A genetic link between Konzentrat- and Konservat-Lagerstätten in the Fezouata Shale (Lower Ordovician, Morocco)

Abstract In the central Anti-Atlas (Morocco; Zagora region), the Fezouata Shale has yielded an extraordinary fossil Lagerstatte documenting the initial stages of the Great Ordovician Biodiversification Event. This Lagerstatte contains abundant and exceptionally well-preserved fossils (EPF) exhibiting soft-bodied preservation as well as “hard”, mineralized parts of extinct organisms. While soft-bodied preservation in the Fezouata Shale is confined to a few stratigraphic intervals, other fossiliferous intervals contain only shelly fossils in sandstone lenses (SL). Placed in the context of a previously established depositional model, EPF are interpreted as autochthonous assemblages buried by storm deposits, close to storm wave-base. There, the current component of storms is reduced compared to proximal settings and weak oscillation is dominant resulting in the record of oscillation ripples. In contrast, bioclastic accumulations were generated upslope in shallower, more proximal lower shoreface environments. There, the current component of storms was significant, as suggested by the disarticulation of skeletal remains. Bioclastic materials are localized in SL, which are lenses a few tens of centimeters in thickness and a few meters in width and length. These lenses commonly display erosive bases and convex tops, resulting in a lobate or a channel-lobe morphology. A large amount of sediment was thus rapidly deposited, resulting in the rapid burial of allochthonous material across a strongly localized surface of the sea floor. Integrating both sedimentological and paleontological evidence, intervals yielding EPF are considered as Konservat-Lagerstatten (KsL), in contrast with bioclastic accumulations defined as Konzentrat-Lagerstatten (KzL). Thus, KsL assemblages are interpreted to approximate biocenoses, in which organisms experienced rapid burial by distal tempestites, while KzL assemblages more likely correspond to thanatocenoses, locally deposited by storm back-currents. Here, a genetic link between KsL and KzL is suggested within the Fezouata Shale. Both types of Lagerstatten were related to storm activity, but at different depths.