The Atlas-Meseta Red Beds basin (Morocco) and the Lower Ordovician rifting of NW-Gondwana

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The Atlas-Meseta Red Beds basin (Morocco) and the Lower Ordovician rifting of NW-Gondwana

HASSAN OUANAIMI¹, ABDERRAHMANE SOULAIMANI², CHRISTIAN HOEPFFNER³, ANDRÉ MICHARD⁴ and LAHSSEN BAIDDER⁵

Key words. – Tremadocian, Floian, Red beds, High Atlas, Meseta, Morocco, Rifting, NW-Gondwana.

Abstract. – The transition from the Cambrian to Ordovician in Morocco is known to be characterized by a frequent Furongian hiatus, restricted extension of the Tremadocian marine deposits, and frequent unconformities at the base of the transgressive upper Floian deposits. In the present work, we first highlight the occurrence of Fe- and mica-rich, red silty/sandy formations in the Central and Eastern High Atlas between the Middle Cambrian and Upper Floian sequences. In the Tislyt type-locality, a synsedimentary hemigraben structure is defined, within which the red beds show frequent slump folds, debris flows and internal unconformities. The correlation with several coeval series of the Meseta domain allows us to define a shallow marine, ferruginous clastic Atlas-Meseta Red Beds (AMRB) basin during the Tremadocian-early Floian. The AMRB basin extended between the Meseta coastal block and the Anti-Atlas domain, being limited by the fault zones that became the West Meseta shear zone and the South Meseta fault, respectively, in the Variscan orogen. The AMRB basin compares with the coeval rifted basins of the central Iberian and Armorican massifs.

The red beds were likely sourced from the east, from both the Precambrian basement and Early Ordovician magmatic rocks, contrary to the Ordovician deposits of the Sahara platform sourced from the south. Subsidence of the AMRB and central Iberian-Armorican basins of the NW-Gondwana border aborted during the Floian, whereas the opening of the Rheic ocean went on more to the west.

Le bassin à couches rouges du domaine atlaso-mésétien (Maroc) et le rifting ordovicien inférieur du Gondwana nord-ouest

Mots-clés. – Trémadocien, Floien, Séries rouges, Haut Atlas, Meseta, Maroc, Rifting, Gondwana nord-ouest.

Résumé.– La transition Cambrien-Ordovicien au Maroc est connue pour présenter de fréquentes lacunes du Furongien, une extension limitée des dépôts marins du Trémadoc et des discordances fréquentes à la base des dépôts transgressifs du Floien supérieur. Dans le présent travail, nous mettons en évidence les formations silto-gréseuses versicolores, riches en Fe et micas, intercalées dans le Haut Atlas central et oriental entre le Cambrien moyen et le Floien supérieur. Dans la localité-type de Tislyt, une structure en hémigraben est reconnue, à l’intérieur de laquelle les couches rouges montrent de fréquents plis de slumping, glissements en masse et discordances intraformationnelles. La corrélation avec plusieurs coupes de même âge du domaine de la Meseta nous permet de définir le bassin marin Atlaso-Mésétien à Couches Rou- ges (bassin AMRC, en anglaisAMRB basin), de faible profondeur d’eau, durant le Tremadocien-Floien inférieur. Le bassin AMRC s’étendait entre le bloc côtier mésétien et le domaine de l’Anti-Atlas, étant ainsi limité par les zones de failles qui deviendront la zone de cisaillement ouest-mésétienne et la faille sud-mésétienne dans l’orogène varisque. Le bassin AMRC est comparable aux bassins contemporains des massifs centraux ibérique et armoricain. La source des couches rouges était vraisemblablement située à l’est et correspondait au socle précambrien et aux roches magmatiques de l’Ordovicien inférieur, au contraire des dépôts clastiques ordoviciens de la plateforme saharienne, alimentés depuis le sud. La subsidence des bassins AMRC et des bassins correspondants ibéro-armoricains de la bordure nord-ouest du Gondwana avorta au cours du Floien, tandis que l’ouverture de l’océan Rhéique se poursuivit plus à l’ouest.

INTRODUCTION

Morocco has suffered the brunt of the Alleghenian-Variscan collision, which built there a system of three juxtaposed mountain belts, namely the Mauritanide belt in the south- west, the Meseta belt in the north (extending in most of the Atlas mountains), and the Anti-Atlas belt in the southeast

(fig. 1, insert). Correlations between the various structural zones of these belts are often difficult, even when the more recent, Atlasian (Neogene) perturbations are resolved. The Paleozoic sequences are well-developed in the Meseta and Anti-Atlas belts, but those of the Meseta belt are fragmented and displaced relative to each other whereas their stratigraphy is frequently obscured by synmetamorphic

o 1. EGE, Ecole Normale Supérieure, Cadi Ayyad University, Marrakech, Morocco.houanaimi@gmail.com

2. Faculty of Sciences Semlalia, Cadi Ayyad University, Marrakech, Morocco.

3. Hon. Pr. Mohamed V University, Rabat, Morocco.

4. Em. Pr. Université Paris-Sud (Orsay), 10 rue des Jeûneurs, 75002 Paris, France.

5. Hassan II University, Faculty of Sciences Aïn Chock, BP 5366, Casablanca, Morocco.

Manuscript received on October 15, 2015; accepted on February 4, 2016.

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folding [Michard, Soulaimani et al., 2010]. Problems arise in particular for the correlation of the formations and sedimentary gaps that occur during the Cambrian-Ordovician transition. This span of time is characterized throughout Morocco by the most frequent lack of Furongian sediments, the restricted extension of the Tremadocian deposits and the occurrence of coarse clastic, undated strata beneath the dated Arenigian (Floian) deposits [Destombes et al., 1985;

Alvaro et al., 2007]. In the present work we focus on the latter type of azoic, clastic ferruginous formations of the Meseta-Atlas domain, labeled hereafter “Red bed series”.

