Page 59 - Sumatra Geology, Resources and Tectonic Evolution

46 CHAPTER 4

Table 4.1. Correlation of formations in the Woyla Group in the Natal area from thicker bedded and more massive limestones, some oolitic. Near
Rock et al. (1983) with the lithotectonic units defined by Wajzer et al. (1991) the top of the section a limestone conglomerate, eroded into the
underlying limestone with basal scours, provides clear evidence
Rocket et al. (1983) Wajer et al. (1991)*
of way-up. Above the limestone there is a break in outcrop, until
1. Langsat Volcanic Formation 1. Langsat Volcanic Formation further downstream and in the road section above, the Golok
2. Sikubu Formation 2. Si Kumbu Turbidite Formation Tuff, a calcareous vitreous crystal tuff is exposed. Although
3. Tambak Baru Volcanic Unit the contact between the breccia and the tuff is not seen, this
4. Simpang Gambir Megabreccia Formation section is regarded as an essentially continuous stratigraphic
3. Belok Gadang Formation 5. Nabana Volcanic Unit sequence McCarthy et al, (2001).
6. Belok Gadang Siltstone Formation In the Ngalau Quarry, near Indarung, McCarthy et al. (2001)
7. Panglong M61ange Formation collected samples from a 15 m section of bedded chert for radi-
8. Ranto Sore Formation olarian determination. In the Karang Putih Quarry, one kilometre
Volcanics in both the Belok 9. Parlampungan Volcanic Unit to the south of lndarung, lenses of chert are associated with
Gadang and Maurasoma massive limestone. McCarthy et al. (2001) report that the lime-
Formations stone in this quarry is completely recrystallized, possibly due to
4. Maurasoma Formation 10. Si Gala Gala Schist Formation the effects of a granitic intrusion which occurs a short distance
Schistose Member to the south (Fig. 4.16). An interpretative cross section shows
11. Simarobu Turbidite Formation the cherts and limestones imbricated together along low angle
12. Batang Natal Megabreccia Unit thrusts (McCarthy et al. 2001).
13. Rantobi Sandstone Formation Rock units in the Indarung area are well dated from fossil
14. Jambor Baru Formation and radiometric age determinations. Radiolaria from chert in the
15. Maurasoma Turbidite Formation
Massive limestones in both the 16. Batu Nabontar Limestone Unit Ngalau Quarry belong to the Transhsuum hisuikoyense Zone,
Belok Gadang and of Aalenian, early Mid-Jurassic age (McCarthy et al. 2001).
Maurasoma Formations Lithologies and fbssil content of the limestones in the Lubuk
Peraku section and in the Ngalau and Karang Putih quarries
*units are listed in approximate order upstream from Langsat with no age were described by Yancey & Alif (1977). The limestones are
relationship implied. biosparites, with abundant bioclasts, oolitic calcarenites and
micrites. Molluscan shell fragments, pellets, calcareous algae,
stromatoporoids and scleractinian corals are common components
Units in central Sumatra correlated with the Woyla Group of the limestones. Among the fossils identified were the (?)
stromatoporoids Actostroma and Lovfenipora. The former is
Outcrops of rock units with similar lithologies to those of the considered to be restricted to the Late Jurassic, while the latter
Woyla Group or which were formed within the same Jurassic- is diagnostic of the Late Jurassic to Early Cretaceous. A K-Ar
Cretaceous age range have been mapped throughout western age date of 105 _+ 3 Ma (Albian, mid-Cretaceous) is reported
Sumatra (Fig. 4. ! 2). Many of these outcrops have been correlated from the Golok Tuff in the Lubuk Peraku by Koning & Aulia
by previous authors with units of the Woyla Group described from (1985) from a Caltex Pacific Indonesia internal report.
northern Sumatra. Pillow lavas and cherts of the Indarung Formation have been
equated with the oceanic assemblage of the Woyla Group of
lndarung Formation. Small outcrops of the Mesozoic Indarung Aceh and with the Belok Gadang Formation of the Natal area
Formation occur near Padang in West Sumatra. These rocks were (Cameron et al. 1980; Rock et al. 1983). Where these rocks are
mapped and described by Yancey & Alif (1977) and were corre- imbricated, deformed and altered to greenschists they may be
lated with the Woyla Group of Aceh by Cameron et al. (1980). interpreted, as is the case in Aceh and Natal, as materials accreted
Outcrops occur 15 km east of Padang in road, river and quarry from a subducted ocean floor. The recent recognition of Middle
sections near Indarung, where they are surrounded and overlain Jurassic radiolaria in the cherts (McCarthy et al. 2001) shows
by Neogene and Quaternary volcanic and volcaniclastic rocks that part of this ocean floor was of Jurassic age. The volcanic brec-
(Fig. 4.16). The area of outcrop is included on the Padang, cias tufts and volcaniclastic sandstones of the Indarung Formation
Solok and Painan Quadrangle Sheets (Kastowo & Leo 1973; are interpreted as the products of seamount volcanism, and the
Silitonga & Kastowo 1975; Rosidi et al. 1976). These rocks massive limestone with its Late Jurassic-Early Cretaceous fossil
have been mapped more recently by McCarthy et al. (2001). fauna is interpreted as part of a fringing reef formed around
Yancey & Alif (1977) described rocks exposed in the Lubuk the seamount (McCarthy et al. 2001). During subduction the
Peraku River, the Ngalau Quarry, the Karang Putib Quarry and seamount with its carbonate cap collided with already accreted
adjacent river sections near lndarung. Rock types in these outcrops ocean floor materials, and the whole assemblage was imbricated
are basic volcanics, which may include pillow lavas, volcanic to form the present complex.
breccia, tuff, volcaniclastic sediments, radiolarian chert and
massive or bedded limestones. The basic rocks are sometimes Siguntur Formation. Mesozoic rocks of the Siguntur Formation are
deformed and metamorphosed to form greenschists. On the exposed in the Sungai Siguntur, 15 km to the south of Indarung
other hand, the limestones and cherts are essentially undeformed, (Fig. 4.16). The area of outcrop is shown on the Painan
although disharmonic folding and small-scale thrusts in the chert Quadrangle Sheet and the lithology is described in the Explanatory
and gentle folds in the limestone are seen in the quarries, and Note (Rosidi et al. 1976). Rock types are quartzites, siltstones
the limestones may be recrystallized (McCarthy et al. 2001). and shales, the latter sometimes altered to slates, and compact
A well-exposed section of limestone and tuff occurs in the limestones. The map shows that the strike of the beds is east-
river section of the Lubuk Peraku and in the road above the west, transverse to the general Sumatran trend. In the report
river (Yancey & Alif 1977; McCarthy et al. 2001). A measured the rocks are described as not intensely deformed or folded, but
columnar section of these outcrops from McCarthy et al. (2001) quartzites interbedded with slates showing bedding-parallel clea-
is given as Figure 4.17. The lower part of the section, described vage, suggest that the rocks are more highly deformed than at
as the Lubuk Peraku Limestone, is a limestone breccia, which first appears. The limestones are reported to contain Lovfenipora,
includes volcanic clasts near the base and is interbedded with and are therefore of a similar age to the limestones at Indarung.
thin tuff bands near the top. The breccia is overlain by a few The 'quartzites' reported from Siguntur were taken to indicate
metres of thin-bedded limestones and shelly marls and then by that these rocks had a continental origin (Barber 2000) but it