The effects of Callianassa bioturbation on the preservation of carbonate grains in Davies Reef Lagoon, Great Barrier Reef, Australia

Coring, air-life excavation, and tracer-sediment experiments in the lagoon-floor sediments of Davies Reef revealed that callianassid shrimp had produced, during their feeding, a 5-60-cm-thick surface layer of moderately to poorly sorted gravel-free sediment, above very poorly sorted gravelrich sediment. The subsurface gravel is epilithic in origin close to reef framework but consists predominantly of infaunal molluscs through most of the lagoon. These infaunal skeletons tend to be buried rapidly in a well-preserved state escaping the microboring that attacks all grains on the surface. Two geologically significant points emerge; first, the surface layer will not get preserved in its present form and any analyses of surface samples can give a false impression of the accumulating sediment pile and the prevailing hydraulic regime, and second, the combination of continuous fine-sediment recycling and surface microboring can lead to a bias in the fossil-reefal limestone record, with epilithic organisms seemingly subordinate to infaunal molluscs.