TEKTONIK SULAWESI SELATAN DENGAN ACUAN KHUSUS CIRI

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 sekitarnya merupakan himpunan-himpunan batuan yang terjadi
dalam lingkungan tektonik yang berbeda sejak zaman Trias
sampai zaman Kuarter. Beberapa sistem tektonik dapat dikenali berdasarkan ciri-ciri himpunan batuan serta strukturnya. Macam-macam himpunan batuan tersebut memberikan
gambaran yang sesuai bila diterangkan kejadiannya dengan teori
tektonik lempeng. Baik macam himpunannya, hubungan stratigrafinya 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 tercampuraduk serta terimbrikasi secara tektonik, terdiri dari "batuan ultramafik Kayubiti", "batuan metamorfosis Bontorio",
"batupasir
dan
Paremba", "basal Dengengdengeng",
"rijang
"breksi
sekis"
Paring", yang secara bersama menyusun
"Kom-
plek Melange Bantimala". Himpunan batuan berumur dari Kapur Akhir sampai Pliosen merupakan himpunan batuan
"au-
tochthone" yang superposisi serta hubungannya dapat diamati 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 volkaniklastik serta volkanik yang menyusun formasi-formasi Benrong, Kunyikunyi, Ceppiye, serta Tondongkarambu, dan diakhiri oleh endapan darat berasal longsoran serta runtuhan 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 Trias. "Batupasir Paremba" adalah endapan cekungan tepi kerak 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 beralaskan "breksi sekis" pada zaman Jura Akhir-Kapur Awal. "Batuan ultramafik Kayubiti" ditafsirkan sebagai kerak samudera yang terjadi di cekungan antar-busur pada zaman
Trias. Berbagai macam himpunan batuan yang lingkungan
terjadinya berbeda itu telah tercampuraduk serta terimbrikasi secara tektonik, dan membentuk "komplek melange" pada 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 busurdepan 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 hasil pendataran pada kala Eosen Awal. Karbonat Formasi Tonasa 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 mengalami pengangkatan dan erosi yang berlangsung hingga sekarang.
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 zaman Kapur Tengah yang menyebabkan berbagai himpunan batuan dari Trias sampai Kapur Awal tercampuraduk serta terimbrikasi di daerah Bantimala, telah terjadi membentang
S-U di sisi timur Kraton Sunda yang kenampakannya sekarang berupa "lajur sutur" TG-BL dari "Komplek Melange Bantimala", anomali aeromagnet tak teratur di Selat Makassar sampai "Komplek Melange Boyan" di Kalimantan Barat.
Dalam perkembangan selanjutnya, daerah yang semula berupa
lajur tunjaman Kapur Tengah itu kemudian menjadi cekungan busur-depan Kapur Akhir di sisi timur Kraton Sunda pada 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 batuan volkanik Formasi Alla pada kala Paleosen. Perputaran dan pengangkatan Kraton Sunda diikuti oleh peretakan
selama Paleosen Akhir-Eosen Awal, sehingga terjadi sedimen terestrial yang sangat luas yang di Sulawesi Selatan 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 sisi selatan Kraton Sunda menjadi melengkung ke arah BD-TL.
Sistem busur-palung di,bagian timur itu kemudian menjadi sistem busur-palung Sulawesi di sisi tenggara Kraton Sunda, dan terpisah dari sistem busur-palung JawaNusatenggara yang mulai berkembang sejak Miosen Awal. Gerakan ke barat Lempeng Pasifik yang tercepatkan sejak Miosen Awal telah menyebabkan di antaranya, selama Miosen
Tengah-Miosen Akhir, Batur Tukang Besi serta Batur Banggai -Sula membentur Busur Sulawesi Timur, dan Busur Sulawesi Timur melanggar sistem busur-palung Sulawesi. Akibat dari benturan serta pelanggaran itu maka Busur Sulawesi Timur menyatu dengan Busur Sulawesi Barat yang keduanya melengkung membentuk huruf K, dan kegiatan magma
di Busur Sulawesi Barat sebelah selatan Katulistiwa mulai 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 Cretaceous 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 observed. Flysch sediments of Balangbaru Formation of Late
Cretaceous in age ovelies unconformably "Bantimala Melange Complex", and successively overlain by volcanic rocks
of Alla Formation, terestrial sediments of Malawa Formation, shelf carbonate of Tonasa Formation, volcaniclastics
and volcanics constructing formations of Benrong, Kunyikunyi, 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 Jurassic period in a basin of continental crust margin, and
"Dengengdengeng basalt" irrupted through continental fisures of the same period. "Schist breccia" is interpreted 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 basin during the Triassic period. Those various rock association occurred at different environment had been tectonically 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 paneplanisation. 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 Formation and Tondongkarambu Formation are parts of a Middle-
Late Miocene magmatic arc. Since Pliocene time the Bantimala area and its surroundings affected regional uplifting and erosion commencing up to the present day.
By comparing similarities of rock association, stratigraphic 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 Bantimala Melange Complex, the irregular aeromagnetic anomalies 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 arctrench system at the southern margin of Sunda Craton
producing among of them the volcanic rocks of Alla Formation 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 Sulawesi 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 Miocene to Late Miocene, the Tukang Besi and the BanggaiSula platforms collided to the Eastern Sulawesi Arc, and
the Eastern Sulawesi Arc overrod the Sulawesi arc-trench
system. As the
re
sult of collision and overriding the
Eastern Sulawesi Arc amalgamated with the Western Sulawesi Arc, bent together to form a K-shape, and the magmatic activity at the Western Sulawesi Arc of south
Equator started to decline since Pliocene.
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