Origin and history of the East Makassar Strait
Stephen Calvert, completed PhD thesis
Fieldwork in the Karama River, Sulawesi: Eocene sedimentary rocks in background
The Lariang and Karama regions of Western Sulawesi, a total area of c. 10,000 km2, were the object of this field-based investigation which had the aim of understanding the Cenozoic evolution of the eastern side of the Makassar Straits. The Cenozoic history of Western Sulawesi was influenced by the Paleogene development of the Makassar Straits to the west, and the Neogene collision of continental, ophiolitic and island arc fragments to the east. The timing of these events has been the subject of considerable debate and it has been suggested that the collisions in Sulawesi caused inversion in Borneo. The study area has never been integrated into models for the region and the new data, therefore, provide significant insights into the evolution of the region.
Sixty days fieldwork involved detailed logging, basic field mapping, and the collection
of 550 rock samples. Data was collected from a series of logging roads in the deforested
coastal lowlands, and from river traverses into the mountainous jungle interior. Samples
were dated using nannofossils, planktonic foraminifera, and pollen. Remote sensing data
have been interpreted and integrated with the field and biostratigraphic data to produce a
new geological map of the region, with a significant revision of the existing
stratigraphy. Detailed sedimentary facies analysis combined with information from the
microfossil assemblages allowed interpretation of Cenozoic environments. Thin section
analysis identified provenance trends in the Neogene sequences. The new mapping, with
interpretation of onshore seismic data, enabled structural interpretation of the region.
Two major unconformities were identified. There is an unconformity between Upper
Cretaceous forearc deposits and Eocene shelf sediments, and a younger unconformity
between Lower Pliocene shelf sediments and Plio-Pleistocene syn-orogenic sediments.
Non-marine sediments at the base of the lower Tertiary sections could be as old as
Paleocene. The oldest dated sediments are marine anddate a transgression in the Middle
Eocene (NP15, 48-44Ma) which may be related to pre-breakup subsidence but must
post-date the initiation of rifting in the region. The Eocene sediments were deposited in
graben and half graben in both marine and marginal marine environments. The
Makassar Straits Eocene rift was highly asymmetric. This is more consistent with a
simple shear, rather than a pure shear, a model for lithospheric extension. The zone of
extension of the western margin (Sulawesi) was approximately twice as wide as that of
the eastern margin (Borneo). Comparison with other rifts shows that such asymmetry is
not unusual.
The post-rift subsidence phase started by the Late Eocene. In the Late Eocene carbonate
shoals and shelf mudstones covered the margins of the Makassar Straits, and by the end
of the Oligocene most of Western Sulawesi was an area of shelf carbonate and
mudstone deposition.
During the Early Miocene microcontinental fragments collided with the SE Arm of
Sulawesi. In South Sulawesi, there is no evidence for a break in marine deposition
although in the study area the lowermost Miocene has not been found. However, there is
no evidence in Western Sulawesi for a significant regional unconformity or input of
orogenic sediment. Instead, throughout the Early Miocene and in places until the
Middle-Late Miocene, carbonates and mudstones were deposited. The Early Miocene
collisions did not cause an orogeny in Western Sulawesi.
In the Lariang and Karama regions volcanogenic input dates only from the late Middle
Miocene. The lack of widespread persistent Cenozoic volcanism indicates the absence
of a nearby arc. Towards the end of the Middle Miocene the region was undergoing
extension and not inversion or crustal thickening. There was inversion and uplift in
eastern Kalimantan during the Miocene but none in Western Sulawesi. The inversion in
eastern Kalimantan was due to tectonic events in Borneo, not collisions in Sulawesi.
In the Pliocene sedimentation across the whole of Western, Central and East Sulawesi
changed significantly. There was uplift and erosion followed by the deposition of coarse
orogenic clastics. In the Late Pliocene the study area changed from a passive margin to
a foreland basin setting, the older Paleogene basins were inverted, and sedimentation
rates doubled. During the Pliocene, there was major orogenic activity to the east of the
study area. To the west of the orogenic belt there was syn-orogenic sedimentation,
inversion of Paleogene basins, folding and thrusting, and development of intra-basinal
unconformities and mini-basins. The deformation offshore Western Sulawesi dates from
the Pliocene whereas deformation offshore eastern Kalimantan dates from the Early
Miocene. The Pliocene orogeny in Sulawesi resulted in the development of the present day
mountains which are up to 3km high.
