New field investigations and stratigraphic studies in East Java have identified the most important features of arc activity in the Southern Mountains.
In the overlying sedimentary rocks volcanic debris and detrital zircons indicate initiation of the arc, and subduction beneath Java, by the Middle Eocene (42 Ma).
Volcanic activity along the Southern Mountains Arc culminated in the Early Miocene in a climactic phase of eruptions preserved in the Batu Agung Escarpment, near Yogyakarta.
The volcanic rocks of the Semilir Formation are characterised by features typical of terrestrial air-fall, pyroclastic surge and flow deposits including dune and antidune structures, crystal layering, well-sorted granular laminations, diffuse bedding, breccias (with metre-scale pumice blocks), thick mantling ashes, and abundant fragments of charcoal.
Locally there are water-laid deposits, with scoured irregular bases, flame, traction and suspension structures, and large slump folds indicating an unstable marine slope.
Fragment imbrication within the breccias records diverging west and southwest flow from a source in the northeast part of the Batu Agung Escarpment.
There is no evidence of any marine influence, and we interpret the Nglanggran Formation as terrestrial deposits formed by sector or repeated dome collapse, marking the end of arc activity in the Southern Mountains.
Widely distributed volcanic quartz-rich sandstones of Early Miocene age exposed in East Java that may be Semilir products but have not been precisely dated.