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Devonian of the Benahmed region, western Moroccan Meseta

Auteur : Becker Ralph Thomas, Aboussalam Zhor Sarah, El Hassani Ahmed ...[et al.]
Année de Publication : 2021
Type : Article
Thème : Sciences de la terre
Couverture : Maroc

Résumé/Sommaire :

Based on new field work, sampling for conodonts, ammonoids, and microfacies analysis, the Lower to Upper Devonian stratigraphy of the Benahmed region is refined. Fossiliferous neritic limestones of the Pragian/lower Emsian represent a continuation of the shallow carbonate platform of the Al Attamna region to the north (Units A/B of Sidi Ahmed Lemdoun Formation). More argillaceous upper Emsian/Eifelian strata are regionally still poorly known. In the southern Chaouїa or Beni Sekten region NW of Benahmed, Givetian deep neritic to shallow pelagic mud-wackestones of the new Oulad Amar Formation were partly reworked and resedimented together with crinoidal grainstones in conglomeratic debris flow beds. Peaks of Eovariscan block faulting and reworking occurred high in the middle (ansatus Zone) and at the top of the Givetian (norrisi Zone).
Lower/middle Frasnian strata are still unknown in outcrop but limestones with corals of possible Givetian/Frasnian age have been reported from E/SE of Benahmed. In the NW, the new upper Frasnian Boudouda Formation is characterized by transgressive hypoxic goniatite shales, which are unique for the Moroccan Meseta and which strongly resemble the contemporaneous Büdesheim Goniatite Shales of Germany. Unfortunately, the top-Frasnian and Frasnian-Famennian boundary are not exposed. Upper/uppermost (“Strunian”) limestones and siliciclastics belong to a neritic succession that requires further research. In the southern Mdakra Massif E of Benahmed, there was a distinctive, middle/upper Famennian basin with richly fossiliferous, hypoxic ammonoid shales, the new Oued Aricha Formation. More than forty goniatite and clymeniid species are recorded, with up to 13 new forms that are currently left in open nomenclature. They fall in the UD III-C t o V -A2 interval of the international ammonoid zonation and include many common taxa with the contemporaneous Fezzou Formation (Lahfira to Jebel el Krabis Members) of the Maїder Basin of the eastern Anti-Atlas. The associated fauna is composed of rugose corals, including the revised Hebukophyllum arichense, nautiloids, bivalves, gastropods, brachiopods, and trilobites. As the Chabet el Baya Formation of the northern Mdakra Massif, the Oued Aricha Formation grades upwards into the poorly fossiliferous Mgarto Formation that straddles the Devonian-Carboniferous boundary.

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