Geoheritage in Morocco: The Neoproterozoic Ophioliteof Bou Azzer (Central Anti-Atlas)

Hassan El Hadi & Abdelfatah Tahiri &José Fernando Simancas &

Francisco González-Lodeiro & Antonio Azor &

David Martínez-Poyatos

Received: 23 July 2009 /Revised: 30 January 2011 /Accepted: 15 March 2011 /Published online: 19 April 2011# Springer-Verlag 2011

Abstract The Bou Azzer ophiolitic complex (Anti-Atlas,southern Morocco) marks the suture of the Pan-Africanorogenic belt. This fragment of old oceanic crust (697±8 Ma) obducted onto the continental margin of the WestAfrican Craton during a collisional event that occurredbetween 650 and 580 Ma. Very well exposed desertoutcrops display serpentinites associated with chromitepods and clinopyroxenolites, ultrabasic and basic cumu-lates, quartz diorite, a sheeted-dyke complex, basalticpillow lavas, and red cherts, i.e., the set of lithologiescomposing an ophiolitic sequence. Other geological units

cropping out in the area are: (1) the underlying continentalmargin of the West African Craton; (2) a volcanic arc nextto the oceanic crust represented by the ophiolite; (3)overlying unconformable volcanic and sedimentary rocks.The unusual character of this Neoproterozoic magmatic andtectonic geoheritage site, together with the excellent qualityof the outcrops and the relatively easy access to the area,make this complex attractive from both a scientific and ageotouristic perspective. Its declaration as a geoheritage sitewill serve as a stimulus for the preservation of this singulararea.

Keywords Morocco . Ophiolite . Neoproterozoic .

Geoheritage . Geotourism . Preservation

Introduction

Morocco conceals a rich geological patrimony, frequentlytied to beautiful landscapes and generally exceptional bythe quality of the outcrops. In this paper, we focus on aspecific geoheritage entity, the Bou Azzer Neoproterozoicophiolite complex. First, however, we provide a shortexplanation of the geology of Morocco (Fig. 1).

In terms of geology, three large geological domains canbe distinguished in Morocco (Michard 1976; Piqué 1994;Michard et al. 2008). From south to north, these broaddomains are:

1) The Anti-Atlas and Sahara domain, where Palaeopro-terozoic and Neoproterozoic rocks crop out in anumber of inliers. These rocks have been affected byEburnean (approx. 2 Ga; e.g., De Kock et al. 2000;Walsh et al. 2002; Thomas et al. 2002) and/or Pan-African (700–600 Ma; e.g., Leblanc and Lancelot

H. El Hadi (*)Department of Geology, Science Faculty of Ben M’Sik,Hassan II University Mohammedia Casablanca,BP 7955( Bd. Driss El Harti,Casablanca, Moroccoe-mail: hassan_el_hadi@yahoo.fr

A. TahiriInstitut scientifique,BP 703, Rabat-Agdal, Moroccoe-mail: tahiri@israbat.ac.ma

J. F. Simancas : F. González-Lodeiro :A. Azor :D. Martínez-PoyatosDpto. Geodinamica, Facultad de Ciencias,Universidad de Granada,Avda. Fuentenueva s/n,18002, Granada, Spain

J. F. Simancase-mail: simancas@ugr.es

F. González-Lodeiroe-mail: lodeiro@ugr.es

A. Azore-mail: azor@ugr.es

D. Martínez-Poyatose-mail: djmp@ugr.es

Geoheritage (2011) 3:89–96DOI 10.1007/s12371-011-0032-2

Fig. 1 a Schema of structural domain of Morocco, b The Anti-Atlasregion, showing the location of Bou Azzer and other inliers cited inthe text, c Sketch map of the Bou Azzer Proterozoic outcrop showingthe three main terranes (arc, ophiolite and continental margin) which

compose the outcrop and the location of figures, d Landsat image ofBou Azzer inlier, e general geological cross-sections illustrating thestructure of the Bou Azzer outcrop (see location in Fig. 1c)

90 Geoheritage (2011) 3:89–96

1980; Saquaque et al. 1989a; Gasquet et al. 2005;D’Lemos et al. 2006) orogenies. Aweakly folded, well-exposed Palaeozoic cover (Cambrian to Permian) liesunconformably over the Proterozoic rocks.

