TEKTONIK SULAWESI SELATAN DENGAN ACUAN KHUSUS

CIRI - CIRI HIMPUNAN BATUAN DAERAH BANTIMALA

A B S T R A K

Batuan yang tersingkap di daerah Bantimala dan seki-

tarnya merupakan himpunan-himpunan batuan yang terjadi

dalam lingkungan tektonik yang berbeda sejak zaman Trias

sampai zaman Kuarter. Beberapa sistem tektonik dapat di-

kenali berdasarkan ciri-ciri himpunan batuan serta struk-

turnya. Macam-macam himpunan batuan tersebut memberikan

gambaran yang sesuai bila diterangkan kejadiannya dengan teori

tektonik lempeng. Baik macam himpunannya, hubungan stra-

tigrafinya maupun strukturnya menandakan suatu pengertian

yang jelas di dalam evolusi geologi yang pendekatannya

berdasarkan teori tektonik lempeng.

Himpunan batuan berumur dari Trias sampai Kapur Awal

merupakan himpunan batuan "allochthone" yang tercampur-

aduk serta terimbrikasi secara tektonik, terdiri dari "ba-

tuan ultramafik Kayubiti", "batuan metamorfosis Bontorio",

"batupasir Paremba", "basal Dengengdengeng", "breksi sekis"

dan "rijang Paring", yang secara bersama menyusun "Kom-

plek Melange Bantimala". Himpunan batuan berumur dari Ka-

pur Akhir sampai Pliosen merupakan himpunan batuan "au-

tochthone" yang superposisi serta hubungannya dapat di-

amati dengan jelas. Sedimen "flysch" Formasi Balangbaru

yang berumur Kapur Akhir menindih tak selaras "Komplek

Melange Bantimala", dan ditindih berturut-turut oleh ba-

tuan volkanik Formasi Alla, sedimen terestrial Formasi

Malawa, karbonat paparan Formasi Tonasa, batuan volkani-

klastik serta volkanik yang menyusun formasi-formasi Ben-

rong, Kunyikunyi, Ceppiye, serta Tondongkarambu, dan di-

akhiri oleh endapan darat berasal longsoran serta runtuh-

an yang berumur Pliosen.

"Batuan metamorfosis Bontorio" ditafsirkan sebagai

hasil metamorfosis batuan sedimen di bagian bawah cekung

an busur-depan pada suatu sistem busur-palung zaman Tri-

as. "Batupasir Paremba" adalah endapan cekungan tepi ke-

rak benua pada zaman Jura Awal-Jura Tengah, dan "basal De

ngengdengeng" ke luar melalui retakan kerak benua pada

zaman itu. "Breksi sekis" ditafsirkan sebagai turbidit

"fluxo" di cekungan tepi kerak-benua pada zaman Jura Akhir,

dan "rijang Paring" sebagai endapan laut dalam beralas-

kan "breksi sekis" pada zaman Jura Akhir-Kapur Awal. "Ba-

tuan ultramafik Kayubiti" ditafsirkan sebagai kerak sa-

mudera yang terjadi di cekungan antar-busur pada zaman

Trias. Berbagai macam himpunan batuan yang lingkungan

terjadinya berbeda itu telah tercampuraduk serta terimbri-

kasi secara tektonik, dan membentuk "komplek melange" pa-

da sistem busur-palung zaman Kapur Tengah.

Sedimen "flysch" Formasi Balangbaru sebagai himpunan

batuan "autochthone" tertua yang beralaskan "komplek me

lange" tersebut, telah diendapkan di dalam cekungan busur-

depan pada sistem busur-palung zaman Kapur Akhir. Batuan

volkanik Formasi Alla adalah bagian dari busur magmatik

kala Paleosen yang menerobos Formasi Balangbaru. Sedimen

terestrial Formasi Malawa diendapkan di darat sebagai ha-

sil pendataran pada kala Eosen Awal. Karbonat Formasi To-

nasa telah diendapkan dalam lingkungan paparan selama ka

la Eosen Akhir-Miosen Awal. Batuan volkaniklastik dan

volkanik dari Formasi Benrong, Formasi Kunyikunyi,

Formasi Ceppiye dan Formasi Tondongkarambu adalah bagian

da-

ri busur magmatik kala Miosen Tengah-Miosen Akhir. Se

jak Pliosen daerah Bantimala dan sekitarnya telah menga-

lami pengangkatan dan erosi yang berlangsung hingga se-

karang.

