Page 43 - Sumatra Geology, Resources and Tectonic Evolution

30 CHAPTER 4

calcareous nodules up to 40 cm in size, around which the slaty Tigapuluh Group
cleavage diverges as the result of compaction. Indeterminate
foraminifers were recognized in one nodule, and an insoluble Pre-Tertiary rocks form the Tigapuluh Mountains, isolated as
residue from another yielded abundant sponge spicules. an inlier 70 km long and 40 km wide among the surrounding
The associated mudstones contain leaf and fungal fragments. Tertiary sediments, east of the Barisan Mountains to the south
These outcrops were distinguished by Turner (1983) as the Tua of Rengat (Fig. 4.5). Three formations have been identified: the
Member. These records of plant fragments, foraminifers and Mentulu, Pengabuhan and the Gangsal formations, interpreted as
siliceous spicules indicate that the less deformed sediments different facies of the Tigapuluh Group. The distribution of
in the Kuantan Formation are very likely to yield age-diagnostic these units are shown on the Rengat and Muarabungo Quadrangle
fossils to a systematic search. Sheets (Suwarna et al. 1991; Simandjuntak et al. 1991) (Fig. 4.6).
On the Pakanbaru Quadrangle Sheet, to the north of Solok, Deformation increases in intensity from NE to SW and in the
Clarke et al. (1982b) distinguish the Pawan and Tanjung Puah aureoles of Triassic-Jurassic granitic intrusions the sediments
members of the Kuantan Formation (Figs 4.2 & 4.4). The are converted to spotted slates or hornfels.
Pawan Member cropping out to the east of Lubuksikaping is
composed of intensely folded muscovite, tremolite, chlorite Mentulu Formation. The Mentulu Formation, defined from out-
and carbonate schist. The very similar Tanjung Puah Member to crops in the upper part of the Mentulu River, occupies large
the SW, also includes quartz schist. Both units show an early areas in the northern and eastern parts of the Tigapuluh Mountains
phase of tight isoclinal folding on vertical or steep SW-dipping (Fig. 4.6). The formation is characterized by pebbly mudstones,
axial planes and east-west or NW-SE axes, and are refolded similar to those of the Bohorok Formation of northern Sumatra.
by later upright folds on NW-SE axes. The latter are probably The mudstones are interbedded with greywacke sandstones
represented by the large-scale folds seen on aerial photographs and shales, the latter generally occurring as slates, or as hornfels
and indicated on the Pakanbaru Quandrangle Sheet (Clarke et al. adjacent to granite contacts. The mudstone matrix contains irregu-
1982b). Again, these more highly metamorphosed rocks may
larly distributed angular to rounded clasts of granite, silicified
represent fragments of an earlier metamorphic basement, or, basalt, vein quartz, slate, quartzite and feldspar. The clasts are
where rock types include tremolite and chlorite schists, may generally of pebble size, up to a few centimetres, but may reach
represent a hitherto unrecognized suture zone. 30 cm in diameter. The pebbly mudstone is usually deformed,
On the Solok Sheet Silitonga & Kastowo (1975) recognized with the matrix altered to slate, and the clasts flattened and
a Limestone Member within the Kuantan Formation (Fig. 4.5), elongated within the cleavage planes. Cordierite is commonly
composed of massive, black, white, grey or reddish limestone, developed where the pebbly mudstones have been converted to
locally containing irregularly-shaped chert nodules, with interbeds spotted slates or hornfels within metamorphic aureoles.
of quartzite and siliceous shale. Detailed petrographic studies The interbedded greywacke sandstones are massive, dense, grey
of samples of limestone have been made by Vachard (1989a, b). sandstones, sometimes conglomeratic, containing folded quartz
He recognized algal structures, including algal mats, oolites and veins. The sandstones are poorly sorted and also contain irregu-
possible pisolites, and concluded that the limestones were depos- larly distributed clasts, of the same rock types as those found in
ited in an intratidal to supratidal environment. From the fossils the mudstones. The conglomerates are polymict and are composed
collected during the mapping survey Silitonga & Kastowo of sub-angular to rounded clasts. Finer sandstone units show
(1975) established that the limestones in the Kuantan Formation parallel lamination and may be poorly graded. Shale or claystone
range in age from Lower Carboniferous to Mid-Permian, although units are well bedded and parallel laminated and contain scattered
the younger limestones are better considered as a separate matrix-supported fragments of quartz and feldspar. Some of
formation. the sandstone units are tuffaceous and andesitic and basaltic
Subsequently the fossiliferous localities were re-examined tuf~ distinguish the Condong Member in Bukit Condong and
by Fontaine & Gafoer (1989). New collections were made and Gunung Endalang (Fig. 4.6).
macro- and microfossils studied to establish the ages of these The pebbly mudstones of the Mentulu Formation, like those
limestone occurrences more precisely. Important localities in the Bohorok Formation in northern Sumatra are considered to
containing Carboniferous fossils occur in the Again River and be of glacio-marine origin, and the lithology of the clasts indicates
the Batang Kuantan Gorge (Fig. 4.5). The limestone outcrops a continental provenance.
to the east of Lake Singkarak (Guguk Bulat) which yielded
Permian fossils are considered by Fontaine & Gafoer (1989)
to be best classified with the Mid-Permian Silungkang Pengabuhan Formation. The Pengabuhan Formation occurs in the
Formation, rather than, as shown on the map of the Solok central part of the Tigapuluh Mountains where it is defined from
Quadrangle, with the Kuantan Formation (Silitonga & Kastowo outcrops in the upper part of the Pengabuhan River (Simandjuntak
1975). et al. 1991) (Fig. 4.6). The formation is composed principally
Limestone outcrops in the Again River near the bridge on the of lithic greywackes or sandstones, quartzites and siltstones.
road from Bukit Tinggi to Pakanbaru yielded the alga Konincko- These lithologies contain irregularly distributed clasts of granite,
pora and the foraminifers Palaeotextularia, Eoendothyranopsis vein quartz and quartzite, similar to those seen in the Mentulu
and Archaediscus, indicating a Mid-Vis6an age. With additional Formation. The quartzites are often feldspathic and are well-
samples the age range was extended from the late Early or sorted, being composed of well rounded grains of quartz and
early Mid-Vis6an to Late Vis6an (Fontaine & Gafoer 1989). A feldspar. The siltstones also contain clasts of feldspar, quartz
Mid-Late Vis6an age was confirmed by the discovery of cono- and lithic fragments. The outcrop patterns in the northern part of
donts, including Gnathodus girO, i rhodesi Higgins, from the Tigapuluh Mountains, as delineated by Suwarna et al.
this locality (cf. the Alas Formation above) (Metcalfe 1983). (1991) (Fig. 4.6), show the Mentulu and Pengabuhan formations
Limestones exposed in a scenic gorge along the Kuantan River interdigitating, suggesting that they are facies variants, distin-
contain large colonies of the tabulate coral Syringopora, the fasci- guished only by the presence or absence of pebbly mudstone.
culate Tetracorallia Siphenodendron and the alga Koninckopora Alternatively the two units may have been imbricated by thrusting.
inflata, indicating a Late Vis~an age (Fontaine & Gafoer 1989;
Vachard 1989a, b). These limestones containing the colonial Gangsal Formation. The Gangsal Formation crops out in the
coral Syringopora and intratidal algal mats, were evidently western part of the Tigapuluh Mountains, and was defined from
deposited in a sub-tropical to tropical, shallow, warm water the upper part of the Gangsal River. The formation is also
environment. shown occupying a small area between the Mentulu and