Siliciclastic and volcaniclastic influences on equatorial carbonates : insights from the Neogene of Indonesia.

In active tectonic areas of humid equatorial regions, nearshore shallow-water environments are commonly sites of near-continuous siliciclastic influx and/or punctuated volcaniclastic input. Despite significant clastic influence, Neogene carbonates developed in SE Asia adjacent to major deltas or volcanic arcs, and are comparable with modern mixed carbonate–clastic deposits in the region. Research into delta-front patch reefs from Borneo and fore-arc carbonate platform development from Java is described and used to evaluate the effects of siliciclastic and volcaniclastic influx on regional carbonate sedimentation, local changes in carbonate-producing biota and sequence development. Regional carbonate development in areas of high siliciclastic or volcaniclastic input was influenced by the presence of antecedent highs, changes in the amounts or rates of clastic input, delta lobe switching or variations in volcanic activity, energy regimes and relative sea-level change. A variety of carbonate-producing organisms, including larger benthic foraminifera, some corals, coralline algae, echinoderms and molluscs could tolerate near-continuous siliciclastic or volcaniclastic influx approximately equal to their own production rates. These organisms adopted various 'strategies' for coping with clastic input, including a degree of mobility, morphologies adapted to unstable substrate inhabitation or shedding sediment, and shapes adapted to low light levels. Local carbonate production was also affected by energy regime, clastic grain sizes and associated nutrient input. Clastic input influenced the inhabitable depth range for photoautotrophs, the zonation of light-dependent assemblages and the morphology and sequence development of mixed carbonate–clastic successions. This study provides data on the dynamic interactions between carbonate and non-carbonate clastic sediments and, when combined with information from comparable modern environments, allows a better understanding of the effects of siliciclastic and volcaniclastic influx on carbonate production