Jurassic ferruginous hardgrounds of the “Dorsale Calcaire” and the Jbel Moussa Group (Internal Rif, Morocco); stratigraphical context and paleoceanographic consequences of mineralization processes

The Jurassic pelagic successions of the Dorsale Calcaire are dominated by reduced or condensed facies. Apart from long-term gaps due to the polyphased paleokarsts in the Internal Dorsale, many omission surfaces are recognizable. Ferruginous crusts cover the most important of them. These hardground episodes, listed herein from HG.1 to HG.6, involved significant hiatuses. Biostratigraphical dating from enclosing strata allows the correlation of them throughout different units of the Rifian Dorsale Calcaire. They are either simple or composite and lie usually: 1) on transgressive facies filling karstic cavities, or else 2) between two distinct facies. They allow us to pinpoint episodes of stratigraphic break that often separates two different depositional conditions. Their genesis in oxygen-poor environments (affecting the shelf) is confirmed here since black-shale deposition is proved to occur coevally in depressions adjacent to the shelf and in the basin. This result is compatible with the nature of the hardground mineralizations that proved to be of bacterial origin (anaerobic Fe-Mn fixing Bacteria).