Marrakech High Atlas Research Articles

Reply to the comment on "Geochemistry, Rb-Sr whole rock age and Sr-Nd isotopic constraints on the Variscan A-type granite from Azegour area in the Marrakech High Atlas (Moroccan Meseta) and their geodynamic implications" by Hadani et al. (2024)

Reply to the comment on "Geochemistry, Rb-Sr whole rock age and Sr-Nd isotopic constraints on the Variscan A-type granite from Azegour area in the Marrakech High Atlas (Moroccan Meseta) and their geodynamic implications" by Hadani et al. (2024)

Comment on "Geochemistry, Rb-Sr whole rock age and Sr-Nd isotopic constraints on the Variscan A-type granite from Azegour area in the Marrakech High Atlas (Moroccan Meseta) and their geodynamic implications" by Hadani et al. (2024): Geologos 30, 1 (2024): 1–16

Comment on "Geochemistry, Rb-Sr whole rock age and Sr-Nd isotopic constraints on the Variscan A-type granite from Azegour area in the Marrakech High Atlas (Moroccan Meseta) and their geodynamic implications" by Hadani et al. (2024): Geologos 30, 1 (2024): 1–16

Geochemistry, Rb-Sr whole rock age and Sr-Nd isotopic constraints on the Variscan A1-type granite from Azegour area in the Marrakech High Atlas (Moroccan Meseta) and their geodynamic implications

In the northern part of the Marrakech High Atlas (MHA), along the southern Variscan segment of the Western Meseta, a Variscan granitic intrusion crops out, intruding metasediments and meta-volcanosedimentary rocks of Early Cambrian to Ordovician age. A new whole-rock Rb-Sr isochron age of 268 ± 9 Ma for the granite, combined with a previously published whole-rock Rb-Sr radiometric dating (271 ± 3 Ma), reveals a post-kinematic (tectonic) character with regard to the main Variscan deformational event, belonging within the tectonic context of the Moroccan Variscan orogenic belt. Geochemically, the Azegour intrusion is metaluminous to peraluminous and exhibits a calc-alkaline affinity with a ferruginous composition. The massif shows an extremely differentiated character (SiO2 = 77.53–78.14 per cent), K2O and high total alkali contents, FeOt/(FeOt + MgO) and Ga/Al ratios, which have typical characteristics of an A-type granite. In addition, the granite contains high concentrations of LREE (LaN/SmN= 7.9–13.67) relative to HREE (LaN/YbN= 4.81–11.61) and a well-defined Eu negative anomaly (Eu/Eu* = 0.44–0.75). The granitic samples exhibit a strong enrichment of the most incompatible elements (RbN/YbN = 69.84–159.98) and a strong depletion of Ba, Sr, Eu, Nb, P and Ti. These characteristics are similar to those of A1-type granites. The absence of mineralogy typical of an S-type granite, combined with its weakly peraluminous character [A/CNK (molar Al2O3/CaO+Na2O+K2O) = 1,013–1,045], suggest that there is little or no significant involvement of supracrustal sources in the petrogenesis of the intrusion studied. Despite the strongly differentiated character of Azegour granitic rocks samples, their multi-element patterns shows many similarities to those of I-type granitoids, which has led to postulate that the parental liquids of A1-type were derived from partial melting of mafic magmas. The representative samples studied show less depleted εNd(t = 270 Ma)values of –0.94 to –4.85 and lower positive to slightly negative εSr(t = 270 Ma) values of –1.45 to 9.32. The isotopic data suggest that the Azegour granite was emplaced 270 myr ago, apparently generated by partial melting of a mafic/intermediate magma source in the lower crust as a result of the underplating of the asthenosphere mantle-derived Oceanic Island Basalt-like magmas. Alternatively, their isotopic signatures also can be attributed to the interaction and/or hybridisation of basaltic liquids derived from the mantle with these lower crust materials. The generated parental magma probably occurred at deep structural levels and involved fractional crystallisation processes by the separation of a mineralogical association composed of plagioclase + potassium feldspar ± biotite ± amphibole ± sphene ± apatite. The whole-rock Rb-Sr age of 268 ± 9 Ma, whole-rock geochemistry and Sr-Nd isotopic compositions of εNd(t = 270 Ma) and εSr(t = 270 Ma), combined with fieldwork data, suggest that the Azegour granite was emplaced.

