Karakteristik Endapan Paleotsunami Berdasarkan Studi Sedimentologi dan Paleontologi, Daerah Adipala, Kabupaten Cilacap, Provinsi Jawa Tengah
ARIF PRAYOGO, Salahuddin Husein, S.T., M.Sc., Ph.D. ; Dr. Eko Yulianto, S.T., M.T.
2018 | Skripsi | S1 TEKNIK GEOLOGIPulau Jawa merupakan salah satu daerah dengan potensi bencana tsunami yang tinggi diakibatkan pesisir selatan Pulau Jawa berada di depan zona subduksi. Hal tersebut mengakibatkan aktivitas tektonik berupa gempa bumi pemicu tsunami. Pencarian endapan tsunami dilakukan untuk mengetahui catatan sejarah tsunami yang pernah terjadi. Indikasi terjadinya paleotsunami terlihat pada stratigrafi Kuarter di morfologi swale daerah pesisir Pantai Bunton, Kecamatan Adipala, Kabupaten Cilacap, Provinsi Jawa Tengah. Endapan ini berada sekitar 1 km dari garis pantai. Untuk mengidentifikasi endapan tsunami dilakukan analisis secara megaskopis dan analisis laboratorium yang terdiri dari analisis granulometri, analisis LOI (Loss on Ignition), analisis mineralogi, dan analisis paleontologi. Dari hasil analisis terdapat dua endapan paleotsunami, yang ditemukan pada kedalaman 63 - 70 cm (lapisan tsunami lanau pasiran) dan 100 -104 cm (lapisan tsunami radiolaria). Lapisan tsunami lanau pasiran memiliki hasil analisis granulometri berupa nilai mean dengan ukuran butir coarse silt - fine silt, sortasi poorly sorted - very poorly sorted, skewness dengan kelas coarse skewed, kurtosis dengan kelas platykurtic - leptokurtic. Hasil analisis LOI menunjukkan material organik mengalami penurunan dan material karbonat mengalami kenaikan dibandingkan dengan lapisan di bawahnya. Hasil mineralogi menunjukkan lapisan memiliki kandungan mineral kuarsa, feldspar, glaukonit, kalsit, litik sedimen, dan mineral berat. Hasil analisis paleontologi tidak ditemukan baik foraminifera plangtonik maupun bentonik pada lapisan ini. Tidak ditemukannya foraminifera diakibatkan akibat variasi sumber sedimen ataupun pelarutan yang diakibatkan oleh air rawa yang menganggu preservasi foraminifera. Lapisan tsunami radiolaria memberikan hasil analisis granulomteri berupa nilai mean dengan ukuran butir medium silt, sortasi very poorly sorted, skewness dengan kelas coarse skewed, dan kurtosis dengan kelas mesokurtic. Hasil analisis LOI menunjukkan lapisan mengalami penurunan nilai material organik dan material karbonat. Analisis mineralogi menunjukkan lapisan tersusun atas mineral kuarsa, feldspar, glaukonit, foraminifera, litik sedimen, dan mineral berat. Hasil analisis paleontologi ditemukan 11 spesies foraminifera juvenile, 1 foraminifera mold, dan 4 spesies radiolaria yang mengindikasikan lapisan berasal dari laut dan terbawa oleh gelombang tsunami. Rendahnya kandungan material karbonat diakibatkan adanya variasi sumber sedimen baik yang berasal dari laut maupun darat.
Java island is one of the areas with a high potential of tsunami disaster caused by the southern coast of the java island is in front the subduction zones, precisely. This has resulted in tectonic activity (earthquake) which triggered in a tsunami. The searching of tsunami deposit was done to find out the historical record of the previous tsunami event. The indication of paleotsunami can be seen on Quaternary stratigraphic of the swale morphology, coastal Beach Bunton, Adipala, Cilacap District, Central Java. These deposits are located about 1 km from the coastline. The tsunami deposits were identified by megascopic analysis and laboratory analysis (granulometric analysis, LOI analysis, mineralogy analysis and paleontology analysis). The analysis results found two paleotsunami deposits and were located at a depth of 63 - 70 cm (layer of sandy silt tsunami) and 100 - 104 cm (layer of radiolarian tsunami). The granulometric analysis of the layer of sandy silt tsunami showed the mean value with grain size as coarse silt - fine silt, sorting value as poorly sorted - very poorly sorted, skewness with coarse skewed class, kurtosis with platykurtic - leptokurtic class. Furthermore, compared with the previous layer, the LOI analysis showed that organic material declined and carbonate material inclined. The mineralogy analysis showed that the layer has a mineral content of quartz, feldspar, glauconite, calcite, sediment lithic and heavy mineral. The paleontology analysis showed that the foraminifera were not found, foraminifera planktonic nor benthonic. The absence of foraminifera was caused by the variation of the sedimentary sources or dissolution caused by swamp water which is disrupting the preservation of foraminifera. Meanwhile, the granulometric analysis of the layer of radiolarian tsunami showed the mean value with grain as medium silt, sorting value as very poorly sorted, skewness with coarse skewed class and kurtosis with mesokurtic class. LOI analysis showed that the both of organic material and carbonate material were shown a decline value. The mineralogy analysis showed that the layer has a mineral content of quartz, feldspar, glauconite, foraminifera, sediment lithic and heavy mineral. The paleontology analysis found 11 species of foraminifera juvenile, 1 foraminifera mold and 4 species of radiolarian which indicates that the layer were from the sea and carried by the tsunami waves. The low content of carbonate material is due to the variation of sediment source from both sea and land.
Kata Kunci : Adipala, Paleotsunami, Sedimentologi, Paleontologi