2) The Meseta-Atlas domain, which is subdivided into:(1) a Palaeozoic basement (Cambrian to Permian)displaying variable Variscan metamorphism/deforma-tion and large-scale intrusion by Carboniferous granites(Michard 1976; Piqué 1994; Michard et al. 2008); (2) aMeso-Cenozoic cover deformed by the Alpine tectonics(Michard 1976; Laville 1985).

3) The Rif domain, which is a fragment of the Mediter-ranean Alpine Orogenic belt that, together with theBetic chain in southern Spain, constitutes a tightorogenic arc in the westernmost Mediterranean region.

Geoheritage in Morocco

The geology of Morocco is very rich in sedimentary,magmatic, palaeontological, tectonic, and geomorphicphenomena of different ages. They are generally exposedin easily accessible good-quality outcrops. Most of thesesites have been the object of scientific studies, but theyremain poorly known by the general public and deserve tobe both popularized and preserved. Outstanding examplesneeding preservation are specific palaeontological sites,notably those of the Palaeozoic of southeast Morocco (e.g.,Erfoud and Tazarine areas), which have been excessivelyexploited as ornamental rock and are at risk of seriousdeterioration. Some of these sites have a stratigraphicinternational dimension (e.g., stratotypes).

The inventorization of geoheritage sites in Morocco isstill in its early stages, despite a few localized attempts bythe universities. Conscious of the importance of inventoriz-ing sites and identifying geological objects as vectors ofregional development, the defenders of this geoheritagemust do not delay any longer in ringing the alarm bells.Scientists (through symposia, specialized seminaries, andnational declarations) must play a leading role in elucidat-ing the significance of the inventory and its impact on thesustainable development of the different regions of thecountry. In this respect, the “Academie Hassan II desSciences et Techniques” supports a large-scale nationalproject on the geoheritage of Morocco, with contributionsfrom all sectors of Moroccan civil society. To this endl’Association Marocaine des Sciences de la Terre (AMST)planned a debate on the “geoheritage and geotourism”during its first organized action held in Marrakesh, 2008and The Association for the Protection of the Geoheritageof Morocco (APPGM) and the Association of the M’GounGeoparc (AGM) agreed to declare the Mgoun region, in thecentral High Atlas (12791 km2 of area), a geopark, thus

promoting both better protection and local tourism. From alegislative point of view, this region falls under thejurisdiction of national governmental authorities (specifi-cally Ministry of Energy, Mines, Environment, and Water)which, by means of numerous texts and regulations,regulate the use of this region with the double aim ofpreservation (geoparks, parks, panoramas, landscapes,fossiliferous and geomaterial sites) and rational exploita-tion. Thus, this increasing awareness of the necessity toprotect the Moroccan geoheritage has led to the classifica-tion of specific geological sites and the creation ofprotected natural areas, such as the stromatolites field ofAman-n'Tourhart, located at approximately 25 km from thecity of Ouarzazate (Fig. 1a) and the parks of Mgoun andToubkal in the High Atlas (Fig. 1a).

Inventorization Criteria and Strategies for Bou Azzer

In this paper, we focus our attention on the exceptionalarea of Bou Azzer–El Graara (henceforth, Bou Azzer;Fig. 1b–d) where a Neoproterozoic ophiolite of excep-tional quality crops out, resting tectonically over rocks ofa continental margin and covered by Palaeozoic series. Apre-desert rocky landscape (Fig. 1c), together with afascinating geological history and excellent outcrops ofsingular rocks, make this area a clear target to beconsidered as a geoheritage site.

