SEDIMENTASI AWAL CEKUNGAN ASRI DAN STRUKTUR YANG BERKEMBANG PADA SEDIMEN TERSEBUT

• Main Author: GURITNO, NUSATRIYO
• Format: Theses
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/69601
• Institution: Institut Teknologi Bandung

Asri Basin locate d 180 kms offshore to the north of North Java coastline and 140 kms offshore to the east of Southeast Sumatra coastline with 20 to 25 meter water depth. Asri Basin form as a half-graben structure with north south strike axis and part of series of sturctures with northwest – southeast trend from the position of Asri Basin to southeast to the Arjuna Basin area and continue to the Central Java direction. The fault boundaries of the half graben perform arcuate shape to the L shape that can be explained as the result of the transtentional movement cause by the wrenching in the area. The wrench movement could be the ma in movement in the Early Tertiary before the compression after Oligocene due to the subduction. The result of the wrench movement generated the nor th-south faults that control the syn-rift sedimentation in the half-graben basin. The simple profile of Asri Basin use to make people think that the basin development is very simple, however, the detail structure analysis of Asri Basin resulted to a more complex basin development. Asri Basin is not created by a simple extensional movement but more of transtensional movement. The development of Asri Basin can be devided into two main phase, which are the Paleogene Rift phase than can be devided again into pre-rift, infra-rift and syn-rift phase and follow by the Neogene Post Rift phase. The pre-rift phase starts by the subduction of the Hindia oceanic crust beneath the Sunda continental crust in the Late Cretaceous to Paleocene. The movements continue in the Early Eocene when the continental crust extended due to the wrench movement that strech the litosfer and developed the sag basin. The fluvial sedimentation and the sagging continue until the surround area vi covered by water and the basin become a lake and deposited the fine-grained sediment of lacustrine shale (Banuwati shale). The syn-rift phase starts by the breaking of the lithosphere and follow by the generation of curves and L shape fractures with northwest-southeast orientation. The fractures crea ted boundary to the development of the half graben rift and alluvial fan can be found along the rift boundary. This phase started with uplifting and erosional that resulted to the development of local angular unconformity, afterward the sedimentation is influence by the movement of the main normal fault of the syn-rift and developed the asymetric thick sediments. After the fault movement ceased, then began the deposition of the sediment in the tidal marine environment due to the sea level rise from Oligocene to Early Miocene. Later on, the Neogen Post –rift began with the development of the limestone (Gantar limestone) that spread out to the basin in the Northwest Java and therefore stratigraphically the areas have the same stratigraphic sequences. Structure analysis resulted an understanding of the interior structure of the Asri Basin that showed the development of a duplex structural pattern or segmented strike-slip pattern that influence the interval of the sediment deposited in the early basin fill or the Banuwati Formation, the Paleogen pre-rift sediment. The structural development then ended by the transtensional movement that can be seen in the seismic section that showed a wrench movement in younger age (Plio-Pleistocene) which created the southern boundary of the Asri Basin.