formatior
~ Sikubu Formation
Kanaikan Peridotite/serpentinite

Pasaman Ultramafic
9 V L.~'~_ Complex
Air Bangi,~ ~-- ~ ~ l ~ ' au ~O X LUBUKSIKAPING
"~)~~(.Talam ~

- O~ uator 0 50km ~176

99 ~ 100 ~
I I
Fig. 4.14. The distribution of the Woyla Group in the Natal area, North Sumatra. Modified from Rock et al. (1983). KFZ, Kanaikan Fault Zone; SGF, Simpang Gambir Fault.

type locality of Belok Gadang, a tributary of the Batang Natal, Intrusions and volcanics in the Natal area. Several large granite
show basaltic pillow lavas, with white clay interbeds and manga- bodies are intruded into the rocks of the Woyla Group in the
nese-rich horizons with braunite, resembling the 'umbers', Natal area. The largest of these is the Manunggal Batholith at
described from the Troodos Ophiolite of Cyprus (Robertson the northeastern end of the Batang Natal Section (Rock et al.
1975). Analysis shows that the pillow basalts are spilites (Rock 1983) (Fig. 4.14). This batholith is a composite body, some
et al. 1982, 1983). In the type locality basalts are overlain by 230km 2 in extent, composed of leocogranite, granodiorite,
red, bedded cherts, but again no identifiable radiolaria have been granite and pyroxene-quartz diorite, with contaminated syenitic
recovered. and monzonitic varieties, and appinites. The granitoid rocks are
intruded by vogesite lamprophyre dykes. The granitoid rocks
Sikubu Formation. The Sikubu Formation, cropping out in the have been dated by the K-Ar method at 87 Ma (Late Cretaceous)
lower part of the Batang Natal section, is composed of massive (Kanao et al. 1971, reported in Rock et al. 1983). In the Aik Soma,
volcaniclastic metagreywackes, with thin shale interbeds. The near Muarasoma, large granitic boulders in the river bed enclose
sandstones show very well-developed sedimentary structures, serpentinite xenoliths, surrounded by reaction zones of amphibo-
including graded bedding, flame structures and convolutions, lite. Limestones in the same area are converted to skarns near
typical of turbidites. Massive porphyritic andesitic dykes and the contact with the granite.
lava flows, with distinctive pyroxene phenocrysts, are intruded A second granitoid, the Kanaikan is intrude into the Woyla
into, or interbedded with, the sediments in the lower part of the Group in the Pasaman area (Fig. 4.14). This body is composed
section. Fragments of porphyritic andesite, identical in compo- of coarse granodiorite and leucogranite cut by microgranitic and
sition to the dykes and lavas, occur as clasts in the sandstones 9 granophyric dykes. This intrusion lies within the Kanaikan Fault
Woyla Group rocks in the Pasaman area include m~langes and Zone, a strand of the main Sumatran Fault, and is much dissected
massive and foliated peridotites (Rock et al. 1983) (Fig. 4.14). by faults and deformed to form cataclasites along shear zones.
Peridotites are well exposed in the Pasaman River where they Granitic rocks outcrop in headlands near Air Bangis along the
are composed mainly of harzburgite with minor dunite pods, coast to the south of Natal (Fig. 4.14). Rock et al. (1983)
pyroxenite dykes, disseminated chromite and rare chromite speculated that these rocks might be of Late Cretaceous age and
pods. Some of the peridotite is foliated, containing orthopyroxenes analogous to the Sikuleh Batholith which intrudes the Woyla
enclosed in augen. Coarse plagioclase-hornblende rocks, found as Group in Aceh. Later age dating showed that these granites
boulders in the float, represent metasomatised gabbro pegmatite were of Eocene-Oligocene age (Wajzer et al. 1991).
which formed dykes in the peridotite. The peridotite is variably
serpentinized, and in shear zones may be completely altered to Age constraints for the Woyla Group in the Natal area are provided
serpentine and talc. Smaller bodies of serpentinite, with chromite by a limestone sample from the Batang Kanaikan in the Pasaman
pods, outcrop at the upper end of the Batang Natal section near inlier which yielded a colonial organism, closely resembling the
Muarasoma (Figs 4.14 & 4.15) where they form spectacular samples of Lovfenipora described and illustrated by Yancey &
serpentinite breccias faulted against slates and limestones of the Arif (1977) from the Indarung area, near Padang, and considered
Muarasoma Formation. Serpentinite also occurs as xenoliths in to be of Late Jurassic to Early Cretaceous age (IGS/British
granite in the Aik Soma. Museum Sample No. TC/J1/Rll01B--Rock et al. 1983).