Dengan memperhatikan kesebandingan himpunan batuan,

kedudukan stratigrafi serta hubungan tektonik antara ber

bagai himpunan batuan di daerah Bantimala dan yang ada di

daerah sekitarnya, maka perkembangan geologi regional wi

layah Sulawesi dapat dikenali. Sistem busur-palung za-

man Kapur Tengah yang menyebabkan berbagai himpunan ba-

tuan dari Trias sampai Kapur Awal tercampuraduk serta ter-

imbrikasi di daerah Bantimala, telah terjadi membentang

S-U di sisi timur Kraton Sunda yang kenampakannya seka-

rang berupa "lajur sutur" TG-BL dari "Komplek Melange Ban-

timala", anomali aeromagnet tak teratur di Selat Makas-

sar sampai "Komplek Melange Boyan" di Kalimantan Barat.

Dalam perkembangan selanjutnya, daerah yang semula berupa

lajur tunjaman Kapur Tengah itu kemudian menjadi cekung-

an busur-depan Kapur Akhir di sisi timur Kraton Sunda pa-

da zaman diendapkannya Formasi Balangbaru.

Pada Kapur Akhir itu Kraton Sunda mulai berputar

lawan-jarum-jam, dan diikuti tumbuhnya sistem busur–pa

lung di sisi selatannya yang di antaranya membentuk ba-

tuan volkanik Formasi Alla pada kala Paleosen. Perputar-

an dan pengangkatan Kraton Sunda diikuti oleh peretakan

selama Paleosen Akhir-Eosen Awal, sehingga terjadi se-

dimen terestrial yang sangat luas yang di Sulawesi Sela-

tan menghasilkan Formasi Malawa. Penurunan perlahan te-

lah menghasilkan endapan karbonat paparan yang sangat lu

as selama Eosen Akhir-Miosen Tengah yang di Sulawesi

Selatan berupa Formasi Tonasa. Perputaran Kraton Sunda

yang menerus dan terjadinya perubahan arah gerak Lempeng

Pasifik, yang semula ke utara kemudian ke barat sejak Eo

sen Tengah, maka bagian timur sistem busur-palung di si-

si selatan Kraton Sunda menjadi melengkung ke arah BD-TL.

Sistem busur-palung di,bagian timur itu kemudian men-

jadi sistem busur-palung Sulawesi di sisi tenggara Kra-

ton Sunda, dan terpisah dari sistem busur-palung Jawa-

Nusatenggara yang mulai berkembang sejak Miosen Awal. Ge-

rakan ke barat Lempeng Pasifik yang tercepatkan sejak Mio-

sen Awal telah menyebabkan di antaranya, selama Miosen

Tengah-Miosen Akhir, Batur Tukang Besi serta Batur Bang-

gai -Sula membentur Busur Sulawesi Timur, dan Busur Su-

lawesi Timur melanggar sistem busur-palung Sulawesi. Aki-

bat dari benturan serta pelanggaran itu maka Busur Su-

lawesi Timur menyatu dengan Busur Sulawesi Barat yang ke-

duanya melengkung membentuk huruf K, dan kegiatan magma

di Busur Sulawesi Barat sebelah selatan Katulistiwa mu-

lai mereda sejak Pliosen.

A B S T R A C T

Rocks exposed in Bantimala area and its surroundings

comprise of some rock association which have originated

in different tectonic environments since Triassic to Qua

-ternary periods. Certain tectonic systems are identifi

able through their characteristic of rock association and

structure. Various rock associations occurred in the

area of Bantimala and its surroundings show features of

matching to be explained with plate tectonic theory. The

rock association and their stratigraphic as well as their

tectonic relationship indicate a clear understanding of

geologic evolution in the light of plate tectonic theory.

Associations of rocks of Triassic to Early Creta-

ceous in age constitute of allochthonous rock association

which tectonically mixed up and imbricated, consist of

"Kayubiti ultramafics", "Bontorio metamorphics", "Paremba

sandstone", "Dengengdengeng basalt", "schist breccia" and "

Paring chert", all together forming "Bantimala Melange

Complex". Associations of rocks of Late Cretaceous to

Pliocene in age constitute of autochthonous units showing

their superposition and relationship to be clearly ob-

served. Flysch sediments of Balangbaru Formation of Late

Cretaceous in age ovelies unconformably "Bantimala Mela-

nge Complex", and successively overlain by volcanic rocks

of Alla Formation, terestrial sediments of Malawa Forma-

tion, shelf carbonate of Tonasa Formation, volcaniclastics

and volcanics constructing formations of Benrong, Kunyi-

kunyi, Ceppiye and Tondongkarambu, and terminated by

terestrial deposits derived from landslide and rockfall

masses at Pliocene.

