Page 55 - Sumatra Geology, Resources and Tectonic Evolution

42 CHAPTER 4

6 ~
Cretaceous granites
BANDA WOYLA GROUP
~ Oceanic assemblage
Arc assemblage
Geumpang
Meukuk Gneiss
Lamno
Bentaro ~ . Larn Minet Undifferentiated
Volcanic~
5 ~ N Peridotite/serpentinite 5 ~

Teunom~ , Batholith
CALANG Lke

Penarum

r undifferentiated
MEULABOH
4 ~ N
~. Faults Babahrot
Thrusts
SFZ Sumatran Fault System
KL Kla Line
GL Geumpang Line
Meukuek
Fig. 4.13. The distribution of the Woyla
0 50 100km '~ Group in Aceh. Modified from Stephenson
9 TAPAKTUAN & Aspden (1982), with data from Bennett
96 ~ 97 ~ et al. (1981a, b) and Cameron el aI.
I I (1982, 1983).

to the SE of Banda Aceh on both sides of the Sumatran Fault. Rock Bale Formation, composed of coloured slates, with minor wackes
types include massive or schistose basic volcanics, pillow basalts, and cherts, limestones and limestone breccias, is shown outcropping
volcaniclastic sandstones and tufts, commonly epidotized and to the NW of the Sumatran Fault, and SE of Takengon.
altered to greenschists or phyllites, and thin grey or black lime-
stones. The phyllites are usually lineated and crenulated, Arc assemblage. The basaltic-andesitic volcanics are interpreted
indicating multiple delbrmation. The rocks of the Geumpang as an island arc assemblage (Cameron et al. 1980) (Fig. 4.27),
Formation are considered to constitute the typical lithological which is represented on the Banda Aceh Sheet (Bennett et al.
and structural assemblage of the Woyla Group. The Geumpang 1981a) by the Bentaro Volcanic Formation, and on the Tapaktuan
Formation also includes a massive limestone member, frequently Sheet (Cameron et al. 1982b) by the Tapaktuan Volcanic
occurring as marble. The very similar Babahrot Formation Formation. The Bentaro Volcanic Formation is composed of
cropping out to the NW of the Anu-Batee Fault towards porphyritic basalts and andesitic basalts with agglomerates,
Tapaktuan (Cameron et al. 1982c) (Fig. 4.13)includes serpenti- which are intruded by basic dykes. Basaltic vents, surrounded by
nites and talc schists, as well as metagabbroic bodies metamor- breccias, tufts and volcaniclastic sediments, have been identified
phosed in the greenschist facies and highly disrupted and near Lain No and north of the Bentaro River on the Banda Aceh
sheared into lenses. Sheet. A chemical analysis of a xenolithic, porphyritic basalt
The Lain Minet Formation (Banda Aceh Sheet--Bennett et al. with pyroxene phenocrysts from this formation is given in Rock
1981a) and the similar Gume Formation (Takengon Sheet-- et al. (1982). The Tapaktuan Volcanic Formation occurs in
Cameron et al. 1983) are composed of basaltic lavas, commonly fault-bounded lenses, within strands of the Anu-Batee Fault
epidotized, basaltic conglomerates and breccias, with volcanic Zone, parallel to the west coast of Aceh north of Tapaktuan
and limestone clasts, but only rarely chert, graded volcaniclastic (Fig. 4.13). It consists of massive epidotized andesites and
wackes, radiolarian cherts with manganese oxide veining, rhodo- basalts, commonly porphyritic, and intrusive dykes of a similar
nite, and calcareous, manganiferous and carbonaceous slates. A composition. An analysis of hornblende microdiorite from this
clast of radiolarian chert, embedded in a volcanic conglomerate formation is given in Rock et al. (1982). The formation also
with flattened clasts, was collected by Nick Cameron (pers. includes agglomerates, breccias, tufts, red and purple volcaniclas-
comm. 1999) in the Kreung Baro, Aceh, from a landslip within tic sandstones and shales, the latter often as slates, and a limestone
the outcrop of this formation. This occurrence indicates that volca- member, composed of sparite and calcilutite, all as lenses and
nic rocks were erupted through ocean floor sediments, perhaps much disrupted by faults.
during the formation of a seamount. The formation also includes Scattered outcrops of gneiss (Meukek Gneiss Complex) occur
a recrystallized limestone member. The Jaleuem Formation crop- within the Tapaktuan Volcanic Formation in the Barisan
ping out 100 km to the SE of Banda Aceh on both sides of the Mountains to the north of Tapaktuan, between strands of the
Sumatran Fault, is composed largely of slates, but red cherts Anu-Batee Fault (Fig. 4.13). They consist of concordant leuco-
occur in float and the unit also includes a limestone member. The granodioritic gneiss, with garnet-biotite amphibolite containing