Page 93 - Sumatra Geology, Resources and Tectonic Evolution

80 CHAPTER 6

I I I %,/ I I
100OE,., 101 ~ 102 ~ 103 ~ 105 ~ 106 ~
, Singkarak

1os ah 50 100km
Siguntu I

~ukgadang

~Tabir
2 ~ BANGKA

3 ~ 9 PALEMBANG
Pendopo

Faults "" 121 +2Ma
JAVA
Thrusts SEA
BENGKL
WOYLA GROUPASSEMBLAGES BATURAJA

~ Oceanic Volcanic Arc
Lampung
High
-~ Accretionary Complex
(ocean-floor material)

Foreland assemblage

[ [ Palaeozoic basement
102 ~ 103 ~ 104 ~

Fig. 6.13. The distribution of the Woyla Group Assemblages in Southern Sumatra and localities mentioned in text.

(Table 6.8) is a tectonic composite of oceanic and foreland Oceanic volcanic arc fragments
lithologies.
Oceanic island arcs, fragments of which are incorporated within
the Woyla Accretionary Complex, originated in Meso-Tethys
probably in the Early Jurassic. The volcanic arcs have been
West Java Sea suggested to have been constructed on continental basement
(Cameron et al. 1980), but Hamilton (1988) and Barber (2000),
To the east of Sumatra, in Java and the West Java Sea, the with more detail, has thrown doubt on this idea, and also on the
Woyla Group is difficult to trace, but lithologies of the Woyla suggestion that these arcs originated as fragments of Gondwana
Accretionary Complex have been recognized in oil well termin- (Metcalfe 1996). It would appear that the Woyla Oceanic Volcanic
ations in the off-shore Sunda oil field, where serpentinite and Arcs originated within Meso-Tethys, although how many island
metasediments, together with Late Cretaceous granites, were arc strings were created, and whether the strings were continuous
encountered beneath Tertiary sediments. In the southern part of is not certain. In Aceh there are three large arc fragments, the
the Sunda Basin, in the East Java Sea, the Woyla Group is overlain Bentaro, Tapaktuan and Sise (Fig. 4.13) of which the latter is poss-
by Late Cretaceous sediments. Ben Avraham & Emery (1973) ibly a different age to the other two, depending upon the nature of
found that the interpretation of magnetic intensity measurements the undifferentiated area of Woyla Group east of the Anu-Batee
in the East Java Sea was problematic, but the magnetic anomalies Fault. In Southern Sumatra the Gumai-Garba Line (Fig. 6.13) of
have large amplitudes (200-600 gamma) and the wavelengths McCourt et al. (1993) links a string of arc fragments (Saling Arc)
(10-30kin) are shorter than, but resemble those of oceanic which appear to be of a similar age. Lithological details of the
crust. In the regional context these anomalies might represent Oceanic Volcanic Arc fragments are given in Table 6.10.
the subcrop of ophiolite from the Woyla Accretionary Complex.
Certain zones within the West Java Sea have the magnetic
signatures of large basic or ultrabasic bodies, one example, on Aceh Province (refer to Fig. 4.13)
the SE margin of the Lampung High of SE Sumatra, has a
similar magnetic signature to the Pasaman Ophiolite Complex in The Bentaro Island Arc (Barber 2000) is the largest of the oceanic
the Natal area. island arc fragments included in the Woyla Group. The Bentaro