Page 91 - Sumatra Geology, Resources and Tectonic Evolution

78 CHAPTER 6

The Nabana Volcanic Unit at the southwestern end of the of megabreccia composed of blocks of metasediment and
Batang Natal section (Fig. 6.12) is composed of vesicular spilitic serpentinite. The serpentinite body is thrust into a turbidite
basic volcanics intruded by dolerite dykes. These vesicular pillow sequence, probably equivalent to the Rawas Formation in the
lavas indicate submarine extrusion at less than abyssal depths. The Tembesi-Rawas Mountains, along strike to the SE.
dolerites are metamorphosed at greenschist facies. The Nabana
Volcanic Unit is interpreted as a tilted slab of oceanic crust with
ocean-floor basalts and dolerite feeder dykes. Again, associated Tembesi-Rawas Mountains
limestones may be part of a seamount carbonate capping
(Wajzer et al. 1991). Two preliminary analyses of spilites from In the Sarolangan Quadrangle the boundary of the Woyla
the Nabana Volcanic Unit/Belok Gadang Formation are given Accretionary Complex is taken at the Rawas Thrust, marking
by Rock et al. (1983) (Table 6.11). the approximate southern boundary of the Asai Formation.
The Tambak Baru Volcanic Unit of andesites and andesite Serpentinite pods are mapped along the thrust (Suwarna et al.
agglomerates and the associated Simpang Gambir Megabreccia 1994). Diabases and basalts are also present, associated with tur-
are the faulted remains respectively of a volcanic centre and associ- bidites and a large limestone body in a pelagic marine sequence,
ated proximal volcaniclastic erosional debris. A sample of andesite which has been affected by thrusts and strike-slip faults. The
yielded a Campanian-Maastrichtian (Cretaceous) K-Ar age of generalized description of the Rawas Formation is fairly typical
78.4 + 2.5 Ma (Wajzer et al. 1991) [N.B. this date should not of the Oceanic and Accretion Complex elsewhere in Sumatra,
be given too much credence, as the rocks are affected by low- but the detail is lacking and it is described by Suwarna et al.
grade metamorphism; Editor]. The unit was suggested by Wajzer (1994) as interleaved within the non-volcanic, shallow marine,
et al. (1991) to represent a collided volcanic arc, but the units are Peneta Formation and perhaps represents a forearc basin deposit.
not highly deformed as might be expected in a collision; a volcanic The Woyla Accretionary Complex is exposed in river sections
centre intruded into the accretionary complex during the Late where tuffaceous shales alternate with meta-limestones to the
Cretaceous is a more probable explanation. west of the Barisan Mountains, in the Sumatra Fault Zone, and
to the east of Danau Kerinci (Kusnama et al. 1993b).

Padang area (refer to Figs 4.16 and 6.13) Subcrop beneath the South Sumatra Tertiary Basin
In the Padang Quadrangle, to the north of the Danau Maninjau vol- The subcrop of the Woyla Accretionary Assemblage beneath
canic centre, the northern margin of the Woyla Accretionary Tertiary sediments between the Gumai and Garba Mountains
Complex is truncated by the Sumatra Fault Zone (Kastowo & and Palembang has been reconstructed from oil company borehole
Leo 1973). Here a zone of serpentinite pods aligned along faults termination records (Fig. 6.13). These were studied by Adiwidjaja
has been emplaced in massive limestones, phyllites, metasand- & de Coster (1973) and de Coster (1974) who distinguished a belt
stones and metasiltstones, occasionally with mafic greenstones. of 'Mesozoics with mafics' south of the 'Mesozoic Metamorphics'
Jurassic fossils were collected from the limestones at Palembanjan of the Tembesi-Rawas area of the Woyla Foreland Assemblage.
by Volz (19 ! 3). Mesozoics with mafics were encountered in exploration drilling
To the east of Padang, McCarthy et al. (2001) recognized of the Tertiary sediments north of Tebingtinggi beneath the
thrusting in the volcanic-sedimentary sequence in the Indarung headwaters of the Sungai Musi (Kikim-Teras High) and east of
Formation of Yancey & Alif (1977) and identified Mid-Jurassic Baturaja (Lematang Sub-Basin). Lithologies encountered corre-
radiolaria in cherts, indicating that part of the accreted ocean spond with those in the Foreland, Oceanic and Accretion
crust was of Jurassic age. The Golok Tuff Formation composed Complex Assemblages of the Woyla Group. The Foreland
of crystal tufts which lies above the Lubuk Peraku Limestone Assemblage sediments are on strike with the Peneta and Asai For-
(Upper Jurassic-Lower Cretaceous, Yancey & Alif 1977) has mations, and the Oceanic and Accretion Complex metavolcanics
been dated using the K-Ar method at 105 _+ 3 Ma (Koning & beneath the Lemat Formation volcanics (Eocene), are recorded
Aulia 1985). McCarthy et al. (2001) interpreted the massive in oil-well terminations as far north as the Sungai Musi.
Lubuk Peraku Limestone as part of a fringing reef to a seamount De Coster (1974) reports a Mid-Cretaceous (?deformation)
which collided during subduction with the Accretionary K-Ar age of 121 _ 2 Ma from tuffaceous clastics at the base of
Complex and was imbricated within it. the Lemat-2 well, south of the Sungai Musi.
The Limestone Member of the Siguntur Formation, on strike
to the SE at Surian in the Painan Quadrangle, is described by
Rosidi et al. (1976) as similar to the Indarung Limestone and Lampung area (refer to Fig. 4.8)
possibly also capped a former seamount. The main outcrop of
the Siguntur Formation south of Padang includes quartzites Two Pre-Tertiary units, the Menanga Formation and the
(McCarthy et al. 2001). Rosidi et al. (1976) remark on the Gunungkasih Complex (McCourt et al. 1993), were mapped in
cherty nature of quartzites, which suggests that they may have the Kotaagung (Amin et al. 1994b) and Tanjungkarang
an oceanic origin. The diverse origins of sediments are typical (Andi-Mangga et al. 1994a) Quadrangles. The Early Cretaceous
of the Oceanic and Accretion Complex, and this poorly exposed, Menanga Formation, which is in thrust contact with the older
but extensive unit includes distal terrestrial, volcaniclastic, (Palaeozoic) Gunungkasih Complex, consists of a mixture of
pelagic and chemical oceanic sediments, probably juxtaposed by lithologies ranging from shales with cherts, sandstones, siltsones
thrusting and movement along strike-slip faults. and claystones and rare porphyritic basalt. The claystones
are tuffaceous and the sandstones include andesite, glassy andesite
and quartz-diorite clasts. The sedimentary environment of the
Danau Diatas to Gunung Kerinci Menanga Formation is interpreted as deep marine, related to a
volcanic arc, and is correlated with the Lingsing Formation of
Between Danau Diatas and Gunung Kerinci to the east of the the Gumai Mountains by Amin et al. (1994b) and Andi-Mangga
Sumatra Fault Zone (Fig. 6.13) a 'serpentinite front' to the et al. (1994a). According to Barber (2000) the depositional
Woyla Oceanic and Accretion Assemblage is marked by serpenti- environment was that of a forearc to an Andean-type volcanic
nite pods (Rosidi et al. 1976). Serpentinite and pyroxenite are also arc, built on continental basement, and he interprets the
present at Galagah (McCarthy et al. 2001). North of Lubukgadang sequence as part of the Foreland Assemblage of the Woyla
a large serpentinised hartzburgite body is associated with a lens Group. The lithological mix suggests that the Menanga Formation