Page 89 - Sumatra Geology, Resources and Tectonic Evolution

76 CHAPTER 6

Table 6.10. Volcanic units in the Oceanic Volcanic Arc fragments of the Woyla Group
Formation Litbological description Ref.

Bentaro arc
Bentaro Volcanic Porphyritic basalts and basalts and agglomerates with andesine, associated with mafic dykes, l
Basaltic vents surrounded by tufts, breccias and volcanic sediments were found near Lam No
and north of the Bentaro river
Lhoong Volcanic wackes, subordinate sandstones and siltstones, mafic volcanics and limestones l
Tapaktuan Volcanic Massive, partly epidotised, frequently porphyritic andesites, subordinate basalts with 2, 3
feldsparphyric varieties and coeval dykes. Agglomerates, breccias and tufts are present in the
southeast. Subordinate shales and slates containing volcanic debris and purple to red tuffaceous
sandstones
Meukuek Gneiss Biotite-hornblende-andesine schists & biotite amphibolites interpreted as syntectonic deformed 2
Complex Tapaktuan Volcanics associated with concordant gneissic leuco-granites
Sise
Kenyaran Volcanic Epidotized intermediate to mafic lavas which are frequently amygdaloidal and porphyritic and 2,3
agglomerates
Saling
Saling Chloritised and prophylitised andesitic and basaltic lavas, tufts and breccias with local limestone 4
intercalations
Lingsing Basalts and andesites interbedded with claystone, siltstone, calcilutite and chert 4
Garba (?) amygdaloidal and porphyritic lavas of basalt and andesite, crystal tufts, chert and rare 5
serpentinite
Insu Member Basalt and andesite lavas with minor lenses or intercalations of chert 5
M~lange Complex Boulders and clasts of limestone, chert, schist and andesite similar to the andesite lava in the Garba 5
Formation, all within a scaly matrix
References: l, Bennett et al. (1981a); 2, Cameron et al. (1982a); 3, Barber (2000); 4, Gafoer et al. (1992e); 5, Gafoer et al. (1994)

140-t- 10 Ma from the Silungkang Formation (Suwarna et al. limestones may represent the carbonate caps to seamounts
2000) indicates that Lower Cretaceous volcanic rocks are more constructed on oceanic crust. Serpentinite is also imbricated into
extensive than previously thought, but were previously included these formations and sometimes occur as diapirs within the
with Permian volcanics. Sumatran Fault Zone. The larger bodies of serpentinite (Tangse,
The Siulak Formation, forming a limited outcrop within Cahop and Beatang Ultramafic Complexes) represent slices of
the Sumatra Fault Zone near the southern margin of the Painan oceanic upper mantle harzburgite incorporated into the accretion-
Quadrangle (Rosidi et al. 1976), includes dacitic lavas and ary complex. The volcanic rocks are often deformed and altered
tufts and a 500 m thick fossiliferous (Cretaceous) Limestone to greenschists, and the ultramafic rocks to talc schists. Garnetifer-
Member. It is suggested that this formation represents forearc ous amphibolites present in the Reunguet River are suggested
sediments and continentally-sourced andesites trapped by strike- by Barber (2000) to have been subducted and metamorphosed at
slip faulting within the fault zone. high pressure before being tectonically exhumed. The large area
Continentally sourced voicaniclastic sediments which occur as of undifferentiated Woyla Group south of the Sumatra Fault
fault packets in the Woyla Oceanic and Accretionary Complex Zone includes intermediate to mafic metavolcanics, cherts and
in the Batang Natal section (Wajzer et al. 1991) may have slates, and may be considered, to be composed mainly of the
been derived from erosion of the contemporaneous Jurassic- Woyla Oceanic Assemblage.
Cretaceous Plutonic-Volcanic Arc. The Upper Permian-Triassic Situtup Formation in the
Takengon Quadrangle (Cameron et al. 1983) composed mainly
of limestones, also includes metavolcanics such as epidotised
Volcanics in the Woyla Accretionary Complex basalts, basaltic breccias and agglomerates and schistose metaba-
salts. The adjacent Toweren Member also contains massive
Volcanic lithologies occur commonly in the Woyla Group, metavolcanics. Barber (2000) points out that the descriptions of
where they are tectonically juxtaposed as fault packets within the volcanic lithologies in the Situtup and Toweren formations
the Accretionary Complex (Tables 6.8 and 6.9). They are best resemble those of the Woyla Group and suggests that Woyla
known from the Batang Natal section, where Wajzer (1986) volcanics may have been tectonically imbricated within the
carried out detailed mapping and documented the variety and Situtup Formation.
discussed the origin of oceanic and pelagic rock types (Wajzer
et al. 1991). Elsewhere in Sumatra the distribution of the major
lithological units within the Woyla Accretionary Complex has
been established by reconnaissance mapping only. Natal area (refer to Figs 4.14 and 6.12)
Oceanic rocks of the Woyla Group in the Natal area were first
Aceh Province (refer to Fig. 4.13) mapped by Rock et al. (1983) as part of the Lubuksikaping
Quadrangle. The rock units and their relationships were described
The Geumpang, Lain Minet and Penarum formations in the Banda in detail from the Batang Natal river and road sections by Wajzer
Aceh and Takengon quadrangles include basaltic lavas, often (1986), with a more accessible summary in Wajzer et al. (1991).
pillowed, basaltic breccias and conglomerates, tufts and volcanic The section shows imbricated slices of massive limestone, serpen-
sandstones, imbricated with limestones, radiolarian chert and tinite, volcaniclastic sandstone, sometimes turbiditic, pillow
argillites of the Woyla Oceanic Assemblage (Bennett et al. basalt, radiolarian chert and m~lange, composed of blocks of
1981a; Cameron et al. 1983; Barber 2000). The more massive these lithologies in a clay matrix, arranged in an apparent