Aspek geologi dan implikasinya terhadap kualitas marmer komersial :: Studi kasus marmer komersial Fatumnasi, Kabupaten Timor Tengah Selatan Propinsi Nusa Tenggara Timur
KOTTA, Herry Zadrak, Promotor Prof. Ir. Sukandarrumidi, M.Sc., Ph.D
2008 | Disertasi |Penelitian ini dilakukan pada batugamping oolit (oleh masyarakat setempat disebut “marmerâ€) yang sejak tahun 1999 hingga saat ini telah ditambang, namun masih terdapat silang pendapat tentang kualitas antara investor, pengrajin dan pemerintah. Hingga saat ini belum ada referensi kualitas standar batugamping oolit sebagai acuan. Guna menjembatani silang pendapat tersebut di atas dan dengan latar belakang bahwa aspek geologi diduga berpengaruh terhadap kualitas batugamping oolit, maka dilakukan studi aspek geologi yang berimplikasi terhadap kualitas batugamping oolit. Batugamping oolit yang dijadikan obyek studi adalah batugamping oolit Tubu Naetapan, Kecamatan Fatumnasi, Kabupaten Timor Tengah Selatan, Propinsi Nusa Tenggara Timur. Tujuan penelitian ini adalah: 1). Menyusun korelasi antara aspek geologi dan aspek kualitas batugamping oolit sesuai SNI.13-0089-1987; 2). Mengetahui aspek geologi dan implikasinya terhadap kualitas “marmer†komersial; dan 3). Menjembantani kriteria kualitas batugamping oolit di penelitian sesuai SNI 13-0089-1987 berdasarkan aspek geologi di lapangan. Metode penelitian yang digunakan adalah pendekatan analisis sifat fisik sesuai SNI.13-0089-1987, analisis petrografi untuk mengetahui tekstur, komposisi, porositas, tipe dan jenis semen, termasuk di dalamnya menyangkut kenampakan diaganesanya (diagenetic feature). Di dalam penelitian ini digunakan Scanning Electron Microscope (SEM) untuk mengetahui alterasi, proses diagenesa, porositas dan hubungan antar pori; dan cathodoluminescence (CL) untuk mengetahui fase sementasi. Hasil penelitian menujukkan bahwa batugamping di daerah penelitian yang disebut “marmer†komersial terdiri dari Ooid Grainstone, Ooid Pack to Grainstone, Coralline Algae Pack to Grainstone dan Indeterminate Bioclastic Mudstone. Lingkungan pengendapan batugamping tersebut di atas adalah pada winnowed platform edge. Analisis diagenesa menunjukan bahwa batugamping di daerah penelitian terjadi pada tiga lingkungan diagenesa, yaitu 1). Marine Phreatic: ditunjukkan oleh sementasi mikrit dan sparit (fibrous dan equant), 2). Fresh Water Phreatic Environment pada zona jenuh aktif dicirikan oleh neomorfisme butiran dengan pengendapan kalsit serta pengisian rongga antar butir oleh kalsit sparit dan 3). Fresh Water Vadoze Environment, yaitu lingkungan diagenesa saat ini. Kenampakkan diagenetic feature terdiri dari: mikritisasi, kompaksi (chemical compaction), sementasi dan neomorfisme (replacement). Analisis petrografi menunjukkan tipe semen terdiri dari isopachous equant calcite rim, isopachous fibrous fringes dan granular atau blocky cement. Porositas visual terdiri dari porositas retakan dan pelarutan. Analisis cathodoluminescence menunjukkan bahwa terdapat 6 fase sementasi yang mengisi ruang antar butir dengan jenis semen kalsit. Analisis SEM menunjukkan struktur asli ooid tidak terlihat lagi, telah mengalami mikritisasi. Analisis dengan metode cathodoluminescence yang umumnya digunakan di dunia perminyakan, terbukti di dalam penelitian ini dapat diterapkan untuk mengetahui kualitas batugamping, semakin banyak fase sementasi, semakin baik pula kualitas batugamping tersebut. Sifat fisik batugamping di daerah penelitian menunjukkan bahwa warna: krem (seragam), tekstur/corak, terdiri dari nilai butriran-butiran ooid yang relatif seragam, tanpa stylolite dan kekar, kilap yang baik, kuat tekan berkisar antara 205 kg/cm2 hingga 1069 kg/cm2, daya serap air berkisar antara 0,482 – 3,722%, keausan berkisar antara 0,3183 – 0,1781 mm/menit. Aspek geologi yang paling berpengaruh terhadap sifat fisik batugamping oolit di daerah penelitian adalah: tekstur batuan yang meliputi: pemilahan butiran, ukuran butir, kontak antar butir atau fabric type akibat kompaksi kimiawi, porositas visual dan komposisi. Marmer†komersial walaupun secara geologi masih termasuk di dalam batugamping oolit, namun secara kualitas beberapa contoh telah memenuhi kriteria sifat fisik marmer sesuai SNI 13-0089-1987 (SNI 13-0089-1987 untuk marmer), namun dalam pemanfaatannya perlu dipertimbangkan sifat batugamping yang mudah larut pada lingkungan basah. Berdasarkan kajian aspek geologi dan sifat fisik batugamping oolit di daerah penelitian, dipandang perlu dibuatkan petunjuk praktis kriteria kualitas batugamping berdasarkan kondisi di lapangan bagi masyarakat non geologi.
