Bivalves of the Menorcan late Tortonian shelf: faunal and taphonomic signals of an upwelling, seafloor current- and storm-dominated dynamic palaeoenvironment

ABSTRACT The Tortonian phosphatic hardground and overlying section of coquinas interbedded with bioturbated bioclastic packstones at S’Algar, Menorca, Spain, record two distinct bivalve faunas. The phosphatic hardground is dominated by infaunal, aragonitic taxa, while the overlying beds comprise epifaunal, calcitic fossils. These two distinct faunas represent mélanges from reworking of shells from different shelf environments. Hardground bivalves are preserved in four preservational forms (denoted ‘A’ to ‘D’) that are mixed together in the mineralized bed. Each taphonomic form is characterized by distinct bivalve associations, as well as by different levels of boring (Entobia isp.) and encrustation. A Glycymeris–Crassatella–Clypeaster association (‘preservation A’) records part of an upper ramp slope fauna that was periodically affected by high-energy and low-oxygen conditions due to upwelling waters. It represents a pre-mineralization fauna, as well as part of an opportunistic hard ground association along with Gastrochaena. A smaller sized contemporaneous fauna (‘preservation B’), characterized by an arcid-carditid-venerid association, is derived from the middle ramp that was affected by storms and a persistent-westward flowing current that intermittently drove upwelling. The other two hardground preservation forms ‘forms C and D’ (unaltered) suggest a multiphase mineralization mechanism. In contrast to the hardground fauna, the overlying shell beds and bioturbated bioclastic packstones contain a distinct assemblage of fossil bivalves, predominantly comprising pectinoids (spondylids, pectinids) and oysters (ostreids, gryphaeids) with varying degrees of disarticulation, fragmentation and wear. The majority of the pectinoids are unencrusted and show no sign of boring, the exceptions being spondylids. Pectinoids dominate the lowermost section with thick shell beds, while gryphaeids dominate the predominantly bioturbated uppermost sediments. These faunas are interpreted as having been derived from a middle ramp palaeoenvironment that was affected by storm activity that punctuated periods of low sedimentation on a current-swept seafloor. Changes in the frequency and directionality of storm and current activity drove environmental change through time.