Kutai Basin
[2] 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.
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.
[5] The Schwaner Mountain area consists of early-mid Cretaceous granitic batholiths intruded into Silurian to Permian age metamorphic units.
[3] There is geologic evidence that suggests Borneo has rotated counter clock wise about 45° from its orentation at the end of the Oligocene while remaining to straddle the equator.
[3] The equatorial perhumid climate provided intense chemical weathering and erosion of the newly uplifted rock and filled the marginal basins of Borneo with sediment.
[3] A period of punctuated compressional events beginning in the mid Miocene affected the continued evolution of these basins, deforming and inverting them.
It is bound to the North by the Mangkalihat High and the Central Kalimantan Ranges, to the south by the Paternoster Platform, Adang fault zone and the Schwaner and Meratus mountains.
Basin formation was initiated in the middle Eocene as extension related to the opening of the Makassar straits and Celebes Sea rifted the crust of Eastern Borneo.
A typical initial graben fill in the Kutai basin is composed of coarse and poorly sorted basement derived material.
[3] Large amounts of clastic sediment derived from the rising central mountains, and the now inverted Paleogene poured into the lower Kutai Basin.
Subsequent tectonic inversion events in the middle Miocene and Pliocene continued to shift the deltaic depocenter of the Mahakam river eastward into the Makassar Strait.
This incision has prevented any lateral migration of the lowermost Mahakam river, creating a point source deltaic depocenter that has been active since the mid Miocene.
[8] The most prominent geologic structure in the Kutai Basin is the Samarinda anticlinorium—Mahakam foldbelt, a series of NNE-SSW trending folds and faults in Miocene deltaic strata that parallel the modern coast line.
When contraction occurs while delta progradation is active, re-activation along these faults produces detached, uplifted anticlines[4] Three suites of intrusive and volcanic rocks are found in the Kutai Basin, and have been used to constrain the Tertiary stratigraphy .
