Structural and sedimentologic relations between the High and the Middle Atlas of Morocco during the Jurassic time

The cartographic, sedimentologic and stratigraphic studies carried out on the Mesozoic deposits in the border zone between the Middle and the High Moroccan Atlas (regions of Naour and Aghbala) led us to specify the lithology of formations, the significant differences of thickness and the angular unconformities as well as stratigraphic hiatuses. All of this indicates a tectonic regime of transcurrent faults from the Bajocian–Bathonian period along the major fault zone “Aghbala–Afourer” in a N70° direction. A sinistral strike-slip movement along this major fault zone induced the development of folded and fractured zones in a N120° direction, which limited a small trough filled by the red continental formations. The whole system is covered thereafter by lower Cretaceous deposits.     

Mid-Jurassic events in the Western High Atlas (Morocco)

Angular unconformities, time gaps, and progressive increase of thickness in the Mesozoic sedimentary pile of the Western High Atlas, Morocco, point to mid-Jurassic tectonic pulses in this area. Red beds, Middle Jurassic in age, were deposited in the nearshore Agadir-Essaouira basin in contrast to transgressive tendencies of the eustatic curve. These phenomena are easily explained by uplift and erosion in the central part of the mountain belt even in the Middle Jurassic. Thus, a rift stage (Permian to Early Jurassic) in the tectonic history of the Western High Atlas was followed by a gulf stage (Late Jurassic to Middle Cretaceous) until later the high mountain range generated by inversion in Late Cretaceous and Neogene to recent times. Tensional forces that created the mobile rift belt were superimposed by vertical uplift during the Middle Jurassic that gave rise to a NNE-SSW stretching high. Even in the present-day high mountain range this area, called the »ancient massif«, is strongly elevated. Thus, the evaluation of the geological history of the Western High Atlas seems much complicated.

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Zusammenfassung Bei Kartierarbeiten und Profilaufnahmen im Westlichen Hohen Atlas in Marokko beobachtete Winkeldiskordanzen, Schichtlücken und progressive Mächtigkeitsentwicklungen deuten auf mitteljurassische tektonische Aktivitäten hin. Unterstützung erhält diese Hypothese durch Rotsedimente des Dogger im Küstenbecken von Agadir-Essaouira, die hier im Gegensatz zur allgemeinen Meeresspiegelentwicklung zum Absatz kamen.In der tektonischen Entwicklung ergibt sich ein Rift-Stadium von Perm bis Unterjura, das von einem Golf-Stadium von Oberjura bis Mittelkreide abgelöst wird, bevor sich durch Inversion in der Oberkreide und seit dem Jungtertiär das heutige Hochgebirge bildete. Im Mesozoikum wird Zerrungstektonik überlagert durch vom Atlantik ausgehende Vertikaltektonik. Sie führte bereits im Mittleren Jura zu einem NNE-SSW verlaufenden Hochgebiet, das auch im heutigen »massif ancien« des Westlichen Hohen Atlas gipfelt und die Aufklärung der sedimentären Vorgeschichte des Gebirges erheblich erschwert.

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Résumé Dans les couches mésozoïques du Haut Atlas occidental (Maroc), des discordances angulaires, des lacunes stratigraphiques ainsi que des variations progressives d'épaisseur, témoignent d'activités tectoniques au Jurassique moyen. C'est ainsi qu'au Dogger, des couches rouges se sont déposées dans le bassin côtier d'Agadir-Essaouira en opposition avec les tendances transgressives générales de la courbe eustatique. Ces phénomènes s'expliquent facilement par des processus de soulèvement et d'érosion dans la partie centrale de la zone mobile atlasique au Jurassique moyen.L'histoire tectonique a comporté une période de rifting (du Permien au Jurassique inférieur) suivie par une période où le golfe marin atlasique s'est constitué (du Jurassique supérieur au Crétacé moyen) et enfin par un processus d'inversion qui, au Crétacé supérieur et depuis le Néogène, a produit le relief moderne du Haut Atlas.Les forces d'extension, responsables de la naissance de la zone mobile du rift atlasique, ont été suivies au Jurassique moyen par un soulèvement vertical en relation avec l'expansion de l'Océan atlantique. Il en est résulté un bombement NNE - SSW, appelé la »Terre des Almohades« par les auteurs français. Le »massif ancien« dans la zone centrale de la haute montagne actuelle est une partie de ce vieux bombement, où on ne trouve guère de séries mésozoïques. C'est de ce fait que la reconstitution de l'histoire du Haut Atlas occidental est particulièrement difficile.

