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The Kutai Basin, Kalimantan, is one of several large Tertiary basins which opened during the mid- to late Eocene in Borneo. Extensional tectonics are responsible for basin formation, and not crustal flexure as has been suggested previously. Extensional faulting did, however, occur within a general foreland setting located to the south of the late Cretaceous/Palaeogene Central Kalimantan fold belt. The basin post-dates late Cretaceous/early Tertiary orogenic activity which affected the Central Kalimantan foldbelt and formed several upper Cretaceous granites. The Palaeogene stratigraphy of the basin includes basal conglomerates, shallow marine clastics and minor clastics and thick sequences of bathyal marine shales. Neogene stratigraphy is dominated by deltaic clastics and carbonate platforms. Several phases of Tertiary igneous activity are seen intruded into and interbedded with Tertiary sediments of the Kutai Basin. Three suites are recognized and have been variously interpreted as the products of melting of an orogenic root, extensional driven melting and/or subduction related melting. We review Tertiary sedimentation in the basin and propose a new model which relates the formation of the Kutai Basin to the opening of Celebes Sea and collapse of an uplifted Late Cretaceous/Palaeogene orogenic belt.

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