Palaeogene and Early Miocene geological evolution of West Java
Ben Clements, completed PhD
Java has considerable proven hydrocarbon reserves and mineral deposits both offshore and onshore. Java is densely populated, has good infrastructure and has been geologically mapped in the greatest detail of all the Indonesian islands. Paradoxically, however, the Cenozoic evolution of the island, history of subduction and associated volcanism, changes in sedimentary environments and correlations between onshore and offshore deposits are poorly understood. As a consequence we began a new project in 2000 which focused on the region of East Java.
The study has shown that the onshore East Java basins began to develop in the Eocene but are not typical extensional or subduction-related basins. Despite subduction beneath Java since the early Tertiary, arc activity has not been continuous. A volcanic arc was active in southern Java from the Middle Eocene, earlier than previously suggested, until the Early Miocene. There was a lull in the Middle Miocene and arc activity shifted about 50 km northwards in the Late Miocene. Previous studies of the basins have focussed on the sedimentary provenance of their fill. They have suggested that quartz was derived primarily from continental Sundaland and the basins were formed by rifting in a backarc setting. The new field studies question these interpretations. Quartz-rich sands in onshore basins are potential hydrocarbon reservoirs but have unusual characteristics. They contain well-sorted but angular and bipyramidal quartz grains, with melt inclusions and embayments. Feldspars show similar features and volcanogenic clays are widespread. There is more volcanic material than previously appreciated and we interpret it as the products of Plinian-type eruptions. Factors traditionally used to assess sediment maturity, such as grain size distribution and morphology are not applicable. New U-Pb SHRIMP analyses of zircons precisely date Cenozoic volcanic episodes but also imply melting of continental material, consistent with dacitic eruptive products, of Precambrian age. Although formed behind a volcanic arc the basins are not the result of backarc rifting and we propose they were formed by flexural loading. Comparisons with historical activity in the Sunda arc suggests volcanic material should be present in all East Java sequences, onshore and offshore, with important implications for correlation, provenance and reservoir properties.
Examination of some of the quartz-rich sands of West Java suggest they too contain a previously overlooked volcanic component. At present we do not know how widespread these are, nor how the volcanogenic contribution varies in time and space. Up to now there has been general acceptance that West Java is underlain by continental crust of Sundaland and that sediment has been derived from Sundaland since at least the Eocene. The work in East Java indicates that these assumptions need to be examined more carefully. We would aim as part of this project to discover the provenance and transport direction of sediment being supplied to West Java, and more recently, being eroded from it. We would also aim to determine how much volcanic material there is in the sequences, and how widespread this is. As noted above, historical activity in the Sunda arc suggests volcanic material should be present in West Java sequences, onshore and offshore. In East Java the drainage divide is far to the south in the Southern Mountains so almost all sediment is being carried north to the Java Sea at the present day. Has this been the situation in the past? The lack of detailed information and difficulties of regional correlation strongly hinders our understanding of the development of the southern margin of Sundaland and the adjacent highly productive basins.
When considering development of Java we noted that there had been suggestions that the subduction trench migrated gradually southwards from a position in Central Java to the current location during the Tertiary. In fact, the recent work in East Java suggests the opposite. The Eocene trench was located south of the present south coast of Java, and the arc moved north not south during the Tertiary. We do not know if was the same in West Java.
There are some obvious important differences between East and West Java. The large broadly east-west gravity low which is associated with the East Java onshore depocentre disappears in central Java. There are considerable thicknesses of Mio-Pliocene turbidites reported from West Java which have no obvious equivalents in East Java. There also seems to be more extensive development of carbonates in East Java than in West Java. The development of east–west trending basins, separated by block-faulted highs, where both clastic and carbonate sediments accumulated, may also be related to changes in the subduction system. Mapping and exploration by a large number of individuals and companies has resulted in a considerable array of names and age interpretations for the different lithologies across Java. Due to the tectonic complexity of the area and the common inter-digitation of carbonate and volcaniclastic/clastic lithologies many of the units are time transgressive, have been assigned lithostratigraphic names, or are only locally developed.