THE POSO BASIN IN GORONTALO BAY, SULAWESI: EXTENSION RELATED TO CORE COMPLEX FORMATION ON LAND

摘要

Gorontalo Bay is a semi-enclosed sea between the North and East Arms of Sulawesi. It is surrounded by land on three sides, separating a northern volcanic province from metamorphic rocks to the south and west, and ophiolites to the southeast. Seismic analysis and literature research suggest a possible Early Miocene origin for Gorontalo Bay, following Sula Spur collision which resulted in terrestrial conditions. In the western part of Gorontalo Bay there are two subbasins: the northern Tomini Basin and the southern Poso Basin, which have different histories. This study presents a new geological interpretation of the Poso Basin based on recent multibeam and 2D seismic data. The seismic stratigraphy of Tomini Basin shows progressive deepening of the area, with deposition largely under marine conditions (Unit B and C), followed by a regional uplift event (Middle-Late Miocene?) that caused local subaerial erosion (east Lalanga Ridge) with development of an unconformity and shallowing of the central part of the basin. Renewed subsidence formed shallow marine environments (Unit D and E). Subsidence later accelerated, causing backstepping of the shelf edge, drowning of pinnacle reefs and subsidence to present depths of 2 km in the basin centre (Unit F). The Poso Basin is much younger than Tomini Basin. The deeper part of the sequence is probably the time equivalent of Unit D in Tomini Basin. However, rapid subsidence is recorded by a thick sequence of up to 3 sec TWT which is the equivalent of the thinner Units E and F in Tomini Basin. Subsidence is interpreted to be related to uplift nearby. A large north-dipping potential low angle normal fault, identified below the southern part of Poso Basin, suggests that the development of Poso Basin may be related to extension associated with the rapid development of metamorphic core complexes on land.