CONTROLS ON FACIES HETEROGENIETY OF A TECTONICALLY ACTIVE CARBONATE SHELF: MIOCENE WONOSARI FORMATION, SOUTH CENTRAL JAVA, INDONESIA

The Miocene Wonosari Fm. occurs along the south coast of central Java. The Wonosari Fm. represents a carbonate platform developed in a forearc position deposited during the transgression over the eroded Oligocene “Old Andesite Arc” and south of the active Miocene volcanic arc. The Wonosari Fm. is truncated at the current shoreline extending south into the subsurface beneath the Indian Ocean. The Wonosari platform has been previously been interpreted to be 1) an isolated platform separated from the Miocene arc by a deep trough, 2) an attached platform connected to the arc, 3) a high energy open-shelf lacking a barrier reef, and 4) a platform bounded by a barrier reef.

The goal of this project is to interpret the factors that controlled the distribution of patch reef and inter-reef facies within the Wonosari Fm. Possible controls on patch reef vs. inter-reef facies include: 1) antecedent topography fostering patch reef environments in local shallow, higher-energy environments on an otherwise deeper open-marine shelf, 2) patch reefs developed in warm-oligotrophic waters whereas inter-reef areas were fostered by upwelling cool, nutrient rich waters that impinged on the shelf, 3) patch reefs developed in shallow wave agitated stenohaline waters whereas inter-reef areas were fostered by euryhaline or low-energy tidal flat environments developed behind a discontinuous barrier.

Patch reef facies include boundstones and rudstones with stenohaline organisms including scleractinian corals, red algae, benthic foraminifers, gastropods, and bivalves. Muddy facies with fragmented reef debris are interpreted to represent patch reefs peripheries. Non-reef facies are molluscan-foraminiferal-red algal skeletal wackestone-packstone and rodolith packstone that contain stenohaline biota, although corals are lacking. One section contains planktonic foraminiferal mudstone representing a deepening event.

Results indicate the reef vs. inter-reef facies distribution is controlled by water depth on an open-marine high energy shelf lacking a barrier reef margin. Wave energy and open-marine circulation facilitated patch reef development on topographic highs whereas intervening non-reef development occurred in relatively deep, stenohaline waters where low energy conditions and occasional wave agitation prevailed.