Print this pageProblematic Interpretation of Mixed Carbonate-Siliciclastic Facies of Rajamandala Limestone, Padalarang, West Java, Indonesia
The Rajamandala Formation of Late Oligocene to Early Miocene age is well exposed in Padalarang area, about 20 km west of Bandung. A collaboration study between Geology Research group of Geological Department Institute of Technology Bandung, Rice University, and ExxonMobil has been focused on the tectonic and sedimentation evolution of this formation.
The Rajamandala Formation forms two structural ridges namely North-west ridge and South-east ridge. Based on field observation, supported by petrography and biostratigraphy analysis, six carbonate facies association can be identified: 1) Bedded foraminifera-red algae packstone to grainstone, with bindstone intercalation, 2) Thick bedded domal platy coral bindstone and massive coral framestone, 3) Thick bedded to massive rudstone-packstone facies, 4) Thin well bedded classic turbidite foraminifera packstone to grainstone, 5) Milliolid Packstone to wackestone facies, and 6) Mixed Carbonate-Siliciclastic Breccia (MC-SB). All facies with the exception of the last where deposited in shallow-water conditions.
Of special importance is the MC-SB facies as they contain carbonate breccia with quartz and sandstone clasts (up to 4 cm in size). This unit is only observed in the upper part of the exposed section of the South- ridge. Similar facies but without quartz/sandstone clasts can be observed in the upper most section of North-west ridge. Based on planktonic foraminifera analysis, the base of Mixed Carbonate-Siliciclastic Deep-Water Breccia is P22 (Late Oligocene) while the youngest section of this facies is N4-N5 (Early Miocene).
The MC-SB deposition marks the deepening and demise of the Rajamandala shallow-water deposition. The carbonate breccia and quartz/sandstone clasts suggest erosion of older than Rajamandala Fm lithologies as well as of Rajamandala equivalent rocks to the south. A preliminary hypothesis is that the drowning of the Rajamandala Formation was due to rapid subsidence related to faulting which caused uplift to the south and reworking of a suit of older and time-equivalent lithologies.
AAPG Search and Discover Article #90100©2009 AAPG International Conference and Exhibition 15-18 November 2009, Rio de Janeiro, Brazil