Becker & El Hassani A project outline Devonian to Lower Carboniferous stratigraphy and facies of the Moroccan Meseta: implications for palaeogeography and structural interpretation – a project outline R. Thomas BECKER & Ahmed EL HASSANI 1. Introduction al., Eds., 2013; HARTENFELS et al., Eds., 2018). The eastern Anti-Atlas is famous for its 1.1. Devonian stratigraphy internationally celebrated geological heritage The Devonian was the first system in which (e.g., EL HASSANI et al. 2017), but, as presented all chronostratigraphic units (series and stages) in this volume, there are equally important and were formally defined, producing a stable time fossil-rich localities in the Western Meseta and frame work for detailed international and Middle Atlas basement. interdisciplinary correlation (e.g., BECKER et al. 2012, 2020). This also applies to the main 1.2. Devonian plate tectonics part (Tournaisian, Viséan) of the Lower The Devonian plate tectonic configuration Carboniferous (e.g., DAVYDOV et al. 2012). has been controversial for a long time and this The most significant fossil groups that provide applies especially to the latitudinal position of an international high-resolution subdivision of Gondwana and Laurussia (= Laurentia + time are conodonts and ammonoids for the Baltica + Avalonia) and to the width of the pelagic facies realm, conodonts, brachiopods oceanic system between both supercontinents, and trilobites for the neritic realm, and variably named as western part of the miospores for terrestrial settings and for Prototethys, Palaeotethys, or the Variscan nearshore clastic sediments with high Oceans (FRANKE et al. 2017). The widely used terrigeneous influx. There are both regional term Rheic Ocean applies only to a specific and global zonation schemes, which part within the dynamic system; it closed supplement each other. Together with other during the Emsian, which was followed by the modern stratigraphic techniques, such as opening of the Rhenohercynian Ocean (e.g., Devonian global event, isotope, and sequence FRANKE 2000). Whilst palaeomagnetic data stratigraphy, it is now possible to establish and reconstructions (e.g., VAN DER VOO 1990; precise time frameworks for fossiliferous BACHTADSE et al. 1987; TAIT et al. 2000; sedimentary successions, which lie far beyond NANCE et al. 2012) postulated a very wide the past research (Fig. 1). This can now be Devonian ocean, palaeobiogeographic applied to regions with difficult outcrop comparisons between cratonic Gondwana and conditions and strong tectonic overprinting, Laurussia (e.g., ROBARDET et al. 1990; where stratigraphic progress has been slow. MCKERROW et al. 2000; GOLONKA 2007; One such example is the Moroccan Meseta, NAZIK & GROOS-UFFENORDE 2016), suggest a which, in terms of Devonian-Carboniferous narrow ocean, as depicted first in the Devonian stratigraphy, has long remained in the shadow continent distribution of HECKEL & WITZKE of the Anti-Atlas (e.g., EL HASSANI & TAHIRI, (1979). Occurrences of calcareous contourite Eds., 1999; EL HASSANI, Ed., 2004; BECKER et deposits in strongly condensed deeper-water Hassan II Academy of Science and Technology Frontiers in Science and Engineering - Vol. 10 - n° 1 - 2020 9 1 Becker & El Hassani A project outline settings (HÜNEKE 2006, 2007) agree with the Mrirt sucession (see BECKER et al. 1997; close-fit palaeogeographic reconstructions. BECKER & HOUSE 2000a). Based on the close similarity of German and Meseta Givetian reefs, EICHHOLT & BECKER 1.3. Prototethys palaeogeography (2016) suggested a ca. 3.000 km wide (N-S) A better understanding of the western “Variscan Sea”. It had no oceanic crust in a Prototethys evolution can be reached by a wide archipelago of crustal blocks and islands much finer (in terms of time resolution) and (BECKER et al. 2016d) ranging from the dynamic comparison of facies, faunal, floral, Moroccan Meseta in the South to a ribbon of and palaeogeographic characteristics of its southern European microplates. These were southern and northern shelf areas. These are represented, for example, by the Betic preserved in the moderately deformed, Cordillera and Catalonia of Spain, Menorca, fossiliferous, southern (= Meseta, SE Portugal) Sardinia, Sicily, and Calabria, as well as the Rif and northern external Variscides. An improved and Kabylie areas, which were then situated understanding of the varied and complex more easterly than today (“Palaeo-Adria” belt sedimentary and faunal successions of the of FRANKE et al. 2017). Clearly, the Meseta Meseta is the key to understand the dynamics faunas and facies developments play a of a seaway, which controlled the East-West significant role in this plate tectonic debate. moving oceanic water masses and, in the form The Devonian plate configuration is