LINGKUNGAN PENGENDAPAN FORMASI TAPAK DAERAH SAMUDRA, KABUPATEN BANYUMAS, JAWA TENGAH
Abstract
ABSTRAK: Secara administratif daerah penelitian terletak di Sungai Arus daerah Samudra, Kabupaten Banyumas, Jawa Tengah. Berdasarkan beberapa penelitian terdahulu, Formasi Tapak terendapkan pada laut dalam dengan sistem sub-marine fan dan zona tidal. Pada daerah penelitian, belum ada penelitian terdahulu yang mengidentifikasi lingkungan pengendapan Formasi Tapak sehingga peneliti tertarik untuk melakukan penelitian. Studi ini bertujuan untuk mengidentifikasi lingkungan pengendapan serta mekanisme pengendapan yang terjadi pada Formasi Tapak. Metode penelitian terdiri dari metode measuring stratigraphy, analisis fasies, dan analisis paleontologi. Metode measuring stratigraphy dilakukan dengan merekam data geometri batuan, tekstur batuan, struktur sedimen, dan karakteristik batuan. Metode analisis fasies digunakan untuk mengelompokkan dan mengkorelasikan fasies batuan berdasarkan pola perubahan litologi batuan. Identifikasi litofasies kemudian diperkuat dengan hasil analisis paleontologi yang bertujuan untuk mengetahui umur relatif batuan dan batimetri. Hasil analisis fasies menunjukkan bahwa Formasi Tapak daerah penelitian terbagi menjadi dua satuan batuan, yaitu satuan batupasir dan satuan batuserpih. Satuan batupasir terdiri dari perselingan antara batupasir sangat kasar - sedang kehijauan, konglomerat, dengan sisipan batupasir halus karbonat dan serpih. Sedangkan satuan batuserpih terdiri dari perselingan batuserpih dan batupasir karbonat. Analisis paleontologi menunjukkan bahwa terjadi dua kali fase kenaikan air laut dan satu fase penurunan air laut yang menyebabkan terjadinya perubahan lingkungan pengendapan. Hasil analisis fasies menunjukkan bahwa Formasi Tapak merupakan bagian dari fasies turbidit pada suprafan lobes di bagian Middle fan dari sub-marine fan system. Terjadi empat kali perubahan lingkungan pengendapan dari channelled portion of suprafan lobes, channelled to smooth portion of suprafan lobes, channelled portion of suprafan lobes, hingga ke smooth portion of suprafan lobes.
Kata kunci: Formasi Tapak, Turbidit, Middle Fan.
ABSTRACT : Administratively the research area is located in the Samudra, Banyumas Regency, Central Java. Based on several previous studies,Tapak Formation was deposited in a deep seawithsubmarine fan system and tidal zone. In the research area, no previous studies have identified the depositional environment of the Tapak Formation so that researchers are interested in conducting research.. This study aims to identify the depositional environment as well as the depositional mechanism that occurs in the Tapak Formation. The research method consisted of measuring stratigraphy, facies analysis, and paleontology analysis. The method of measuring stratigraphy is done by recording data of rock geometry, rock texture, sedimentary structures, and rock characteristics. The facies analysis method is used to classify and correlate rock facies based on patterns of changes in rock lithology. The identification of lithofacies is then strengthened by the results of paleontological analysis aimed at determining the relative age of rocks and bathymetry. The results of the facies analysis show that the Tapak Formation in the study area is divided into two rock units, namely sandstone units and shalestone units. The sandstone unit consists of alternating between very coarse – coarse greenish sandstones, conglomerates, with fine carbonate sandstones and shale inserts. Where as the shalestone unit consists of interlocking shalestone and carbonate sandstones. Paleontological analysis shows that there were two phases of sea level rise and one phase of sea water descent which caused changes in the depositional environment. The results of facies analysis show that the Tapak Formation is part of the turbidite facies in suprafan lobes in the Middle fan section of the submarine fan system. There have been four changes in the depositional environment from channeled portion of suprafan lobes, channeled to smooth portion of suprafan lobes, channeled portion of suprafan lobes, to smooth portion of suprafan lobes.
Keywords: Tapak Formation, turbidite, Middle fan.
