Contourite-drift archive links periodic anoxic shelf water cascading with Late Devonian bioevents (Tafilalt, Morocco)

The analysis of Devonian cephalopod limestones in the eastern Anti-Atlas of Morocco (Tafilalt) reveals the formation of a contourite terrace on the uppermost slope of northern Gondwana. The inner terrace was bounded by a contourite channel and an associated mounded drift. The Eifelian–Frasnian record includes pelagites and bioclastic contourites with coquinas of planktonic and nektonic fauna, forming integral parts of bi-gradational sequences. Frequent hiatuses at sequence midpoints are marked by lag -deposits of intraclasts, ferromanganese nodules, and conodonts draping erosional surfaces. Deposition was primarily driven by oxic, clear-water bottom currents.

Of particular palaeoceanographic interest are widespread erosional hiatuses and associated organic-rich calcarenitic contourites (black styliolinid coquinas), many formed penecontemporaneously with Devonian evolutionary events. These deposits record intensified bottom currents coinciding with the expansion of a dysoxic–anoxic water mass. Facies- and drift-scale features indicate northwest-directed along-slope circulation. We attribute these palaeocirculation events to repeated overflows of anoxic water from the North African Epicontinental Sea via the Ougarta Trough, likely driven by dense (saline) shelf water formed on the northern Gondwana margin. These overflows cascaded downslope until reaching a density equilibrium, probably forming an intermediate water mass.

This evidence for dense shelf-water cascading supports the photic-zone eutrophication (top-down) model proposed for the Kellwasser Crisis and related Devonian anoxic events. We therefore propose a direct link between these anoxic overflows and Devonian evolutionary events.