Abstract

The Lariang and Karama regions of Western Sulawesi, an area of approximately 10,000 km², were the subject of a field-based investigation with the aim of understanding the Cenozoic evolution of the eastern Makassar Straits. Western Sulawesi was influenced by the development of the Makassar Straits to the west, and the 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 Neogene collisions in Sulawesi caused inversion in Borneo. A new stratigraphy for the Lariang and Karama regions of Western Sulawesi, based on fieldwork, is presented here and provides new and significant insights into the evolution of the Makassar Straits region. The oldest sediments are non-marine and could be as old as Paleocene; they include coals, sandstones and mudstones. Rifting had started by the Middle Eocene and continued into the Late Eocene. Eocene sediments were deposited in graben and half graben in both marine and marginal marine environments. The Eocene Makassar Straits rift was highly asymmetrical; the Kalimantan margin was approximately twice the width of the Sulawesi margin. Thermal subsidence had started by the Late Eocene and by the end of the Oligocene most of Western Sulawesi was an area of shelf carbonate and mudstone deposition. Carbonates and mudstones were deposited throughout the Early Miocene and in places until the Middle or Late Miocene. During the Pliocene the character of sedimentation changed significantly. Uplift and erosion was followed by the deposition of coarse clastics derived from an orogenic belt to the east of the study area. Early Miocene collisions to the east had little effect on Western Sulawesi.

Orogenic deformation, a regional unconformity and input of orogenic sediment are Pliocene. Deformation offshore Western Sulawesi dates from the Pliocene whereas deformation offshore Eastern Kalimantan dates from the Early Miocene.