Late Permian to early Middle Triassic palaeoenvironmental reconstructions of the High Atlas, Morocco: Geodynamic and climate implications in the intertropical western Peri-Tethyan domain

This study characterizes the depositional environment and climate evolution from the late Permian to the early Middle Triassic, based on detailed sedimentological and palaeosol analysis. Sequence stratigraphy and palaeoenvironmental reconstructions were used to define the Permian-Triassic (P-T) evolution of the High Atlas basins as well as to make a comparison with European basins.The late Permian in Morocco is dominated by distal fluvial fan environment, under arid to semi-arid paleoclimate conditions. Above the P-T unconformity (BS1), the Lower Triassic deposits, record a similar climate evolution from arid to subhumid conditions. In the Argana Basin, the Early Triassic starts by an aeolian deflation lag, followed by stacked debris flow lobes and sheetflood ephemeral alluvial fan deposits, showing immature to locally mature calcretes. The Marrakech High Atlas basins are further characterized by well-developed aeolian dunes in alluvial fan environment. These arid environments evolve through time from proximal to distal deposits, as a retrogradational trend, until well-developed floodplain or lacustrine deposits under semi-arid to subhumid climate (maximum flooding surface, MFS). From MFS, the evolution from distal to proximal environments, testified the progradational trend which is capped by a newly defined angular unconformity (BS2). Above this tectonically induced BS2, the sedimentary section, dated as Anisian is characterized by palaeoenvironments that include meandering river fluvial packages deposited in a semi-arid floodplain.Basin configuration, border fault direction, palaeoenvironments and palaeocurrents recorded in the Argana Basin confirm an independent system, separate from the Marrakech High Atlas (MHA) area during the Triassic. Above BS1, remnant topographies resulted in discrete catchment and discrete drainage systems are evidenced by palaeocurrent and palaeowind measurements controlled by border faults. Palaeoenvironment reconstructions show that the MHA was made up of numerous independent basins during the Early Triassic and the remnant topographies are progressively infilled until BS2. After the BS2, the palaeo-drainage became axial and basins were probably connected in the MHA. Above the P-T unconformity, a common climate evolution is observed in Morocco during the Lower Triassic until BS2, which could correspond to the European Hardegsen unconformity. The sedimentation record shows a similar change in climate and geodynamics to that recorded from both Europe and Morocco.

Mapping and analysis of structural lineaments using SRTM radar data and Landsat-8 OLI image: an example from the Telouet–Tighza area, Marrakech High Atlas, Morocco

This study provides the first evaluation of the potential of both the Landsat-8 Operational Land Imager (OLI) sensor images and the Shuttle Radar Topography Mission (SRTM) Digital Elevation Model (DEM) data for automated lineament extraction in the south side of the Marrakech High Atlas (Telouet–Tighza area). After image corrections, enhancement methods such as principal component analysis, band composite (BC), and directional filtering were adopted to create new images that provided high visibility of linear structures. The new Landsat BC image used in this study was selected based on the calculation of the optimum index factor and correlation index. In addition to the Landsat image, the SRTM DEM was used to detect structural lineaments in the area by generating shaded relief images. Multisource data, such as band ratio image, geological maps, and fieldwork, were used to eliminate the nongeological lineaments extracted. The results indicate that an automated method was applied successfully for lineament mapping in this area, by detailing the main tectonic faults. Moreover, new lineaments are identified and are validated by fieldwork. Structural lineaments extracted show compatibility in their direction, length, distribution, and density with the tectonic evolution of the study area. A total of 2945 lineaments were extracted with major ENE–WSW and predominant E–W directions. The new structural map shows more structural information compared with the geological map of this area and exemplifies the performance of Landsat-8 OLI bands and SRTM data in this kind of study.