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Faulting history at the eastern termination of the High Atlas Fault (Western High Atlas,Morocco)

At its eastern termination, the High Atlas Fault in the Western High Atlas in Morocco, consists of a splay of three faults. In the interjacent fault blocks, Neo- and Paleoproterozoic basement, forming the northernmost extremity of the NW-African Craton, is cropping out. The Precambrian basement witnesses a long history of brittle deformation starting at the end of the Pan-African Orogeny. A subsequent episode of normal faulting can be related to the development of a Hercynian basin along the northern passive margin of the cratonic promontory. With regard to the main tectonic activity in the Western High Atlas, basically two models exist: one emphasising block tectonics reflecting Mesozoic rifting followed by Alpine uplift and inversion, the other emphasising Late Paleozoic dextral wrench tectonics. The analysis of the fault activity along the splay faults reveals a predominantly Alpine history, consisting of the Triassic development of the Atlas Rift along the axial zone of the orogen, followed by uplift and inversion. The Late Jurassic to Cenozoic fault activity took place in a sinistral transpressive regime and was partitioned over the three splay faults. Dextral strike-slip fault activity could not be demonstrated in the fault blocks nor along the splay faults. Therefore the faults were probably not involved in Late Paleozoic dextral wrench tectonics.     

Inversion tectonics of intracontinental ranges: High and Middle Atlas,Morocco

The High and Middle Atlas are intracontinental mountain belts situated within the mobile foreland of the Mediterranean Rif orogen. They developed in three stages. The first period (Permian — Bathonian) culminated during the Lias with extended rift grabens and tholeiite extrusions. From Callovian to Eocene, the tectonic activity and the rates of sedimentation were reduced, both pointing to a cooling of the lithosphere. Since the Oligocene, the whole region is submitted to compressional stress. The High and the Middle Atlas were uplifted within two phases, which were correlated with main phases of Rif orogenesis. Refraction seismic measurements have recently revealed there a flat layered structure of the crust with several low velocity zones. The deepest one coincides with a layer of high electric conductivity, which is interpreted as a zone of detachment.From the geotectonic evolution of the High and Middle Atlas and from the structure of the crust, the following model was deduced: During Early Mesozoic rifting, the crust on top of the mantle elevations was thinned by both extensional fracturing and by gliding along intracrustal detachment planes. During the Cenozoic collisions of the Rif, these shear planes were reactivated by thrusting in opposite directions. Compressional deformation of the graben fillings led now to a moderate thickening of the crust, e.g. up to 40 km beneath the High Atlas. Subsequent uplift and inversion was not only caused by isostasy, but also by squeezing upward due to thick- and thin-skinned tectonics.

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Zusammenfassung Der Hohe und der Mittlere Atlas sind intrakontinentale Gebirge im mobilen Vorland des mediterranen Rif-Orogens. Ihre Entwicklung weist drei Perioden auf: Die erste (Perm-Bathonium) kulminierte im Lias mit der Bildung von Riftgräben entlang spätvariskischer Bruchzonen und Tholeiit-Ergüssen. Im Intervall Callovium-Eozän deuten tektonische Beruhigung und geringere Sedimentation auf eine allmähliche Abkühlung der Lithosphäre hin. Seit dem Oligozän steht die Region unter Kompression. Der Hohe und der Mittlere Atlas haben sich zeitgleich mit den Hauptphasen der Kompression im Rif herausgehoben. Refraktionsseismische Untersuchungen haben einen flachen Lagenbau der Kruste mit mehreren low-velocity-Zonen aufgewiesen, deren tiefste mit einer Zone hoher elektrischer Leitfähigkeit zusammenfällt und als bedeutende Abscherungszone gedeutet wird.Aus der geotektonischen Entwicklung des Hohen und des Mittleren Atlas und aus der heutigen Krustenstruktur wird folgendes Modell abgeleitet: In der frühmesozoischen Rift-Phase wurde die Kruste über den Mantel-Aufwölbungen durch Zerrungsbrüche und durch Zergleiten an subhorizontalen Scherflächen ausgedünnt. Während der känozoischen Kollisionen im Rif-Atlas wurden diese Scherflächen dann gegenläufig bewegt, die Riftgraben-Füllungen dabei bis zu geringer Krustenverdickung eingeengt und anschließend herausgehoben. Die Inversion der beiden Atlas-Gebirge ist somit nicht nur isostatisch bedingt, sondern auch durch Aufpressung bei thick-and-thin-skinned-Tektonik verursacht.

