Polychaete macrobioerosion in the Messinian (Late Miocene) Rhodolith beds of the western Mediterranean

The Rhodolith beds constitute one of the main components of the Messinian carbonate platforms of the Mediterranean marginal basins. In the northwestern Algeria, they are widely distributed on the Tessala – Beni Chougrane carbonate platform, which occupies the southern edge of the Lower Chelif basin. The Rhodolith beds of the Tafraoui region are here studied with respect to their ichnological content. Four ichnotaxa are reported: Balanoglossites isp., Caulostrepsis cf. taeniola, Sulcichnus isp. and Trypanites isp. All these trace fossils are produced by polychaete annelids. Depending on the degree of lithification, polychaetes were able to burrow firmgrounds (Glossifungites ichnofacies), as evidenced by the presence of Balanoglossites isp., and to bioerode hardgrounds (Trypanites ichnofacies), e.g., Caulostrepsis cf. taeniola, Sulcichnus isp. and Trypanites isp. Also, Balanoglossites-bearing levels can be interpreted as omission surfaces which are considered as discontinuity surfaces. However, the development of an unconformity indicates changes in the sediment characteristics from softground through firmground to hardground.