Climate of the Marrakech High Atlas, Morocco: Temperature lapse rates and precipitation gradient from piedmont to summits

ABSTRACT Understanding mountain climates poses many challenges, because difficult terrain leads to a sparsity of weather stations and therefore poor data availability, meaning the detailed information required to understand these complex systems is lacking. Here, we analyze eleven years of half-hourly climate observations from the Joint International Laboratory LMI-TREMA (Télédétection et Ressources en Eau en Méditerranée semi-Aride) network of weather stations in the Marrakech High Atlas, Morocco, providing detailed information about the climate in this area. Our analysis shows the mean annual near-surface temperature lapse rate is −4.63°C km−1, with an uncertainty range of −4.39 to −4.85°C km−1, lower than the standard environmental temperature lapse rate. Mean temperature lapse rates vary from −3.67°C to −5.21°C km−1 monthly, and throughout the day from −2.75°C to −7.1°C km−1, which has important implications for understanding snowpack variations at the highest elevations. Understanding precipitation is inherently complex, but our analysis shows that mean annual precipitation increases by 166 mm km−1 (150.6 to 183.7 mm km−1) with a significant snow component at the highest elevations. This analysis improves our understanding of the mountain climate system with new regional temperate lapse rates and precipitation gradients, having the potential to improve gridded climatologies and climate models, with relevance for the wider High Atlas region.

Palynological evidence from a sub-alpine marsh of enhanced Little Ice Age snowpack in the Marrakech High Atlas, North Africa

The grazing lands of the High Atlas are vulnerable to climate change and the decline of traditional management practices. However, prior to the mid-20th century, there is little information to examine historical environmental change and resilience to past climate variability. Here, we present a new pollen, non-pollen palynomorph (NPP) and microcharcoal record from a sub-alpine marsh (pozzine) at Oukaïmeden, located in the Marrakech High Atlas, Morocco. The record reveals a history of grazing impacts with diverse non-arboreal pollen assemblages dominant throughout the record as well as recurrent shifts between wetter and drier conditions. A large suite of radiocarbon dates (n = 22) constrains the deposit to the last ~ 1,000 years although multiple reversed ages preclude development of a robust age-depth model for all intervals. Between relatively dry conditions during the Medieval period and in the 20th century, intervening wet conditions are observed, which we interpret as a locally enhanced snowpack during the Little Ice Age. Hydrological fluctuations evidenced by wetland pollen and NPPs are possibly associated with centennial-scale precipitation variability evidenced in regional speleothem records. The pollen record reveals an herbaceous grassland flora resilient against climatic fluctuations through the last millennium, possibly supported by sustainable collective management practices (agdal), with grazing indicators suggesting a flourishing pastoral economy. However, during the 20th century, floristic changes and increases in charcoal accumulation point to a decline in management practices, diversification of land-use (including afforestation) and intensification of human activity.

A new ichnofauna from the Permian of the Zat Valley in the Marrakech High Atlas of Morocco

A new ichnofauna from the Permian of Morocco is described in details: it is the first Palaeozoic ichnofauna from the Zat Valley in Marrakech High Atlas. The new tracksite was found in the Tighdouine region, in the middle-upper Permian of the Cham-el-Houa Siltstone Formation. An abundant and diverse ichnoassemblage composed of both protostomian (probably arthropods and annelids) traces and vertebrate tracks is recorded. The presence of protostomian burrows and traceways, associated with tetrapod tracks corresponds to the Scoyenia ichnofacies. Protostomian traces are ascribed to Diplichnites gouldi, Diplopodichnus biformis, Scoyenia cf. gracilis and Spongeliomorpha carlsbergi. Tetrapod tracks include more than 70 tracks attributed to Amphisauropus, Erpetopus, Hyloidichnus, Characichnos, and indeterminate tracks. The co-occurrence of tetrapod tracks (both walking and swimming tracks), protostomian traces, mudcracks, ripple marks, as well as the lithological features of the track-bearing levels, indicate regularly inundated depositional environments during periods of high discharge under a local seasonal climate. A sedimentological analysis shows that the depositional environments evolved from braided-meandering systems to alluvial floodplains. The track-bearing surfaces are mainly preserved in crevasse splays, levees and pond deposits. This newly discovered ichnofauna helps to better reconstruct the palaeoenvironments of the Marrakech High Atlas in Morocco during the Permian, and enlarges the palaeogeographic distribution of important ichnotaxa.

