Doctor of Philosophy
School of Geosciences
Susilohadi, Late tertiary and quaternary geology of the East Java Basin, Indonesia, Doctor of Philosophy thesis, School of Geosciences, University of Wollongong, 1995. http://ro.uow.edu.au/theses/1973
The East Java Basin is classified as a retro-arc basin, situated on the southeastern margin of the Sunda Shelf, Indonesia. A detailed Late Tertiary and Quaternary sedimentary evaluation of this basin is based primarily on seismic stratigraphic interpretation, combined with onshore outcrop studies of equivalent geological units. The primary aim of this study was to assess the sedimentation patterns and sedimentological history in the East Java Basin.
During the Middle Miocene and earlier, the northern part of the East Java Basin was strongly controlled by northeast-trending half graben systems which occurred along sutures between the earlier Late Cretaceous-Early Tertiary subduction systems. Little is known about the basin configuration in the southern part of the basin prior to the Middle Miocene.
Since the Late Miocene, east-trending anticlinal zones developed and were superimposed on the previous northeast-trending structures. The anticlinal Rembang Zone, which extends from the Blora area to Madura Island, became a dominant structure which divided the East Java Basin into two major basinal (synclinal) areas. In the north, a basinal area occurred between the Karimunjawa and Bawean Arches and the Rembang Zone. In the south, a basin developed between the Rembang Zone and the chain of volcanism along the median line of Java. Peak sedimentation in these east-trending basins occurred during the Late Miocene and Pliocene. The sedimentation patterns, such as local thickening, in the north basinal area (southern Java Sea) suggest that the basin development ceased in the Early/Middle Pleistocene. In contrast, rapid basin subsidence still occurred in the south basinal area at least until the late Middle Pleistocene, when extensive east-trending folding accompanied by diapiric movement occurred to form the anticlinal Kendeng Zone.
Middle Miocene and earlier sedimentation have not been comprehensively studied in the East Java Basin, particularly because of the paucity of outcrop and well data. The present seismic stratigraphic study of the onshore Rembang Zone revealed that the Middle Miocene sedimentation (Tawun Formation) patterns were strongly controlled by half graben systems, the southern extension of the northeast-trending stmctures indicated earlier. These structures still partly controlled the Late Miocene and Pliocene sedimentation in the Rembang Zone and the southern Java Sea.
Hemipelagic marl deposition (Wonocolo Formation) occurred in the southern part of the East Java Basin (onshore area) during a sea level highstand in the early Late Miocene, but little is known about equivalent deposition in the southern Java Sea at this time. The marl deposition was contemporaneous with the deposition of volcaniclastic turbidites (Kerek Formation) derived from Late Miocene volcanism along the median line of Java. This volcanism appears to have ceased in the late Late Miocene. A shallow shelfal facies (Ledok Formation) occurred in the southern part of the Rembang Zone (onshore area) which southward, grades laterally into the marl of the Kalibeng Formation.
During the Pliocene, the sedimentation was confined within the northern and southern basinal areas. Widespread pelagic marly deposition occurred during a prolonged highstand of sea level in the Pliocene which reflects a global eustatic sea level rise recognised by Haq et al. (1988). In the southern basinal area, the deposit is represented by the Mundu (seismic unit PL1), Kalibeng and Atasangin Formations respectively; whereas in the northern basinal area (southern Java Sea) the deposit is represented by seismic units JP1 and JP2. A relative sea level fall occurred in the Late Pliocene and led to the development of an anoxic lacustrine deposit (Lidah Formation) in the southern basinal area.
During the Quaternary, the seismic stratigraphic study indicates that sediment stratal patterns were highly controlled by frequent relative sea level changes, and sedimentation was still confined within the two major basinal areas indicated earlier. In the southern basinal area, seismic sections clearly display an intertonguing between volcaniclastics derived from the Quaternary volcanism along the median line of Java, and shallow marine sediments derived from the exposed Rembang Zone in the north. Field observations indicate that volcaniclastic deposits are coarse-grained, whereas on seismic sections from Madura Strait these deposits are associated with relatively steep depositional slopes (of alluvial fan/fan delta systems). The seismic stratigraphic study in Madura Strait indicates that the depositional timing of these deposits was strongly controlled by the relative sea level changes in this area.
In the southern Java Sea (northern basinal area), the basin filled during the Pliocene as revealed from the seismic stratigraphic study, giving a flat basin topography at the end of the Pliocene. Nine thin seismic subunits are recognised in this area and are believed to represent at least nine Quaternary sea level fluctuations. Each subunit is relatively thin, and tends to be distributed widely because of deposition on a relatively flat lying area. The seismic characters of these subunits are very similar, with subparallel reflection or almost reflection free patterns representing marine deposits, topped by extensive fluvial channelling. This repetitive succession represents highstand and lowstand of sea level respectively.
The integration of seismic sequence stratigraphic analysis and outcrop sedimentological studies has provided a comprehensive view of the East Java Basin on the timing of deposition and deformation of the basin fill, mode of deposition and sedimentation patterns within the sub-basins, and the influence of sea level changes on stratal patterns.