The "Bontorio metamorphics" is interpreted as a re-

sult of metamorphism of sedimentary rocks at the deep

zone of fore arc basin of a Triassic arc-trench system.

The "Paremba sandstone" is deposits of Early - Middle Ju-

rassic period in a basin of continental crust margin, and

"Dengengdengeng basalt" irrupted through continental fi-

sures of the same period. "Schist breccia" is inter-

preted to be fluxo turbidite irk basin of continental mar-

gin during Late Jurassic, and "Paring chert" is deep sea

sediments of Late Jurassic-Early Cretaceous deposited on

top of "schist breccia". "The Kayubiti ultramafics" is

interpreted as oceanic crust occurred in an interarc ba-

sin during the Triassic period. Those various rock asso-

ciation occurred at different environment had been tec-

tonically mixed up and imbricated forming melange complex

at a Middle Cretaceous arc-trench system.

Flysch sediments of Balangbaru Formation, as the

oldest autochthonous unit, has been deposited overlying

melange complex in a forearc basin of Late Cretaceous

arc-trench system. The volcanic rocks of Alla Formation

is part of Paleocene magmatic arc intruded the Balangbaru

Formation. The terestrial sediments of Malawa Formation

was deposited on land as a product of Early Eocene pane-

planisation. The carbonate of Tonasa Formation deposited

on a shelf environment during Late Eocene-Early Miocene.

The volcaniclastic and volcanic rocks constructing the

Benrong Formation, Kunyikunyi Formation, Ceppiye Forma-

tion and Tondongkarambu Formation are parts of a Middle-

Late Miocene magmatic arc. Since Pliocene time the Ban-

timala area and its surroundings affected regional up-

lifting and erosion commencing up to the present day.

By comparing similarities of rock association, stra-

tigraphic position and tectonic relationship between the

various rock units of Bantimala area and of which occurred

at surrounding areas the evolution of regional geology of

Sulawesi could be identified.. The Middle Cretaceous arc

-trench system which brought about a mixture and imbrica

tion of various rock association of Triassic to Early

Cretaceous age in Bantimala area occurred stretching S-N

at the eastern margin of Sunda Craton. It's appearance

at present is a suture zone trending SE-NW from the Ban-

timala Melange Complex, the irregular aeromagnetic ano-

malies in Makassar strait, to the Boyan Melange Complex

in West Kalimantan. Further development indicates that

the previous Middle Cretaceous subduction zone had changed

into a forearc basin at the eastern margin of Sunda Craton

during the Late Cretaceous when then Balangbaru Formation

was deposited.

Since the Late Cretaceous, Sunda Craton rotated

counterclockwisely and followed by development of arc-

trench system at the southern margin of Sunda Craton

producing among of them the volcanic rocks of Alla Forma-

tion during the Paleocene. Rotation and uplifting of

Sunda Craton was followed by rifting commenced during the

Late Paleocene to Early Eocene, subsequently a very broad

terrestrial sediments occurred which part of them is Ma-

lawa Formation in South Sulawesi. Gradually subsidence

resulted a very extensive shelf carbonate deposits during

Late Eocene - Middle Miocene which part of them is Tonasa

Formation in South Sulawesi. Continue rotation of Sunda

Craton and change of direction of movement of Pacific

Plate, originally to the north and then to the west since

Middle Eocene, the eastern part of arc-trench system at

the southern margin of Sunda Craton bent northeastward.

Later the eastern part of arc-trench system became Sula-

wesi arc-trench system at the southeast margin of Sunda

Craton, and separated from Java-Nusatenggara arc-trench

system which has developed since Early Miocene. Westward

movement of Pacific Plate which was . accelerated since

Early Miocene affected among of them, during Middle Mio-

cene to Late Miocene, the Tukang Besi and the Banggai-

Sula platforms collided to the Eastern Sulawesi Arc, and

the Eastern Sulawesi Arc overrod the Sulawesi arc-trench

system. As the result of collision and overriding the

Eastern Sulawesi Arc amalgamated with the Western Sula-

wesi Arc, bent together to form a K-shape, and the mag-

matic activity at the Western Sulawesi Arc of south of

Equator started to decline since Pliocene.