Reconstructing Extensional Basin Architecture and Provenance in the Marrakech High Atlas of Morocco: Implications for Rift Basins and Inversion Tectonics

AbstractThe Moroccan Atlas developed through a multiphase tectonic history including Permian‐Jurassic continental rifting and Cretaceous‐Cenozoic intracontinental shortening. New detrital zircon (DZ) U–Pb geochronologic results from the Ourika‐Zat region in the Marrakech High Atlas constrain the sediment provenance of prerift sediment sources and synrift and postrift basin fill. Prerift Precambrian‐Cambrian units contain Neoproterozoic DZ from the Pan‐African orogeny. Ordovician rocks contain diagnostic 0.95‐ to 1.28‐Ga DZ that are absent in the West African Craton, suggesting distal sediment sources. Permian rift basin fill contains Permian zircons that constrain maximum deposition age and diagnostic of northern Meseta domain sediment sources. New mapping and cross‐section construction demonstrate that Permian basin fill accumulated in a half‐graben above a southward dipping normal fault that likely guided subsequent inversion. Triassic rift basin fill thickens southward, above a likely northward dipping normal fault. The change in rift basin geometry was accompanied by catchment reorganization, the introduction of southern rift flank sediment sources, and subbasin integration. The southern catchment expanded to encompass Meso‐Archean rocks of the Reguibat Shield situated >800 km southward. Existing and new provenance results from postrift strata document onset of recycling of Atlas rift basin fill during the mid‐Cretaceous and before the onset of compressional inversion. These results highlight the interaction between deformation and sediment routing in an extensional basin and sediment dispersal patterns across continental margins during multiphase deformation. Stratigraphic, structural, and provenance characteristics reveal that the Atlas rift shares similarities with high‐angle and supradetachment basin models, emphasizing the diverse nature of extensional styles.

Detrital zircon U–Pb provenance and palaeogeography of Triassic rift basins in the Marrakech High Atlas

AbstractThe Marrakech High Atlas contains some of the best exposures of the Triassic early‐rift strata related to Atlantic opening in NW Africa. We present the first detrital zircon U–Pb data of five Triassic redbed samples from the Tizi n'Test basin to quantify sediment provenance, transport and dispersal patterns during early rifting. These U–Pb ages document dominant sediment sourcing from the south, the Anti‐Atlas domain, with very limited to absent input from the Variscan Meseta domain to the north. This combined with stratigraphic and thermochronologic information points to a highly asymmetric palaeogeography during Triassic rifting. Furthermore, the occurrence of Archaean detrital zircon grains in Triassic sandstone, likely recycled from the Reguibat shield, suggests the presence of a fully developed regional drainage system with rivers and catchments reaching hundreds of kilometres into the hinterland of the rift flank.

Polyphased Inversions of an Intracontinental Rift: Case Study of the Marrakech High Atlas, Morocco

AbstractThe High and Middle Atlas intraplate belts in Morocco correspond to Mesozoic rifted basins inverted during the Cenozoic during Africa/Eurasia convergence. The Marrakech High Atlas lies at a key location between Atlantic and Tethyan influences during the Mesozoic rifting phase but represents today high reliefs. Age and style of deformation and the mechanisms underlying the Cenozoic inversion are nevertheless still debated. To solve this issue, we produced new low‐temperature thermochronology data (fission track and [U‐Th]/He on apatite). Two cross sections were investigated in the western and eastern Marrakech High Atlas. Results of inverse modeling allow recognizing five cooling events attributed to erosion since Early Jurassic. Apart from a first erosional event from Middle/Late Jurassic to Early Cretaceous, four stages can be related to the convergence processes between Africa and Europe since the Late Cretaceous. Our data and thermal modeling results suggest that the inversion processes are guided at first order by the fault network inherited from the rifting episodes. The sedimentary cover and the Neogene lithospheric thinning produced a significant thermal weakening that facilitated the inversion of this ancient rift. Our data show that the Marrakech High Atlas has been behaving as a giant pop‐up since the beginning of Cenozoic inversion stages.

