Karakteristik Endapan Paleotsunami Berdasarkan Studi Sedimentologi dan Geokimia di Teluk Pacitan, Kabupaten Pacitan, Provinsi Jawa Timur

MUHAMMAD FAISAL, Salahuddin Husein, S.T., M.Sc., Ph.D; Dr. Eko Yulianto, S.T., M.T

2018 | Skripsi | S1 TEKNIK GEOLOGI

Wilayah Teluk Pacitan, Jawa Timur berhadapan langsung dengan Samudera Hindia dimana terdapat batas lempeng tektonik aktif. Beberapa gempa berskala besar pernah terjadi dan diantaranya menghasilkan tsunami di pesisir selatan Pulau Jawa. Pencarian endapan paleotsunami bertujuan untuk mempelajari perulangan kejadian tsunami guna penyiapan mitigasi bencana yang lebih baik. Endapan paleotsunami ditemukan pada swale yang berjarak 1.918 meter dari garis pantai saat ini. Untuk mengetahui karakteristik endapan paleotsunami, maka dilakukan analisis sedimentologi (granulometri, loss on ignition, pemisahan mineral ringan dan berat dengan larutan yang memiliki densitas 2,89 gr/cm3) dan geokimia (Portable X-Ray Fluoroscence, X-Ray Diffraction). Endapan paleotsunami ditemukan berwarna coklat terang setebal 4 cm diantara endapan swale (gambut). Memiliki struktur menghalus ke atas dengan batas bawah yang tegas dan melampar sepanjang swale yang sejajar garis pantai dengan ketebalan berkisar 3-7 cm. Nilai-nilai granulometri berubah secara gradual ke atas, nilai mean 1,079-5,660 phi (pasir sedang-lanau kasar), sortasi poorly sorted-very poorly sorted, kurtosis very leptokurtic-platykurtic, dan skewness very fine skewed-symmetrical dengan bentuk butir unimodal - bimodal. Kandungan material organik pada lapisan pasir paleotsunami 8-10%, lebih rendah dari lapisan pasir lainnya dan diikuti penurunan kandungan unsur S (sulfur). Terjadi pengkayaan mineral berat dengan persentase 16-49% dibandingan pada lapisan pasir lainnya yang berkisar 9-11%. Kadar unsur-unsur asal darat seperti Fe (besi), Zr (zircon), dan Ti (titanium) mengalami kenaikan akibat pengkayaan mineral berat magnetit, hornblende, zircon, dan rutil. Pada endapan paleotsunami terdapat mineral glaukonit yang juga terdapat pada endapan permukaan dasar laut pada kedalaman 40 m.

The Pacitan Bay, East Java was directly interfacing Indian Ocean where there was an active continental margin. Severe 'tsunami earthquakeâ' have been noted ever happened on Java's South Coast. The exploring paleotsunami deposit is aim to reconstruct recurrent intervals of tsunamis and may help in assesing tsunami hazard. Paleotsunami deposit has been found in swale morphlogy with 1.918 meters inland that beyond the present coastline. For understanding characteristics of paleotsunami deposit, some analyses such as sedimentology (granulometry, loss on ingnition, separated heavy minerals content with 2,83 gr/cm3liquid density) and geochemistry (Portable X-Ray Fluoroscence, X-Ray Diffraction) have been done. Within core sample BRJ-15, paleotsunami deposit has been found between swale deposits with 4 cm thicks. It has clearly lower boundary, fining upward structure and lateral distribution along swale with thick about 3-7 cm. Granulometry values are gradually changing upward with 1,079-5,660 phi (medium sand-coarse silt) mean, poorly sorted-very poorly sorted sortation, very leptokurtic-platykurtic kurtosis, very fine skewed-symmetrical skewness, and unimodal-bimodal size distribution. Paleotsunami deposit has 8-10% organic matters content that the lowest in others sand deposit and it was followed by declining S (sulpur) content. There was enrichment of heavy minerals content with value 16-49% that the highest than others sand with value 9-11%. Continental elements like Fe (iron) Zr (zircon), and Ti (titanium) have inclined because heavy minerals enrichement like magnetite, hornblende, zircon, and rutile. There has glauconite mineral that has been also found in surface of sea floor sediments about 40 m depth.

Kata Kunci : Teluk Pacitan; paleotsunami; sedimentologi;geokimia