Carbonate Depositional Systems Miocene carbonate shelf and knoll reef deposits, Sulawesi
Alit Ngakan, PhD Thesis – completed

Alit Ngakan studying carbonates of the Neogene Tacipi Formation, Sulawes

Alit Ngakan studying carbonates of the
Neogene Tacipi Formation, Sulawes

South Sulawesi is located in the centre of a complex tectonic region where Indo-Australian, Eurasian and Pacific plates interact and collide. The Tacipi Formation is inferred to have been deposited in an intra-arc or forearc setting. Although outcrops of the Tacipi Formation are located in a present day N-S trending strike-slip system, seismic and field evidence suggests that the strike-slip regime was not active until the Pliocene. In middle Miocene times block faulting, localized uplift and gentle folding of the lower Tertiary strata occurred and carbonate production of the Tacipi Formation began in relatively shallow-marine areas.

The Tacipi Formation is up to 300 m thick and surface outcrops cover an area of 1,500 km2 in eastern South Sulawesi. Lithologies of this formation are composed of wackestones, packstones, floatstones, rudstones and framestones and these facies are dominated by corals, coralline algae, molluscs, benthic and planktonic foraminifera.

Detailed facies mapping, logging and petrography indicate that by middle Miocene times shallow-water carbonates of the Tacipi Formation were deposited in the south, whilst knoll-reef mostly displaying a north-south trend developed to the north. The knoll-reef comprise deeper-water mid-Miocene facies at the base and shallow upwards into late Miocene reef complexes. In the southern part, middle Miocene normal faulting resulted in segmentation of the platform, causing the formation of a localised unconformity and the contemporaneous development of shallowing and deepening upward sequences.

During the late Miocene shallow water carbonates developed throughout much of the area, whilst towards the west shallow-water material was reworked into deeper-water environments. Up sequence, in the southern shelfal area a number of reefs developed with a distinct NW-SE trend. There is evidence for probable upper Miocene karstification of the northern mound structures. During early Pliocene times shallow-water reefs developed towards the north surrounded by deeper water facies, whilst redeposited facies contain reworked Eocene and Miocene volcanic clasts and clasts derived from upper Cretaceous sandstones of the Marada Formation occur in the western area.

Tectonic activity controlled facies development and distribution of the Tacipi Formation during the middle to late Miocene. Localized unconformities and reworking of basement clasts in the middle Miocene and Pliocene respectively, indicate further tectonic activity. The widespread extent of shallow-water facies during the late Miocene suggests a relative transgression whilst extensive karstification surfaces suggests a relative sea level fall. A final phase of reefal buildup occurred in the north prior to Pliocene drowning. Stress regime associated with strike-slip motion on the East Walanae Fault systems may have caused uplift in the restraining bend (southern area), whilst subsidence occurred adjacent to releasing bend (northern region).

The Tacipi Formation was deposited during the middle Miocene to early Pliocene in an intra-arc or forearc setting. Deposition, diagenesis and hydrocarbon potential of these carbonates was strongly influenced by the tectono-stratigraphic setting, faulting, subsidence and possibly eustatic sea-level variations. Outcrops of the Tacipi Formation cover an area of 1,500 km2 and the thickness of the formation varies between 300-700 m. Subcrops of the this these carbonates also occur in the Sengkang Basin and form economic gas reservoirs. Dominant lithologies within the formation include reef related facies (framestones, rudstones and floatstones) and packstones and wackestones. These contain a wide variety of shallow marine bioclasts, such as corals, coralline algae, molluscs and large benthic foraminifera, as well as planktonic foraminifera. Interpretation of seismic data, detailed facies mapping, logging and petrography has allowed differentiation of the carbonate deposits into three different regions. These have a markedly different arrangement of facies, depositional environemnt and corresponding hydrocarbon potential.

The northern outcrops of the Tacipi Formation (Northern Bone Region) consist of isolated knoll-reefs, mostly displaying a north-south trend, surrounded by deeper-water facies. These buildups are composed of deeper-water middle Miocene facies at the base and shallow upwards into late Miocene reef complexes with locally developed karstification. These outcrops are comparable with knoll reefs forming gas reservoirs in the subsurface. However, differential subsidence resulted in variations in the timing of drowning of these reefs. Fine grained clastic sediments, including a high volcaniclastic component, cover the knoll reefs and act as seals.

Ascaria, N. A., Harbury, N. A. & Wilson, M. E. J. 1997. Hydrocarbon potential and development of Miocene Knoll-Reefs, South Sulawesi. In: Proceedings of the Petroleum Systems of SE Asia & Australasia, Indonesian Petroleum Association, 569-584