Pustaka Ilmiah Universitas Padjadjaran

Abstrak

Studi Fasies Dan Lingkungan Pengendapan Satuan Batupasir Formasi Halang Berdasarkan Asosiasi Litofasies Di Daerah Ajibarang, Kecamatan Ajibarang, Kabupaten Banyumas, Propinsi Jawa Tengah (Sebagian Lembar Peta Rupabumi Digital Indonesia (Bakosurtanal) No.1308-611)

Penelitian ini dilakukan di sungai Tajum, Desa Ajibarang, Kecamatan Ajibarang, Kabupaten Banyumas, Propinsi Jawa Tengah. Pengambilan data dilapangan dilakukan dengan mengukur ketebalan perlapisan singkapan batuan. Daerah penelitian didominasi oleh perlapisan batulempung dan batupasir disebelah selatan, namun didominasi oleh perlapisan batupasir pada sebelah utara. Hal ini menimbulkan dugaan adanya perubahan fasies dari arah selatan menuju utara daerah penelitian. Hasil dari analisis penampang litologi, singkapan batuan menunjukkan karakteristik struktur sedimen berupa massive sandstone, graded bedding, upper parallel lamination, lower parallel lamination, cross lamination, climbing-ripple cross lamination, dan convolute lamination yang diendapkan dengan media air. Sedangkan dari hasil analisis mikropaleontologi, batuan diendapkan pada lingkungan Batial Atas-Tengah. Litofasies yang teridentifikasi pada penampang litologi adalah B, C, D dan F (berdasarkan endapan submarine fan Mutti & Ricci Lucchi, 1972) dan Classical Turbidite (C.T.), Massive Sandstone (M.S.) dan Debris Flow (D.F.), (berdasarkan fasies turbidit Walker, 1978). Analisis asosiasi fasies menunjukkan daerah penelitian berada pada sistem kipas bawahlaut (submarine fan) dengan fasies berupa Mid-fan Suprafan lobes dan Slope hingga Upper-fan yang merupakan hasil pengendapan laut dalam dengan sistem pengendapan arus turbidit dan aliran debris. Pada penampang litologi juga menunjukkan adanya perubahan fasies yang dipengaruhi oleh naik- turunnya permukaan air laut, yaitu pada dua fase transgresi dan satu fase regresi. Analisis arah arus purba (paleocurrent) dilakukan dengan indikator struktur sedimen sekunder berupa flute cast pada bidang perlapisan batulempung. Restorasi lineasi sumbu memanjang flute cast dengan menggunakan stereonet menghasilkan arah arus dengan azimuth N118oE, yang mengindikasikan arah arus pengendapan pada saat material sedimen diendapkan berasal dari arah Baratlaut menuju ke arah Tenggara. Secara keseluruhan, maka lingkungan pengendapan pada daerah penelitian adalah laut dalam, khususnya pada sistem pengendapan kipas bawahlaut.

This research is conducted in river Tajum, Ajibarang village, district Ajibarang, Banyumas, Central Java province. Field data is done by measuring the thickness of bedding rock outcrops. The research area is dominated by mudstone and sandstone bedding in the south , but is dominated by sandstone bedding on the north. This has led to interpretation of facies environment change from south to north area of research. Results from cross-sectional analysis of lithology, rock outcrops shows the characteristic of sedimentary structures in the form of massive sandstone, graded bedding, parallel lamination upper, lower parallel lamination, cross- lamination, climbing- ripple cross lamination, and convolute lamination precipitated with aqueous media. While the results of the micropaleontology analysis, rocks deposited on the Middle – Upper Bathyal environment. Litofacies identified in the lithological cross section are B, C, D and F (based on submarine fan deposition, Mutti & Ricci Lucchi,1972) and the Classical Turbidite (CT), Massive Sandstone (MS) and Debris Flow (DF), (based on turbidite facies Walker,1978). Facies association analysis shows the study area is located on the submarine fan system (submarine fan) with a Mid-fan Suprafan lobes facies and Slope to the Upper-fan that result in the deposition of marine turbidite deposition system flow and debris flow. In cross-section also shows the lithological facies changes are influenced by the rise and fall of sea level, which is the two phases of the transgression and regression phases. Analysis of ancient current direction (Paleocurrent) performed with the secondary indicators of sedimentary structures such as flute casts on bedding plane of claystone. Lineation restoration of longitudinal axis of the flute cast using stereonet analysis generate current direction with azimuth N118oE, which indicate the direction of flow during the deposition of sediments material deposited comes from the Northwest heading Southeast towards. Overall, the depositional environment in the study area is deep marine, especially in submarine fan depositional systems.