Syntectonic Eocene to Miocene carbonate platform development in Sulawesi
Moyra Wilson – completed PhD project
South Sulawesi is located in the midst of a complex tectonic region where three major plates interact and collide. The Tonasa Limestone Formation was deposited initially as part of a transgressive sequence in an extensional backarc setting. Tectonics, oceanography and the nature of carbonate producing organisms exerted a major control on carbonate sedimentation, whilst eustatic and autocyclic affects are difficult to differentiate.
Detailed facies mapping, logging and petrography reveal that by late Eocene times shallow-water carbonates were being deposited over much of South Sulawesi forming a widespread (100 km length) platformal area, named here as the Tonasa Carbonate Platform. Normal faulting resulted in segmentation of the platform, causing basinal graben formation or localised subaerial exposure in areas proximal to faulting. On other parts of the platform, shallow-water carbonate sedimentation continued unabated until the middle Miocene. During this period the syntectonic tilt-block platform had a steep faulted northern margin, a gently dipping southern ramp-type margin and more complex areas of block-faulting lay to the east and west.
Platform top facies belts trend E-W and are composed of wackestones, packstones and grainstones dominated by large benthic foraminifera. These facies belts were relatively static through time, indicating subsidence and aggradation of the platform top. In fault bounded graben areas, basinal marls are interbedded with thick sequences of texturally immature redeposited carbonate facies containing basement clasts. These redeposited facies were derived from the footwall areas of syn-depositional faults and provide a unique record of tectonic activity which is consistent with more regional tectonic events.
Redeposited carbonate facies and interbedded marls from the Barru area are described and interpreted in this study. The immaturity and provenance of clasts indicate that the redeposited facies were derived from the faulted northern margin of the Tonasa Carbonate Platform. A relay ramp between at least two major NW-SE trending faults is the inferred configuration of this margin. Three main phases of faulting are indicated by the redeposited facies: late Eocene to early Oligocene, middle Oligocene, and early to middle Miocene. This is consistent with other outcrop and seismic data from the region and with the inferred plate tectonic situation during the Tertiary.
Wilson, M. E. J. & Bosence, D. W. J. 1996. The Tertiary evolution of South Sulawesi: a record in redeposited carbonates of the Tonasa Limestone Formation. In: Tectonic evolution of Southeast Asia. Hall, R. & Blundell, D.J. (eds.). Geological Society Publication No 106. 365-389.
Temporal and spatial variations have been used to construct a palaeogeographic reconstruction of the platform and to evaluate controls on carbonate sedimentation The late Eocene to mid Miocene shallow-water and outer ramp/basinal deposits of the Tonasa Carbonate Platform, from the Pangkajene and Jeneponto areas of South Sulawesi respectively, formed initially as a transgressive sequence in a probable backarc setting. The platform was dominated by foraminifera and had a ramp-type southern margin. Facies belts on the platform trend east-west and their position remained remarkably stable through time indicating aggradation of the platform top. In comparison outer ramp deposits prograded southwards at intervals into basinal marls. Tectonics, in the form of subsidence, was the dominant control on accommodation space on the Tonasa Carbonate Platform. The location of ‘barriers’ and the resultant deflection of cross-platform currents, together with the nature of carbonate producing organisms also affected sedimentation, whilst eustatic or autocyclic effects are difficult to differentiate from the affects of tectonic tilting. Moderate to high energy platform top or redeposited carbonate facies may form effective hydrocarbon reservoirs in otherwise tight foraminifera dominated carbonates, which occur widely in SE Asia, and have not been affected by extensive porosity occlusion.
Wilson, M. E. J. & Bosence, D. W. J. 1997. Platform-top and ramp deposits of the Tonasa Carbonate Platform, Sulawesi, Indonesia. In: Petroleum Geology of Southeast Asia. Fraser, A. J., Matthews, S. J. & Murphy, R. W. (eds.). Geological Society Publication 126, 247-279.