Page 42 - Sumatra Geology, Resources and Tectonic Evolution

PRE-TERTIARY STRATIGRAPHY 29

limestones have undergone deformation with the development of which the argillaceous red brown shale and phyllite component
alternating zones of high and low strain and the formation of press- is dominant, with intercalations of quartzite, siltstone, dark grey
ure-solution cleavage, as illustrated by distortion of the bryozoan chert and andesitic to basaltic lava flows.
networks. The limestones are interbedded with sandstones and No systematic sedimentological study has been carried out on
associated with slates of the Kluet Formation. Unfortunately, the Kuantan Formation and outcrop details are not given in the
when they were examined at the Natural History Museum the Explanatory Notes for the GRDC Quadrangle sheets. Descriptions
bryozoa were found to be too decalcified, and the other fossils too of the lithological features of the Kuantan Formation by Peter
fragmentary, to provide a precise age determination for this unit. Turner (Turner 1983) from three outcrops near Rao (Fig. 4.4)
The age range suggested for the fossil assemblage is from Late Car- are therefore particularly valuable. The first is on the Auk
boniferous to Early Permian with the balance of opinion favouring Mangkais to the west of the Batang Sumpur, where massive
an Early Permian age (Aldiss et al. 1983). The collection of further grey quartzite beds, 1-6 m are interbedded with blue-grey and
fossil and limestone samples from this unit are required for a more black phyllites and fine siltstones 10-80 cm thick. The quartzites
precise age determination. show both sharp tops and bases and the siltstones may show
cross-lamination. Tight folds of the slaty cleavage are seen in
Kuantan Formation. As the Kluet Formation was mapped south- loose blocks in the stream bed.
wards towards the equator it became obvious that it was the Steeply dipping (100~176 black slates outcrop in the
same unit as the Kuantan Formation, previously defined on the Sungai Nior to the east of the Batang Sumpur, showing isoclinal
Solok Quadrangle Sheet in West Sumatra, from outcrops along folds to which the cleavage has an axial plane relationship
the Batang Kuantan by Silitonga & Kastowo (1975) (Fig. 4.5). (Turner 1983). The slates are interbedded with rippled, laminated
On the Padangsidempuan Quadrangle Sheet to the north, the siltstones containing ribbed plant stems of Calamites type. The silt-
change from Kluet to Kuantan Formation was set arbitrarily stones are sometimes deformed by slump folds. A section in the
where there is a break in the outcrop at 99~ longitude (Aldiss fiver bank shows several lenses of matrix-supported conglomerate,
et al. 1983) (Fig. 4.4). up to 1 m thick, with bases eroded into the underlying slate. Angular
The outcrop of the Kuantan Formation extends along the core to rounded clasts in the conglomerate include vein quartz, microgra-
of the Barisan Mountains from Padangsidempuan to the latitude nite, phyllite, greywacke, quartzite and chert. Siltstone clasts show
of Padang (Figs 4.4 & 4.5). Silitonga & Kastowo (1975) distin- both cleavage and crenulation cleavage, indicating two earlier
guished a Lower Member dominated by quartzites and quartz phases of deformation These conglomerates are interpreted as
sandstones, rarely conglomeratic, with interbedded shales, debris flows (Turner 1983). Further upstream, greywacke sandstone
usually metamorphosed to slates or phyllites. Finer-grained sand- beds 30 cm thick are folded into upright folds, 2-3 m in amplitude.
stone units may show graded beds, small-scale cross lamination, These rocks have been identified as distal turbidites and are distin-
ripples and slump structures. Subordinate components include guished by Turner (1983) as the Nior Member.
brown chert, chloritized tufts and volcanic rocks. The lower unit Black, micaceous mudstones and slates in a small tributary
was distinguished from an upper Phyllite and Shale Member in of the Auk Lajang to the NE of Ciranting contain ellipsoidal

~
'
~
Formation
Equator J 100{~Ex_,r~ ~, - i - ~ ~ ~ "< 1 ~t .':<-"-~Tuh ur 1[~2 ~
~"
PAYAKUMBUH~
BUKIT
O RENGAT ~~~--~~

~_~rigapuluh
raO,c,

Major Faults
-

Tabir Formation ~ '"*" ~ ii
"
-
Certain Recent
PERMO-TRIASSIC \ %~ "'-~::!i!::iii!i::: 9 MUARABUNGO ~ Volcanoes
Triassic Patepat Permo-Triassic
Formation Intrusions
Permian with \ )_~k ~
,. ~ volcanics ~ Duabelas S Serpentinite
k
- ?EARLY PERMIAN Formation Mountains 2,"
2 o C A R B O N I F E R O U S
J 2
Mentulu Fm etc.with pebbly mudst
Kuantan Formation
Limestone Units L 0 t ' 50 . . . . 100km
III
100~'E 101 ~ (~ 103 ~
I
Fig. 4.5. Distribution of Carboniferous, Permian and Triassic stratigraphic units in central Sumatra from GRDC map sheets, showing lithologies and critical localities as
well as Late Permian to Early Triassic intrusives. Areas left blank are covered by Late Mesozoic to Quaternary sediments and volcanics.