General criteria for the geoheritage inventory in the areaof Bou Azzer are linked to: (1) the geological andgeographical value of these sites (outcrops, cross sections,landscapes) as well as their usefulness for teaching; (2) thescientific and academic value of these rocks, including rarerock-types of an ophiolitic suite; (3) the aesthetic value ofthe sites, which together with their scientific value makethem potential socio-economical and cultural centers ofcultural tourism (geotourism).

The beauty of the landscapes of the Moroccan Anti-Atlas, which is visited by 3–5 million tourists a year, willbe enhanced with simple didactic explanations placed in aprecise but accessible geologic frame.

A number of recommendations have been proposedbased on research and assessment of the Moroccan geologicheritage. These include:

(1) To reveal—on an international scale—the existence ofa complete, although tectonically dismembered ocean-ic crust (ophiolite) of an ancient age (approx. 700 Ma)in Morocco.

(2) To attract attention to the exceptional characteristics ofthis outcrop as a main geosite which displays: (I) asingular old geological body, the ophiolite; (ii) avolcanic arc associated to the ophiolite; (iii) the

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continental margin on which the ophiolite and volca-nic arc have been emplaced; (iv) syntectonic sedimen-tary units (Tiddiline Formation) attesting to theprocess of ophiolite emplacement onto the continent.

(3) To prevent against the risk of destruction of this siteby a non-conservative exploitation of rocks as indus-trial geomaterial and to promote effective actions forthe protection of this geoheritage.

(4) To elaborate on legislation of banning unauthorizedfree access to the most important and most vulnerablegeologic sites.

(5) To regulate the informal trade of rocks and minerals.

The Neoproterozoic Ophiolitic Complex of Bou Azzer

Geological Framework

Geographically, the Bou Azzer inlier is located at thenorthern border of the Moroccan Sahara desert, in thecentral part of the Anti-Atlas Mountains, about 300 km eastof the town of Agadir. Geologically, it belongs to the Anti-Atlasic and Saharan domain defined above (see Introduc-tion and Fig. 1a, b), exhibiting a variety of geological unitsof Proterozoic age. The Bou Azzer ophiolite complex isexposed over an area of approximately 60 × 5 km, trendingWNW–ESE along the boundary (orogenic suture) betweenthe West African Craton to the south and the Pan-Africanorogenic belt to the northeast.

The schists, gneisses, and metagranites of the WestAfrican Craton, older than 2000 Ma, are well displayedjust to the west of Bou Azzer, in the neighboring inlierof Zenaga (Ennih et al. 2001). The West African Cratonis the continental basement over which the oceanic rocksof the ophiolite were tectonically emplaced during thePan-African convergence (Leblanc 1975; El Hadi et al.2010).

The ophiolite complex (oceanic rocks; Fig. 1c) includesthree distinct geological units: (1) the true ophiolite, i.e., anoceanic-type crust and upper mantle; (2) a northern belt ofvolcanic-arc affinity made up of calc-alkaline siliceousmetavolcanics, tuffs, epiclastites, and metabasites (Saquaqueet al. 1989b); (3) metaluminous, diorite to granodiorite syn-kinematic plutons, intruding both oceanic and arc units(Beraaouz et al. 2004).

The Bou Azzer ophiolite was originally interpreted byLeblanc (1975) as a fragment of oceanic lithosphere (seealso Leblanc 1981; Bodinier et al. 1984). The age of ametagabbro of this ophiolite has been recently beendetermined as 697±8 Ma (SHRIMP U-Pb on zircons; ElHadi et al. 2010). All of the typical lithologies of anophiolitic suite can be found at Bou Azzer, although due tofaulting and dismembering of the ophiolite body, not all