These formations have been noticed in some regional monographies [Ouanaimi, 1989; Hoepffner, 1987; Ramirez Merino et al., 2008] or unpublished reports [Ovtracht, 1976], but never considered globally. Based on our field studies and the literature, we are able to show for the first time that a large faulted, mostly shallow marine basin fill with red beds occupied most of the Meseta-Atlas domain during the Tremadocian-early Floian, and to discuss its geodynamic control in the frame of the evolution of the NW-Gondwana margin.

GEOLOGICAL SETTING

The three major segments of the Moroccan Variscan belt (fig. 1, insert) display strongly distinct structural characters. The Mauritanide segment, not considered in this paper, is defined by stacked thrust nappes made up of poorly iden- tified Precambrian and Cambrian-Ordovician (?) metamorphic formations [Villeneuve et al., 2006; Michard, Soulaimaniet al., 2010; Beaet al., 2015]. The thin-skinned

fold belt at the front of the Mauritanide nappes continues to the north into the Anti-Atlas.

The Anti-Atlas Paleozoic belt corresponds to the foreland fold belt of both the Mauritanide and Meseta Variscan segments. The belt displays a thick-skinned tectonics with uplifted Precambrian blocks cropping out as large inliers overlain by the folded Cambrian-Lower Carboniferous series (fig. 1) [Michard, 1976; Burkhardet al., 2006]. Syntectonic foliation and greenschist-facies recrystallization occur only in the Cambrian formations of western Anti-Atlas whereas flexural, disharmonic folding in anchimetamorphic to diagenetic conditions dominates elsewhere in the belt [Michard, Soulaimani et al., 2010]. The Variscan deforma- tion resulted from the inversion of paleofaults inherited, firstly from the Ediacaran collapse of the Pan-African belt and subsequent Cambrian rifting of the NW Gondwana [Soulaimani et al., 2014], and secondly from the Devonian dislocation of the Sahara platform [Baidder et al., 2008;

Frizon de Lamotteet al., 2013]. Inversion occurred during the Serpukhovian-Bashkirian in the western and central Anti-Atlas (330-320 Ma) whereas it lagged up to the Early Permian in the eastern Anti-Atlas [Michard et al., 2008].

The Anti-Atlas foreland belt is limited in the north by the South Meseta fault zone (SMF) within which the Neogene South Atlas fault systems is included [Ouanaimi and Petit, 1992; Michard, Ouanaimi et al., 2010].

North of the SMF, the Variscan Meseta domain includes several structural zones derived from the distal NW-Gond- wana margin amalgamated during the Late Carboniferous collision (fig. 1). From east to west, five major zones are classically described [Piqué et Michard, 1989; Hoepffneret

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FIG.1. – Variscan domains of Morocco.Upper insert: Overview from the Mauritanides to the Alpine Rif allochthons.Lower insert: Summary of chronos- tratigraphy for the Moroccan Lower and Middle Ordovician, after Destombeset al.[1985], Gutiérrez-Marcoet al. [2003], Videtet al.[2010] and Van Roy et al. [2015].Main map: Anti-Atlas and Atlas-Meseta domains, after Michard, Soulaimaniet al.[2010], with location of the stratigraphic sites described in this work. Numbers in empty circles: sites without red beds; numbers in black circles: sites involving Lower Ordovician red beds. CHA: Central High Atlas; EHA: eastern High Atlas; MAAF: Main Anti-Atlas fault; RTFZ: Rabat-Tiflet fault zone; SAF: South Atlas fault; SMF: South Meseta fault;

SOF: Smaala-Oulmes fault; TBF: Tazekka-Bsabis fault; TTF: Tizin’Tretten fault; WHA: western High Atlas; WMSZ: West Meseta shear zone.

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al., 2005, 2006; Michardet al., 2008; Michard, Soulaimani et al., 2010]:

– the eastern Meseta is typified by its early diastrophism that begins during the Late Devonian-Tournaisian.

The stratigraphy of the Cambrian-Ordovician formations is poorly known in the central part of this zone, due to amphi- bolite-facies metamorphism;

– the Nappe zone of western Meseta corresponds to the frontal part of the eastern Meseta, thrust westward over the central Meseta and currently isolated by the Mesozoic cover of the Middle Atlas;

– the central Meseta displays an important bimodal volcanism during the Late Devonian-Early Carboniferous. Syn- metamorphic shortening of the Central Meseta basin started during the late Visean and was followed by the emplacement of a number of granitic plutons between 330-270 Ma.

The central Meseta structural zone continues southward in the eastern part of the western High Atlas Paleozoic massif;

– the Coastal block [Michard, 1976; Piqué and Michard, 1989] is remarkable by its moderate diastrophism and the lack of pre- and post-orogenic magmatism. The block is limited in the east by the West Meseta shear zone (WMSZ) and continues southward in the western part of the western

High Atlas Paleozoic massif, where the WMSZ is labeled West Atlas fault zone (WAFZ).

– the Sehoul block in the north is typified by its greenschist-facies Cambrian-Ordovician formations intruded by Eo-Variscan granite dated at ca. 367 Ma [Tahiri et al., 2010]. This terrane is bounded in the south by the Rabat- Tiflet fault zone (RTFZ), which roughly parallels the SMF.

Neither the tiny Sehoul terrane nor the Paleozoic nappes of the Rif Alpine belt are considered in this work as they do not offer any clear structural correlations with the Meseta and Anti-Atlas domains.

THE CAMBRIAN-LOWER ORDOVICIAN SERIES IN THE ANTI-ATLAS AND COASTAL BLOCK Both the Anti-Atlas domain and the Coastal block of the Meseta domain display Cambrian-Ordovician sequences de- void of red beds, whether the stratigraphic record is complete (Central Anti-Atlas, fig. 2, site 1) or not (sites 2-6 and 7-9, respectively). The only (and partial) exception is site 15, at the very eastern border of the Coastal block.

