TEKTONIK SULAWESI SELATAN DENGAN ACUAN KHUSUS CIRI - CIRI HIMPUNAN BATUAN DAERAH BANTIMALA

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1 TEKTONIK SULAWESI SELATAN DENGAN ACUAN KHUSUS CIRI - CIRI HIMPUNAN BATUAN DAERAH BANTIMALA

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3 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 Paremba ", "basal Dengengdengeng ", " breksi sekis " dan " rijang Paring ", yang secara bersama menyusun " Komplek Melange Bantimala ". Himpunan batuan berumur dari Kapur Akhir sampai Pliosen merupakan himpunan batuan " autochthone" 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 batuan 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.

4 "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-

5 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-

6 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 Jawa- Nusatenggara 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.

7 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.

8 The " Bontorio metamorphics " is interpreted as a result 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 margin 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-

9 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-

10 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 Banggai- Sula platforms collided to the Eastern Sulawesi Arc, and the Eastern Sulawesi Arc overrod the Sulawesi arc-trench system. As the r e 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 of Equator started to decline since Pliocene.

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