Cartographie Des Risques De L’erosion Hydrique Par L’equation Universelle Revisee Des Pertes En Sols, La Teledetection Et Les Sig Dans Le Bassin Versant De L’ourika (Haut Atlas, Maroc)

Covering an area of 576 square kilometers, the Ourika watershed is a sub-watershed of the large Tensift basin, located on the northwest slopes of the Marrakech High Atlas, in mid-west Morocco. This basin of dramatic topography, with sparse vegetation cover and friable substrates, is under increasing human action exacerbated by a variable and changing climate. Its vulnerability to water erosion is quite high, increasing risks of wadi flows with significant sediment loads. The aim of this work was to quantify soil loss in the basin using the Revised Universal Soil Loss Equation (RUSLE) and Geographic Information System. The results showed that the Ourika watershed was subject to strong climatic aggressiveness ranging from 55.22 to 100.57 MJ.mm/ha.h. The average soil erodibility value, K, was 0.48 t.ha.h/ha.MJ.mm, with a standard deviation of 0.28 t.ha.h/ha.MJ.mm. Slopes with values higher than 35% represented 72% of the watershed’s area, with the topographic factor, LS, values ranging from 0.01 to 94.5. The vegetation factor was high throughout the Ourika watershed while C values were higher than 0.5 for 73% of the watershed’s area. The average soil loss obtained in the basin was 380 t/ha/year. These results indicated that 48% of the watershed’s area was subject to a soil loss between 50-400 t/ha/year, and between 400 and 1000 t/ha/year for 30% of the watershed. Soil loss below the tolerance level (<7 t/ha/year) represented only 4% of the watershed area. These findings served in highlighting the significance of erosion in the Ourika watershed.

Magnitude of rift‐related burial and orogenic contraction in the Marrakech High Atlas revealed by zircon (U‐Th)/He thermochronology and thermal modeling

AbstractThe Atlas of Morocco is a continental rift developed during the Triassic‐Jurassic and moderately inverted during the Cenozoic. The High Atlas south of Marrakech, with exposures of basement and Triassic early synrift deposits, has been viewed as a high during the Mesozoic rifting. First zircon (U‐Th)/He ages and thermal models obtained from 42 samples in the Marrakech High Atlas following two NNW‐SSE transects across the mountain belt reveal that in contrast to previous models, the Triassic‐Jurassic rift was well developed in the Marrakech High Atlas (with more than 4.5–6 km of rift‐related deposits). Middle Jurassic‐Early Cretaceous zHe cooling ages obtained indicate that rift‐related subsidence in the Marrakech High Atlas finished in the Middle Jurassic and was followed by a period of exhumation where 2–3 km of rock were eroded. Thermal models from zHe data provide the first thermochronologic clue for a Late Cretaceous initiation of the Atlas compression‐driven exhumation in the inner parts of the Marrakech High Atlas. The Triassic‐Jurassic basin reconstruction assisted by thermochronology highlights a key role of inherited basement anisotropy in rift orientation and evolution, and on its subsequent inversion. Comparison of present‐day and restored sections to the rifting stage aided by thermochronology suggests minimum values of total orogenic shortening in the Marrakech High Atlas of 13 to 14 km (21 to 17%), with exhumation of 1 to more than 5 km of rocks. Similar zHe ages on both sides of the Tizi n'Test fault evince minor vertical movements along the fault during the Atlas orogeny.

Environmental implication of subaqueous lava flows from a continental Large Igneous Province: Examples from the Moroccan Central Atlantic Magmatic Province (CAMP)

The Late Triassic-Early Jurassic volcanic sequence of the Central Atlantic Magmatic Province (CAMP) of Morocco is classically subdivided into four stratigraphic units: the Lower, Middle, Upper and Recurrent Formations separated by intercalated sediments deposited during short hiatuses in volcanic activity. Although corresponding to a Large Igneous Province formed in continental environment, it contains subaqueous lava flows, including dominant pillowed flows but also occasional sheet flows. We present a study of the morphology, structure and morphometry of subaqueous lava flows from three sections located at the Marrakech High-Atlas (regions of Aït-Ourir, Jbel Imzar and Oued Lhar-Herissane), as well as an analysis of the sediments, in order to characterize them and to understand their environmental meaning.The analysis of clays by the diffraction method X-ray revealed the presence of illite, mica, phengite, céladonite, talc and small amounts of quartz, hematite, calcite and feldspar, as well as two pairs of interbedded irregular (chlorite Smectite/chlorite-Mica). Fibrous minerals such as sepiolite and palygorskite were not detected.The peperite of Herissane region (Central High Atlas) provided an excellent overview on the factors favoring the magma-sediment interaction. These are the products of a mixture of unconsolidated or poorly consolidated sediments, low permeability with a low viscosity magma. The attempt of dating palynology proved unfortunately without results.

