5th Swiss Geoscience Meeting, Geneva 2007
An overview of Upper Triassic carbonate deposits of
Indonesia: Palaeogeographic and geodynamic
implications
Rossana Martini*
* Dep. Geology and Palaeontology, University of Geneva, 1205 Geneva, Switzerland
(Rossana.Martini@terre.unige.ch)
Present-day structural framework of Indonesia has been generated by the
relative convergence of three main plates since Mesozoic times: Philippine
Plate; Indian-Australian Plate, and Eurasian Plate.
In Indonesia this convergence strongly dismembered the margins of the
continental plate into “microcontinents” and several marginal basins opened,
especially during Miocene times.
One of the ways to better understand the origin of the Banda Sea blocks is to
investigate the Mesozoic pre-rift sequences using fossil assemblages in wellconstrained depositional environments. The aim of this project was to study the
Upper Triassic carbonate deposits in different islands, and to compare them
with series of surrounding areas (e.g., Australian Plate), in order to provide
information for palaeogeographic and geodynamic reconstructions.
The existence of an extensive carbonate platform was already identified in
various localities all around the Banda Sea: Sinta Ridge; Central-East Sulawesi;
Buru; Misool, and the Wombat Plateau (off NW Australia). News results from
Eastern Sulawesi; Central and Western Seram, and West Timor islands are
presented.
Sulawesi Island is composed of four main structural zones: the Western Zone
(a Cainozoic volcanic arc which developed on a Lower Cretaceous collision
complex); the Central Zone (made up of metamorphosed sedimentary rocks
and ophiolites, including blueschist facies); the Ophiolitic Zone (composed of a
dismembered ophiolitic complex), and the Banggai-Sula Zone (composed of a
continental basement overlain by passive margin type sedimentary rocks). The
investigated area is located on the western margin of the Ophiolitic Zone. Upper
Triassic rocks in Kolonodale area are represented by a wide carbonate platform
that is 200 to 250 meters thick and at least 20 km long. The Late Triassic (Late
Norian-Rhaetian) age of this carbonate complex is based on biostratigraphic
data (foraminifers and palynomorphs).
Seram Island is one of the continental fragments belonging to the outer Banda
Arc, mainly composed of strongly imbricated metamorphic, ophiolitic and
sedimentary sequences. It rims the active inner volcanic Banda Arc related to
the Indian-Australian north-dipping subduction. In Seram, the Triassic deposits
are found in the parautochthonous, as well as in the allochthonous series.
The facies of the two series are of Gondwanian-Australian type in the
Parautochthonous, and of Laurussian-Asian type in the Allochthonous. The
Asinepe Limestone, about 1500 m thick, extends in the main central ranges,
from the Manusela Mountains, to the Loemoete Mountains, forming one or more
overthrust sheets. The complete Manusela Mountains section is comprised of
an Upper Carnian to Norian - Rhaetian interval. This range is confirmed by
5th Swiss Geoscience Meeting, Geneva 2007
foraminiferal and palynological data, and the coral framebuilders give evidence
for a Norian-Rhaetian age.
Timor Island is the result of collision between the Australian continent and the
geological complex of Banda. From a structural point of view Timor is classically
subdivided into three tectonic complexes: the Parautochthonous (Permian to
Oligo-Miocene); the Allochthonous, of unknown origin (Jurassic to Oligocene),
and the Autochthonous (Lower Miocene to Recent). Triassic deposits are found
in the Parautochthonous, as well as in the Allochthonous complexes. The facies
of the two series are pelagic in origin with radiolaria and filaments. Sporadically,
highly fossiliferous condensed limestone with ammonites and conodonts also
occur. The whole stratigraphic data (palynomorphs, radiolaria, ammonites,
conodonts) lead to the suggestion that the Allochthonous complex of Timor,
classically interpreted as a tectonic melange of the accretionary prism of the
Island Arc of Banda, is a tectonically dismembered part of an Upper Triassic
(Carnian-Norian to Rhaetian) lithostratigraphic succession.
The biostratigraphic results are consistent with reconstructions that place the
Seram-Buru Block in a palaeogeographic zone distinct from that of Sulawesi,
and therefore from the Kolonodale Block. For these reasons we propose that,
during the Upper Triassic, the Seram-Buru Block and the Kolonodale Block
were two separated entities, the former located in a more tropical position
(Onslow-type flora) than the latter (Ipswich-type flora).
Microflora data suggest that during the Late Triassic-Early Jurassic times, the
Seram microcontinent moved progressively to the north crossing the boundary
between the temperate and the warm subequatorial belts.
It is proposed that the Seram-Buru Block did not originate in the same area of
the Australian shelf as the Kolonodale Block (East Sulawesi): the former
originated from the Irian Jaya area and the latter from the Argos Abyssal Plain.
The presence of foraminiferal associations, showing close affinities to each
other in Seram, East Sulawesi, Wombat Plateau, and Sinta Ridge, indicates
that these geological entities were parts of the same marine
paleobiogeographical wide province, the northern Australian margin, which
extended from the Wombat Plateau to Irian Jaya.
The island of Timor was classically considered to be an external part of the
Australian margin. However, comparing the Triassic of Timor with that of the
many regions of the intensively studied Australian margin (Papua-New Guinea,
NW coast of Australia, Wombat Plateau), similarities do not clearly appear.
Conclusively, in the current state of knowledge, the initial adherence of the
Triassic series of the Allochthonous of Timor to the Australian margin is highly
questionable. The studied area in Timor underwent an original sedimentary
evolution, rather different from that of the Australian margin, and of the
microcontinents of the Banda Sea.
REFERENCES
Martini, R., L. Zaninetti, B. Lathuilière, S. Cirilli, J.-J. Cornée & M. Villeneuve
(2004) - Upper Triassic Carbonate Deposits of Seram (Indonesia):
palaeogeographic and geodynamic Implications. Palaeogeography,
Palaeoclimatology, Palaeoecology, 206, p. 75-102.