In the western and central areas of the Anti-Atlas domain, the Lower Cambrian sequences consist of carbonates

o FIG. 2. – Lithostratigraphy of the Cambrian-Ordovician transition in the Anti-Atlas and Atlas-Meseta domains. Same legend and abbreviations as figure 1, with Fm: formation. The red background corresponds to the extension of the red beds in the Atlas-Meseta domain. Dashed lines: main Variscan faults.

Stratigraphic symbols after the geological map of Morocco, scale 1:200,000, with Km1: “Schistes àParadoxides”(Middle Cambrian); Km2: Tabanit Sandstones and El Hank Quartzites (Middle Cambrian); Ks: Upper Cambrian (Furongian); Or1: Lower Fezouta Fm (Tremadocian); Or2: Upper Fezouata Fm (Floian); Or3: Tachilla Fm (Dapingian-Darriwilian); C-O: undifferentiated Cambrian-Ordovician series. Sources: see text.

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and fine-grained clastics whereas carbonates are no longer present in coarse clastic sequences of the eastern areas. The depositional setting was controlled by the rifting of NW-Gondwana, as testified by the associated subalcaline magmatism [Benssaou and Hamoumi, 2003; Michardet al., 2008; Pouclet et al., 2008]. The subsequent Middle Cam- brian pelites and greywackes of the “Schistes à Paradoxides” Formation (Fm) accumulated throughout the entire Anti-Atlas, being followed upward by the Tabanit Sandstones Group (Gp) [Destombeset al., 1985; Ouanaimi, 1992]. The Middle Cambrian sedimentation was accompa- nied by the emplacement of scattered alkaline basalts, tra- chytes and dolerites [Destombes, 2006; Benziane et al., 2002; Malusà and Schiavo, coordinators, 2007; Raddiet al., 2007]. Furongian deposits are preserved in the Alougoum area (site 1, figs. 1-2). They begin with tidal sandstones and continue upward with 50 m-thick offshore argillites and fine HCS-bearing sandstones [Destombes and Feist, 1987;

Bleinet al., 2013], suggesting a large flooding event. After an erosional episode recorded by unconformable, up to 10 m-thick conglomerates and coarse sandstones, Tremadocian deposits of the Burgess Shales-type [Van Roy et al., 2010, 2015; Martin et al., 2015] extended transgressively over a large area including the central and eastern part of Anti-Atlas and the Algerian Sahara [Destombes et al., 1985; Videt et al., 2010]. The Tremadocian formation (Lower Fezouata Fm) thins westward, eastward and northward [Imini inlier, Destombes, 1963] of the basin depocenter (Zagora) where its thickness reaches about 400 m. Tremadocian deposits no longer exist west of the J. Tachilla (fig. 2, site 3) and in the northeast of the J. Saghro-Ougnat axis (fig. 2, sites 4-6). At site 6, the Upper Fezouata Fm (Floian; fig. 2, insert) overlies directly the Middle Cambrian Tabanit sandstones through a well-marked unconformity (fig. 3). The Upper Fezouata Fm (upper Floian) begins frequently with channelized, ferruginous conglomeratic sandstones that change laterally into thin sandy beds or carbonate ferruginous sandstones, (e.g. site 6, fig. 3; southwestern part of site 10).

The Coastal block of the Meseta domain and the High Atlas Paleozoic massif west of the WAFZ show about the same Cambrian-Ordovician evolution than the Anti-Atlas, beginning with a complete Lower-Middle Cambrian succession involving abundant volcanic intercalations. The Bouznika greenish greywackes and siltstones and the

overlying El Hank Quartzites are the equivalents of the Anti-Atlas “Schistes à Paradoxides” and Tabanit Forma- tion, respectively [Destombes et al., 1966; Destombes et al., 1985]. The thickness of these Middle Cambrian deposits increases westward (Mansouria, site 15) relative to the less subsiding Bouznika area, about 10 km in the east [Bernardin et al., 1988; Corsini, 1988]. Mafic magmatism occurred during this extensional evolution, with an alkaline character at Sidi Saïd Maachou (site 8) [Oualiet al., 2000;

Álvaro et al., 2008] and a calc-alkaline tendency at Bouznika [El Hadiet al., 2006]. Deposits of Furongian age are known in the neighbouring Casablanca area [André et al., 1987; Corsini, 1988], in the Imfout syncline 80 km fur- ther south [Merglet al., 1998] and in the western High At- las (site 9) [Cornéeet al., 1987]. In contrast, Tremadocian deposits are unknown in this zone, and the earliest Ordovi- cian series above the Cambrian ones are of Arenigian (upper Floian-early Middle Ordovician) age [Destombeset al., 1985; Cornée and Destombes, 1991]. In the Mansouria pro- file (site 15), the El Hank Quartzites are followed upward by reddish-violet, glauconite-bearing sandstones and microconglomerates (60 m), in turn overlain by the greenish micaceous shales of the Chabet el Oukaref Fm, which is ascribed to the Arenigian.

THE LOWER ORDOVICIAN RED BEDS OF THE ATLAS AND MESETA DOMAINS

The peculiar sedimentary sequences described hereafter consist of multicolored beds, purple, greenish or brick red, 100 m to 400 m-thick, intercalated between the Middle Cambrian and well-dated upper Floian sequences. They mainly crop out in the High Atlas immediately north of the SMF (figs. 1, 2; sites 10, 11) and in the eastern Meseta (sites 12, 13), but one outcrop is also found in the central Meseta (site 14). Some other outcrops further in the west can be compared with the typical red beds sequences; all of them are located along the WMSZ (sites 15-18).

