Macrofauna associations and formation of shell concentrations in the Early Eocene Roda Formation (southern Pyrenees, Spain)

The invertebrate macrofauna (mainly molluscs) of the Early Eocene Roda Formation (southern Pyrenees, Spain) is reported and classified in seven biofacies associations, representing ecologically related groups of macro-invertebrates of shallow marine fan-delta environments ranging in depth from intertidal mudflat to shallow carbonate shelf settings (maximum water depth c. 60 m). The most common association is indicative of mixed siliciclastic-carbonate substrates with the characteristics of an abandonment surface in shallow subtidal settings above wave base. The second most frequent biofacies association is found in subtidal normal marine outer-bay carbonate environments with a relatively low sedimentation rate. However, the most prominent associations, although seldomly occurring, are a (par)autochtonous polytypic coral-mollusc hardground association on top of the last major fandelta sandstone body, an autochtonous monotypic oyster association adapted to soft muddy bottoms representing a time span of c. 500 years, and a parautochtonous (probably) brackish water, intertidal mudflat association dominated by Potamididae. The occurrence of the associations is related to the autocyclic shifting of depocentres, waterdepth, and the palaeotopography of the seafloor as evident from lateral transitions between biofacies associations. This facilitated the formation of calcareous horizons of restricted spatial extent. These are explained as a response to local starvation of clastic sediment supply rather than being a manifestation of relative sea-level changes.