Bioerosion in Late Permian Rugosa from reefal blocks (Hawasina Complex,Oman Mountains): implications for reef degradation

Summary Rugose corals are known from allochthonous Late Permian reefal blocks of the A1 Jil and Ba’id Formation (Hawasina Complex), Oman Mountains. In contrast to many Late Permian Rugosa found elsewhere in the Tethys, they occurred in sponge reefs and contributed to reef construction. The waagenophyllid warm water coral fauna is moderately diverse comprising cerioid, thamnasterioid, and fasciculate taxa. In contrast to sponges, chaetetids, and low-growing reefbuilders, the corals secreted diagenetically stable, most probably Mg-calcitic skeletons. Borings in coral skeletons are consequently well preserved providing important data for the interpretation of reef destructive processes. Thin-section analysis revealed three taxa of infaunal borers includingEntobia Bronn 1837, uncertain thallophyte borings, and borings of unknown bioeroders. Macroborers were more important than microborers, because of the dominance of clionid sponges. Good evidence exists also for the occurrence of two types of undetermined grazers which destroyed the coral surfaces. The amount and distribution of bioerosions is variable among different coral taxa. The fasciculate coralPraewentzelella regulare Flügel 1995 was the favorate substrate. Up to 33% of the calices were bored. Dendroid and compound corals were bored subordinately. Bioerosion of these colonies does not exceed 2%. There is good evidence for substrate preference amongst the borers. Major controlling factors affecting borer distribution are believed to be variations of skeletal density and gross morphology. The borer assemblage could not limit reef accretion significantly. Factors controlling boring activity might have been quality of substrate, sedimentation rate, rapid incrustation of substrates, and competition for food with reef constructors including sponges, chaetetids, and rugose corals.