Eocene to Miocene basin history and volcanic activity in East Java, Indonesia
Helen Smyth, completed PhD project
East Java is located on the SE margin of the Mesozoic continental core of SE Asia (Sundaland). In this study the region is divided into four east-west trending zones. The Southern Mountains Zone contains the eruptive products of an Eocene to Miocene volcanic arc. North of this arc is an (?Eocene) Oligocene to Early Miocene depocentre, the Kendeng Zone, now a fold-thrust belt. The most northerly zone, on the edge of the Sunda Shelf, is the Rembang Zone where Eocene to Miocene shelf and slope clastic and carbonate rocks dominate. About 50 km north of the Eocene to Miocene volcanic arc is the modern (Late Miocene to Recent) volcanic arc, products of which cover many older Cenozoic deposits.
This field-based study, aims to understand the Cenozoic development of East Java, particularly the basement character, nature of overlying sedimentary and volcanic sequences and their relationships to one another, and timing and causes of basin formation. A range of field techniques were utilised, including detailed section logging, palaeocurrent analysis and sampling. Samples have been studied using petrographic, SEM, XRD, and heavy mineral analytical methods. Isotopic dating has been carried out in collaboration with CSIRO, Australia. Geophysical data sets and remotely sensed images have been used to complement field and laboratory studies.
The exposed basement in East Java includes rocks of arc and ophiolitic character accreted during the Cretaceous; there are no outcrops of continental rocks. However, the new field observations and U-Pb SHRIMP analysis of zircons indicate the presence of continental crust at depth beneath SE Java.
In the Early Paleogene, basins developed on the Cretaceous and older East Java basement. Sedimentary rocks in the East Java basins have previously been interpreted to be continental clastic sediments. However, there has been subduction at the Sunda-Java Trench since the Early Cenozoic and volcanic activity in East Java was important. This study shows that Eocene-Miocene explosive Plinian-type volcanic activity was much more important and more acidic than previously recognised. New provenance studies show that many of the sediments have a volcanic origin or a significant volcanic component. There is no evidence to suggest sediment transport from Sundaland. Many of the sedimentary rocks previously thought to be “mature” have a major volcaniclastic component, and their “mature” character reflects pyroclastic processes and tropical weathering. This study shows that maturity or discriminant diagrams should be used with caution in tropical volcanic settings.
New U-Pb SHRIMP analysis of zircons has enabled precise dating of volcanic events. A potential super-eruption has been identified within the Southern Mountains (19 Ma ±1), likely to be a Toba-scale event. This previously unrecognised eruption may have had a significant influence on global climate during the Early Miocene. The U-Pb SHRIMP analysis also identified the presence of inherited zircons, which reveal basement structure and indicate the presence of Gondwanan continental crust beneath SE Java.
Basin development appears to be closely related to volcanism. Modelling in this study suggests that the Kendeng Zone depocentre was a flexural response to loading by the volcanic arc.