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Résumé Le Haut Atlas et le Moyen Atlas sont des chaînes intracontinentales situées dans l'avant pays mobile de l'orogène méditerranéen du Rif. Elles se sont développées en trois périodes. La première (du Permien au Bathonien) a culminé au Lias avec la formation de fossés de rift accompagnés d'effusions tholéiitiques. Au cours de la deuxième période (du Callovien à l'Eocène), l'activité tectonique et les taux de sédimentation étaient réduits, indice d'un refroidissement de la lithosphère. Depuis l'Oligocène, la région est soumise à une compression. Le Haut Atlas et le Moyen Atlas se sont soulevés en deux phases, coïncidant avec les phases principales de compression du Rif. Les sondages sismiques ont mis en évidence une structure de la croûte en couches subhorizontales comportant plusieurs zones à faible vitesse dont la plus profonde coïncide avec une zone de haute conductivité électrique interprétée comme un vaste décollement.En conclusion, nous proposons le modèle suivant pour le Haut et le Moyen Atlas: pendant la phase de rifting du Mésozoïque inférieur, la croûte a été amincie au-dessus de bombements du manteau, non seulement par des fracture d'extension mais aussi par glissement le long de décollements subhorizontaux. Durant les collisions cénozoïques du Rif, ces décollements ont été réactivés en charriages par glissement en sens inverse. Ce processus, en rétrécissant les remplissages des grabens, a provoqué un léger épaississement de la croûte (jusqu'à 40 km sous le Haut Atlas) et le soulèvement final. L'inversion des chaînes atlasiques n'a donc pas seulement été l'effet de l'isostasie, mais aussi d'une tectonique »thin-and-thick- skinned«.

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Plate tectonics and intracratonic mountain ranges in Morocco - The mesozoic-cenozoic development of the Central High Atlas and the Middle Atlas

Plate tectonic processes in the Atlantic and western Tethyan realm directed the post-Variscan sedimentary and structural evolution of the High Atlas and Middle Atlas intracontinental mountain ranges of Morocco. Plate movements caused a reactivation of an inherited pan-African or Hercynian fault pattern by the variation of stress regimes through time. This resulted in strike-slip as well as vertical tectonics. During times of relative tectonic quiescence eustatic sea-level changes governed the sedimentary development. The most important, often interacting, global tectonic determinants are: taphrogenesis of the NW-African continental margin lasting until the Early Cretaceous (Triassic rifting and subsequent mid-Atlantic spreading), strike-slip-faulting at the Newfoundland-Gibraltar fault zone (Liassic — earliest Eocene), and continental convergence between Europe (Iberia) and Africa which started in the Late Cretaceous and reached its acme in the Neogene. In the realm of the Central High Atlas and the Middle Atlas the interaction of these processes triggered continental rifting (Triassic) and subsequent marine flooding of the intergrown riftgrabens prograding from the Tethys realm (Early Jurassic — earliest Middle Jurassic). After its abortion, the former Atlas rift was filled up with marine sediments (Bajocian — Bathonian), followed by continental redbeds and final uplift (late Mid Jurassic — late Early Cretaceous). Eustatic sea-level changes mostly governed the sedimentary evolution from Aptian to latest Mid Eocene. After a first weak uplift of the central High Atlas during the Senonian major uplift of the intracontinental chains commenced at the Mid/ Late Eocene transition. Diastrophism of the Atlas ranges during the Miocene and Pliocene coincided with the main orogenic movements of the Betico-Rifean arc.