The Tislyt Formation

The most typical red bed formation is defined around the Tislyt village, 10 km north of Tizi n’Tichka, but it extends with the same characters in the Ait Hkym massif about 5 km further in the NE (fig. 1, quadrangle site 10). The

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FIG. 3. – Unconformity and hiatus at the Cambrian-Ordovician transition in the northeastern Anti-Atlas (site 6, fig. 1). A: Panoramic view; notice the lack of Furongian and Tremadocian-Lower Floian deposits, as shown in figure 2. Km2: Upper part of Middle Cambrian; Or2: Upper Floian; Or3: Darriwilian;

Or4: Darriwilian-Sandbian. - B: Close-up view on the unconformity.

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occurrence of thick multicolored beds has been previously noticed in the area [Gigout, 1937; Ouanaimi, 1989]. The overall succession can be described as follows from bottom to top (fig. 4B, unformal members 1 to 7):

1) siltstones and fine-grained sandstones with tiny lenticular ripples, flaser bedding and laminations. They form two parasequences at the top of the “Schistes à Parado- xides” that outcrop as a large anticline in the Tislyt area;

2) dark green sandstone bed (1-3 m) with carbonate lenses or discontinuous layers, by place breccias and/or clus- ters of tiny fragmented fossils, and by place small synsedimentary normal faults and accumulations of slumped, rusty carbonate lenses with stromatolite laminae. This is the lower strata of the Tislyt Fm as defined here. The hiatus of the Tabanit Sandstones, otherwise well-characterized in the west, east and north (fig. 2), is to be noticed;

3) green-blue sandy siltites and argillites more or less rich in bioturbations (100 m). The upper part of this sequence is affected by slumpings that erase bedding over several meters vertically. Conglomerates occur at the top of the sequence with pebbles and cobbles of greenish sandstones or sandy pelites, and accumulations (debris flows) of rusty carbonate lenses (fig. 5A);

4) a pair of 0.40 m-thick ferruginous limestones that can change laterally into a single limestone bed about 1 m-thick or into a thinner conglomeratic layer with sandstone and carbonate pebbles. This sequence is generally transgressive on the siltites (3) and overlies unconformably the slump folds they may contain. Again, synsedimentary movements are recorded above this level (4) at Tislyt village where a 10 m-thick red bed horizon showing an internal unconformity and a thin conglomeratic bed with reworked purple and ferruginous elements can be seen (fig. 5B);

5) grey or black shales whose weathered outcrops show a yellow color make up a decametric marker horizon, followed upward by sandy pelites that reach 100 m thickness westward, but thin down to zero eastward. There, the shales are replaced by 10 m-thick red siltstones containing thin conglomeratic beds reworking red silty elements;

6) above these basal deposits a 300 m-thick multicolored sequence begins, consisting of alternating violet-red and green sandy pelites. The thinly rhythmic deposits (fig. 5C) form decametric, thickening and coarsening upward parasequences. Sedimentary structures such as thin laminations, bioturbations, small lensoid ripples and lenticular flaser bedding are frequent and suggest a tidal flat envi- ronment (fig. 5C, D). The green or red color of a given bed remains constant over long distances along strike, suggesting a synsedimentary control. Slump structures are also frequent in the purple or violet-red beds;

7) sandstones and lenticular conglomerates followed upward by fine-grained graded siltstones with cone-in-cone layers and black shales [Ouanaimi, 1989], dated as Arenigian (i.e. upper Floian-lower Darriwilian) in the Tizi n’Tichka outcrops [Destombes, 1971];

The Tislyt Formation is defined here conventionally by the succession (2) to (6) described above. In thin section, the violet-red siltites show alternating light quartzose beds rich in opaque minerals (iron oxides) and undifferentiated ferruginous cement, and dark micaceous beds rich in chlorite, muscovite, altered biotite and iron oxides. The latter occur as exsolutions along the cleavages of the mica grains or as bedding-parallel filaments. It seems likely that the reddish color results from a synsedimentary process within a muddy deposit rich in altered biotite and iron oxides, poor in potential fossils, but with occasional burrowing activity of soft body animals. The synsedimentary character

o FIG. 4. – Extension and stratigraphy of the Tislyt Fm in site 10 (location: fig. 1).

A: Structural map showing the location of the Tislyt red beds between the Ourika-Taddert and Meltsene faults (two branches of the SMF system), east and north of the uplifted Ourgouz and Taddert blocks. B: Lithostratigraphy of the Middle Cambrian-Lower Ordovician succession in the Tislyt basin. 2-6: Tislyt Fm. Location of the fig. 5 (A, B, C, D and E) views. See text for details. C: Restoration of the Tislyt red beds hemigraben in cross-section (schematic).

Symbols as figure 2.

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of the violet-red color is confirmed by the occurrence of red clasts and pebbles reworked in some coarse layers intercalated near the base of (6). The bioturbations, ripples and flaser beddings associated with slump structures, small synsedimentary faults and debris-flows suggest that sedimentation occurred in shallow marine conditions, likely at the front of a prograding delta.

From the structural point of view, the Tislyt Fm crops out in a block bounded by two branches of the SMF, namely the Ourika-Taddert and Meltsene faults in the south and north, respectively, and in the west by the Assif Iswal fault zone (fig. 4A, C). The latter is a set of synsedimentary faults that allowed subsidence to increase southeastward (hemigraben). One of these faults passes next to Tislyt-village (fig. 5E). Consistently, a number of slumps, debris flow and intraformational unconformities occur at the bottom of the Tislyt Fm. Likewise, such synsedimentary structures occur frequently along the Ourika-Taddert fault [Ouanaimi, 1989;

Laville, 1980]. The Tislyt Fm also occurs in the Ait Hkym massif (fig. 2) within two anticlines striking broadly E-W.

