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The Devonian and Viséan transgression in the Eastern Jebilet (Moroccan Meseta) – review and new data

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 allochthonous Lower/Middle Devonian stratigraphy and facies history of the Eastern Jebilet NNE of Marrakech is revised and refined. Silurian to Pragian partly siliceous black shales are assigned to the new Jebel Smaha Formation, which yielded HUVELIN (1977) in its upper part graptolites of very different age (lower/middle Lochkovian in the W, up to the upper Pragian in the E). In the Jaïdet region, upper Lochkovian to upper Emsian nodular limestones with deep neritic to pelagic faunas form the new Jaїdet Formation, which deposited in an outer shelf setting. In the Middle Devonian, this basin deepened by increased subsidence, resulting in a thick, poorly fossiliferous siltstone succession, initially and locally with hypoxic Zoophycos-Chondrites ichnofauna, later with limestone turbidites (new El Kahla Formation). Currently, there is no biostratigraphic proof of Upper Devonian strata in the Eastern Jebilet; the youngest known conodonts fall in the top middle Givetian ansatus Zone but the thick upper El Kahla Formation is not dated. With the help of corals, foraminifers, and bryozoans, detrital and microbial coral limestones at the base of the thick carbonate platform at Koudiat Lahmara (Tekzim Formation) can be dated as upper Viséan (Cf6γ1, upper Asbian). An associated basal conglomerate contains reworked pebbles of an older (lower/middle Viséan) platform. The transgressive oldest allochthonous limestones of the Tekzim Formation correlate probably with goniatite shales of the (par)autochthonous upper Kharrouba Formation, which also record an upper Asbian deepening episode. With shallowing upwards in the top Asbian/Brigantian, the carbonate platform progradated basinwards, leading to local top-Kharrouba inner shelf limestone deposition before the onset of wildflysch conditions.
The allochthonous Lower Devonian of the eastern Jebilet shows significant similarities with the Skoura region at the southern margin of the High Atlas and, slightly less distinctive, with the Azrou-Khenifra Basin in the east. They belonged to the same outer shelf basin but the timing and effects of Eovariscan block movements were regionally different. The Eastern Jebilet source region and Skoura successions experienced major facies changes and synsedimentary tectonic events around the Lower/Middle Devonian transition. Originally, both may have represented different sides of the same tilted block. There were no close palaeogeographic relationships of the Jaїdet source area with the western Jebilet (Jebel Ardouz) or the Rehamna to the north. Only the upper Viséan transgression led to a more uniform palaeogeography characterized by shallow-water carbonate platforms throughout the southern and eastern parts of the Western Meseta.

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