Eocene to Pliocene coralline algae in the Queensland Plateau (northeastern Australia)

Sediments containing coralline algae in the Queensland Plateau range from the middle Eocene to the early Pleistocene in age. In the middle Eocene sediments, the corallines occur as highly fragmented and eroded particles in temperate, platform carbonates (grainstones and rudstones) with abundant bryozoans, benthic foraminifers, and bivalves. They occasionally interbed with thin intervals of tropical to subtropical sediments that contain abundant Halimeda, coral debris, and coralline fragments, mainly of Lithoporella and Mesophyllum. In the upper Oligocene sediments, similar temperate carbonates, in which small, scarce, unidentifiable coralline fragments can be seen, also occur. The lower Miocene platform carbonates consist of grainstones and packstones having a tropical assemblage with corals and Halimeda, together with a shallow-water coralline association of geniculate (Jania, Corallina, Amphiroa) and encrusting forms (Lithoporella and Spongites). Allochthonous fragments of geniculate and encrusting corallines {Mesophyllum and Lithothamnion) of early Miocene age occur in clasts in bioclastic floatstones (debris-flow deposits) of middle Miocene age. The algal composition points to an outer-shelf origin for these clasts. The middle Miocene carbonate sediments that contain corallines are also tropical and consist of bioclastic packstones to wackestones, with Halimeda, and corals. Coralline-algal debris is predominantly composed of loose, abraded, branching thalli of Sporolithon and Lithothamnion, and small rhodoliths (up to 2 cm). The nuclei of most of the rhodoliths consist of a small branch fragment. They are encrusted by laminar growths of Lithothamnion, Mesophyllum, Hydrolithon, and Sporolithon. Some fragments of the rare coralline genus Aethesolithon also have been found. The depositional environment is that of a low-energy, neritic, open platform, as can be deduced from the predominance of delicate branching growths, the smaller size of the rhodoliths, the abundance of fine-grained sediments, and the scarceness of reworking. Lower Pliocene corallines occur as redeposited elements in debris flows. They appear mainly as very thin laminae in foralgaliths that are intimately associated with encrusting foraminifers. The most common alga is Lithoporella. The present-day, shallow-water, reefal coralline associations dominated by members of the subfamily Mastophoroideae were not detected in any of the sediments drilled in the Queensland Plateau