No more than the upper part of the formation can be seen

there, consisting of alternating, meter- to decameter-thick sequences of green sandy pelites and violet-red rhythmic siltites. These sequences are overlain by Floian deposits similar with those of the Tizi n’Tichka area [Ouanaimi, 1989].

The Oued Rhmat Fm

This formation is defined here from the eastern part of the Mougueur massif (figs. 1, 2, site 11), next to the Ait Ihya Ou Aissa village. The same formation can be also seen in the E-W trending shear zone (a branch of the SMF) at the western tip of the massif. The outcrops are typified by alternating green and purple strips, several decameters-thick by place, and the sequence was ascribed provisionally to the Tremadocian-lower Arenigian (Floian) by Ovtracht [1976].

The Oued Rhmat Fm consists of a mix of argillaceous rhythmic siltites, and of fine-grained, centimeter- to decimeter-thick beds of graded sandstones (fig. 5F). Undulated laminaes, lenticular ripples and flaser beddings are numer- ous. The sandy beds are generally micaceous and show cur- rent ripples by place. The sedimentary context is that of a

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Fig. 5. – Selected views of the AMRB deposits from the High Atlas. A-E: Tislyt type-formation (see fig. 4 A, C for location); F: Mougueur massif.

(A) Chanelled debris flow with silty matrix and various elements, especially rusty carbonate lenses, unit 4 of the Tislyt section. (B) Unit 5 conglomerate with various ferruginous gravels. (C) Unit 6 purple siltstones with lenticular sandy beddings (arrow). (D) Polished section of a sample from unit 6 showing small quartz rich (Q) wavy bedding with lenticular (L) and flaser (F) beddings; (E) View of the N-S to NNW-SSE synsedimentary fault next to Tislyt-village (see Google earth at GPS 31°24'24.78"N/7°19'52.93"W for plane view); (F) Sandy/silty violet strata with graded beddings in the red beds of the Oued Rhmat Fm from the Mougueur massif.

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shallow marine basin. Thin sections show quartzarenites or quartzwackes microfacies with ferruginous matrix. Some of the beds are entirely oxidized and rich in hematite whereas others display alternating light quartz-rich and dark oxide- rich laminae. Muscovite and chlorite grains occur in both type of laminae, but bedding-parallel, ferruginous mica grains (altered biotite) are concentrated in the dark laminae.

The Oued Rhmat red beds sequence overlies the Tabanit Sandstones whose upper beds show frequent structures of synsedimentary deformation as in the Tizi n’Tichka and Tislyt area. The top of the red beds have been eroded and partly resedimented in the form of red flattened pebbles within a ca. 20 m-thick, sandy conglomeratic bed equivalent to the bottom conglomerate of the Floian series of the Tizi n’Tichka-Tislyt area.

Tazekka Red beds Fm

This formation is defined in the Tazekka massif (figs. 1, 2, site 12) beneath greenish shales dated from the Lower Ordo- vician [Rauscheret al., 1982]. It consists 300-400 m-thick, violet or wine-red argillites intercalated with millimeter- thick laminae rich in quartz and chlorite [Hoepffner, 1987, 1989; Ramirez Merino et al., 2008]. The violet layers are made up of clay minerals, tiny quartz, chlorite and muscovite grains with abundant opaque minerals either as isolated grains or grouped in thin layers parallel to bedding. The violet argillites pass upward to olive-green pelites and lithic grauwackes that contain clasts of felsic volcanic and subvolcanic rocks. Flaser bedding has been described in the dominantly fine-grained, tidal to infra-tidal varicolored formation.

Zekkara Red beds Fm

The Zekkara red beds appear above undated green greywackes and pelites (figs. 1, 2, site 13) intruded by a Variscan granodiorite [Hoepffner, 1987]. The green greywackes and pelites compare with other series of East- ern Meseta and High Atlas (Midelt, Mougueur, respectively) and could belong to the Middle Cambrian. The red beds formation consists of about 400 m of violet or olive- green micaceous pelites with decimeter-thick sandstone layers, which contain quartz, plagioclase, scarce micas and abundant ferruginous grains. The red beds end with 20 m-thick reddish sandstone bed overlain by olive-green micaceous pelites more or less rich in bioturbations. Tec- tonic slices of similar green pelites have been dated as Early Llanvirnian [Elaouad-Debbaj et al., 1985], i.e. Dapingien- Darriwillien, south of the Zekkara red beds outcrops. A gabbro sill occurs in the lowermost red beds at the top of the green greywackes, being affected by the Variscan diastrophism as the country rocks. This isolated sill could repre- sent a Lower Ordovician magmatic event, possibly coeval with the Rabat-Tiflet basalt outpours (see below).

Goaida Red beds Fm

This sequence of red beds has been described in the Moroc- can Central massif (figs. 1, 2, site 14) by Cailleux [1994].

The red beds overlie directly the Middle Cambrian green pelites in the absence of the El Hank quartzites or Tabanit sandstones. The red beds are in turn overlain by the Zaian Quartzites whose age is at least Darriwillian. The following

levels are distinguished in the red beds formation [Cailleux, 1994]: i) sandstone and microconglomerates (20 m);

ii) black pelites with thin sandstone beds (20 m) and a quartzite bed on top; iii) black or wine-red micaceous slates more and more rich in sandstones beds upward (20 m);

iv) grey and multicolored siltites with alternating pelites and ferruginous mica-rich sandstones showing frequent ripples (60 m); the importance of the reddish sandstone beds rich in bioturbations increases upward.

