The Kutai sedimentary basin extends from the central highlands of Borneo, across the eastern coast of the island and into the Makassar Strait. With an area of 60,000 km2, and depths up to 15 km, the Kutai is the largest and deepest Tertiary age basin in Indonesia. Plate tectonic evolution in the Indonesian region of SE Asia has produced a diverse array of basins in the Cenozoic. The Kutai is an extensional basin in a general foreland setting. Its geologic evolution begins in the mid Eocene and involves phases of extension and rifting, thermal sag, and isostatic subsidence. Rapid, high volume, sedimentation related to uplift and inversion began in the Early Miocene. The different stages of Kutai basin evolution can be roughly correlated to regional and local tectonic events. It is also likely that regional climate, namely the onset of the equatorial ever wet monsoon in early Miocene, has affected the geologic evolution of Borneo and the Kutai basin through the present day. Basin fill is ongoing in the lower Kutai basin, as the modern Mahakam River delta progrades east across the continental shelf of Borneo.
Plate tectonic setting
The Cenozoic plate tectonics of the Indonesian region have generated a complex assemblage of micro-continental blocks and marginal ocean basins surrounded by extensional margins, subduction zones and major transcurrent faults. The island of Borneo and the Kutai basin are located on the Sunda micro-plate, which is bounded to the north and west by the Eurasian plate, to the south by the Indo-Australian plate and to the west by the Philippine and Pacific oceanic plates. In the Cenozoic, the Indo-Australian plate has been moving north and subducting under Eurasia. The collision of the Indian continent with Eurasia halted subduction and uplifted the Himalayas. In between the continents of India and Australia, the oceanic crust is still subducting under the Sunda plate, forming the Sunda trench and Sunda Arc. Australia and Australian derived micro-plates collided with the Sunda plate and Pacific plate in the Pliocene, creating a complex of subduction zones and island arcs. The Philippine plate has been obliquely subducting the Sunda plate for most of the Cenozoic.
The complex interaction of the Sunda, Eurasian, Indo-Australian, Philippine and Pacific plates in the Cenozoic has controlled the evolution of approximately 60 Tertiary sedimentary basins in the Indonesian region. Many of these basins, including the Kutai, have formed in a back arc extensional setting, driven by passive or active subduction rollback. The mid Miocene episode of inversion in the Kutai can be linked to collision of continental fragments from the South China Sea with NW Borneo. The Pliocene inversion episode is contemporaneous with the collision of Australia with the Banda arc, with structural connections provided by strike-slip fault systems through Sulawesi.
Geology of Borneo
The basement rock of Borneo is a complex mosaic of geologic terrains, commonly interpreted as the product of primarily Mesozoic accretion of micro-continental fragments, island arc material, oceanic crustal material and marginal basin fill onto the Paleozoic core of the Schwaner Mountains in the southwest of the island. The Schwaner Mountain area consists of early-mid Cretaceous granitic batholiths intruded into Silurian to Permian age metamorphic units. NW of the Schwaner Mountains is a small area of older continental basement consisting of Permo-Triassic granite and metamorphic rocks. SE of the Schwaner Mountains, volcanic island arc and ophilitic rocks emplaced in the late Cretaceous comprise the Meratus Mountains. The basement terrain of eastern and northern Borneo is interpreted to be Cretaceous subduction melange, mostly covered by tertiary sediment. The basement of Western Borneo is an accreted melange of upper cretaceous to Paleocene age that formed the central Kalimantan Ranges as the result of SW directed subduction beneath the continental core of Borneo.