The Miocene Wonosari Formation, Java, Indonesia: Volcaniclastic influences on carbonate platform development
Stephen Lokier, completed PhD project
Tropical active volcanic settings are sites for carbonate platform development. Although this association has long been recognized, very few studies into the interactions between volcaniclastic and carbonate lithologies have been undertaken. An ideal site for such a study is the Early to Late Miocene Wonosari and Kepek Formations of south Central Java, Indonesia. During the Miocene, andesitic volcanism in Java occurred along an extensive island-arc associated with the subduction of the Indo-Australian oceanic plate.
To the south of this island-arc an isolated shallow-water carbonate platform, here named the Wonosari Platform, developed on an uplifted fault block under moderate to high energy conditions. The Wonosari Formation was deposited as a single protracted transgressive sequence resulting in platform aggradation. Coralline algae and larger benthic foraminifera rudstones, packstones and grainstones dominated the carbonate lithologies on this platform. High-energy regimes and unstable substrates resulted in the development of a rhodalgal assemblage with corals being rare on the platform.
Situated to the north of the carbonate platform was a relatively deep (~200-400 m) trough. This trough, here named the Wonosari Trough, received volcaniclastic sedimentation derived from the volcanic-arc to the north and carbonate deposition from the shallow marine platform to the south. There is some interdigitation of the two sediment types.
Periodic inputs of volcaniclastic material, either as sub-aerial ash-falls or sediment gravity flows, into the marine environment had dramatic effects on the development of the shallow water platform and its slope deposits.
Where volcaniclastic input was small there was an increase in the numbers of species and the numbers of individuals within the palaeoenvironment. Larger, or more sustained periods of volcaniclastic sedimentation resulted in a decrease in the numbers of species but increased numbers of individuals. These responses can be attributed to a number of factors including: ability to tolerate physical burial; a reduction in the photic zone; an increase in nutrients; changes in water chemistry; and changes in substrate.
The Wonosari limestone in therefore interpreted as a carbonate platform developed in a high-energy forearc setting.