Garuda - Garba Rujukan Digital

Purna Sulastya Putra

Research Center for Geotechnology Indonesian Institute of Sciences

Author-ID : 2819571

Earth & Planetary Sciences

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KARAKTERISTIK ENDAPAN TSUNAMI KRAKATAU 1883 DI DAERAH TARAHAN, LAMPUNG Purna Sulastya Putra; Eko Yulianto
JURNAL RISET GEOLOGI DAN PERTAMBANGAN Vol 27, No 1 (2017)
Publisher : Indonesian Institute of Sciences

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. AbstrakLokasi penelitian terletak di sebelah selatan Kota Tarahan, Lampung merupakandaerah yang terkena dampak gelombang tsunami letusan Gunung Krakatau 1883. Penelitian inidilakukan 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.

A Reinterpretation of the Baturetno Formation: Stratigraphic Study of the Baturetno Basin, Wonogiri, Central Java Purna Sulastya Putra; Eko Yulianto
Indonesian Journal on Geoscience Vol 2, No 3 (2015)
Publisher : Geological Agency

DOI:10.17014/ijog.2.3.125-137This paper focuses on the Quaternary Baturetno Formation. An earlier research concluded that the black clay of the Baturetno Formation formed as a ‘palaeolake’ deposit. The ‘palaeolake’ was interpreted to form due to the shifting course of the Bengawan Solo Purba River in relation to Pliocene tectonic tilting in the southern Java. The stratigraphy of the Baturetno Formation was observed in the western part of the Baturetno Basin, and based on marker beds, the Baturetno Formation was classified into three units: (1) Gravel unit (GR) in the upper part, (2) clay unit (CU) in the middle part, and (3) sand-gravel unit (SG) in the lower part. There are floating gravel fragments of andesite, claystone, coral, and limestone with diameters of up to 10 cm in the clay unit. The particle size of sediment reflects the environment, but the lake deposition occurs under very quiet conditions. The occurrence of these fragments within the clay cannot be explained if the clay was deposited within a lake environment. The occurrence of floating fragments in the black clay of Baturetno Formation can best be explained through mudflow process. The cohesive strength of the mudflow is responsible for the ability of large fragments to float within the mud matrix. In general, the Baturetno Formation is inferred to be an alluvial fan deposit. The presence of sand, gravel, and mud are characteristics of alluvial fan deposits.

Sedimentological and Micropaleontological Characteristics of the Black Clay Deposit of the Baturetno Formation, Wonogiri, Central Java Purna Sulastya Putra; Eko Yulianto
Indonesian Journal on Geoscience Vol 3, No 3 (2016)
Publisher : Geological Agency

DOI:10.17014/ijog.3.3.163-171A previous research on black clay of the Baturetno Formation identified that this clay was a palaeolake deposit. In this study, the black clay deposit was characterized by using sedimentological and micropaleontological approaches to reveal its origin. The black clay of the Baturetno Formation has a low organic matter (mostly less than 8%) and almost no pollen content. These characteristics are not representative of a lake deposit. This black clay was suggested a mud flow deposit. This conclusion is supported by the occurrence of dispersed rock fragments in the clay deposit. In addition, the previous research has identified that the formation of a palaeolake was related to the Late Pliocene tilting in South Java. The carbon dating indicates that the deposition of this clay has no relationship to the Late Pliocene tilting in South Java as the age of this clay was determined to be much younger (around 7000 BP; early Holocene).

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