Page 40 - Sumatra Geology, Resources and Tectonic Evolution

PRE-TERTIARY STRATIGRAPHY 27

I I I I
96 ~ 97 ~ 98 ~ 99 ~
BA " NDA ' A'-CEH
Major Faults

Recent Volcanoes

Unit @ Permo-Triassic Intrusions

,E~::~Ujeuen
tion

Sormation
Tawar "~
Formation 9 LANGSA
LATE PERMIAN - LATE TRIASSIC |N
(Peusangan Group)
Uneun Unit, Tawar Lst Fro, Simpang Kiri
Situtup Lst Fm, Sembuang Lst Fm,
Ujeuen Lst Fm, Kaloi Lst Fm,
Batumilmil Lst Fm (mainly limestones) Gnei Kaloi
Kualu Formation (cherts & clastics) Formation
CARBONIFEROUS - ?EARLY PERMIAN
(Tapanuli Group) ~.
,U.:0 XkJ'~i
Bohorok Formation
(pebbly mudstones)
Atas Formation (Vis6an) Bohorok
limestone member
,:-,,C.e-. -.<-
N alvvampu Toba
Alas Formation - clastic sediments \ tumilmil Tufts
('m'- metamorphosed) TA PAKTUAN
Kluet Formation --.. (._~ Kualu
(turbidites with limestone %') Formation
9 -.,....,_
Ktuet Formation o Toba
(metamorphosed) Tufts ~-~ I~j
=_ i i lU... =. lOOk~
96 < , 97 ~
1 I
Fig. 4.3. The distribution of Carboniferous, Permian and Triassic stratigraphic units in northern Sumatra, showing rock types and critical fossil localities, together with
Late Permian to Early Triassic intrusions (after Stephenson & Aspden 1982, with additions from GRDC map sheets, Cameron et al. 1982a, b, 1983). Areas left blank are
occupied by Late Mesozoic to Quaternary sediments and volcanics.

Alas Formation. The Alas Formation was defined by Cameron areas of metamorphic rocks are closely associated with igneous
et al. (1982a) in the valley of the lower Alas River on the intrusions and some, particularly where the rocks are garneti-
Medan Sheet (Fig. 4.3). It is distinguished by its geographical ferous, may be of regional metamorphic origin and may even
location, occupying a graben within the Sumatran Fault System, represent an earlier, Pre-Carboniferous, basement. The occurrence
between the outcrops of the Bohorok and Kluet formations, of mylonites and cataclasites suggests that some of the rocks
and by a preponderance of limestones and meta-limestones. included in the Alas Formation have undergone major shearing.
Otherwise, in the remainder of the outcrop, shales, siltstones, sand- A fossiliferous limestone locality within the Alas Formation at
stones, sometimes calcareous, quartz wackes and conglomerates, the junction of the Lau Pakam and the Sungai Alas north of
are identical to those of the Bohorok Formation, without the Laubaleng has yielded a rich fauna (Fig. 4.4). Cameron et al.
pebbly mudstones, and to the Kluet Formation as well. Cameron (11980) reported the coral Allotriophyllum chinense, known from
et al. (1982a) also report the occurrence of possible green tufts. the Lower Permian Chiksa Limestone of southern China, but
The outcrop is much dissected by faults and the rocks are intensely this coral has been re-identified by Fontaine (1989) as the solitary
folded locally, intruded by granites and migmatised. horn-shaped rugose coral Zaphrentites, indicative of a Carboniferous
Limestones in the Alas Formation are sometimes oolitic, may age. Brachiopods, which include Cleiothyridina (?) and Margina-
show cross-bedding and are locally fossiliferous with abundant tia, indicate a Vis6an age and Metcalfe (1983) obtained a conodont
productid and spiriferid brachiopods and some corals. However, fauna from this same locality which included Gnathodus girtyi
the limestone is frequently metamorphosed to massive, coarsely rhodesi Higgins, Gnathodus sp., Hindeodella sp., Spathognathodus
crystalline and sometimes graphitic marble with phlogopite, campbelli Rexroad and Spathognathodus scitulus (Hinde), con-
and deformed to form calcareous schist. The marbles and calc- firming the Vis6an age of the limestones. The form Gnathodus
schists are associated with slate, phyllite, mica schist, locally girtyi rhodesi, in particular, is restricted to the Bollandian Stage
containing garnets, biotite hornfels with cordierite and/or chiasto- of the Late Vis6an, defining the age of this outcrop of the Alas
lite, quartzite and more rarely gneiss, migmatites, mylonites and Formation even more precisely (Metcalfe 1983).
cataclasites (Cameron et al. 1980). Much of this metamorphism
may be attributable to the contact effects of intrusive granites, Kluet Formation. The Kluet Formation was defined by Cameron
affected synchronously or subsequently by shearing, but not all et al. (1982b) from outcrops along the Krueng Kluet in the