Facies characterization and mechanism of termination of a tertiary carbonate platform: Rajamandala Fm., West Java

Abstract

The objective is to better understand an Oligocene coral reef ecosystem, exposed in SW Java as a scenic ridge of karst towers. Extensive mining threatens to destroy the spectacular scenery and important geological history. Depositional environments were interpreted from field and laboratory studies. Previous studies established the system as a fringing reef in a deep marine back-arc basin. It has been debated whether the reef formed an isolated platform or if it was attached to the island arc. Results indicate an attached shelf receiving sedimentation from a landward source, due to the presence of quartz sand. Initial reef development in a high-energy environment is indicated by fractured grains. The biologically diverse reef contains massive corals, algae, and foraminifera. The slope contains delicate platy corals and experienced episodic debris-flows. Extinction of the reef is represented by deepwater limestones, indicating platform drowning. The geologic context suggests rapid subsidence. The presence of photosynthetic red algae suggests submergence was not to subphotic depths. Abundant giant foraminifera indicates delayed reproduction under stressed conditions, which may have included cool waters, increased nutrients, algal blooms, and/or influx of sedimentation. Results will improve the understanding of similar ancient and modern reefs and their response to environmental change.