Kutai Basin

KUTAI BASIN

The Kut Kutai ai sedim sedimentar entary y basin ext extend ends s fro from m the cen centra trall hig highla hlands nds of  of Borneo Borneo,,   across across the eastern coast of the island and into the Makassar Strait. Strait. With an area of 60,000 km , and de!ths u! to

"# km,

the Kutai is

the largest

and

dee!est Tertiary age

basin

in $ndonesia $ndonesia..%"& 'late

tectonic e(olution in the $ndonesian region of S) *sia has !roduced a di(erse array of basins in the +enooic +enooic..%& The Kutai is an extensional basin in a general foreland setting. $ts geologic e(olution begins in the mid)ocene mid )ocene and in(ol(es !hases of extension and rifting rifting,, thermal sag, and isostatic subsidence. -a!id, high (olume, sedimentation related to u!lift and in(ersion began in the )arly Miocene.%"& The different stages of Kutai basin e(olution can be roughly correlated to regional and Miocene. local tectonic e(ents. %& $t is also likely that regional climate, namely the onset of the euatorial e(er  /et monsoon in early Miocene Miocene,, has affected the geologic e(olution of Borneo and the Kutai basin through the !resent day.%& Basin fill is ongoing in the lo/er Kutai basin, as the modern Mahakam -i(er  delta  delta !rogrades east across the continental shelf of shelf of Borneo.

PLATE TECTONIC SETTING

The +enooic !late tectonics of the $ndonesian region ha(e generated a com!lex assemblage of micro1continental blocks and marginal ocean basins surrounded by extensional margins, subduction ones and ma2or transcurrent faults. %3& The island of Borneo and the Kutai basin are located on the Sunda micro1!late, /hich is bounded to the north and /est by the )urasian !late, to the south by the $ndo1*ustralian !late and to the /est by the 'hili!!ine and 'acific oceanic !lates. $n the +enooic, the $ndo1*ustralian !late has been mo(ing north and subducting under )urasia.  The collision of the $ndian continent /ith )urasia halted subduction and u!lifted the 4imalayas. $n

%&

bet/een the continents of $ndia and *ustralia, the oceanic crust is still subducting under the Sunda !late, forming the Sunda trench and Sunda *rc. *ustralia and *ustralian deri(ed micro1!lates collided /ith the Sunda !late and 'acific !late in the 'liocene, creating a com!lex of subduction onesand island arcs. The 'hili!!ine !late has been obliuely subducting the Sunda !late for most of the +enooic. The com!lex interaction of the Sunda,

)urasian,

$ndo1*ustralian,

'hili!!ine and 'acific !lates in the +enooic has controlled the e(olution of  a!!roximately 60 Tertiary sedimentary basins in the $ndonesian region. Many of  these basins, including the Kutai, ha(e formed in a back arc extensional setting, dri(en by !assi(e or acti(e subduction rollback. The mid Miocene e!isode of  in(ersion in the Kutai can be linked to collision of continental fragments from the South +hina Sea  /ith 5W Borneo. The 'liocene in(ersion e!isode is contem!oraneous /ith the collision of   *ustralia /ith the Banda arc, /ith structural connections !ro(ided by strike1sli! fault systems through Sula/esi.%&

GEOLOGY OF BORNEO

The basement rock of Borneo is a com!lex mosaic of geologic terrains, commonly inter!reted as the !roduct of !rimarily Mesooic accretion of micro1continental fragments, island arc material, oceanic crustal material and marginal basin fill onto the 'aleooic core of the Sch/aner  Mountains in the south/est of the island. %#& The Sch/aner Mountain area consists of early1mid +retaceous granitic batholiths intruded into Silurian to 'ermian age metamor!hic units. %6& 5W of the

Sch/aner Mountains is a small area of older continental basement consisting of 'ermo1Triassic granite and metamor!hic rocks. S) of the Sch/aner Mountains, (olcanic island arc and o!hilitic rocks em!laced in the late +retaceous com!rise the Meratus Mountains. The basement terrain of  eastern and northern Borneo is inter!reted to be +retaceous subduction melange, mostly co(ered by tertiary sediment. %#& The basement of Western Borneo is an accreted melange of u!!er cretaceous to 'aleocene age that formed the central Kalimantan -anges as the result of SW directed subduction beneath the continental core of Borneo. %#&%6& The +enooic e(olution of Borneo is !redominantly controlled by acti(e regional and local tectonics and climate. $n the 'aleocene, Borneo /as a !romontory of S) *sia, !artially se!arated by oceanic crust of the !roto1South +hina Sea. %&There is geologic e(idence that suggests Borneo has rotated counter clock /ise about 3# from its orentation at the end of the 7ligocene /hile remaining to straddle the euator. This /ould indicate most of the 'aleogene sediment in 5orth Borneo /as sourced from $ndochina. %& $n the Mid )ocene formation of the +elebs Sea and Makassar Strait rifted the eastern margin of Borneo /hile subduction of oceanic crust occurred on the /estern margin, !roducing dee! basins on both sides. $n the late 7ligocene to )arly Miocene, the central mountain ranges of Borneo began to rise.%& The euatorial !erhumid climate !ro(ided intense chemical /eathering and erosion of the ne/ly u!lifted rock and filled the marginal basins of Borneo /ith sediment. 5eogene sediments are u! to 8 km thick in sections of some basins. %6& -econstruction of  the sediment (olume indicates that at least 6 km of crust /as remo(ed from the interior of Borneo in the 5eogene.%& * !eriod of !unctuated com!ressional e(ents beginning in the mid Miocene affected the continued e(olution of these basins, deforming and in(erting them. $gneous acti(ity continued throughout the +enooic but /as !articularly more acti(e in the northern region of Borneo in the 5eogene.

BASIN MARGIN

The Kutai basin tra(erses the eastern slo!e of the island of Borneo do/n from the central highlands, across the modern coastline to the basin floor of the Makassar Straits. $t is bound to the 5orth by the Mangkalihat 4igh and the +entral Kalimantan -anges, to the south by the 'aternoster  'latform, *dang fault one and the Sch/aner and Meratus mountains. The Muller mountains form the /estern basin margin. $n its !resent configuraton, the basin can be di(ided into t/o !arts. The /estern, or u!!er Kutai /hich has been in(erted "#001009 abo(e sea le(el, and the eastern, or  lo/er Kutai /hich is still recei(ing sediment.

BASIN FORMATION AND EVOLUTION

Basin formation /as initiated in the middle )ocene as extension related to the o!ening of the Makassar straits and +elebes Sea rifted the crust of )astern Borneo. %"& This rifting created a broad system

of half grabens that

re(erse

!olarity along

55)1SSW

and

51S

trending normal

faults. Thermal subsidence in the late )ocene and early 7ligocene induced minor reacti(ation along the existing faults. :uring the late 7ligocene there /as a brief rene/al of extension and rifting along the northern margin of the basin, /hile the other basin margins ex!erienced u!lift. %6& $n(ersion of the basin began in the ;ate 7ligocene. Tectonic u!lift of Borneo in the earliest Miocene in(erted the