FACIES CHARACTERIZATION AND MECHANISM OF TERMINATION OF A TERTIARY CARBONATE PLATFORM: RAJAMANDALA FORMATION, WEST JAVA

The Oligocene, Rajamandala Fm. consists of a northeast trending belt of limestone in the Padalarang and Sukabumi areas of southwest Java. The belt, exposed along a north-verging thrust, forms a scenic ridge of karst towers in an area dominated by volcanic and siliciclastic rocks. Extensive mining for building stone and concrete production threatens to destroy the spectacular scenery and the important geological history of this deposit.

The Rajamandala Fm. occurs north of an Oligocene volcanic arc and faces a deep-marine, back-arc basin to the north. Koesoemadinata and Siregar (1984) established the regional stratigraphic framework consisting of a shallow platform with restricted lagoon facies, and a fringing reef that changes northward and eastward into slope and basinal facies. It is uncertain whether the reef formed an isolated carbonate platform within the back-arc basin or if it was a shelf attached to the southerly arc. The presence of sandstone layers in the base of the Rajamandala and the presence of quartz sand in reef and lagoon facies suggests it formed as an attached shelf.

Lagoon facies consist of skeletal-peloidal packstones with foraminifera, miliolids, red algae, minor corals and echinoderms. The reef facies are massive limestones with large head corals, sheet and ‘staghorn' corals embedded in a packstone matrix containing large forams, red algae, rhodoliths, intraclasts, mollusks, and possibly calcareous green algae. The slope facies contains delicate platy corals and debris-flow breccias with clasts of boundstone and packstone. In the distal slope and basin, carbonate turbidites are graded packstones with benthic and planktonic foraminifers, red algae fragments and minor coral fragments, interbedded with foraminifer marls and shales.

The top of the Rajamandala, marking the termination of the platform, changes to a dark brown, argillaceous foraminifer packstone followed upward by siliciclastic turbidites of the Citarum Fm. The geologic context indicates tectonic subsidence causing the platform to sink into deep waters, as indicated by the dark colored marl. The presence of exceeding large foraminifers (up to 5.5 cm across) indicates stressed conditions and delayed reproduction. The stressed conditions that contributed to termination may have included cool waters or siliciclastic flux.