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Zusammenfassung Die post-variskische sedimentäre und strukturelle Entwicklung der intrakontinentalen Gebirgsketten des Hohen und des Mittleren Atlas wurde durch plattentektonische Prozesse im atlantischen und westmediterranen Raum gesteuert, die auf ein ererbtes panafrikanisches bzw. variskisches Störungsmuster einwirkten. Zu Zeiten relativer tektonischer Ruhe bestimmten eustatische Meeresspiegelschwankungen die sedimentäre Entwicklung. Die von horizontaler und vertikaler Bruchtektonik beherrschte Atlas-Tektonik wurde durch in der Zeit wechselnde Streßfelder bestimmt. Wichtigste großtektonische, oft zusammenwirkende Steuerungsfaktoren waren: die bis in die Unterkreide wirkende Taphrogenese im Bereich des NW-afrikanischen Kontinentalrandes (triadisches Rifting und anschließendes Spreading im Atlantik), Transform-Bewegungen an der Neufundland-Gibraltar-Störungszone zwischen Lias und ältestem Eozän sowie die Konvergenz zwischen Europa (Iberia) und Afrika, welche nach ihrem Beginn in der späten Kreide ihren Höhepunkt im Neogen erreichte. Das Zusammenspiel dieser Faktoren bewirkte im zentralen Hohen Atlas und im Mittleren Atlas die Entstehung kontinentaler Riftgräben während der Trias, die von der Tethys ausgehende Flutung der zusammengewachsenen Riftgräben im Lias und untersten Dogger (Sinemurium — Aalenium) sowie die zuerst marine (Bajocium — Bathonium), dann kontinentale Füllung und Heraushebung der stillgelegten Riftzone zwischen spätem Mitteljura und später Unterkreide. Zwischen Apt und Ende des Mitteleozän kontrollierten vor allem eustatische Meeresspiegelschwankungen die sedimentäre Entwicklung. Nach ersten Hebungen im zentralen Hohen Atlas während des Senon begann die eigentliche Heraushebung der intrakontinentalen Ketten an der Wende Mittel-/Obereozän. Die mio-/pliozäne Hauptphase der Atlas-Tektogenese war zeitgleich mit der Orogenese im Betico-Rif-Bogen.

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Résumé L'évolution sédimentaire et structurale postvarisque des chaînes intracontinentales du Haut et du Moyen Atlas a été régie par les processus de la tectonique des plaques qui se déroulaient dans l'Atlantique et dans la Méditerrannée occidentale. Ces mouvements de plaques ont réactivé un ensemble de failles héritées des phases pan-africaine et hercynienne, en y produisant des déplacements verticaux et horizontaux en réponse aux fluctuations temporelles des champs de contraintes. Au cours des périodes de calme tectonique relatif, les changements eustatiques du niveau de la mer ont déterminé le régime de la sédimentation. Les facteurs de la tectonique les plus importants, à action d'ailleurs souvent combinée, ont été: la traphrogenèse, active jusqu'au Crétacé inférieur, de la marge nord-ouest du continent africain (rifting triasique et ouverture subséquente de l'Atlantique), les mouvements de décrochement le long de la zone transformante de Terre Neuve-Gibraltar depuis le Lias jusqu'au début de l'Eocène, ainis que la convergence entre l'Europe (Ibérie) et l'Afrique qui débuta au Crétacé supérieur pour culminer au Néogène. Dans le Haut Atlas central et dans le Moyen Atlas, le jeu combiné de ces facteurs a provoqué la formation de grabens continentaux (Trias) puis leur envahissement par la mer à partir de la Téthys (Lias-Dogger inférieur). Après la fin de son activité, le rift atlasique a été comblé par des sédiments, d'abord marins (Bajocien-Bathonien), puis continentaux jusqu'au soulèvement final (fin du Jurassique moyen — fin du Crétacé inférieur). De l'Aptien à la fin de l'Eocène moyen, l'évolution sédimentaire a été régie par les changements eustatiques du niveau de la mer. Après un premier soulèvement modéré du Haut Atlas central au cours du Sénonien, le soulèvement majeur des chaînes intracontinentales débuta à la limite Eocène moyen-Eocène supérieur. Le diastrophisme de l'Atlas au cours du Mio-Pliocène a coïncidé avec les mouvements orogéniques de l'arc bético-rifain.

