A model for the development of rhodoliths on platforms influenced by storms: middle Miocene carbonates of the Marion Plateau (northeastern Australia)

Abstract

Middle Miocene carbonates of the Marion Plateau consist of dolomitized bioclastic floatstones and rudstones with rhodoliths (up to 6 cm in size) as the most prominent component. These rhodoliths are embedded in a bioclastic matrix with Halimeda, echinoids, bivalves, gastropods, bryozoans, small coralline fragments, benthic (and planktonic) foraminifers, and rare dasyclads. Corals (not abundant) occur only as fragments and may serve as nuclei for some of the rhodoliths. Two main types of rhodoliths are evident. The commonest type is formed by Lithothamnion and Sporolithon, together with minor Hydrolithon, Mesophyllum, Spongites, and Lithoporella. The other type is made up mainly of Mesophyllum. Laminar growths are always predominant inside the rhodoliths. Both the growth types and the algal associations are characteristic of rhodoliths that formed at depths of some tens of meters and below the normal wavebase. Similar coralline associations presently occur in the Indo-Pacific area at depths between 30 and 80 m. Clearly, these depths are below the normal wavebase, but within the reach of storms. Encrusting foraminifers, serpulid worm tubes, bryozoans, and vermetids are sometimes important elements within these rhodoliths and occur either as more-or-less discrete layers interbedded with the coralline growths or in their nuclei. Bioclastic sediment is also incorporated within the rhodoliths. Some of the rhodoliths now appear partially broken and, presumably, were reworked in the environment of deposition. Others exhibit several phases of growth and reworking. Some of them have also been bored. The context in which these rhodoliths developed was that of a neritic, open-platform environment. They were reworked and partially broken and abraded during storms and then grew once more during the intervening calm periods. The internal structure of the rhodoliths is complex in detail, with successive coralline laminae encrusting more-or-less eroded former growths, and in turn being partially destroyed during the next storm event.