Page 64 - Sumatra Geology, Resources and Tectonic Evolution

PRE-TERTIARY STRATIGRAPHY 51

I
103o00 ' 103~ '

Lm
Qv Qv

Qv

to Bengkulu 60km
Qv
9 ,
9 , .
Qv
, ,
-- 3o45 '
9 , 9 . . . . . . .
._.,_____-.- F
. . . . . . . .
. . . . . . . . . .
Quaternary Volcanics
Q v
PI Pliocene
Lm Late Miocene 9 ,, .,, .,, ,., ,,.
9 , ,
Middle Miocene
-..,, ::
Tom Oligo-Miocene
Eocene - ' - F ~ ~ ............ .%<..:..
Qv ~iiilil
. . . . .
Late Cretaceous Granites
Sepingtiang Limestone Formation
Lingsing (sedimentary) Formation Qv ~ .J
Saling (volcanic) Formation = ------- Faults 0 5 10 15 20km
|
Pyroxenite I
103o00 ' 103o15 '
I I
Fig. 4.19. The distribution of the Saling, Lingsing and Sepintiang Formations, correlatives of the Woyla Group, in the Gumai Mountains, South Sumatra, after GRDC
map of Bengkulu (Gafoer et al. 1992c).

(Gafoer et al. 1994). Neither the cherts nor the limestones have so Intrusions in the Garba lnlier. Both the metamorphic Tarap and the
far yielded age-diagnostic fossils. Garba formations are intruded by the Garba Pluton (Fig. 4.7), a
The Garba Formation has been compared to the Woyla Group composite body in which an older component has been dated
of Natal (Gafoer et al. 1994) and certainly lithological descrip- by the K-At method at 115 • and 102_3Ma (mid-
tions of this formation and its Insu and Situlanglang members, Cretaceous) and a younger component at 79 • 1.3 Ma and
correspond very well with those from Aceh and the Batang 89.3 + 1.7 Ma (Late Cretaceous) (Gafoer et al. 1994). Since the
Natal section. The basaltic and andesitic lavas of the Garba Garba Pluton (115-79 Ma) intrudes both the Tarap and the
Formation correspond with those of the Bentaro Arc, and may Garba formations, the accretion of the Garba Formation to
similarly be interpreted as part of a volcanic arc sequence. the margin of Sundaland took place before the mid-Cretaceous.
Limestone blocks within the m61ange may represent fragments The age of the younger component of the Garba Pluton is com-
of fringing reefs or the collapsed carbonate cappings of seamounts, parable to that of the Sikuleh Batholith in Aceh (98 Ma) and the
the latter now represented by volcanics in the Garba Formation, as Manunggal Batholith (87 Ma) in Natal.
has been suggested for the Natal and Indarung areas (Wajzer et al.
1991; McCarthy et al. 2001).
Descriptions of the m61anges of the Insu Member of the Garba Menanga Formation. The Menanga Formation occurs in scattered
Formation (Gafoer et al. 1994) are identical to those from Natal outcrops between Bandar Lampung and Kotaagung to the SW of
(Wajzer et al. 1991). The interlayering of the Insu Member with the schists and gneisses of the Gunungkasih Complex (Fig. 4.8).
lavas, chert and m61ange (Gafoer et al. 1994) suggests that these The Menanga Formation consists of tuffaceous and calcareous
rocks are deformed and imbricated in the same way as the claystones, sandstones and shales with intercalated radiolarian-
Woyla Group in the Batang Natal section, and similarly represent bearing cherts, manganese nodules and coral limestones and rare
an accretionary complex formed by subduction of an ocean floor. porphyritic basalt. The sandstones contain clasts of glassy andesite
It may be that some of the low-grade metamorphic schists mapped and lithic fragments of andesite, quartz-diorite and quartzite. The
within the Insu Member as Tarap Formation, are part of this accre- cherts have not so far yielded diagnostic radiolaria, but Zwierzijcki
tionary complex, as metamorphic rocks, up to greenschist facies, (1932, confirmed in Andi Mangga et al. 1994a), reports the occur-
are incorporated in the accretionary complex at Natal 9 Rock rence of Orbitolina sp. of Aptian-Albian (mid-Cretaceous) age
units within the Garba inlier are cut and bounded by NW-SE- fi'om limestones in the Menanga river section. The bedding
trending faults. Although these faults are parallel to the Sumatran strikes NW-SE with dips of 35o-60 ~ to the NE. The rocks
Fault System they do not appear to affect significantly the Tertiary are folded and cut by faults, with slickensides indicating reverse
rocks and must be largely of Pre-Tertiary age. movement.