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The Selloum Basin: new element of the Middle Liassic paleogeography in the southern Middle Atlas (Morocco)

A comparative study of the Middle Liassic series in the south western part of the Middle Atlas system leads to the definition of three new formations called Fellat 1, 2 and 3. They mainly involve gravity-flow deposits related to the breakup of the regional carbonate platform in a subsiding area, the Selloum Basin. The creation of this basin provides evidence for a rifting stage during the Carixian–Domerian interval, inducing the separation of the Selloum unit with respect to the Middle Atlasic trough and its possible connection with the High Atlasic sedimentary domain.     

The Plio-Pleistocene evolution of the Southern Middle Atlas Fault Zone (SMAFZ) front of Morocco

The South Middle Atlas front constitutes a northeast-trending shear zone, located north of the Neogene Missour basin and east of the Taza Guercif basin. This paper analyses the Southern Middle Atlas Fault Zone (SMAFZ) deformation since the Pliocene. The set of structures observed suggests that reverse and thrust faulting along the central part of the SMAFZ are combined with left-lateral slip along N–S striking faults of its south-western termination and right-lateral faulting along E–NE striking faults of the east–northeast termination. Thrusts and oblique thrust-related anticlines of the two lateral ramps partly accommodate north-west directed motion of the African plate. The Thrusts probably resulted from rejuvenation of Jurassic normal faults; they were active during the Upper Miocene–Pliocene and the Pleistocene. The geometries of positive inversion structures and buttressing effects are clearly dependent on the geometry and sedimentology of the original basin-controlling fault system and on the presence of a décollement level. Field mapping is integrated with Landsat imagery and a digital elevation model to investigate the morphotectonic evolution of the south-eastern range front of the Middle Atlas. Geomorphological features provide significant information on the processes that govern lateral propagation of active anticlines. Both suggest that the deformation front may have been active since Pliocene.     

Geosites inventory of the northwestern Tabular Middle Atlas of Morocco

Across the northwestern Tabular Middle Atlas of Morocco there are many examples of landscapes, rocks and fossils that provide key evidence of a particular moment or period in Earth history. Such Earth heritage sites are important for educating the general public in environmental matters. They also serve as tools for demonstrating sustainable development and for illustrating methods of site conservation as well as remembering that rocks, minerals, fossils, soils, landforms form an integral part of the natural world. The significance of certain sites for aesthetic or tourism reasons is obvious. There are numerous geosites, which could contribute to effective exploitation of geotourism, often in conjunction with ecotourism. The strategy employed to such sites involves close consultation with all communities in the vicinity of the respective geosite and is not only aimed at tourism and education, but also at sustainable improvement of the infrastructure of the people of this area. Geological heritage sites, properly managed, can generate employment and new economic activities, especially in regions in need of new or additional sources of income.     

Synsedimentary faulting and palaeocurrent patterns in the Triassic sandstones of the High Atlas (Morocco)

The influence of syndepositional fault patterns on palaeocurrents is demonstrated in fluvial to shallow-marine sandstones of Upper Triassic basins in the High Atlas. The synsedimentary nature of faults is deduced from hydroplastic slickensides, thickness variations due to block tilting and dislocation of layers next to the fault scarp. On a regional scale, it is shown that the major normal fault trend of N050–070° controlled the overall palaeocurrent pattern which was directed towards the west-southwest, i.e. in the direction of the future Atlantic ocean. Some anomalies in the palaeocurrent pattern could be related to an increase in subsidence which induced a general coarsening of sediment towards the top of the Triassic.     

Neogene exhumation of the Marrakech High Atlas (Morocco) recorded by apatite fission-track analysis

Apatite fission-track (AFT) data have been obtained along a traverse across the Marrakech High Atlas to constrain its tectono-thermal evolution. AFT ages vary between 212 ± 15 Ma and 20 ± 4 Ma. An Early Miocene AFT age accompanied by long mean track length from the central part of the chain has been interpreted as the timing of the main inversion of this region with the creation of relief because of the shortening induced by the interplay between the African and Eurasian plates. Thermal modelling of samples collected south of the South Atlas Fault Zone indicates a Middle-Late Miocene or even later cooling that has been attributed to the component of the uplift of the chain related to the thermal anomaly present beneath the Atlas Mountains.     