Other Ordovician red beds outcrops

Several outcrops of the western Meseta and western High Atlas Paleozoic massif located within or close to the WMSZ corridor offer analogies with the preceding red beds formations. In the Rehamna massif, the Draa Guessaa Fm (figs. 1, 2, site 16) consists of several hundred meters-thick, sandy micaceous phyllites with wine-red levels overlain by quartzites with violet sandy pelites and ferruginous sandstone intercalations [Piqué, 1972]. A Lower/Middle Ordovi- cian age was suggested for these red beds [Piqué, 1972;

Destombes et al., 1982], but this age is controversial and may be Devonian as that of the Skhour Rehamna Quartzites [Baudin et al., 2003 ; Michard, Soulaimani et al., 2010].

Further in the south, between the Central and Western Jebilet (figs. 1, 2, site 17), the Skhirat Unit contains violet pelites and multicolored argillites overlain by the Ordovi- cian (Arenigian?) quartzites [Tahiri, 1984]. These red beds are regarded as equivalent to the red shales with Ordovician acritarchs described in the area by Poutchovsky [1978].

Eventually, the Addouz multicolored Fm of the western High Atlas (site 18), here noticed for the first time, is a 400 m-thick, typical red beds series stratigraphically sandwiched between the Cambrian Tabanit Sandstones and the Ordovician sandy pelites and quartzites. The latter red beds are located east of the WAFZ (southern WMSZ)

o FIG. 6. – Sketch map of the Atlas-Meseta Red Beds (AMRB) basin in its post-Variscan configuration. The Lower Ordovician red beds are bounded by the SMF, the WMSZ and the RTFZ that correspond to inverted paleofaults of the pre-Variscan setting. The AMRB basin is interpreted as a rift domain developed southeast of the earlier Cambrian rift. A-B and C-D:

Approximate trace of the schematic cross-section figure 7.

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whereas the equivalent Skhirat red beds extend within the fault zone itself.

At last a poorly known occurrence of possible red beds of similar age must be quoted here, which is the Aïn Chair Fm of the Tamlelt Massif (eastern High Atlas). This easternmost sequence consists of ca. 800 m of dominantly greenish micaceous pelites with some red brown layers, and was dated as “Skiddavian” [Du Dresnay and Willefert, 1960], i.e. Floian-Dapingian. Then the dated part of the formation does not strictly correspond to the typical red beds described above, which predate the Upper Floian, but possibly the lower strata of this very thick sequence could repre- sent an eastern equivalent of the Tislyt and Mougueur red beds series.

INTERPRETATION AND DISCUSSION The Atlas-Meseta Red Beds basin (AMRB)

In spite of a part of indetermination about some of the red bed occurrences reported above (particularly in the Rehamna, figs. 1, 2, site 16, and the Tamlelt massif, fig. 1, right), the data presented here allow us to reconstruct a large, broadly homogeneous basin filled up with red beds deposits during the Tremadocian-Lower Floian (?). This At- las-Meseta Red Beds (AMRB) basin occupied most of the domain of the future Meseta orogen, which includes the At- las domain north of the SMF and basically east of the WMSZ (figs 5, 6). We assume that the AMRB basin was subsequently fragmented into scattered minor outcrops of red beds formations as a result of the Variscan tectonics.

The major Variscan faults of the SMF and WMSZ systems can be regarded as inherited from Pan-African lineaments (SMF), or at least as inverted paleofaults linked to the Ediacaran-Cambrian rifting evolution of northwest Gond- wana [Piqué and Michard, 1989; Michard et al., 2008].

Thus it is reasonable to assume that these fault corridors also controlled the extension, opening and evolution of the AMRB basin.

Overall, the sedimentation in the AMRB basin is typical of a shallow marine basin with feldspath and mica-rich clastic influx. The iron richness of the deposits may result both of the abundant mica and chlorite input and of Fe-hy- droxides input from some of the weathered sources. Thick- ness ranges from a few meters to 400 m. On the borders of

the basin, ferruginous sandstones and microconglomerates are observed, particularly developed at Mansouria (figs 1, 2, site 15), and thinner in the west of the WMSZ (sites 7 and 9) and south of the Ourika-Taddert fault (sites 1, 6, and southern part of the site 10 quadrangle). Within the basin, intraformational faults, slumps and debris-flows may be ascribed to the gravity-controlled deformation of a prograding delta in a tide-dominated basin. The extensional tectonic context is attested in the case of the Tislyt hemigraben (fig. 4) and, at a larger scale, by the location of the AMRB basin between two major paleofaults, i.e. the SMF and the WMSZ (fig. 7). Indeed, lithosphere extension is attested as early as the Early Cambrian in the western High Atlas and neighbouring Anti-Atlas [Soulaimani et al., 2003, 2004;

Poucletet al., 2008; Álvaroet al., 2015]. During the Middle Cambrian, mafic, dominantly alkaline magmatism occurred in western Meseta (fig. 2, sites 8, 14, 15), western High At- las [Poucletet al., 2008] and eastern Anti-Atlas [Destombes and Hollard, 1986; Raddiet al., 2007]. The climax of extension would correspond to the emplacement of the pillow basalts and gabbro sills intercalated in the Lower Arenigian (Floian) series of the Rabat-Tiflet anticline [El Hassani et al., 1988a, b; Piqué et al., 1993; El Hadi et al., 2014].

This is the time (about 475 Ma) when the Rheic ocean opened, after a long-lasting extensional evolution of the NW-Gondwana margin [Murphyet al., 2006; Nance et al., 2012].