Pleistocene fluviatile deposits in the Ourika drainage basin (Marrakech High Atlas, Morocco): indicators of climatic variations associated with base level change

Aggradational packages of alluvial sediment are preserved in the Ourika river basin. Field mapping and DEM analysis allowed us to identify a model sequence of river terraces. We focus on sites where the study of sedimentary sections allow us to decipher the sedimentary evolution of the Ourika watershed. Quaternary evolution of the drainage basin is characterized by major phases of sediment accumulation and erosion, forming alluvial fans and cut-fill terraces. More intense rainfall events during the Middle Pleistocene Ourika drainage basin resulted in increased erosion and transport of sediment from the hillslopes into the trunk river. First, the cut-fill terraces near the sub-basins outlets are formed by a large-scale aggradation, followed by a main vertical incision and lateral erosion. Then, the second sedimentation period was probably a result of increased precipitation that caused landsliding in steep sub-basins. Finally, a last stage of incision in the Ourika Valley is linked to a base level lowering due to climatic fluctuations. We suggest that cyclic climatic fluctuations superimposed on a continuous uplift of the High Atlas are responsible for the generation of stepped terraces along the Ourika River. Sub-basins steep affected by erosion processes dominated by landslides rocky shallow were accompanied by debris flows along convex profiles at their downstream end and associated with steep knickpoints. We interpreted erosion of the Pleistocene deposits as the result of an autocyclic negative feedback such as exhaustion of the hillslope sediment stocks and the resulting increase of the relative capacity of the trunk stream to bring and transfer sediment towards the Ourika Valley.

The three main steps of the Marrakech High Atlas building in Morocco: Structural evidences from the southern foreland, Imini area

The timing of Cenozoic deformation in the intraplate orogen of the High Atlas of Morocco is still a matter of debate. In this work, the deformation calendar has been studied in the southern foreland of the Marrakech High Atlas. Using a structural approach and the mapping of the Cenomanian–Turonian reference layer, we propose a coherent chronology of deformation that proceeded in three stages: (i) a Late Eocene event that set local and minor structures, barely visible nowadays, but important enough to trigger the record of a first detrital pulse; (ii) an Early(?)–Middle Miocene event that is not related to important shortening, setting gravity “sheets” that provoke cover deformation and feed a second detrital pulse; (iii) a Late Pliocene–Quaternary event that is mainly observed through the activity of basement features like the South Atlas Front of the Imini Fault in our studied area and reworks earlier structures. This calendar can partially fit to the geodynamic changes occurring in the western Mediterranean realm during the Cenozoic. Specifically, the Early(?)–Middle Miocene stage of deformation is suggested to be intrinsic to Morocco.

The inverted Triassic rift of the Marrakech High Atlas: A reappraisal of basin geometries and faulting histories

The High Atlas of Morocco is an aborted rift developed during the Triassic–Jurassic and moderately inverted during the Cenozoic. The Marrakech High Atlas, with large exposures of basement and Triassic early syn-rift deposits, is ideal to investigate the geometries of the deepest parts of a rift, constituting a good analogue for pre-salt domains. It allows unraveling geometries and kinematics of the extensional and compressional structures and the influence that they exert over one another. A detailed structural study of the main Triassic basins and basin-margin faults of the Marrakech High Atlas shows that only a few rift faults were reactivated during the Cenozoic compressional stage in contrast to previous interpretations, and emphasizes that fault reactivation cannot be taken for granted in inverted rift systems. Preserved extensional features demonstrate a dominant dip-slip opening kinematics with strike-slip playing a minor role, at variance to models proposing a major strike-slip component along the main basin-bounding faults, including faults belonging to the Tizi n'Test fault zone. A new Middle Triassic paleogeographic reconstruction shows that the Marrakech High Atlas was a narrow and segmented orthogonal rift (sub-perpendicular to the main regional extension direction which was ~NW–SE), in contrast to the central and eastern segments of the Atlas rift which developed obliquely. This difference in orientation is attributed to the indented Ouzellarh Precambrian salient, part of the West African Craton, which deflected the general rift trend in the area evidencing the major role of inherited lithospheric anisotropies in rift direction and evolution. As for the Cenozoic inversion, total orogenic shortening is moderate (~16%) and appears accommodated by basement-involved large-scale folding, and by newly formed shortcut and by-pass thrusting, with rare left-lateral strike-slip indicators. Triassic faults commonly acted as buttresses.