Comprehensive physicochemical study of dioctahedral palygorskite-rich clay from Marrakech High Atlas (Morocco)

This study is devoted to the physicochemical and mineralogical characterizations of palygorskite from Marrakech High Atlas, Morocco. The raw clay and its Na⁺-saturated <2 μm fraction were characterized using chemical, structural, and thermal analytical techniques. Measurements of specific surface area and porous volume are reported. The clay fraction was found to be made up of 95 % of palygorskite and 5 % of sepiolite. An original feature of this palygorskite is its deficiency in zeolitic H₂O. The half-cell structural formula of its dehydrated form was determined on the basis of 21 oxygens to be (Si_7.92Al_0.08)(Mg_2.15Al_1.4Fe_0.4Ti_0.05 $ \square_{1} $ )(Ca_0.03Na_0.08K_0.04)O₂₁, while the hydrated form could be formulated as (Si_7.97Al_0.03)(Mg_2.17Al_1.46Fe_0.40Ti_0.05)(Ca_0.03Na_0.07K_0,03)O_20.18(OH)_1.94(OH₂)_3.88·2.43 H₂O. These formulas show that the (Al³⁺+Fe³⁺)/Mg²⁺ ratio is around 0.84, revealing a pronounced dioctahedral character. Further, inside its octahedral sheet, it was determined that the inner M₁ sites are occupied by vacancies, whereas the M₂ sites are shared between 90 % of trivalent cations (78 % for Al³⁺ and 22 % for Fe³⁺), 7.5 % of Mg²⁺, and 2.5 % of Ti⁴⁺, all of them linked to 1.94 of structural hydroxyls. The two remaining Mg²⁺ by half-cell occupy edge M₃ sites and are coordinated to 3.88 molecules of OH₂. Channels of this palygorskite are deficient in zeolitic H₂O since they contain only 2.43 H₂O molecules. A correlation was found between these results and the observation of very intense and well-resolved FTIR bands arising from dioctahedral domains (mainly Al₂OH, Fe₂OH, and AlFeOH) along with very small responses from a trioctahedral domain (Mg₃OH). Accordingly, a schematic representation of the composition of the octahedral sheet was proposed. The cation exchange capacity, specific surface area, and total pore volume were also assessed to be ca. 21.2 meq/100 g, 116 m²/g, and 0.458 cm³/g, respectively.     

Crustal structure under the central High Atlas Mountains (Morocco) from geological and gravity data

Seismic wide angle and receiver function results together with geological data have been used as constraints to build a gravity-based crustal model of the central High Atlas of Morocco. Integration of a newly acquired set of gravity values with public data allowed us to undertake 2–2.5D gravity modelling along two profiles that cross the entire mountain chain. Modelling suggests moderate crustal thickening, and a general state of Airy isostatic undercompensation. Localized thickening appears restricted to the vicinity of a north-dipping crustal-scale thrust fault, that offsets the Moho discontinuity and defines a small crustal root which accounts for the minimum Bouguer gravity anomaly values. Gravity modelling indicates that this root has a northeasterly strike, slightly oblique to the ENE general orientation of the High Atlas belt. A consequence of the obliquity between the High Atlas borders and its internal and deep structure is the lack of correlation between Bouguer gravity anomaly values and topography. Active buckling affecting the crust, a highly elevated asthenosphere, or a combination of both are addressed as side mechanisms that help to maintain the high elevations of the Atlas mountains.     

