Abstract

Both ancient and recent tropical active volcanic settings have acted as sites for carbonate 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 of an ancient example is the Miocene Wonosari/Punung Formation 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 a shallow-water carbonate platform developed under moderate to high-energy conditions.

Calcareous algae and larger benthic foraminifera packstones, rudstones and grainstones dominated the carbonate lithologies on this platform. Although the mid-Miocene was a period of abundant global coral development, corals and other marine biota occurred as tertiary elements within the lithologies.

Situated to the north of the carbonate platform was a relatively deep (~200–400 m) fore-arc basin. This basin experienced 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 high-density flows, into the marine environment had dramatic effects on the development of shallow carbonate platforms. Where inputs were small, an increase in nutrients resulted in 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. This response can be attributed to an increase in nutrients, a lack of competitors and changes in substrate.