Facies variability and depositional architecture of bioclastic bottom-current deposits formed on a Devonian contourite terrace (Tafilalt Platform, Morocco)

This study investigates bioclastic carbonate contourite deposits from the Eifelian–Frasnian of the Tafilalt Platform (Morocco) to enhance the understanding of carbonate sedimentation under bottom-current influence. In contrast to the classical bi-gradational contourite facies model, which is based on continuous deposition of fine-grained siliciclastics, these contourites reflect intermittent accumulation, resulting in thin, condensed successions with abundant hiatuses.

Five facies associations are identified, recording pelagic sedimentation and contourite deposition on a contourite terrace, an inclined section of the uppermost slope of northern Gondwana shaped by a shifting water-mass interface. Deposition was primarily controlled by oxic, clear-water bottom currents, as evidenced by bioturbated limestones with hydrogenetic ferromanganese nodules and low organic-carbon contents. In contrast, episodes of anoxic bottom-current activity are recorded by interbedded organic-rich coquinas featuring well-preserved traction structures, minimal bioturbation, and syngenetic pyrite framboids.

The large-scale depositional architecture includes an abraded surface, an alongslope contourite channel, and a small mounded drift. Microfacies analysis reveals frequent alternations between suspension settling, winnowing, bedload traction, dynamic sediment bypassing, and concomitant seafloor cementation. Coquinas composed of planktonic and nektonic organisms are integral components of bi-gradational sequences, which also feature lag deposits of carbonate intraclasts, conodonts, and ferromanganese nodules draping hardgrounds.

This high-resolution record provides a unique opportunity to investigate the spatial and temporal variability of bed-scale contourite features in conjunction with drift-scale depositional architecture—both shaped by palaeoceanographic processes during the Devonian greenhouse climate.