rock types can be recognized in a single undisrupted cross-section (Fig. 1c). From the bottom to top of the originaloceanic sequence, the ophiolite includes serpentinizedharzburgites and dunites, ultramafic and mafic cumulates,gabbros, dykes, basalts, and minor volumes of volcano-clastic rocks, detritic limestones, and red cherts. Thechromites in the serpentinites strongly resemble those frommantle peridotites of modern ophiolites (Leblanc 1981). Inthe border of the serpentinite, there are deposits of cobaltthat are being mined in the district of Bou Azzer. Thecumulative sequence can be divided in two parts, namely,an ultramafic and a mafic part (El Hadi 1988; El Hadi et al.2008); both are well exposed in the massif of Oumarou, atthe core of the Bou Azzer inlier, and in the Assemlil riverwhere very spectacular and well-conserved mafic units andflaser gabbros crop out. Gabbroïc cumulates, relativelymore abundant than ultramafic cumulates, form a layeredseries about 500 m thick. In general, there is a transitionfrom coarse-grained melanocratic gabbros (more clinopyr-oxene) at the bottom to fine-to-coarse leucocratic facies(more amphibole) at the top. In gabbros, structures ofmagmatic cumulate (microfolds, intraformational fault,igneous lamination, etc.) are typically present.

To the north, the ophiolite lithologies are bounded bycalc-alkaline siliceous volcanics, tuffs, epiclastites, andmetabasites; these are interpreted to represent a volcanicarc adjacent to the oceanic crust represented by theophiolite (Saquaque et al. 1989a; Fig. 1c). Metaluminoussyntectonic diorite-to-granodiorite plutons ranging in agefrom 641 to 654 Ma (Inglis et al. 2005; El Hadi et al. 2010)intrude the ophiolite and the volcanic-arc. The calc-alkalinegeochemistry of these granitoids is typical of arcmagmatism, suggesting that they were formed duringsubduction preceding continental collision (Beraaouz etal. 2004). The ophiolite and the arc-volcanics areirregularly covered by the syn-orogenic clastic TiddilineFormation, with the whole area being blanketed by a post-orogenic Ediacaran–Early Cambrian sequence of ande-sites, dacites, and volcanoclastic rocks of calc-alkalinegeochemical nature (Ouarzazate Formation; Leblanc1981). Consideration of the tectonic setting of theophiolite has led to a number of controversial hypotheses(Leblanc and Lancelot 1980; El Hadi 1988; Saquaque etal. 1989a; Naidoo et al. 1993; Thomas et al. 2002;Gasquet et al. 2005; Soulaimani et al. 2006). However,given the tectonic polarity (from NE to SW) of the arc–ophiolite continent, the most probable interpretationplaces the original oceanic crust of the ophiolite in afore-arc setting. Preceding the Pan-African collision inthis region, the ophiolite rocks would have undergonesome degree of subduction before experiencing exhuma-tion and obduction onto the continental margin (Hefferanet al. 1992; El Hadi et al. 2010).

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The Bou Azzer Inlier as a Geoheritage Site

The Bou Azzer ophiolite can be considered to be anexceptional geological entity from many different view-points. It constitutes a former oceanic crust that is easilyaccessible for viewing, which makes it extremely suitablefor being visited by many people, including students,researchers, and nature-lovers. It testifies to the existenceof an ancient “Pan-African” ocean that disappeared bysubduction during the Pan-African convergence. In addi-tion, it allows some features that we have knowledge ofonly indirectly through geophysical techniques, such as theboundary between the crust and the mantle under theoceans, i.e., the oceanic Moho, to be viewed and studied insitu (Fig. 2).

Unlike other important geological sites of of local scale,it is the entire Bou Azzer inlier what should be considered ageoheritage site. The Bou Azzer inlier is a WNW–ESEerosional window surrounded by Palaeozoic strata that formscarps around the relatively low Neoproterozoic terrains ofthe interior (Fig. 3). This landform, apart from giving aparticular beauty to the area, partially restricts motorizedaccess, which has to be channeled through the NW passageconnecting the Bou Azzer mine (Fig. 1c) to the locality ofTazenakht, in the Zenaga inlier (Fig. 1b). The local roadrunning from the mine of Bou Azzer to the locality ofBleida, all along the inlier, is the main (but not unique)access to specific outcrops.