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 lm 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 (Si7.92Al0.08)(Mg2.15Al1.4Fe0.4Ti0.05h1)(Ca0.03 Na0.08K0.04)O21, while the hydrated form could be formulated as (Si7.97Al0.03)(Mg2.17Al1.46Fe0.40Ti0.05)(Ca0.03Na0.07K0,03) O20.18(OH)1.94(OH2)3.8812.43H2O. These formulas showthat 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.

Transpressional tectonics in the Marrakech High Atlas: Insight by the geomorphic evolution of drainage basins

The Ouzzelarh Massif extends across the Marrakech High Atlas (MHA) and forms the highest elevated mountain belt. To better understand the evolution of collision-related topography, we present the results of a geomorphological study in which elevation changes generated by reactivated pre-Alpine (Variscan and Triassic–Jurassic) faults drive a landscape evolution model. We aim to evaluate the relationship between the geometry of the drainage network and the main fault systems in this region. New insight into geomorphological changes in drainage patterns and related landforms is based on geological fieldwork combined with DEM analysis. To quantitatively measure landscape features we used several classical geomorphic indices (spacing ratio, hypsometric curves and integral, stream frequency drainage, stream length-gradient). The Ouzzelarh Massif is bounded to the north by the Tizi N'Test Fault Zone (TTFZ) and to the south by the Sour Fault Zone (SFZ). These faults delimit a pop-up structure. By using the above geomorphic parameters, we ascertained that the Ouzzelarh Massif is affected by a high spatial variability of uplift. The actual landscape of the Ouzzelarh Massif reveals remnants of an uplifted ancient erosional surface and the heterogeneity of exposed rocks in the range explaining the possibility that the topographic asymmetry between north and south flanks is due to differences in lithology-controlled resistance to erosion. Drainage, topography and fault pattern all concur to show uplifted rhomboidal-shaped blocks. It exhibits high stream frequency drainage and uplift in separate tectonically-uplifted blocks such as Jebel Toubkal which is characterized by asymmetric drainage basins.

Quaternary evolution of the Marrakech High Atlas and morphotectonic evidence of activity along the Tizi N'Test Fault, Morocco

Here, we consider the Ourika and Rheraia drainage basins on the Tizi N'Test Fault zone in the mountains of the Marrakech High Atlas (western Morocco) with regard to Late Pleistocene tectonic activity. New insights into geomorphological changes in drainage patterns and related landforms are based on geological fieldwork in conjunction with DEM analysis. Lithological and structural data combined with certain geomorphometric indices provide clues to the ongoing uplift of Quarternary surfaces in this region. Five geomorphological indices are utilized: 1) drainage network, 2) shape of stream long profiles, 3) hypsometric integral and curves, 4) valley-floor width valley-height ratio (Vf index), and 5) stream gradient index (SL index). We also considered the temporal evolution of alluvial-deposit complexes with diverse lithofacies, such as debris flows, channel gravels, rockslide-debris avalanche, stratified slope deposits, terrace gravels, and fan deposits in the Ourika and Rheraia valleys. Pleistocene talus deposits and fluvial sediments are offset by the Tizi N'Test Fault in the Upper Ourika and Upper Rheraia valleys. Such deformation of thick, continental deposits strongly points to thrust reactivation along the Tizi N'Test Fault. We define the chronology and overall aggradation phases, or lateral incision phases, showing how they are the consequences of variations in tectonic uplift and climate. As a result, we are better able to access recent morphotectonic evolution in part of the Marrakech High Atlas.