Page 56 - Sumatra Geology, Resources and Tectonic Evolution

PRE-TERTIARY STRATIGRAPHY 43

garnets up to 8 cm in diameter, and biotite-hornblende-andesine the Teunom Formation and 'undifferentiated Woyla Group rocks
schist (Cameron et al. 1982b). The occurrence of high-grade are altered by contact metamorphism. Lithologies resembling
metamorphic rocks with garnets suggests that some of the units those of the Lhoong Formation occur as roof pendants within
of the Woyla Group were deeply buried and were subsequently the batholith.
exhumed. These rocks warrant investigation to determine the The Sikuleh Batholith is a complex intrusion composed of an
origin of the protolith and the environment of metamorphism. 'older complex' of migmatised gabbros and diorites locally gneis-
Units containing a high proportion of volcaniclastic material are sose and sheared and intensely veined. A 'younger complex' is
associated with the island arc assemblage. These include the more homogeneous coarser grained and unfoliated biotite-
Lho'nga Formation, which outcrops to the west of Banda Aceh, hornblende granodiorite. The younger complex has been dated,
composed of grey and coloured slates and phyllites, with inter- from the mean of K-Ar analyses of two biotites and one horn-
bedded volcaniclastic sandstones, thin limestones and (?)radio- blende, as 97.7 _+ 0.7 Ma (early Late Cretaceous).
larian-bearing siltstones and the Lhoong Formation, which forms
a large outcrop to the SW of the Sumatran Fault, and also occurs Age of theWoyla Group in Aceh. Fossils from the Lamno Limestone
as roof pendants in the Sikuleh Batholith (Bennett et al. 1981b). and Sise Formations indicate that the fringing reefs around the
The formation consists of basaltic lavas with cherts in the lower volcanic arc were being formed during Late Jurassic to Early
part of the sequence, followed by conglomeratic wackes with Cretaceous times. The K-Ar ages of c. 97 Ma from the Sikuleh
volcanic and limestone clasts, and subordinate sandstones, Batholith which intrudes the limestones and the oceanic assem-
siltstones and limestones. blage show that the lithological units which make up the Woyla
Group were in their present positions and had their present
Limestone units. Massive limestones, o/ten recrystallized, are also structural relationships by the early Late Cretaceous.
associated with the island arc assemblage and are interpreted
as fringing reefs to volcanic islands. These units include the
Lho'nga and Raba Limestone formations which crop out along
the coast and in the Barisan Mountains to the south and west of Woyla Group in Natal
Banda Aceh (Bennett et al. 1981a) (Fig. 4.13) and consist of
massive calcarenite and calcilutite and dark thin-bedded cherty Lithological units correlated with the Woyla Group of Aceh
limestones and shales. The massive limestone is designated a were mapped over an extensive area inland from Natal in North
'Reef Member' which is closely associated in the field with the Sumatra during the Integrated Geological Survey of Northern
Bentaro Volcanic Formation. The Lamno Limestone Formation Sumatra as part of the Lubuksikaping 1:250 000 Quadrangle
also crops out along the west coast of Aceh, south of Banda Sheet (Rock et al. 1983) (Fig. 4.14). The outcrop is limited to
Aceh, and is also associated with outcrops of the Bentaro Volcanic the NE by the Sumatran Fault System and is much dissected
Formation. It consists of dark limestone, with a reef-like facies, internally by faults with a similar trend. The Woyla Group is
and contains volcanic clasts near the base. The limestone is com- intruded by Late Cretaceous granites and overlain unconformably
by the Miocene Barus Group, by Miocene volcanic rocks, and by
monly fossiliferous, with: corals--Actiastraea minima, S(vlosmi-
lia corallina; algae--Clypeina sp., Permocalculus ampullacea, the products of Quaternary volcanism from the volcanoes of Sorik
Merapi, Malintang and Talamau, as well as by recent alluvium.
Lithocodium, Bacinella sp., Boueina sp., Thaumatoporella porvo-
siculifera; foraminifers--Pseudocyclammina lituus, indicating a Units within the Woyla Group strike NW-SE and are very well
Late Jurassic to Early Cretaceous age (Bennett et al. 1981a). exposed in the valley of the Batang Natal, both in the river
The Teunom Limestone Formation crops out along the southwes- section and in the parallel road section, which both cut across
tern margin of the Sikuleh Batholith. It is composed of massive the strike (Fig. 4.15). The main outcrop of the Woyla Group is
separated from a smaller outcrop in the Pasaman inlier to the
dark limestones, which are metamorphosed and recrystallized
along the contact with the granite. The Sise Limestone Formation south by Malintang Volcano (Fig. 4.14).
(Fig. 4.13) resembles the limestone units to the south of Banda In the DMR/BGS report of the Lubuksikaping Quadrangle
Aceh, but anomalously crops out to the NE of the Sumatran (Rock et al. 1983) lithological units in the Batang Natal
Fault. Its present position may be due to some 200 km of dextral section were classified, from NE-SW, into three formations: the
displacement along the fault. The unit consists of massive or Muarasoma, Belok Gadang and the Sikubu formations (Fig. 4.14).
bedded limestones, biocalcarenites and calcilutites with fossils:
corals--Montlivaltia sp., Myriopora sp.; foraminifers--Pseudocy- Muarasoma Formation. The Muarasoma Formation outcrops in the
clammina sp. indicating a Late Jurassic to Early Cretaceous age upstream part of the Batang Natal section and in its tributary, the
(Cameron et al. 1983). Aik Soma. Thicknesses of the rock units in this section were
measured perpendicular to the strike for a distance of 5.5 km
'Undifferentiated' Woyla (Fig. 4.13). On the geological map of the (Rock et al. 1983). The rock types in the measured section
Takengon Quadrangle a large area of 'Undifferentiated' Woyla include cleaved argillaceous units, shale or slate, which may
Group rocks is shown between the main strand of the Sumatran include calcareous concretions, laminated siltstones, and gritty
Fault and the Anu Batee Fault. This area is poorly known, but sandstones showing sedimentary structures, indicating younging
these rocks are described in the Explanatory Note as intermediate in a downstream direction, massive limestones, sometimes
to mafic metavolcanics, slates and chert. 'Undifferentiated' Woyla forming karstic limestone pinnacles, epidotic volcanic breccias
is also shown in the Calang Quadrangle in the area to the south of and volcaniclastic sandstones, chloritic greenschists and musco-
the Sikuleh Batholith in Gunung Paling and as roof pendants vite-chlorite quartz schists. A 10 m 'conglomerate' (?m61ange)
within the outcrop of the batholith (Bennett et al. 1981b). These at the upstream end of the section, with elongated clasts of greens-
rocks are said to resemble the Kluet Formation, which crops out chist in a chloritic matrix, is probably of tectonic origin, formed in
extensively to the NE of the Sumatran Fault, and should not be a fault or a shear zone (Rock et al. 1983).
considered as part of the Woyla Group.
Belok Gadang Formation. The Belok Gadang Formation crops out
Sikuleh Batholith. The Woyla Group in Aceh is intruded by grani- in the central part of the Batang Natal section and is composed of
toids. The largest of these is the Sikuleh Batholith shown on the sandstones, sometimes calcareous, and argillaceous rocks, often
Banda Aceh and Calang sheets (Bennett et al. 1981a, b). It is an cleaved and containing bands and lenses of chert. The chert is
elliptical body (c. 55 x 35 kin) elongated in a NW-SE direction radiolarian, but no identifiable radiolaria have so far been recov-
(Fig. 4.13). Around the margins of the batholith limestones of ered which could be used to date the sequence. Outcrops in the