Despite the convenience of considering all of the BouAzzer as one geoheritage entity, it may be appropriate hereto describe a few local sites in order to illustrate thegeological value of the area. these are examples taken froma broad number of possibilities.

The Eburnean/Latest Proterozoic Unconformity

This site does not belong (strictly speaking) to the BouAzzer inlier, but it is only a few kilometers to the west andnext to the road from Tazenakht to the Bou Azzer mine.The UTM coordinates are 29R 0680098/3380902. Here, themajor unconformity between the rocks of the West AfricanCraton (intensely deformed migmatitic schists older than2000 Ma) and the poorly deformed andesites–dacites of thelatest Proterozoic age (580–600 Ma) can be easilyappreciated (Choubert and Faure-Muret 1970; Thomas etal. 2002; Gasquet et al. 2005; Fig. 4). The latest Proterozoicvolcanic rocks are followed in continuity by sandstones,limestones, and pelites of Cambrian age.

Bou Offroh: Ophiolitic Lithologies, Synorogenic Sediments,and Postorogenic Volcanics

The Bou Offroh sector is the westernmost part of Bou Azzerand might be considered to be the door of this inlier (Fig. 1c).Leaving the road from Tazenakht to the Bou Azzer mine, ashort walk (only a few hundred meters) to the northeastbrings us to a location from which we can observe aremarkable variety of lithologies, including ophiolite andsupraophiolite rocks. First, we observe ophiolite serpentinites(e.g., UTM 29R 0694626/3381120), which give the ground abeautiful green color. A fault separates the serpentinites fromthe ophiolite basalts and minor outcrops of ophiolitesediments, namely slates, metasandstones, limestones, andcherts. Another fault repeats the basic rocks and red cherts,which are irregularly covered by syntectonic microconglom-

Fig. 3 Ophiolitic slices surrounded by the Ediacaran and Cambriansediments in the background. To the SE of the Bou Azzer inlier: aunconformity between the Ophiolitic slices (790 My; El Hadi et al.2010) and the Ediacaran conglomerates, b conformity between theEdiacaran conglomerates and the Lower Cambrian

Fig. 2 The very spectacular gabbro cumulate-ultramafic rocksboundary (the Moho discontinuity). This photo was taken at the southof the Inguejjem Mount, SE of Bou Azzer mine. Location is given inFig. 1c

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erates formed during the collisional Pan-African event(Tiddiline Formation; Leblanc 1975). A second unconformi-ty marks the end of the Pan-African deformation, passingupwards to undeformed dacites of the latest Proterozoic age.

Although the faults are recurrent here and the ophiolitebody lacks “stratigraphic” continuity, this site is goodstarting point for a short walk during which somelithologies of the ophiolite and the two unconformableformations over it can be observed.

A View of the Oceanic Moho

The contact between the serpentinized ultramafic rocksand the lower metagabbros (see Fig. 2) represent theoceanic petrologic Moho. This main geological boundaryis beautifully exposed along the northern limb of arecumbent fold cored in the serpentinites, in the centralpart of the Bou Azzer inlier (Fig. 1c). As an example, thespecific location with UTM coordinates 29R 0729166/3375617 can be chosen, but the contact crops outcontinuously for more than 5 km in the NW–SE direction.It is easily accessible from the local track connecting theBou Azzer mine and the Bleida locality. Figure 2 is a viewtaken from the south and looking towards the NE of thismain boundary.

