KARAKTERISTIK ENDAPAN TSUNAMI KRAKATAU 1883 DI DAERAH TARAHAN, LAMPUNG

Purna Sulastya Putra, Eko Yulianto

Abstract


Our research location is situated at the south of Tarahan city, Lampung. This area has been drowned by tsunami waves that were generated by the eruption of Krakatau Volcano in 1883. This study is to understand the characteristics of the 1883 Krakatau tsunami deposit by observing the stratigraphic profile of 12 outcrops in the river overbanks and trenches, in 3 (three) transects. Laboratory analysis include grain size, microfauna (foraminifera and mollusk), and heavy mineral analysis. The identified tsunami deposit is a sand layer that mixed with shell and coral fragments. This sand layer is associated with pumice and volcanic ash layer, product of the 1883 Krakatau volcano eruption. The benthic foraminifera and mollusk content in the tsunami deposits show that the tsunami waves erode the sea floor sediments up to 30 to 40 meters depth. According to the outcrop observation and grain size analysis, there are four fining upward patterns can be observed in the tsunami deposit layer. This is indicates that at least four tsunami waves inundated the studied area. The vertical variation of heavy mineral shows the similar trend with the grain sizes.

Abstrak
Lokasi penelitian terletak di sebelah selatan Kota Tarahan, Lampung merupakan
daerah yang terkena dampak gelombang tsunami letusan Gunung Krakatau 1883. Penelitian ini
dilakukan untuk mengetahui karakteristik endapan tsunami Krakatau 1883 dengan mengamati profil stratigrafi pada dinding sungai dan parit uji pada 12 lokasi di 3 (tiga) transek. Analisis laboratorium meliputi analisis besar butir, mikrofauna (foraminifera dan moluska) dan mineral berat. Endapan tsunami yang teridentifikasi berupa lapisan pasir dengan pecahan cangkang dan fragmen karang. Lapisan pasir ini berasosiasi dengan lapisan abu vulkanik dan lapisan batuapung yang merupakan material hasil letusan Krakatau 1883. Kandungan foraminifera bentik dan moluska memperlihatkan bahwa gelombang tsunami menggerus dasar laut hingga kedalaman 30 - 40 m. Berdasarkan pengamatan singkapan dan analisis besar butir, terdapat empat pola perulangan struktur penghalusan besar butir ke arah atas dalam satu lapisan endapan tsunami. Hal itu menunjukkan bahwa kemungkinan ada empat gelombang tsunami yang terjadi pada saat itu. Hasil analisis mineral berat cenderung memperlihatkan pola perubahan komposisi ke arah atas mengikutiperubahan besar butir.


Keywords


endapan tsunami, Krakatau 1883, Tarahan

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DOI: http://dx.doi.org/10.14203/risetgeotam2017.v27.301

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