Chellai El Hassane (Morocco), Ghanmi Mohamed (Tunisia) and Doblaas Miguel (Spain)
The Triassic terrestrial deposits at the northern edge of the High Atlas near Marrakech are mainly represented by thick sequences of massively layered, red sandstone. These are topped by a formation of silt and pink-brown clay containing large deposits of evaporites consisting mainly of rock salt and gypsum. The silt and clay formations form domed structures characterised by intruded gypsum and irregular (disharmonic) folds capped by fine sandstone beds, as well as by small, isolated anticlines only a few metres in scale. The direction of folding shows no relationship to that of the major tectonic folding that gave rise to the Atlas Mountains. In contrast, the folding is closely linked to the deposition of rock salt and gypsum in the High Atlas near Marrakech during the Late Triassic. The same phenomenon is observed in the passive margins of the Atlantic of western Morocco.
These Triassic formations are the most prominent features of the landscape, with thicknesses that can reach up to 400m. They consist essentially of two formations: F5 (the Oukaïmeden sandstone) and F6 (the Superiors Silts), which correspond to the uppermost part of the Triassic, as defined in the Ourika valley by Biron (1982). The first formation consists of thick (400m) beds of detrital sandstones with fine to medium-sized, diamond-shaped sedimentary bodies, interbedded with layers (a few centimetres to several metres thick) composed of red clay as well as red and brown silts.
These are overlain by the second formation (50 to 90m thick) consisting of red silts, clays and brown argillites with significant deposits of evaporites, including rock salt and gypsum. These two formations are both of Late Triassic age. Formation F5 is placed within the Carnian at the base of the Late Triassic (Benaouiss et al., 1996), while formation F6 is placed at the boundary of the Carnian and Norian, as seen in the Argana Valley in southern Morocco (Tourani et al., 2000).
On the northern side of the High Atlas near Marrakech, the upper silt formation is characterised by large deposits of rock salt and gypsum. These evaporites consist of domes, folded sandstone beds and isolated small anticlines. Such dome structures can be found near Ait Ourir, a town close to Marrakech (Ferrandini & Le Marec 1982). They are formed by beds of red siltstone and brown claystone with cores made up of alternating salt and/or different types of gypsum in the form of massive slabs, tens of centimetres in thickness. The overall size of these structures is large – from metres to decametres in diameter. In places, the internal structure of the domes in formation F6 is formed by diffusion of gypsum through the silt, as can be seen in Fig. 1.
In some localities, especially near Tighadwyine (which is north of the High Atlas, 50km by road from Marrakesh) the Upper Triassic red silts contain patches of fine to medium sandstone some tens of centimetres thick, which are folded in all directions. These structures have the appearance of being caused by slumping as a result of synsedimentary tectonic instability.
In some localities, sandstone is intercalated with red fine silt evaporites, to form small anticlines a few metres across. The interiors of these are filled with silts, clays and gypsum. Gypsum occurs occasionally in the form of whitish stains from a few centimetres in size to a few metres. The anticlines are completely isolated and are not separated by synclinal structures.
Measurement (in 3D) of the different directions of the axes reveals that these structures do not show any uniform direction, but are ‘disharmonic’. This can even be seen at the scale of a single structure, which may show contrasting directions of stratification. Similarly, isolated anticlines have axes that plunge in all directions, each with different axial planes. These structures are found in large basins near Ait Ourir (Ferrandini & Le Marec 1982; Fig. 3), which can be described as ovoid, ‘synclinal megastructures’. These have traditionally been regarded as aquifers that have slipped from their original positions during the uplift of the High Atlas chain in the south. However, the axes of these structures show very different directions from what might be expected if they were the result of orogenesis.
We suggest instead that the massive deposits of evaporites of halite and gypsum in F6 facies are responsible for the creation of groups of dome structures, disharmonic folding of sandstone beds, and isolated anticlines. These structures result from the movement of enormous masses of salt and gypsum at depth, due to the pressure from the enormous mass of overlying sediments. This type of ‘salt tectonics’ is clearly not consistent with the folding axes from the Alpine orogenic phase responsible for the uplift of the Atlas range.
This phenomenon can also be seen in the halokinetic (that is uplift of hills by salt) level of seismic profiles in the Essaouira Basin west of Marrakech (Hafid et al. 2006) (Fig. 4).
About the authors
Chellai El Hassane is at the Cadi Ayyad University, Faculty of Sciences Semlalia, Geology department, PO Box: 2390 Marrakech, Morocco (firstname.lastname@example.org). Ghanmi Mohamed is at the University Tunis El Manar, Faculty of Sciences, Geology department, Campus Universitaire, 1060, Tunis, Tunisia and Doblas Miguel is at the Departamento de Geologıa, Museo Nacional de Ciencias Naturales, Consejo Superior de Investigaciones Cientıficas, 28006 Madrid, Spain.
Benaouiss N., Courel L. & Beauchamp J. (1996) Rift-controlled fluvial/tidal transitional series in the Oukaïmeden Sandstones, High-Atlas of Marrakech (Morocco). Sedimentary Geology, 107, p. 21-36.
Biron P. & Courtinat B. (1982) Contribution palynologique à la connaissance du Trias du Haut-Atlas de Marrakech, Maroc. Geobios, no 15, fasc. 2, p. 231-235, 1 pl.
Hafid M., Balley A.W., Salem A., Mridekh A. et Toto M. (2006) Rôle de la tectonique salifère dans l’évolution méso-cénozoique de la terminaison occidentale du Haut-Atlas. Notes et Mém. Serv. Geol, Maroc, n° 514, p. 103-120, 14 fig.
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Tourani. A., Lund J.J., Benaouiss N. & Gaupp R. (2000) Stratigraphy of Triassic syn-rift deposition in western Morocco in: G. H. Bachman, I Lerche (Eds.), Epicontinental Triassic. Zentrabl. Geol. Paläonto. p. 1193-1215.