Substrate Topography Control on Initiation and Evolution of Tertiary Reefs and Carbonate Platforms in a Volcanic-Arc Setting of Southern Java, Indonesia

The Miocene Wonosari Formation forms a series of broad, high energy platforms with patch reefs developed along the south-central and southeastern Java. Open platform facies consist of bioclastic, peloidal and rhodolith packstones containing large foraminifers, mollusks, coralline red algae, echinoids, and fragmented skeletal material. Patch reef facies contain diverse assemblages of corals with head, stag-horn and finger morphologies, coralline red algae and large foraminifera.

The platforms were nucleated on irregular topography of the eroded “Old Andesite” volcanic arc. Maps depict the unconformity as a simple surface that truncates intrusive and volcanic rocks of the arc as well as intervening siliciclastic strata. Our mapping reveals a complex surface that truncates intrusive and volcanic rocks on topographic highs that may be overlain by alluvial conglomerates or upon which coral-rich carbonates were nucleated. In topographic lows no discrete truncation surface was found between the Wonosari and the underlying Sambipitu Fm.

In topographic lows siliciclastic sedimentation of the Sambipitu Fm. shallowed upward from turbidites to shoreline sands and fanglomerates containing clasts eroded from the arc and coral clasts reworked from reefs developed as advancing seas flooded southern Java. Although the bulk of coral clasts found within the fanglomerates were likely derived from coeval reefs that grew and were reworked during transgression, the presence of large lithified carbonate blocks of varied lithologies also indicate tectonic uplift and erosion of an older carbonate platform.

Reworked corals occur everywhere in the basal Wonosari indicating abundant patch reefs developed during the initial transgression, whereas later establishment of vast carbonate shelves limited reef development.

Surprisingly we found no evidence of subaerial exposure horizons within the Wonosari Fm. except for soil horizons developed in volcaniclastic sediments formed near landward attached areas of the platforms. In these areas depositional cyclicity was produced by pulses of volcaniclastic influx repeatedly filling accommodation space.