Distribution and abundance of macroborers in an upper Miocene reef system, Mallorca, Spain; implications for reef development and framework destruction

The importance of boring organisms within reef environments, as agents of framework destruction, and thus as controls to rates of reef accretion, has been widely recognised. Relatively little, however, is known about variations in the distribution of macroborers (principally sponges, bivalves and worms), within individual reef systems. The Upper Miocene reefs of Mallorca present a unique opportunity in which to assess such distributional variations. Borings are preserved as casts within the molds of former corals, and these trace fossils typically exhibit highly characteristic morphologies. Based on the distribution of different bore morphologies, it has been possible to assess variations in both the distribution and abundance of individual borers, and the intensity of boring activity across the reef system. Different parts of the reef are characterised by different associations of boring organisms, with the highest diversity of borers, and the most intense boring activity, observed within corals from the lagoonal facies. The principal control on distributional patterns appears to have been the morphology and structure of the reef framebuilders. Massive corals are always more extensively bored, and typically contain a higher diversity of boring organisms than branched or platy corals. On a more local scale, variations in sediment accumulation rates, and degree of substrate encrustation appear to have exerted some degree of control. Rates of reef accretion would have been influenced not only by the abundance of different boring organisms in different parts of the reef, but on the response of different framebuilders to boring activity. Platy and branched corals, whilst typically less intensely bored, would have been more vulnerable to the effects of boring, while massive corals, which were often extensively bored, were more capable of surviving such damage. This preservational bias has important implications for interpreting styles and rates of reef accretion.