Bioclastic contourites: depositional model for bottom-current redeposited pelagic carbonate ooze (Devonian, Moroccan Central Massif)

Devonian limestone successions of the eastern Moroccan Central Massif provide an example to study the characteristics of well-preserved bottom-current deposits that synchronously occur in many condensed and reduced limestone sequences in southern Europe and Northern Africa. They were mainly deposited within the narrow oceanic passage between the approaching continents of Laurussia and Gondwana during the late Givetian and early Frasnian. Calcarenites, laminated calcisiltites, and mottled calcisiltites and calcilutites analogous to recent contourite formations are the main lithotypes. Calcarenites are represented by styliolinid grain- to packstones with rare horizontal and cross-lamination. Laminated calcisiltites are particularly rich in non-carbonate components (conodonts, phosphatic intraclasts). Calcarenites and laminated calcisiltites are interpreted as current-induced accumulations, whereas mottled calcisiltites and calcilutites formed under conditions of weak current influence or pelagic rain. All lithotypes were accumulated in a pelagic environment. The vertical stacking of the three main lithotypes and the transitions between them vary considerably, but define two characteristic stacking patterns: (1) coarsening-upward micro-sequences und (2) fining-upward micro-sequences. These micro-sequences are 2-10 cm thick and, in many cases, are separated by omission surfaces. The vertical facies variation is interpreted in terms of fluctuations in velocity of the transporting bottom-current or shifts in flows of the current axis. Calculations suggest that winnowing and bedload transport occur preferentially as a response to varying current strength and a range of sediment-specific properties. Calcareous silt was stirred in suspension at current speeds of 13-20 cm/s ((U) over bar (100)), whereas whole styliolinid tests were mobilised at similar to 19 cm/s. At velocities of at least 23 cm/s styliolinid shells were broken and denser grains were moved. The associated phosphorites and phosphoritic hardgrounds provide additional evidence of temporarily raised hydraulic energy within the depositional environment of the exemplified deep-sea sediments.