The AMRB basin developed eastward relative to the Cambrian rift of western Morocco [Bernardinet al., 1988;

Piquéet al., 1995; El Hadiet al., 2006; Poucletet al., 2008;

Michard, Soulaimani et al., 2010]. The Coastal block and the northern fringe of the Anti-Atlas were paleogeographic highs that can be interpreted as rift shoulders during the early Tremadocian (figs 6, 7). Most of the infilling of the AMRB basin would have accumulated during the upper Tremadocian-early Floian prior to the Upper Floian trans- gression. Therefore, the rifting evolution of northwestern Gondwana in Morocco extended from the Early Cambrian up to the Tremadocian-early Floian, being sealed by the Floian post-rift sedimentation. Erosional steps of the rift shoulders occurred after the Furongian and during the Tremadocian-early Floian. The NW-Gondwana rifting dura- tion (about 70 m.y.) compares with that of the rifting of Pangea from the Late Permian to the Hettangian- Sinemurian [Olsen et al., 2003;Veratiet al., 2007]. During

o

FIG. 7. – Diagrammatic cross-section of the AMRB basin (red background) and adjacent domains in their post-Variscan setting, showing the Cam- brian-Ordovician tilted blocks and the erosional unconformities, as well as the role of the major inverted paleofaults as boundaries of the varied domains.

See approximate trace A-B and C-D in fig. 6. Not to scale.

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this long period, the location of the subsiding basins evolved, which is observed in the hyper-extended margins [e.g. Mohnet al., 2015] as well as in the magma-rich margins [Stab et al., 2016].

Iberian-Armorican correlations

Clear correlations can be recognized further in the north of the Gondwana margin, i.e. in the Central Iberia zone (CIZ) and the northern and central zones of the Armorican massif (fig. 8A).

In the central zone of the Iberian Meseta, a number of

“basal red beds series” (“capas intermedias”, “púrpurea series”, etc.) are described beneath the Armorican quartzites [Gutiérrez-Alonso et al., 2007] and ascribed to the Tremadocian or Tremadocian-Arenigian [Hammann, 1983;

Julivert and Truyols, 1983; Guttierrez Marco et al., 1990].

These varied sequences consist of wine-red or purple conglomerates, pelites and sandstones deposited in fluvial, intertidal or subtidal environments. Their thickness shows rapid changes from zero up to 1000 m and their accumula- tion is then considered as controlled by synsedimentary extensional faulting [Diez Balda and Vegas, 1992]. To- gether with the broadly coeval magmatism (see below), these red beds formations would record a passive margin-type rifting phase, whose age is younger by about 40-50 m.y. with respect to the arc-type magmatism of the more external Ossa-Morena zone [Sànchez-Garcia et al., 2008, with references therein].

In the Armorican massif, the equivalents to the AMRB series are the 0-800 m-thick clastic formations (e.g. red deposits of Plouézec-Plourivo and Erquy-Fréhel in northern Armorica; red beds formations of Pont-Réan and Cap de la Chèvre-Crozon in Central Armorica) that overlie the

Brioverian basement and predate the Grès Armoricain quartzites [Ballèvreet al., 2009, 2013, and references therein].

They consist of pink, greenish to wine-red conglomerates, gritty feldspathic sandstones, purple or violet sandy pelites and argillites deposited in fluvial, lacustrine, deltaic or tidal environments. As in Central Iberian zone, these red beds are ascribed to the Tremadocian-Floian as they are overlain transitionally by the “Grès Armoricain” (Floian). Moreover, a Rb-Sr whole-rock isochron date of 472 ± 5 Ma is reported from the Plouézec trachy-andesites [Auvray et al., 1980]

whereas a U-Pb zircon age of 465 ± 1 Ma was obtained from interbedded felsic volcanoclastics in the Cap de la Chèvre Fm [Bonjouret al., 1988]. The tectonic context of sedimentation was crustal extension [Ballard et al., 1986;

Suireet al., 1991] with block tilting and hemigraben subsidence [Brunet al., 1991]. The Tremadocian-Floian red beds of the Armorican massif are then interpreted as syn-rift deposits related to the opening of the Rheic ocean [Dabard and Simon, 2011; Ballèvre et al., 2013].

The AMRB, the CIZ and the Armorican massif red beds domains are shown in the sketch of the peri-Gondwana Variscan belt (fig. 8B) restored at 300 Ma following Simancas et al. [2005] and Michard, Soulaimani et al.

[2010]. We may suggest that all the red beds series were parts of the same large basin or system of small basins before the climax of the Variscan collision, i.e. at about 340 Ma (fig.7C). The whole Moroccan and West European red beds basins would result from the rifting process linked to the opening of the Rheic ocean, a process that culminated along a more external zone (Sehoul and Ossa-Morena zones, fig. 8c) and led to the drifting apart of the future Avalonia terrane [e.g. Nance et al., 2012]. In this reconsti- tution we admit in line with Edel et al. [2015] that Iberia and Armorica were subject to anti-clockwise and clockwise

o FIG. 8. – Extension of the Lower Ordovician red beds in the Variscan belt of western Europe and Morocco. A: Present-day geography. Structural background after Ballèvreet al. [2013] and Michard, Soulaimaniet al. [2010], modified. B: Restoration at about 300 Ma, i.e. by the end of the Variscan orogeny. C: Restoration at about 340 Ma, i.e. during the early stages of the Variscan collision. Background maps for B and C after Ballèvreet al.[2009, 2012, 2013] and Michard, Soulaimaniet al.[2010]. AMRB: Atlas-Meseta Red Beds; CB: Meseta Coastal block; CM: Central Meseta; CIZ: Central Iberian zone;

CZ-WALZ-GZ: Cantabrian, West Asturian-Leonese and Galician zones; EM: eastern Meseta; FCM: French Central Massif; Me: Mazagan escarpment;

NAM/CAM/SAM: Armorican massif (northern/central/southern); OMZ: Ossa-Morena zone; Sh: Sehoul block; SPZ: South Portuguese zone; WAC: West African craton.

