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Devonian and the Carboniferous transgression in the Skoura region, Sub-Meseta Zone, Morocco

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 :

The Sub-Meseta Zone forms the transition between the southern Meseta of the main Variscides and the Anti-Atlas realm. The Skoura region at the southern foot of the High Atlas includes a west-east, discontinuous sequence of Devonian and Lower Carboniferous outcrops, from Tizi-n´Tichka East in the NW of Ouarzazate to Asserhmo in the NE. Based on detailed logging, microfacies, conodont and macrofauna sampling, especially at Taliouine and Tizi n-Ouourti, the regional Devonian litho, bio- and event stratigraphy and facies development is revised. The Lochkovian is marked by a change from unfossiliferous black shales (Lower Member of new Tizi-n-Tichka Formation) to condensed, discontinuous, black, detrital Orthocone Limestones (Upper Member, middle Lochkovian) deposited under strong bottom current influence (contourites). Slumps and a subsequent erosional disconformity mark the regional tectonic episode Sk-TP-1a, followed by a distinctive “Antevariscan” conglomerate and reworking interval (Sk-TP 1b) that falls in the lower Pragian (Member 1 of new Imi-n-Tazaght Formation). The higher Pragian and lower Emsian are represented by cyclic, coarsening upwards successions of nodular limestones with trilobites and early goniatites (Members 2 and 3). At the top, Member 4 contains a Mimagoniatites marker level, partly interbedded within cross-bedded dacryoconarid calcarenites. The global Daleje Event is regionally very pronounced and led to an abrupt change to poorly fossiliferous green silty shales of the new Tizi-n-Ouourti Formation (Lower Member). The anarcestid-rich, nodular Upper Member is only locally well-developed, especially at Tizi-n-Ouourti. It is truncated at the top by the next regional Eovariscan erosion, unconformity, reworking and slumping phase that began in the Eifelian (Sk-TP 2a). The sedimentary record ended locally near the top of the Eifelian (Tizi-n-Ouourti) or a second slumping phase (Sk-TP 2b) began after the top-Eifelian Kačák Event Interval (Member 1 of new Taliouine Formation). The lower Givetian (Member 2) consists of well-bedded to nodular mudstones with some brief reworking episodes (Sk-TP 2c) near the base. The middle Givetian Member 3 includes hypoxic marls with maenioceratid faunas that strongly resemble the succession of the Tata region (Ahrerouch Formation) in the eastern Dra Valley. From the top of the middle to the upper Givetian, a third regional phase of Eovariscan block faulting (Sk-TP 3) caused the intercalation of thick, polymict conglomerate units (Member 4), followed by still poorly known Frasnian strata. Only in the east, the Asserhmo Formation is developed, a thick sequence of polymict conglomerates with reworked Ordovician to upper Famennian pebbles and isolated reef corals. It represents Sk-TP 4 and its origin may correlate with the similar breccia units of the Tinerhir region in the eastern Sub-Meseta Zone. In all of the Skoura region, a significant premiddle Viséan erosion cut variable into Devonian or even Silurian beds. The oldest Carboniferous beds differ from place to place. At Taliouine, ca. 10 m of very fossiliferous neritic limestones with abundant brachiopods, large euomphalid gastropods, and bryozoans yielded very abundant foraminifers that provide a correlation with the middle Viséan V2a (Cf5α) fauna from Assif n´Tanzouzmine. The Skoura Devonian and Lower Carboniferous records an individual crustal development at the Meseta-Anti-Atlas transition, with lateral facies similarities fluctuating strongly in time, constrained by the repeated synsedimentary tectonic movements.

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