Cenozoic sedimentary dynamics of the Ouarzazate foreland basin (Central High Atlas Mountains, Morocco)

Cenozoic continental sedimentary deposits of the Southern Atlas named "Imerhane Group" crop out (a) in the Ouarzazate foreland basin between the Precambrian basement of the Anti Atlas and the uplifted limestone dominated High Atlas, and (b) in the Aït Kandoula and Aït Seddrat nappes where Jurassic strata detached from the basement have been thrust southwards over the Ouarzazate Basin. New biostratigraphic and geochronological data constraining the final Eocene marine regression, the characterization of the new "Aït Ouglif Detrital Formation" presumed to be of Oligocene age, and the new stratigraphic division proposed for the Continental Imerhane Group clarify the major tectonogenetic alpidic movements of the Central High Atlas Range. Four continental formations are identified at regional scale. Their emplacement was governed principally by tectonic but also by eustatic controls. The Hadida and Aït Arbi formations (Upper Eocene) record the major Paleogene regression. They are composed of margino-littoral facies (coastal sabkhas and fluviatile systems) and reflect incipient erosion of the underlying strata and renewed fluvial drainage. The Aït Ouglif Formation (presumed Oligocene) had not been characterized before. It frequently overlies all earlier formations with an angular unconformity. It includes siliciclastic alluvial deposits and is composed predominantly of numerous thin fining-upward cycles. The Aït Kandoula Formation (Miocene-Pliocene) is discordant, extensive, and represents a thick coarsening-upward megasequence. It is composed of palustro-lacustrine deposits in a context of alluvial plain with localized sabkhas, giving way to alluvial fans and fluviatile environments. The Upper Conglomeratic Formation (Quaternary) is the trace of a vast conglomeratic pediment, forming an alluvial plain and terraces. The second and third formations correspond to two megasequences engendered by the uplift of the Central High Atlas in two major compressive phases during late Oligocene and Miocene-Pliocene times. These two geodynamic events were separated by a tectonically calm phase, materialized by palustro-lacustrine sedimentation (Görler et al. 1988). Tectono-sedimentary analysis of the two megasequences shows that the basin structure and depositional processes were controlled by the compressive tectonic context generated by the collision of North Africa and Iberia in Tertiary times (Jacobshagen et al. 1988). The Quaternary Formation was apparently controlled by a tectonic continuum and by climatic variations.     

Fractures network mapping using remote sensing in the Paleozoic massif of Tichka (Western High Atlas,Morocco)

The Paleozoic massif of Tichka in the southern part of the Western High Atlas of Morocco constitutes a structural transition between the Meseta and the Anti-Atlas domains. It was affected by a complex network of fractures noticeable at different scales. Using Landsat ETM+ imagery permits detecting the main fracture directions. Various techniques of lineament’s extraction were applied, including the colored compositions, spectral band ratios, and directional filters applied to the principal component analysis. Lineament’s extraction is based on visual interpretation and completed by field observations. The resulted map allows recognizing at least four trending fracture system, with average N-S, NE-SW, E-W, and NW-SE orientations. The surrounding rocks of the granitic massif show a high fracture density. Tectonic indicators show that this massif is initially affected by NW-SE Variscan tectonic extension, followed by NW-SE Variscan compression. This regime is being maintained until the late Variscan period corresponding to the relaxation of the NW-SE major Variscan stress. A clockwise rotation of the latter stress, which became N-S to NNE-SSW, related to the late Variscan deformation, is responsible for reworking preexisting faults.     

Ba- and Ti-rich oxymica from nephelinites in the Middle Atlas Volcanic Province,northern Morocco

Mineralogy and Petrology - A Ba- and Ti-rich mica (up to 14.0&nbsp;wt% BaO and 13.1&nbsp;wt% TiO2) occurs in nephelinites from the Middle Atlas Volcanic Province, Morocco. The rocks show a...     

Biological metal accumulation in the red beds (Guettioua Member) of the Bathonian (Dogger) of Msemrir,Central High Atlas,Morocco

A fluviatile sequence is described from the Dogger (Bathonian) of Msemrir (Central High Atlas, Morocco). The base comprises laminites with fine clouds of galena and some zinc minerals, while the rest of the profile consists of point bars with conglomerates. Due to their limitation on lower energy levels, the sulphide minerals are interpreted as biological metal accumulations of bacterial metabolic processes.

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Zusammenfassung Aus dem Dogger (Bathonium) vom Msemrir (Zentraler Hoher Atlas, Marokko) wird ein vorwiegend fluviatiles Profil beschrieben. An der Basis besteht es aus laminierten Serien, in denen feinverteilter Bleiglanz vorkommt. Am Top des Profils finden sich vor allem große Rinnenkörper mit Konglomeraten und vereinzelten Knochenresten. Die sulfidischen Vererzungen in den niedrig-energetischen Bereichen werden als biogenetische Anreicherungen, hervorgerufen durch bakterielle Umsetzungen, interpretiert.

