Page 50 - Sumatra Geology, Resources and Tectonic Evolution

PRE-TERTIARY STRATIGRAPHY 37

A Pangunjungan Member is distinguished in the river section of to Early Jurassic age, most probably Jurassic. Although this unit is
the same name and is traced along the southwestern side of the included in the Permo-Triassic Peusangan Group by Rock et al.
main outcrop (Fig. 4.4). This unit shows the same lithological (1983) they suggest that it might better be classified with the
assemblage as described above, but the rocks are finer grained Jurassic Rawas Formation of Central Sumatra which will be
and include thin bedded limestones and grey to pale brown discussed later.
radiolarian cherts. The radiolaria from these rocks have not been
identified. Irregular disharmonic folds are interpreted as sedimen- Tuhur Formation (Figs 4.4 & 4.5). Silitonga & Kastowo (1975)
tary slumps (Clarke et al. 1982a). defined the Tuhur Formation forming extensive outcrops to the
To the east and south of Lake Toba a Sibaganding Limestone SE of Lake Singkarak in the Solok Quadrangle. This outcrop
Member has been distinguished (Fig. 4.4). The limestones are was later extended southwards into the Painan-Timurlaut
pale to dark grey biocalcilutites and have yielded an ammonite Muarasiberut Quadrangle to the east of Lakes Dibawah and
Alloclionites aft. timorensis (Early Norian--Ishibashi 1975), Diatas (Rosidi et al. 1976). A further outcrop was mapped to the
corals, brachiopods, gastropods and conodonts; the latter include NE of Payakumbuh and this outcrop was traced northwards,
the zonal form Metapolygnathus polygnatoformis (Late using aerial photographic interpretation, across the equator into
Carnian). At the type locality in the road section along the the Pekanbaru Quadrangle (Clarke et al. 1982b). Silitonga &
eastern side of Lake Toba 3 km to the north of Prapat, limestones Kastowo (1975) distinguished a Slate and Shale Member,
of the Sibaganding Member with Daonella and Halobia overlie forming the greater part of the outcrop, composed of grey to
shales of the Kualu Formation (Metcalfe et al. 1979; Fontaine & dark grey slate, black shales, and brown cherts with thin grey-
Gafoer, 1989, Fig. 22). The microfauna and flora from the wacke sandstones, and a Limestone Member composed of
limestone outcrop has been identified and illustrated by Vachard poorly bedded sandy limestone and massive fossiliferous
(1989c) and the microfacies have described by Beauvais et al. conglomeratic limestone, with thin intercalated shale and slate.
(1989). Although the fossils include corals, calcisponges and Limestone pebbles in the conglomerates contain fusulinid forami-
encrusting bryozoa, and other reef-building organisms, these are nifera of Permian age. Musper (1930) suggested that this
scattered in a micritic matrix and do not form reef structures. formation is of Triassic age. The Tuhur Formation may be
The environment of deposition is interpreted as a mud mound. correlated with the Kualu Formation, described above.
