Page 54 - Sumatra Geology, Resources and Tectonic Evolution

PRE-TERTIARY STRATIGRAPHY 41

9;~ . 9;~ 1'00o , !
A Active Volcanoes
Quaternary-Recent Volcanics
~ ; .~k~, : >,
Tertiary sediments and volcanics
Woyla Group and correlatives
Palaeozoic Basement

22 ~ ~ '7:. 9 9 v v .... "",--... j__~ 2 ~

k '\.~ NataI]~I~)~i i i i i i .O2 ."~-3" (7.

.
- " o ~ , i ~,.,,~ X;~. Rawas- 9 ...... 7)
" ~ ~ . ":..:'\,; ~'~,~'~l~,A~a~: : : : : : : : : .--. 7":.-,.
nUN A NI O X %.,. \;~ \~.[ L:~ ~.'-:-Gumai ........... .)
.,o.,
o k
~,,~,_.L,t"~• ~
~
-"._~ ?'~b ~A,~ " .........
-X~. -~.~\:jv,~ ...........
/
~ ~ ,~5 \ .~',,~ ~ '~,~ .'~'.~. >: ............ :~
.
/ / -"X "'~%. .... Garba ..... ',
7cm/yr / X " >%~O~n!ma;ung
"~.~ ~.: ~ 6 ~
Fig. 4.12. Simplified geological map of
Sumatra, showing the distribution of the
o?o o?~ ,oo0 ,o,,0 ,ooo Woyla Group and correlated units, with
localities mentioned in the text.

of the actual lithologies within each formation are, on the whole, Serpentinite units occur as lenses along the Sumatran Fault and
too small to be represented on the scale of the map. along the Geumpang Line (Fig. 4.13). Several serpentinite bodies
The stratigraphic units can be classified into three lithological are shown on the Takengon Sheet (Cameron et al. 1983), including
assemblages: an oceanic assemblage; a basaltic-andesitic arc the largest of these lenses, the Tangse Serpentinite, which extends
assemblage; and a limestone assemblage (Cameron et al. discontinuously for 27 km to the NW of Tangse, the Cahop
1980). All of the units generally occur as fault-bounded lenses, Serpentinite and the Beatang Ultramafic Complex. These units
distributed on both the northeastern and southwestern sides of are composed of massive serpentinite, representing altered
the Sumatran Fault, and are elongated in a NW-SE direction, harzburgite. Here and elsewhere, serpentinite is locally sheared,
parallel to the Sumatran trend. The oceanic assemblage in par- schistose, twisted and contorted. Sheared serpentinite may also
ticular is broken by a large number of minor faults and thrusts form the matrix to m61ange, i.e. the Indrapuri Complex on the
and has been interpreted as imbricated in an accretionary Banda Aceh Sheet (Bennett et al. 1981a). The m61ange encloses
complex formed above a subduction zone (Barber 2000). The blocks of cumulate gabbro, basalt, red chert and limestones,
arc assemblage and the associated limestones are interpreted as derived from other units in the Woyla Group. Fossils collected
a volcanic arc with fringing reefs (Cameron et al. 1980). The from limestone blocks within the m61ange include: corals--
Woyla Group is affected by several large scale thrusts; Latoceandra ramosa, Stylina girodi; foraminifers--Pseudocycla-
the Geumpang, Takengon and Kla lines, which also affect the mmina sp.; stromatoporoid--Stromatopora japonica, indicating a
Miocene rocks in the area and are attributed to movements on Late Jurassic to Early Cretaceous age. In the Takengon
the Sumatran Fault System. The distribution of these units and Quadrangle large blocks of limestone enclosed in sheared serpen-
their relationships to the faults and thrusts are shown on tinite along the Geumpang Line, contain Late Miocene fossils
Figure 4.13. (Cameron et al. 1983).
Other units of the oceanic assemblage include the Penarum
Formation, which outcrops to the northeast of the Sumatran
Oceanic assemblage. The oceanic assemblage includes serpenti- Fault south of Takengon (Cameron et al. 1983) (Fig. 4.13), and
nites, gabbros, either massive or layered, and often altered consists of serpentinites, basalts, red cherts with radiolaria and
to amphibolite, basalts, often as pillows, hyaloclastic breccias, slates. Volcanic rocks in this unit are commonly altered to greens-
volcaniclastic sandstones and siltstones, bedded cherts, black or chists. The Geumpang Formation (Banda Aceh Sheet--Bennett
purple shales and minor bedded or massive limestones. et al. 1981a; Tapaktuan Sheet--Cameron et al. 1982c) crops out