Fig. 4 Unconformity between Palaeoproterozoic rocks of the WestAfrican Craton (2000 My) and an upper Neoproterozoic andesites–dacites (560My)

Fig. 5 Beautiful Assemlil valley, south of the Bou Azzer inlier. Notethe unconformity between the Ediacaran volcanic conglomerate andthe Proterozoic platform sequence

Fig. 6 Sheeted dyke complex, located some 10 km SE of the BouAzzer mine

Fig. 7 Flaser gabbro displaying the deformation of the igneouslayering (Assemlil River)

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Bleida Cross-section

The new small settlement of Bleida, at the southern part ofBou Azzer (Fig. 1c) is a good starting point for anotherinteresting geological walk. This short trip starts at UTMcoordinates 29R 0743890/3362396, where the Bleidagranodiorite and its host rock, the schists of the continentalmargin, come into contact. The Bleida granodiorite ispoorly deformed and has been dated as 579 Ma (Inglis etal. 2005). Accordingly, that age is considered to be the timewhen the collisional convergence ended at Bou Azzer.Around 300 m to the northeast, the schists are tectonicallyoverlain by metabasalts, amphibole-garnet schists, calciticschists, and chert, which represent the upper part of theophiolite. The ophiolitic rocks here form an imbricate stack,which is folded by open upright folds. The outcrop iscontinuous and oddly beautiful.

Cross-section East of Tamaliout: Folding and Thrustingof the Ophiolite

The access to this site is not as easy as that to the other sitesmentioned above. The best access is from a local road whichcurls around the Bou Azzer inlier to the south, deviating atUTM coordinates 29R 0688196/3378294 from the local roadto the Bou Azzer mine. At coordinates 29R 0707267/3360359, it is necessary to leave this road and follow a tracktowards the small locality of Assemlil. The track continues tothe north along a beautiful valley (Fig. 5), until the point atcoordinates 29R 0724893/3368999 from where the walk onfoot starts. At this point, a dyke complex (Fig. 6), layeredgabbros, and cumulates of the lower oceanic crust crop out(Fig. 7), showing high dips to the SW. This is the southernlimb of a fold affecting the ophiolite and the underlyingrocks of the continental margin (Fig. 1e), these latterstructures being schists and augen-gneiss (metagranites)which crop out at the core of the anticline. Still to thenortheast, 3.5 km from the starting point, the schists of thecontinental margin dip NE (northern limb of the above-mentioned fold) and take a phylonitic appearance (Fig. 8). Awell-exposed contact separates the mylonitic schists of thecontinental margin from mylonitic metabasalts (greenschists)of the ophiolite. The mylonitic metabasalts show a clearstretching lineation (Fig. 9) and kinematic microstructures,indicating the thrusting of the ophiolite over the continentalmargin. Further north, ophiolitic basalts and gabbros crop outwithout the intense deformational fabric observed near thebasal thrust of the ophiolite.

Conclusion

Morocco is recognized by many people as a geological"paradise" because of the nature and variety of thegeological phenomena, the good conditions of outcrops,and the easy accessibility to these entities . In this context,the site of Bou Azzer stands out as a singular area deservingboth the world's attention and careful control by theMoroccan government. The existence of an ophiolite thatbears witness to the collisional Pan-African event, the highquality of the outcrops, and the beauty of the landscapecome together to form a geological and geographical sitefull of interest. To date, the Bou Azzer area has been wellpreserved due to its remoteness from city areas, but aformal agreement on its great value is needed to prevent thepossibility of any future mis-use.

Acknowledgments The research reported in this paper was madepossible through financial support by “l’Académie Hassan II desSciences et Techniques" and the Agencia Espanola de CooperacionInternacional (AECI), the Inter-Academic Moroccan-Spanish Programprojects 17/P/03, 65/04/RE, A/3008/05, A/4938/06 and A/011499/07.

Fig. 8 Phylonitic appearance of the schists of the continental margin

Fig. 9 Kinematics of the Bou Azzer ophiolite emplacement. Themylonitic metabasalts show a stretching lineation, indicating thethrusting of the ophiolite over the continental margin

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