A Miocene (Tortonian, Menorca, Spain) benthic macrofauna preserved in a phosphatic hardground: A difficult but invaluable record of upwelling palaeoenvironments

Abstract

This field study records that the Tortonian phosphatic hardground at S'Algar, Menorca has a diverse fossil macrobenthic fauna dominated by bivalves. These faunal elements occur in four preservational forms (A–D). Each taphonomic form within the hardground is characterised by different concentrations of Entobia isp. (clionaid sponge boring) and encrustation. The preservational forms have specific fossil macrofaunas associated with them, and the hardground represents a melange of bioclasts derived from different environments on the Menorcan Miocene shelf. These habitats are influenced by the upwelling event that provides the conditions for hardground mineralisation.

A Glycymeris-Crassatella-Clypeaster association (A) records part of a fauna that lived on the upper ramp slope that was periodically affected by high energy and low oxygen conditions due to upwelling waters. The smaller sized fauna (B), characterised by an arcid-carditid-venerid association is derived from the middle ramp that was affected by storms and a more persistent westward flowing current that intermittently drove upwelling. The fragmentary and disarticulated nature of both faunas indicates that they were mixed within the hardground via reworking processes driven by these seafloor energy regimes.

Some of the large gastropod taxa, dentition-up Glycymeris and Gastrochaena suggest that the hardground had its own fauna. Preservation A fauna and other opportunistic taxa inhabited an environment where the hardground was exposed on the Miocene seafloor as ‘islands’ surrounded by thin layers of looser sediment. Phosphatic bioclasts in the underlying limestone indicate that similar mineralised beds were formed elsewhere on the shelf prior to the formation of the studied outcrop.