Page 88 - Sumatra Geology, Resources and Tectonic Evolution

PRE-TERTIARY VOLCANIC ROCKS 75

Table 6.8. Volcanic lithologies in the Oceanic Assemblage of the Woyla Group.
Formation Lithological description Reference

Aceh Province
Geumpang -t-2000 m massive, green to grey, deformed mafic to intermediate volcanics, Bennett et al. (1981a)
frequently epidotized, uralized or silicified, some pyroclastics, amygdaloidal
basalts, minor phyllites and pods of siticified metalimestone
Lam Minet Calcareous, carbonaceous to manganiferous slates and meta-argillites, green Bennet et al. (1981a)
volcanic wackes and chert/basalt beds; the Bengga Limestone Member is
composed of metalimestones, coarse marbles and metavolcanics
Penarum Basalts, red cherts, argillites, metavolcanic wackes and greenschists Cameron el al. 1983)
Situtup Partially epidotized basalt breccias & agglomerates; schistose metabasalts Cameron et al. 1983); Barber(2000)
Undifferentiated Includes intermediate to marie metavolcanics, cherts and slates Cameron et al. 1983)
Woyla Group
Babahrot Metavolcanics, metalimestones and serpentinites and metagabbro intrusions Cameron et al. 1982a)
Padang area
Indarung Basic volcanics, including pillow lava, volcanic breccia, tufts, volcaniclastic Yancey & Alif (1977);
sediments, radiolarian chert and massive or bedded limestone McCarthy et al. (2001)
Siguntur Quartzites, shales, siltstones, slates and volcaniclastics Rosidi et al. (1976)
Tembesi- Rawas Mountains
Rawas Diabases and basalts, associated with turbidites and a large limestone body Suwarna et al. (1994)
'Mesozoics with mafics' Limestone, quartzite, slate, schist, tuff, igneous breccia, tuff breccia, metavolcanic, De Coster (1974)
diabase and serpentinite
Lampung area
Menanga Tuffaceous and calcareous claystones, sandstones and shales with intercalated Barber (2000)
radiolarian-bearing cherts, manganese nodules, coral limestones and rare
porphyritic basalt. The sandstones contain clasts of glassy andesite and lithic
fragments of andesite, quartz-diorite and quartzite
See Table 6.9 for the Natal area.

Table 6.9. Volcanic units and volcaniclastic sediments of oceanic and continental affinity within the Woyla Group Accretionar~, Complex in the Natal area
Formation Lithological description Environment Ref.
Tambak Baru Volcanic Unit Altered, purple, quite strongly sheared, porphyritic andesites and Volcanic centre & proximal volcaniclastics 1
andesite agglomerates and proximal debris flows
Simpang Gambir Megabreccia Dark green, foliated megabreccias with basic volcanic and limestone Proximal sediments and olistostromes derived 1
megaclasts interbedded with poorly sorted conglomerates and from volcanic centre
greywacke sandstones
Nabana Volcanic Unit Vesicular basic lavas, keratophyes and dolerite dykes Ocean-floor basalts, Seamount
Panglong Mdlange Breccias with basic volcanics, radiolarian cherts, limestones with Mn- Mdlange ?olistostrome, of ocean-floor materials
mineralisation and pelagic sediments
Belok Gadang Siltstone Volcaniclastic siltstones, fine siltstones and rare conglomerates ?lower trench slope basin fill
Ranto Sore Volcaniclastic sandstones and unsorted conglomerates (lahars) Fluviatile intra-arc deposits
Parlumpangan Volcanic Unit Undeformed porphyritic andesites with amygdales and altered matrices Volcanic arc or local volcanic centre fragments
and andesitic tufts
Si Gala Gala Schist Unit Greenschist facies banded quartz, muscovite, chlorite schists Metasediments derived from an acid-
intermediate arc or centre of continental type
Simarobu Turbidite Volcaniclastic turbidites with minor calcareous siltstones Ocean-floor or trench deposit
Batang Natal Megabreccia Large clasts of limestone, rare clastic sediments and igneous rocks in a Olistostome or mud diapirs in accretionary
slaty matrix complex
Rantobi Sandstone Thin bedded volcaniclastic sandstones and siltstones Forearc basin deposits
Jambor B aru Volcaniclastic conglomerate, sandstone, siltstone, limestone and tuff Shallow marine and deeper water forearc basin
deposits
Muarosoma Turbidite Thin bedded volcaniclastic turbidites with noticeable quantities of Upper trench slope basin sediments
quartz clasts and less mafic and chlorite material
Mdlange Unit House & room sized fragments of greenstones, greenish wackes, M61ange ?olistostrome of ocean-floor materials,
cleaved metatuffs, sheared fossiliferous limestones and 50% by pelagic sediments & limestone
volume cherts; the blocks are disrupted by serpentinite and invaded
by dykes
Pasaman Ultramafic Complex Variably serpentinized, massive to foliated hartzburgite, with minor Ocean-floor volcanics and basement slices;
dunite pods and stringers, and pyroxenite dykes seamount
Igneous rocks in Batu Serpentinized dunites and hartzburgites intruded by thin dykes, now Slices of ocean-floor basement
Nabontar Limestone rodingites
Undifferentiated Banded metavolcanics, slates and limestones in north of Lubuksikaping ?Accretionary complex
Quadrangle
References: 1, Wajzer et al. (1991); 2, Rock et al. (1983).