This research was done in oolithic limestone (was interpreted as “marble†by local people), which since 1994 till now have been mined, but still found doubts opinion about quality between investor, worker and government and there is no standard quality reference. To get a solution of the doubts opinion and also the background geological aspects might be influences the quality of oolithic limestone, then the research of geological aspects and its implication to the quality of oolithic limestone, was done.The studied object is oolithic limestone at Tubu Naetapan, Fatumnasi Sub district, South Central Timor Regency, Nusa Tenggara Timur Province. The research aims are: 1). To compile the correlation between geological aspects and physical properties under SNI.13-0089-1987; 2). Knowing the geological aspects and its implication to the quality of oolithic limestone; and 3). To get a solution of the criteria of oolithic limestone quality under SNI 13-0089-1987 based on the geology aspect at field. The research method used a physical properties analysis under SNI.13-0089-1987 approach, petrographic analysis to know the texture, composition, porosity, type and cement kind, and the diagenetic feature. In this research utilization of the Scanning Electron Microscope (SEM) is to know alteration, diagenetic processes, porosity and the relationship between pore, and utilization of the cathodoluminescence analysis to know the cementation phase. The result of this research shows that the oolithic limestone at the study area which was interpreted as commercial “marbleâ€, consists of Ooid Grainstone, Ooid Pack to Grainstone, Coralline Algae Pack to Grainstone and Indeterminate Bioclastic Mudstone. The depositional environment of oolithic limestone is interpreted as winnowed platform edge. Diagenetic analysis of the thin section indicates the sequence of the characteristic for each diagenetic environment i.e. 1). Marine phreatic; cementation of micrit and spar (fibrous and equant) 2). Fresh water phreatic environment in active zone: grain neomorphism with calcite spar and pore filling by calcite spar, 3). Fresh water vadoze environment is recent diagenetic environment. The diagenetic feature consists of micritization, compaction (chemical compaction), cementation and neomorphism. Petrographic analysis show that the type of cement consists of isopachous equant calcite rim, isopachous fibrous fringes and granular or blocky cement, and also visual porosity are caused by fracture and solution. Cathodoluminescence (CL) analysis indicates that there are six sequences of calcite cementations. The Scanning Electron Microscope analysis indicates that the original structure of ooid was not so clear, showing that all of those are already micritic. The analysis by cathodoluminescence method was generally used in petroleum, however in this research it was applicable to know the oolithic limestone quality: more and more cementation phase, good progressively also the quality of oolithic limestone. The result of physical properties test are: cream colored, ooids pattern without stylolites and joint, shine to gloss gleam, range of unconfined compressive strength value are 205 kg/cm2 to 1069 kg/cm2, absorption range from 0,482 to 3,722%, abrasiveness range from 0,3183 to 0,1781 mm/minute, under Indonesian National Standard (SNI.13-0089-1987). The grain sortation, grain size, grain contact or fabric type are caused by chemical compaction, visual porosity and composition, those are the geological aspects which strong influenced the physical properties of the oolithic limestone at the study area. However, geological term of the commercial "marble" is categorized as oolithic limestone, but in quality, some example have fulfilled the criteria of the marble physical properties under SNI 13-0089-1987 (SNI 13-0089-1987 for marble). For the application, the solubility of oolithic limestone should be considered since it easily dissolves in wet environment. According to the geological aspects and physical properties of the oolithic limestone in the study area, it is necessary to propose a practical guide of oolithic limestone quality criteria based on the field condition for non geologist.
Kata Kunci : Aspek geologi, Sifat fisik, Batu gamping oolit (marmer komersial), Fatumnasi
