BIOSTRATIGRAFI DAN STUDI LINGKUNGAN PENGENDAPAN FORMASI NGRAYONG DI DAERAH CEPU

• Main Author: FIRMAN SJAMSUDDIN , IMAN
• Format: Theses
• Language: Indonesia
• Subjects: Geologi, hidrologi & meteorologi
• Online Access: https://digilib.itb.ac.id/gdl/view/8207
• Institution: Institut Teknologi Bandung

Depositional environment of Ngrayong Formation is still debatable. Research about depositional environment of Ngrayong Formation with comprehensive data, such as biostratigraphic and wireline log, will give a better understanding about interval, age and depositional environment of Ngrayong Formation, especially at Cepu area. Data, which used in this research, are description of cutting from 6 wells, biostratigraphic and seismic. The method is started by analyzing biostratigraphic and bathymetri. Biostratigraphic analyzing is looking for first and last appearance of foraminifera planktonic fossil, which can used for biomarker in well correlation. Fasies, age and depositional environment analysis of Ngrayong Formation is also doing. Based on analysis in Cepu-1, Cepu-5 and Cepu-6 wells, there are five planktonic foraminifera zone, they are Orbulina universa-Globorotalia peripheroacuta zone, Globorotalia peripheroacuta-Globorotalia praefohsi zone, Globorotalia praefohsi-Globorotalia fohsi zone, Globorotalia fohsi-Sphaeroidinellopsis subdehiscens zone and Sphaeroidinellopsis subdehiscens-Globigerina nepenthes zone. Ngrayong Formation, at Cepu area, is deposited in Middle Miocene or placed into N9-N12 (Blow, 1979). Based on biostratigraphic analysis in Cepu-1, Cepu-5 and Cepu-6 wells, three biomarker are identified, they are first appearance of Sphaeroidinellopsis subdehiscens, first appearance of Globorotalia cultrata and first appearance of Sphaeroidinellopsis semminulina kochi. Ngrayong Formation at Cepu has contact with Bulu Formation limestone at upper and Tawun Formation limestone at lower. It consist three facies, they are sandstone, limestone and shale. Ngrayong Formation at Cepu is deposited in shelf environment. Depositional of Ngrayong Formation is stopped at 100-200 meter water depth or outer neritic zone, after that, siltstone and shale of Tawun Formation is deposited until bathyal environment. Sand shale ratio of Ngrayong Formation at Cepu area, relatively decrease to southward, refer to bathymetry changing to deeper. Dominating of shallow marine to deep marine fossil at Cepu-1 well, indicating storm process in Ngrayong Formation depositional at Cepu area. While at Cepu-5 and Cepu-6 wells, which biostratigraphically has same age with Ngrayong Formation at Cepu-1 (N9-N12), dominating of deep marine to shallow marine fossil, indicating turbidite process in Tawun Formation depositional at Cepu area.