Amusium cristatum shell beds in the lower Pliocene deposits of Almería-Níjar Basin (SE Spain)

Abstract

Densely packed monospecific-paucispecific Amusium cristatum-dominated shell beds are signifi cant features in the lower Pliocene deposits of the Almería-Níjar Basin (SE Spain), embedded in distal prodelta fi ne-grained sand and silt. The shells in these concentrations are exceptionally well preserved: articulation is relatively high, fragmentation is low, abrasion is almost lacking, and many shells preserve the original colour pattern. These taphonomic signatures suggest in situ deposition forming autochthonous fossil assemblages, i.e. biological concentrations. Benthic foraminiferal assemblages found in the prodelta deposits indicate that they formed in mesotrophic mid-shelf settings.The A. cristatum accumulations show similarities withrecent concentrations of A. balloti from the Shark Bay (NW Australia). The latter species form dense populations in restricted positions of the bay from 10 to 75 m depth. The lowermost depth range of A. balloti is similar to the palaeobathymetry inferred for the A. cristatum beds. Furthermore, the shell beds studied occur in the centre of a relatively enclosed palaeobay. Apart from these coincidences, the proximate ecological causes accounting for the recent concentrations of A. balloti are still obscure. Thus, no straightforward actualistic comparison explains the biological origin of the A. cristatum shell beds. Based on benthic foraminiferal assemblages in the prodelta deposits, A. cristatum concentrations are most likely related to intermittent nutrient and food-particle supply from several deltas feeding the palaeobay. The advance of the deltas to the centre of the basin guaranteed rapid burial and the excellent preservation of the shells.