Becker & al. Allochtonous nappes at Mrirt Devonian and basal Carboniferous of the allochthonous nappes at Mrirt (eastern part of Western Meseta) – review and new data RALPH THOMAS BECKER, ZHOR SARAH ABOUSSALAM, AHMED EL HASSANI, SVEN HARTENFELS & HEIKO HÜNEKE Fig. 1: The Eifelian (left minor cliff) to top-Frasnian (bent limestone in the middle ground) at Anjadam S of Mrirt, with Sarah ABOUSSALAM (white cap) and Lahssem BAIDDER (red cap) for scale. 1. Introduction phase onto the autochthonous Tanadra-Bou Tazert Unit (or Kasba Tadla-Azrou Anticline, The Mrirt region lies in the eastern part of autochthonous part of the Azrou-Khenifra the Western Meseta or of the Variscan Central Basin), which consists of contemporaneous Massif, close to the northwestern margin of the strata (e.g., HUVELIN 1970; ALLARY et al. Middle Atlas. A locally dominant geographic 1972; RIBEYROLLES 1976; FAIK 1988; feature is the Gara de Mrirt mountain (Fig. 2: BOUABDELLI 1989). The Ziar-Mrirt Nappe Al Gara, 1534 m) ca. 6 km ESE of the Mrirt overlies the southern margin of the Azrou town, which is formed by tabular Miocene Nappe and ranges from the NW of Mrirt to the carbonates that overlie a strongly folded and Ziar (= Ziyyar) region in the SW. South of faulted Devonian to Lower Carboniferous Mrirt, it is overlain by the northern margin of succession. The Palaeozoic of the Azrou-Mrirt- the Khenifra Nappe (Fig. 3). The largest areas Khenifra region belongs to a system of nappes of the Ziar-Mrirt Nappe are occupied by that originated from the East. These were Ordovician strata and Quaternary cover. Close thrusted during the main Variscan deformation to Mrirt, the folded Devonian is best exposed at Hassan II Academy of Science and Technology Frontiers in Science and Engineering - Vol. 10 - n° 1 - 2020 87 Becker & al. Allochtonous nappes at Mrirt the SW foot of the Gara de Mrirt and at NW of Mrirt at the Jebel Tanoualt (Fig. 4). Ca. Anajdam to the SW (Figs. 2, 4). Viséan 20 km SW of Mrirt, near El Krad, lies the carbonates form a narrow strip of exposure at Devonian of the Akellal (or Brouha Ahallal) the Jebel Aouam (= Jbel Awam). An important Plateau, which forms a steep eastern slope Upper Viséan wildflysch outcrop is situated towards the main road from Mrirt to Khenifra. Fig. 2: Geographic position of the Tibouda/Bou Ounebdou OuN) area at the SW slope of the Gara de Mrirt (“Al Gara”), of the Anjadam section SSW of Mrirt town (Anaj), and of the Jebel Aouma (= Jbel Awan) W of Mrirt (extract from topographic map 1 : 50 000, sheet NI-30-VII-1b Mrirt). 2. Research history limestones with the peculiar, relatively large- sized rhynchonellid genus Dzieduszyckia. The Mrirt region became famous for These are supposed to represent “cold vent” Devonian fossils and strata since descriptions (deeper marine methane seap) ecosystems in the monograph by H. TERMIER (1936) on the (e.g., SANDY 1995; BALINSKI & BIERNAT geology of Central Morocco and the western 2003). Equally famous are good exposures of Middle Atlas. Our summary concentrates on the two Kellwasser Limestones near the the allochthonous nappe succession, excluding Frasnian-Famennian boundary (e.g., LAZREQ the more western Devonian exposed in the 1992a, 1999; RIQUIER et al. 2005, 2007). The Tighza (= Tirza) to Agarad-n-Azdaїt (= Dechra Famennian provides text-book examples for re- Aїt Abdallah) belt. The Devonian of the Jebel sedimentation events related to Eovarican Bouchot shown in Fig. 4 belongs to the block faulting (e.g., WALLISER et al. 2000; southern part of the Dechra Aїt Abdallah zone. HÜNEKE 2006, 2007). Especially famous for the allochthonous Mrirt The regional research history can be Devonian are Famennian brachiopod summarized as follows: 88 Hassan II Academy of Science and Technology Frontiers in Science and Engineering - Vol. 10 - n° 1 - 2020 Becker & al. Allochtonous nappes at Mrirt TERMIER (1927a, 1927b): First reference to the HOLLARD (1967): Brief summary of the Bou Gara de Mrirt area as a Devonian fossil locality Nebedou succession (in fig. 4). based on the record of “Gephyroceras HOLLARD et al. (1970): Stratigraphic position intumescens” (which may refer to any species of the Dzieduszyckia limestones, giving details of Manticoceras). for the Famennian of the Touchhent section TERMIER (1936): Pioneer study of Hercynian (Akellal Plateau). Morocco with a description of Bou Ounebdou HUVELIN (1970, 1973): Structural geology and (= Bou Nebedou). Faunas include Frasnian synsedimentary tectonics in the Mrirt region. Manticoceras, middle Famennian (Upper HOLLARD & MORIN (1973): New details for Devonian III) Sporadoceras, and upper Dzieduszyckia occurrences in the Mrirt region, Famennian (UD IV/V) goniatites (“cf. re-illustrating a cross-section for Bou Aganides sulcatus = Prionoceras