Devonian of the Mechra Ben Abbou region (Rehamna) – new data on the reef succession, microfacies, stratigraphy, and palaeogeography

Abstract

The Devonian of the northeastern Rehmna, from Mechra Ben Abbou in the NW to the area north of Aïn el Melah in the southeast, has been dealt with in numerous publications and theses, but the precise ages, sedimentology, and terminology of numerous lithostratigraphic units were far from being resolved. Based on sampling for conodonts, brachiopods, other macrofauna, and microfacies, we report some progress. At Sakhrat et Taïra in the SE, a new shallow-water faunule with Howellella cf. mercurii confirms that significant synsedimentary block movements occurred locally in the lower Lochkovian. It is followed by a level with limestone turbidites and hostile shelf basin facies before quartzites indicate regression. The facies development differs clearly from the Lochkovian of Koudiat ed Diab in the north. The Pragian and Emsian of the whole region is characterized by neritic, storm-influenced, non-reefal shelf successions. This includes in the NE the Emsian Oued Kibane Formation, which may range just into the lower Eifelian. But the Foum el Mejez region lacks so far evidence of undoubted Middle Devonian fossils. The Middle Devonian Mechra Ben Abbou Formation is defined as a compact succession of shallow-water limestones. It is subdivided into the thin- to medium-bedded, upper Emsian to middle Eifelian, mostly crinoidal Bouchhada Member, the cherty and bioclastic upper Eifelian or lower Givetian Koudiat El Gara Member, and the lower/middle Givetian, massive, reefal Sidi bou Talaa Member. At Sidi bou Talaa, the latter is assigned to four facies assemblages: reef core limestone, seismically induced inner platform reef breccias, fenestral limestones, and thin-bedded lagoonal limestones. 38 species of reef builders (tabulate corals, chaetetids, stromatoporids, calcimicrobes) are regionally identified, 17 of them for the first time in Morocco. Due to regional uplift, non-deposition, condensation or reworking, there is no in-situ Rehamna record of upper Givetian and Frasnian strata. The silty-sandy middle part of the Oued Ater Member of the Foum el Mejez Formation yielded a new middle Famennian brachiopod fauna. It represents a fault-controlled narrow basin with prodelta facies and high subsidence that has no equivalents anywhere else in the Rehamna. At Gare Mechra Ben Abbou, the Givetian reef limestone was erosionally truncated in the upper Famennian, with stromatolite encrustations, karstification, and breccia formation by rockfall, followed by quartzites with Mesoplica praelonga coquinas. This Douar Nahilat Formation correlates with the quartzites of the Dalaa Member of the Foum el Mejez region to the east. A survey of the clast spectrum in the at least 500 m thick reddish breccia at Koudiat ed Diab yielded limited evidence for a Givetian and upper Frasnian age of eroded reefal limestone. Carboniferous clasts are rare but a brachiopod-bryozoan pebble with Rhabdomeson suggests that a part of the regional upper Viséan carbonate platform was reworked, too. This supports the long assumed post-orogenic age of the thick unit that represents massive rockfall talus after the main Variscan relief had formed.