Page 66 - Sumatra Geology, Resources and Tectonic Evolution

PRE-TERTIARY STRATIGRAPHY 53

As the DMR/BGS Survey extended southwards, the model Muarasoma Formation are evidently of oceanic rather than of
developed in Aceh was used to interpret the Jurassic-Cretaceous continental margin origin. The bedded cherts and manganiferous
rocks correlated with the Woyla Group in the Natal area (Rock sediments in the Belok Gadang Formation were interpreted as
et al. 1983). The Muarasoma Formation at the northeastern representing the floor of an extensive ocean, rather than the floor
end of the Batang Natal section, with its turbidites and massive of a restricted marginal sea. A limestone block in m61ange, inter-
limestones was interpreted as shelf sediments formed on the preted as a collapsed carbonate capping to a sea mount, was found
continental margin of Sundaland. The Belok Gadang Formation, to contain a foraminifer of late Triassic age. Evidently the ocean
with pillow lavas manganiferous sediments and cherts, was inter- floor accreted into the Woyla accretionary complex was already
preted as the imbricated floor of the marginal basin, and the in existence in the early Mesozoic. An earlier date for the origin
Langsat Volcanics at the southwestern end of the section were of the Woyla ocean floor has been confirmed by the discovery
interpreted as the volcanic arc overlying a continental basement. of early Middle Jurassic radiolaria from cherts in the Indarung
The underlying basement was inferred from the Air Bangis Formation (correlated with the Woyla Group) near Padang
granites which intrude the volcanics, analogous to the situation (McCarthy et al. 2001). At the southwestern end of the Batang
at Sikuleh (Rock et al. 1983, Fig. 8). In the 'Tectonic Map of Natal section the Langsat Volcanics and the associated
Northern Sumatra' prepared by Aspden et al. (1982a) the conti- volcanoclastics were dated isotopically as of Late Eocene to
nental fragments in Aceh and Natal were identified as the Early Oligocene age (Wajzer et al. 1991). They are not, therefore,
Sikuleh and Natal Microcontinental Blocks. A further block, the a Late Jurassic-mid-Cretaceous arc analogous to the Bentaro
Bengkulu Microcontinental Block was subsequently proposed in Volcanic arc of Aceh.
southern Sumatra. The concept of microcontinents was taken up The concept of microcontinental blocks accreted to the margin
by Metcalfe (1996, Fig. 15) who suggested that these microconti- of Sundaland in the mid-Late Cretaceous has not been proven.
nental fragments separated from the northern margin of Gondwana The arc volcanics of the Bentaro Formation and the granitoids
in the Late Jurassic and were accreted to the Sumatran margin in of the Sikuleh Batholith require detailed geochemical study to
the mid-Late Cretaceous. determine whether they represent arc volcanics extruded through
The study by Wajzer et al. (1991) necessitated the re-interpret- a continental basement. There is no evidence either at Natal or
ation of the Batang Natal section and the reassessment of the mar- Bengkulu for a microcontinental block, the Langsat Volcanics
ginal sea model. It was found that the turbidites of the Muarasoma and the Air Bangis granites have been shown to be part of an
Formation were volcaniclastics, with no significant proportion Eocene to Early Oligocene volcanic arc emplaced against the
of quartz, and that the massive limestones did not contain Natal section by late (Neogene or Quaternary?) strike-slip faulting
any material of continental derivation. The sediments of the (Barber 2000).