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Résumé Nous présentons et décrivons une séquence fluviatile du Bathonien de Msemrir (Haut Atlas Central, Maroc). La base est constituée de laminites dans lesquelles se trouve de la galène finement répartie. La couche suivante comporte de grands chenaux contenant des conglomérats et, ça et là, des débris osseux et des restes de végétaux. Les minéralisations sulfurées dans les zones de basse énergie sont interprétées comme des accumulations biogénétiques dues à une activité bactérienne.

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Lithostratigraphy,biostratigraphy and sedimentary dynamics of the Lower Cretaceous deposits on the northern side of the western High Atlas (Morocco)

The analysis of a sedimentary succession on the northern side of the western High Atlas, from Essaouira to Amizmiz, has enabled us to identify, and describe, thirteen formations ranging in age from Tithonian to Clansayesian. The collection and determination of ammonite and echinoid faunas has allowed us to attribute a precise age to most of these units. Lower Cretaceous formations are organized into six main depositional sequences: Late Jurassic-Berriasian; Valanginian; Hauterivian-Barremian; Bedoulian; Gargasian; Clansayesian-Albian. This organization, the characteristics of the major discontinuities and the geometry of sedimentary succession allows recognition of the following events: a Late Berriasian distensive phase; a period of sea-level change and basin infilling during Valanginian, Hauterivian, Barremian and Bedoulian times under essentially eustatic control; an intra-Aptian tectonic phase; and a period of relative sea level rise and topographic levelling during Gargasian-Albian. This is essentially linked to the evolution of the North Atlantic ocean during the Early Cretaceous.     

Interplay of phosphogenesis and hydrothermalism in the latest Ediacaran rift of the High Atlas,Morocco

Two dissolution surfaces, capped by phosphatic crusts that contain up to 40 wt.% of P₂O₅, are well exposed in the latest Ediacaran Tabia Member (Adoudou Formation) of the Ouneïn region, High Atlas. Both the Tabia dolostones and capping phosphorites are crosscut by several generations of synsedimentary faults, fissures, and hydrothermal dikes hosting Cu ore bodies, in a scenario of down-faulted blocks. Stratigraphic and facies relationships suggest the preservation of a major pulse of intra-cratonic rifting, responsible for the development of an unstable carbonate platform subjected to sharp uplift and tilting perturbations and dissolution processes, associated with hydrothermal dike-swarm injection. The whole framework is sealed with the Tamjout Bed (Tifnout Member), which marks the end of abundant regional tilting (syn-rift phase) and the beginning of a thermal subsidence-dominated regime (post-rift phase) in the Moroccan margin of West Gondwana.     

Stable isotope study of base metal deposits from the Eastern High Atlas,Morocco

Tertiary epigenetic lead ± zinc and copper mineralizations occur in the Mesozoic carbonate cover and stibnite, barite and copper mineralization in the Precambrian to Palaezoic basement in the Eastern High Atlas, Morocco. The carbon isotope data from host carbonates range from +3 to -1‰ PDB. The data are typical for marine carbon. There is no difference between surrounding dolomites and younger vein dolomites. The oxygen isotopic composition of the samples is enriched in ¹⁶O; the range is from -3 to -11‰ PDB with respect to that of carbonates in equilibrium with marine water. It is postulated that the isotopic composition was changed during dolomitization being caused by transition of connate formation waters diluted to some degree by meteoritic water. The bacteriogenically reduced sulphur from syngenetic sulphides and Mesozoic marine sulphate mobilized during Tertiary orogenesis are thought to be the source of H₂S and SO₄. Cambro-Ordovicien sulphate is also supposed to be the source of sulphur in the Jebel Zelmou barite deposit in the basement. Isotopic composition of lead from galenas in the Tertiary deposits from the Mesozoic cover are strikingly homogenous. Model ages vary between 230–180 Ma. It is suggested that important mobilization processes are responsible for Tertiary metallogeny in Morocco.