Origin and history of the East Makassar Strait
Stephen Calvert, completed PhD thesis
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 and date 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, 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 are. 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 event.