not of stepping stones, the North-South migration only crucial for the geodynamic understanding of biota. Rather enigmatic and incompatible of the Variscan Oceans and later orogeny, but with the sedimentary and palaeobiogeographic critical for the reconstruction of global record (e.g., ROBARDET et al. 1990; ROBARDET palaeoclimate, oceanic currents (e.g., BALBUS 2003; ERNST & RODRÍGUEZ 2010) are 2015), and paleobiogeography, including, for Devonian maps with a position of Morocco far example, the early dispersal of tetrapods and to the east and completely outside of the freshwater biota (e.g., YOUNG 1981, 2003 southern European/Variscan realm (e.g. BLIECK et al. 2007). Both equivocal and crucial DALZIEL et al. 1994, COCKS & TORSVIK 2002). is the Devonian plate tectonic position of Morocco. Many reconstructions (e.g., HOUSE 1.4. Devonian synsedimentary tectonics 1975; NEUGEBAUER 1988, SIMANCAS et al. The Middle Devonian to Lower 2006; GOLONKA 2007, VON RAUMER et al. Carboniferous was the time when Eovariscan 2009; PÉREZ-CACÉRES et al. 2017) place it tectonic events occurred throughout the slightly or far to the west of Iberia, directly Variscan oceans. This led to distinctively facing the Appalachian belt. Other authors different sedimentary and palaeoecological (e.g., HECKEL & WITZKE 1979; ZIEGLER 1989; patterns on separate, often closely adjacent MCKERROW et al. 2000; MATTE 2001) place crustal blocks, microplates, and on the the Moroccan Meseta just to the south of the marginal shelf sees. However, the lack of southern European Variscides. The latter view precise biostratigraphic dating for such is strongly supported by palaeobiogeographic movements and re-sedimentation events similarities (e.g. ROBARDET et al. 2000; hampered palaeogeographic and structural EICHHOLT & BECKER 2016) and the trans- reconstructions. Even further, it has been Variscan tracing of thin marker units from the claimed (TRIBOVILLARD et al. 2004; Meseta Devonian to southern France. An AVERBUCH et al. 2005) that Eovarican important example is given in our chapter on tectonics played a major role as a trigger mechanism for major marine eutrophication 10 Hassan II Academy of Science and Technology Frontiers in Science and Engineering - Vol. 10 - n° 1 - 2020 2 Becker & El Hassani A project outline events, such as the Kellwasser Events near the complex framework of structural units (e.g., Frasnian-Famennian boundary. This can be PIQUE & MICHARD 1981; PIQUE 1994; tested (and rejected) based on the precise MICHARD et al. 2010), each with a distinctive dating of Eovariscan uplift and erosion faunal and synsedimentary tectonic history. episodes and their relationships in time and There are both autochthonous and space with the Kellwasser Beds, which are allochthonous units, including nappes and partly well-developed in the Meseta. large olistostromes, that experienced There are links between synsedimentary significant westward displacement and re- tectonics and palaeobiogeography. More equal embedding during the “middle” Carboniferous facies and biota across the ocean system can be peak of deformation. The Western Meseta expected in times of tectonic quiescence, when clearly belong to an extensional tectonic similar depositional conditions could spread regime, from the basal Devonian well into the over large areas and when faunal exchanges Carboniferous (e.g., PIQUE 1975; BEAUCHAMP were not limited by deep and wide troughs or & IZART 1987; OUANAIMI et al. 2019). subaerial arc barriers. Fluctuating biodiversity However, Famennian metamorphism and gradients and peaks of endemism, for example compression has been suggested for the Midelt of (sub)tropical shallow-water corals, area (Middle Atlas; e.g., PIQUE & MICHARD ammonoids, and conodonts, are good 1981). Towards the Anti-Atlas in the South, indicators of stable or changing latitudinal and there is no evidence for a significant terrane or climatic differences. The Meseta localities facies boundary at the tectonically complex provide insights into latitudinally intermediate southern Variscan Front (e.g., JEANETTE & but tectonically influenced ecosystems at the PIQUE 1981; BAIDDER et al. 2008; FERONI et al. southern margin of the palaeotropics. 2010; RYTINA et al. 2013; Sub-Meseta Zone of MICHARD et al. 2010). 1.5. The Meseta Variscides A wide range of previous research dealt As part of the southern Variscides, with post-sedimentary tectonic styles, the main Hercynian Morocco includes the strongly structural framework, and tectonic models tectonized Palaeozoic successions north of the (summaries in PIQUE & MICHARD 1981, 1989; more or less stable cratonic part of NW PIQUE 1994, 2001; EL HASSANI & TAHIRI, Gondwana, the Anti-Atlas, which experienced Eds., 1994; EL HASSANI et al. 2003; MICHARD only moderate