Heterozoan carbonates in oligotrophic tropical waters: The Attard member of the lower coralline limestone formation (Upper Oligocene, Malta)

Photozoan and heterozoan skeletal assemblages are controlled by their trophic requirements (i.e. light-based photosynthesis versus other sources) and temperature. Photozoan associations tend to dominate tropical and subtropical waters, whereas heterozoan carbonate systems tend to occur in cooler waters and in localised areas that are affected by nutrient-rich upwelling or terrestrial runoff. Because of the wide climatic spectrum in which heterozoan carbonates are found, their interpretation is often problematic. We present a high-resolution analysis of the Attard Member of the Lower Coralline Limestone Formation (Upper Oligocene, Malta). The biotic associations and palaeolatitudinal reconstructions suggest that carbonate sedimentation took place in tropical waters under oligotrophic conditions. An important factor controlling the spread of heterozoan assemblages in the Late Oligocene of Malta seems to be related to the palaeoecology and evolution of zooxanthellate. The limited capacity of corals to thrive in high-light conditions and to form a wave-resistant reef promoted the diffusion of heterozoan assemblages.