The growth and demise of a Middle Miocene carbonate platform in Central Luconia, offshore Malaysia

Abstract

The evolution and facies distribution of relatively small carbonate platforms, ∼30 km2, are not well documented, even though they are common in ancient and modern carbonate systems. This study investigates depositional facies and evolution of a Miocene carbonate platform in Central Luconia, offshore Malaysia. An integrated approach, including core-described lithofacies, well-log correlation, and seismic analysis, allowed for the mapping of facies through time. The platform was divided into five stratigraphic zones (1 [top], 2 Upper, 2 Lower, 3 Upper, 3 Lower [bottom]) separated by low porosity intervals. Five depositional facies groups were recognized in core samples: (1) bioclastic wacke-packstone facies deposited in a platform interior, (2) bioclastic packstone facies deposited in backreef environments, (3) coral boundstone facies deposited at the reef margin, (4) interbedded shale and carbonate facies deposited on the slope, and (5) argillaceous carbonate facies deposited across the platform as flooding layers. Well D penetrated almost the entire platform, and facies in that well changed upward from mainly bioclastic wacke-packstone at the bottom to mainly bioclastic packstone in the middle to coral boundstone near the top before being covered by interbedded shale and carbonate. Six seismic facies were identified and combined with well data to map the five facies groups through time. The platform shows repeated backstepping of facies at flooding layers followed by aggradation of facies belts up to exposure surfaces, resulting in a long-term backstepping of depositional environments until the platform drowned. Sr-isotopes dating near the top of the platform yielded an age of approximately 12 Ma. This suggests that the platform drowned earlier than other carbonate platforms in Central Luconia (8–10 Ma). More tectonic subsidence in this area during the Middle Miocene is proposed as the main cause of the early demise of this carbonate platform compared to other Luconia platforms.