sp.) and Ounebdou from 1936, which (wrongly) clymeniids (Laevigites hoevelensis = Clymenia suggests a synclinal structure at the top. laevigata Group; Gonioclymenia speciosa), as TERMIER et al. (1975): Brief reference to the well as nautiloids, bivalves, and brachiopods. Bou Ounebdou Upper Devonian and its faunas. TERMIER (1936, 1938a) and TERMIER & RIBEYROLLES (1976): Tectonics of the TERMIER (1948, 1949, 1950a): Taxonomy and Aguelmous to Mrirt region. phylogeny of Dzieduszyckia species, then BENSAID (1979): Reference to the Mrirt assigned to the genus Halorella; Bou Devonien succession. Ounebdou as the type locality of H. FRANCOIS et al. (1986): Silurian-Devonian crassicostata, H. tenuicostata, and H. transition at the Jebel Aouam. intermedia (with three subspecies). FAIK (1988): Unpublished Ph.D. Thesis on the TERMIER & TERMIER (1950): Illustration of the general stratigraphy and structural geology of suture of a Gonioclymenia speciosa from Bou the Mrirt region, with a subdivision of the Ounebdou (Bou Nebedou; Upper Devonian V- regional Devonian into 10 units. B). BOUABDELLI (1989): Unpublished Ph.D. ROCH (1950, p. 161): Reference to the Thesis on the structural geology of the Azrou- occurrence of supposed Manticoceras Mrirt-Khenifra region. carinatum and M. intumescens at Bou LAZREQ (1992a, 1992b, 1999): Upper Ounebdou, reaching partly giant size (> 30 cm Devonian lithostratigraphy, conodont diameter). stratigraphy, and biofacies of sections at Bou AGARD et al. (1955): Recognition of uppermost Ounebdou and Anajdam, with a focus on the Famennian (“Strunian”) strata overlying Frasnian-Famennian boundary. Frasnian-Famennian cephalopod limestones on BECKER (1993a): Brief summary of Upper the Akellal Plateau. Devonian goniatite occurrences in the Mrirt AGARD et al. (1958): Silurian-Devonian region, mentioning a new record of transition and Middle/Upper Devonian paratornoceratids. succession of the Akellal Plateau, and Upper ZAHRAOUI (1994): Schematic lithostratigraphic Viséan beds of the Jebel Aouam. column for the Emsian to Famennian assigned WILLEFERT (1963): Description of graptolites to the Bou Ounabdou Formation (sensu FAIK from around the Silurian-Devonian boundary 1988). of the Touchchent section (Akellal Plateau). FADLI (1994): Reference to the Frasnian of BIERNAT (1967): Re-assignment of the Bou Anajdam and the Famennian and Tournaisian Ounebdou Halorella species to the Polish of Touchchent. genus Dzieduszyckia. Hassan II Academy of Science and Technology Frontiers in Science and Engineering - Vol. 10 - n° 1 - 2020 89 Becker & al. Allochtonous nappes at Mrirt BOUBDELLI (1994): Tectonics of the Azrou- Touchchent and Bou Ounebdou (= Bou Khenifra region, showing a geological cross- Nebedou). section, including Anajdam. FEIST (2003): Re-assignment of the proetid BECKER (1993b, 1994), BECKER et al. (1997), trilobite Pteroparia ziegleri maroccensis from and BECKER & HOUSE (2000): Upper Frasnian Mrirt to the new genus Chlupaciparia. to middle Famennian nappe reconstruction, DOPIERALSKA (2003, 2009) and DOPIERALSKA litho- and goniatite stratigraphy at Gara de et al. (2015): Use of 17 Frasnian/Famennian Mrirt (= Bou Ounebdou), including the two samples from Gara de Mrirt for the analysis of Kellwasser Events, emphasizing similarities neodymium isotopes in conodont phosphate, as with the Montagne Noire succession of a tool to reconstruct palaeoceanographic southern France. changes. WALLISER et al. (1995, 2000): Detailed litho- CRONIER et al. (2004): Use of Acuticryphops and conodont stratigraphy of the Tibouda-Bou (Phacopida) material from Bou Ounebdou in a Ouenebdou (sections Mrirt I-III) area and comparative morphometric study of trilobite precise dating of Eovariscan xenosediments eye ontogeny and variability. and reworking phases. GIRARD et al. (2005): First record of SANDY (1995): Re-interpretation of the Palmatolepis linguiformis, index-species of Dzieduszyckia mass occurrences of the Mrirt MN 13b Zone (Pa. linguiformis Zone), at Mrirt area as examples for Mid-Palaeozoic cold seep (Bou Ounebdou). chemosymbionthic habitats. RIQUIER et al. (2005, 2007): Isotope and trace GIRARD & ALBARÈDE (1996): Investigation of metal geochemistry and magnetic trace element geochemistry of F-F boundary susceptibility of the Frasnian-Famennian conodonts, comparing trends in sections of boundary beds at Bou Ounebdou and Anajdam. southern France and at Mrirt (Bou Ounebdou). AVERBUCH et al. (2005): Whole rock HÜNEKE (2001, 2006, 2007, 2013): Detailed geochemistry across the F-F boundary. description, conodont biostratigraphy, and GIRARD & RENAUD (2007): Use of conodonts microfacies of three Tibouda-Bou Ounebdou from the Kellwasser Beds at Gara de Mrirt in a sections (MT, MG, and M), with a focus on quantitative study of morphometry,