Java

Java from the space shuttle, its magnificent volcanoes and beautiful coast

Java is a calc-alkaline volcanic island arc formed by the northwards subduction of the Indian plate. Java's geological evolution has been influenced both by abundant volcanism, supplying copious amounts of volcaniclastic material, and an equatorial position, providing an ideal setting for carbonate production. The geological history of Java is surprisingly poorly known. Western Java includes Mesozoic rocks accreted to the Sundaland core and offshore are plutonic rocks representing the termination of the Late Cretaceous Asian active margin which extends from South China. There was a marked change in the Eocene and shallow water Eocene sediments rest on older rocks in a few places in Java. A land connection persisted through the central Java Sea into south Kalimantan until the early Miocene. West of this NE-SW-trending ridge is now an almost flat shelf, about which little is known due to absence of drilling, where Quaternary sediments rest unconformably on pre-Cenozoic rocks. The offshore northwest Java Basins are asymmetric half-graben with thick Paleogene syn-rift sequences which are mainly Oligocene but may locally be as old as Eocene overlain by thick Neogene post-rift sequences. The sediments were derived from the north and northwest. East of the ridge sedimentation began earlier. In the East Java Sea there are Lower Eocene non-marine clastic sediments which pass into Middle Eocene and younger marine sequences. The Paleogene basin history is dominated by extension and subsidence whereas regional contraction and uplift began in the early Miocene. Southern Java is much less well-known due to the absence of significant oil exploration. There are a small number of K-Ar ages from the 'Older Andesites' of van Bemmelen indicating Eocene-Oligocene volcanic activity and relatively deep water conditions in south Java. In south and west Java the 'Older Andesites' and other rocks are overlain by shallow marine limestones and there appears to be only limited volcanic activity during the Early and Middle Miocene. Volcanic activity became extensive again only during the late Middle or Late Miocene. Relatively deep marine sediments of Mio-Pliocene age are folded and thrust in west and north Java, and in south Java the 'Older Andesites' and overlying sequences were elevated to form the mountains now more than 3500 m above sea level.Eko Lelono studied palynomorphs found in the Nanggulan Formation in order to establish palynological zonation within the formation, to interpret vegetation changes during deposition, and the regional distribution of lower Paleogene palynomorphs across the region. Stephen Lokier's project attempted to evaluate the complex interplay of factors affecting carbonate sedimentation, sequence development and biotic assemblage in an area of active volcanism. Although volcanic arcs commonly provide sites for extensive shallow-water carbonate production, there have been few studies of carbonate sedimentation contemporaneous with volcanic activity. Helen Smyth's current project is examining the early development of east Java.

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Last modified: 09 July, 2002