The Devonian of Jebel Ardouz (Mzoudia region, SW Moroccan Meseta) – new data on stratigraphy, facies, and palaeogeography

Abstract

The long-known Devonian of the Jebel Ardouz west of Marrakesh, and just north of Mzoudia, is composed of an allochthonous stack of clastic and carbonate rocks that were thrusted onto each other from the northeast. New biostratigraphic data prove an age range of sedimentation from the lower Eifelian to ?upper Famennian. The lowest, western thrust unit is composed of reddish sandstones and conglomerates/breccias (new Ardouz Formation) deposited originally by rockfall and debris flows on the slope of a repeatedly active fault scarp. Limestone clasts yielded sandstones of unknown age, Eifelian, Givetian and lower/middle Frasnian conodonts, and encrusted reef corals. Re-sedimentation may have occurred in the upper Frasnian or post-dated the Famennian. The “red conglomerates” record a block that was strongly tilted by Eovariscan extensional tectonics, forming on the uplifted side a small island. Exhumation, erosion down into Eifelian carbonates, and a long phase of reworking (pebble formation, hematite impregnation and encrusting) occurred in an arid, lateritic, terrestrial-fluvial to coastal high-energy setting. The overlying middle unit consists of a lower Eifelian to middle Givetian, shallowing upwards carbonate ramp (new Mzoudia Formation, with the new Koudiat Ferjane and Koudiat Kébir Members). Middle Givetian regression resulted in the growth of a biostrome with patch reefs. The middle thrust unit experienced no Eovariscan reworking but upper Givetian uplift resulted in an episode of non-deposition. Following poorly known non-reefal Frasnian strata (still un-named Upper Member), the upper thrust unit on the eastern side of Jebel Ardouz (new Oued el Biad Formation) consists of shallow-water, open marine sandstones/quartzites with brachiopod coquinas, which originally transgressed unconformably the carbonate platform. Shedding of sand from a W/NW source (Imfout Ridge) balanced subsidence. The Jebel Ardouz Devonian differs considerably from the Devonian of the High Atlas Basement (to the south), the Safi region (to the west), and allochthonous eastern Jebilet (in the east). A similar association of carbonate platform blocks truncated by conglomerates or brachiopod-rich quartzites is developed in the Mechra Ben Abbou succession of the rather distant northern Rehamna. But comparable, poorly studied Devonian blocks have been mentioned from the geographically intermediate Skhirat region of the Jebilet.