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rotations, respectively, during the early development of the Iberian-Armorican arc. It is worth noting that the red beds faulted basins formed in a more internal position than the Rheic passive margin itself, then aborted before the upper Floian. In that sense, they compare with the intra- continental, Triassic-Early Jurassic Atlas rift that aborted whereas the Alpine Maghrebian Tethys opened further in the north during the Late Jurassic-Cretaceous [e.g. Frizon de Lamotte et al., 2000].

The source issue

In order to envision the possible origin of the clastic and ferruginous deposits of the AMRB basin, the critical data concern the tectonic setting of the basin, the type of the clastic, feldspath- and mica-rich ferruginous input, and the nature of the neighbouring uplifted areas of NW Gondwana.

The red beds basin likely corresponds to juxtaposed hemigrabens born in a rifting context, which differs deci- sively from the slightly uneven mega platform developed during the Middle-Upper Ordovician in the Sahara and Anti- Atlas domains [Boeufet al., 1971; Destombeset al., 1985;

Booteet al., 1998; Ghienneet al., 2007], up to the Murzuq basin [Ghienne et al., 2013]. The broad North Gondwana platform had an off-shelf gradient towards the NNW during the Early Palaeozoic, with source areas located in the SSE, but this paleogeographic setting cannot be a priori applied to the northwest-most areas, whose tectonic setting was clearly distinct. As far as the sedimentary facies are concerned, we may notice that the ferruginous inputs observed from the middle Floian to the Darriwilian (Upper Fezouata Fm to 1^st Bani Fm) in the Anti-Atlas occur in the form of oolithic iron and glauconite in poorly clastic layers interbedded with dominantly quartz-rich formations [Destombes et al., 1985; Michard et al., 2008]. This strongly contrasts with the ferruginous input in the red beds basin, which is closely associated with feldspath and mica-rich clastic influx.

Therefore, we may suggest that the AMRB basin sediments was sourced in an actively eroded area characterized by abundant outcrops of dominantly felsic magmatic rocks and located close to the other red beds basins of northwestern Gondwana, i.e. those of Central Iberia and Armorica (figs 7B, C), and possibly in the Paleozoic areas hidden beneath the Algerian Atlas domain or lost in the Mediterra- nean Alpine domain. In the Iberian Meseta, such source areas correspond to Proterozoic basement uplift located on both sides of the Central Iberian zone and intruded by Cam- brian-Ordovician magmatic rocks. The latter rocks are dated (U-Pb zircon ages) between ~495 Ma and 470 Ma, with a maximum (Ollo de Sapo Fm) between 492-483 Ma [Monteroet al., 2009; Lopez Sanchezet al., 2015, and references therein]. In the Armorican massif, during the widespread Ordovician rifting, a large amount of subaerial to subaquatic volcanics emplaced (e.g. Vendée porphyroids), some of them having an alkaline chemistry, as well as gran- ites intruding the Proterozoic to Cambrian sequences [Ballèvre et al., 2009, 2012 and 2013]. These magmatic rocks together with the Brioverian basement itself are

supposed to have yielded the bulk of the infill of the neighbouring red beds basins.

It is worth noting that Late Cambrian-Lower Ordovician metagranites with both intermediate to felsic peraluminous rocks and felsic peralkaline rocks are widespread in the Pa- leozoic belts of western Europe and the External Massifs of the western Alps [Von Raumer et al., 2002; Helbing and Tiepolo, 2005; Cocherieet al., 2005; Monteroet al., 2009;

Guillot et al., 2009; Liesa et al., 2011]. The peraluminous associations have been interpreted by some authors as evidence for subduction and associated back-arc basins [Carmignaniet al., 1994; Von Raumeret al., 2002], particularly in the easternmost part of the northern Gondwana margin [Stampfli et al., 2002, 2011; Helbing and Tiepolo, 2005]. However, this interpretation has recently been ques- tioned, at least for the western part of the north Gondwana margin [Beaet al., 2007]. In that part (here considered), the duality between peralkaline and peraluminous Cambro-Or- dovician magmas would result of the same phenomenon of rifting with crustal melting caused by the arrival of small batches of hot mantle magmas at the base of the crust [Monteroet al., 2009]. Likewise, a rifting setting is also fa- vored in the same area by Von Raumer et al. [2013] and Lopez-Sanchez et al. [2015].

CONCLUSION

The Cambrian-Ordovician transition in Morocco is marked by frequent hiatuses and conglomerates almost everywhere, but it strongly differs between the Anti-Atlas and Atlas- Meseta domains. We described here for the first time the occurrence of widespread Tremadocian-early Floian red beds series in the domain of the Meseta Variscan belt, which includes most of the Atlas domain. These series, likely con- nected one to the other before the Variscan orogeny, define the Atlas-Meseta Red beds (AMRB) basin that we interpret as a system of juxtaposed hemigrabens limited by the South Meseta fault in the south, the West Meseta shear zone in the west, and the Rabat-Tiflet fault zone in the north. The AMRB basin closely compares with the coeval basins described in the Iberian and Armorican massifs. Pending further researches based on detrital zircon dating, we propose that the red beds deposits were sourced from the east and/or northeast from both basement and Early Ordovician magmatic rocks, contrary to most of the Ordovician deposits of the Sahara and Anti-Atlas platform domain sourced in the south from the basement of the WAC. The Early Ordovician geodynamic framework along the NW-Gondwana is currently interpreted as dominated by passive margin rifting and subsequent opening of the Rheic Ocean. This is consis- tent with the extensional tectonic setting of the partly coeval red beds basins that aborted before the Upper Floian.

Acknowledgements. – We are grateful for the fruitful comments of the re- viewers, Jean-François Ghienne and an anonymous colleague that helped to greatly improve the manuscript. We are indebted to E.C. Rjimati for his help during a common trip to the Tislyt area. A.M. also acknowledge sup- port from the Direction of Mines, Ministry of Energy and Mines, Rabat.

SGF associate editor: Nicolas Bellahsen

o

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