Heterozoan carbonates in oligotrophic tropical waters: The Attard member of the lower coralline limestone formation (Upper Oligocene, Malta) Marco Brandano a,b, ⁎, Virgilio Frezza a , Laura Tomassetti a , Marco Cuffaro b a Dipartimento di Scienze della Terra, Università di Roma “La Sapienza”, P. Aldo Moro 5, I-00185 Roma, Italy b IGAG-CNR, P. Aldo Moro 5, I-00185, Roma, Italy and ISMAR-CNR, Via Gobetti 101, I-40129, Bologna, Italy abstract article info Article history: Received 21 April 2008 Received in revised form 8 December 2008 Accepted 23 December 2008 Keywords: Heterozoan carbonates Carbonate ramp Biofacies Trophism Oligocene 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. © 2008 Elsevier B.V. All rights reserved. 1. Introduction The environmental conditions that control biogenic sediment formation on carbonate platforms are determined by ocean circulation, which affects water temperature (Lees and Buller, 1972; James, 1997), seawater composition and carbonate saturation state (Hallock, 1996; Stanley and Hardie, 1998), and nutrient availability (Hallock and Schlager, 1986; Birkeland, 1987; Carannante et al., 1988; Hallock, 1988, 2001; Mutti and Hallock, 2003). In turn, the biotic community determines accumulation rates and facies distribution, thus controlling platform geometry (Pomar, 2001a). James (1997) introduced the terms photozoan and heterozoan to relate skeletal assemblages to 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 (temperate to polar latitudes) and in lower- latitude settings affected by nutrient-rich waters resulting from upwelling or terrestrial runoff (Lees and Buller, 1972; Carannante et al., 1988; James, 1997; Mutti and Hallock, 2003; Pomar et al., 2004). Because heterozoan carbonates can be found across a wide climatic spectrum (Carannante et al., 1988; Henrich et al., 1995; James, 1997) their interpretation is often problematic. This is shown, for example, by the different reconstructions proposed for various Oligocene and Miocene carbonates of the Mediterranean area (Pomar et al., 2004; Braga et al., 2006). Recently, the role of trophic conditions has been stressed by Mutti and Hallock (2003), Pomar et al. (2004), and Wilson and Vecsei (2005). According to the model of Hallock and Schlager (1986), excess nutrient input may terminate accumulation on a chlorozoan (photozoan) carbonate platform or cause a shift towards heterozoan carbonate production (Pomar, 2001b; Brandano and Corda, 2002; Mutti and Hallock, 2003; Pomar et al., 2004). Our detailed analysis of the stratigraphy, sedimentology and biofacies of the Attard Member of the Lower Coralline Limestone Formation (Upper Oligocene of Malta) shows that it was deposited on a carbonate ramp dominated by heterozoan and, subordinately, photo- zoan skeletal assemblages. The biotic associations and palaeo- latitudinal reconstructions suggest that carbonate sedimentation took place in tropical waters under oligotrophic conditions, a typical environment for photozoan assemblages. The dominance of hetero- zoan assemblages under such conditions add a further complication to the interpretation of ancient carbonates dominated by this skeletal association. The aim of this work is to highlight the role of the different factors controlling the spread of heterozoan assemblages in the late Oligocene of Malta. 2. Geological setting The Maltese Islands and the Iblea Plateu of Sicily were part of a geographically extensive carbonate ramp located on the distal segment of the Late Oligocene (Chattian) to Miocene African Palaeogeography, Palaeoclimatology, Palaeoecology 274 (2009) 54–63 ⁎ Corresponding author. Dipartimento di Scienze della Terra, Università degli Studi di Roma, “La Sapienza”, P.le Aldo Moro 5, I-00185 Roma, Italy. Tel.: +39 06 49694240; fax: +39 06 4454729. E-mail address: marco.brandano@uniroma1.it (M. Brandano). 0031-0182/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2008.12.018 Contents lists available at ScienceDirect Palaeogeography, Palaeoclimatology, Palaeoecology journal homepage: www.elsevier.com/locate/palaeo