The rocks are moderately to tightly folded about NW-SE trending
sub-horizontal axes with easterly dipping axial planes (Aldiss Silungkang Formation (Figs 4.4 & 4.5). The type locality for the
et al. 1983). Silungkang Formation (Klomp6 et al. 1961) is the road and river
sections around the village of Silungkang, between Solok and
Cubadak Formation (Fig. 4.4). The Cubadak Formation is named Sawahlunto to the SE of Lake Singkarak. The formation also
from the Air Cubadak on the western side of the Rao Graben to crops out discontinuously along Lake Singkarak and northwest-
the north of Lubuksikaping (Rock et al. 1983). It is composed of wards across the equator towards Muarasipongi. A lower Volcanic
dark grey, well-bedded mudstones with interbedded siltstone Member is composed of hornblende and augite andesites with
laminae and volcaniclastic sandstones, frequently yielding the intercalated tufts, limestones, shale and sandstone. An upper
pelecypod Halobia flattened on bedding surfaces. A section of Limestone Member is also recognized, composed of massive
the Cubadak Formation in the Aek (Air) Cubadak to the south grey limestone interbedded with shales, sandstones and tufts
of Limau Manis was described by Turner (1983). This section (Silitonga & Kastowo 1975). The rocks are commonly highly
contains limestones which were not mentioned in the description fossiliferous with large foraminifers: Doliolina lepida Schwager,
of the formation given by Rock et al. (1983). About 100 m of Pseudofusulina padangensis, Neoschwagerina multiseptata
blue-grey calcareous mudstones are interbedded with cm thick Deprat and Fusulinella lantenoisi Deprat, at Silungkang (Katili
tuffaceous limestones, sometimes containing ooliths nucleated 1969). Large fusulinacean foraminifers, Nankinella, Parafusulina
around mineral grains. The oolitic limestones show cross lami- and Pseudodoliolina and the porcellaneous foraminifer Hemogor-
nation. The sequence yielded Halobia sp. and several ammonites: dius were also collected from an outcrop in the Aek Cubadak
Trachyceras sp. ind. and ?Ceratites sp. This faunal assemblage near Rao (Rock et al. 1983); these fossils indicate an Artinskian
indicates that the sequence is of Ladinian age (Late Triassic). to Kazanian age for this outcrop. Waagenophyllid corals
(Pavastehphyllum sp.) occur in limestones intercalated with volca-
Limau Manis Formation. Turner (1983) also defined the Limau nics and shales at Silungkang and in limestones at Guguk Bulat
Manis Formation from outcrops in the Air Cubadak to the north (Ipciphyllum and Wentzzelloides) where the Ombilin River flows
of Limau Manis. These outcrops were mapped as part of the out of Lake Singkarak; the latter indicating a Murghabian age
(Permian) Silungkang Formation by Rock et al. (1983). The lithol- (Fontaine 1982). The Guguk Bulat locality was classified with
ogies include breccio-conglomerates with clasts of limestone and the Kuantan Formation by Silitonga & Kastowo (1975) but
acid and basic igneous material, followed by tuffaceous mud- is more reasonably correlated with the Silungkang Formation
stones, cross-bedded volcaniclastic sandstones, the cross beds (Fontaine & Gafoer 1989).
indicating derivation from the NW, and bioclastic turbidites.
These calciturbidites are rich in reworked fusulinids and corals Barisan Formation (Fig. 4.5). Rosidi et al. (1976) defined the
of mid-Late Permian age. The mudstones contain abundant Barisan Formation from outcrops of phyllite, slate, arkosic sand-
ammonites Acanthinites sp., Helictites sp., ?Tibetites sp. ind. indi- stone, limestone and cherts south of Solok and NE of the Sumatran
cating a Ladinian, Carnian to Norian age (Mid-Late Triassic) Fault. The foliation in the phyllites and slates trends NNW-SSE,
(Turner 1983). parallel to the fault. Rosidi et al. (1976) also defined a Limestone
Member which forms linear outcrops trending in the same
Telukkido Formation (Fig. 4.4). Rock et al. (1983) defined the direction. The limestones cropping out at Bukit Cermin have
Telukkido Formation cropping out between Pasirpengarayan and yielded fusulinid foraminifers including Schwagerina sp. of
Lubuksikaping from a stream of the same name. The rocks are Early Permian age. In the eastern part of its outcrop the Barisan
dark grey quartzose sandstones and shales with minor limestones Formation is equivalent to the Silungkang Formation, and
and thin coals. A Limestone Member composed of recrystallized Fontaine & Gafoer (1989) recommend that its designation as a
or argillaceous limestones is also recognized. In the type locality separate formation should be discontinued.
these rocks yielded plant remains from pyritic quartzite, with
leaf impressions identified as Otozamites sp. (possibly Pterophyl- Palepat Formation (Fig. 4.5). Rosidi et al. (1976) defined the
lum) and Ptilophyllum sp. The flora is identified as of Late Triassic Palepat Formation composed of andesitic, basaltic and rhyolitic