Carte Geologique du Maroc au 1/50 000, feuille Imtir - Notice explicative.

The 1/50,000 Imtir sheet includes two different structural domains: the Anti-Atlas to the South, with the precambrian basement and its paleozoic cover outcropping in the Saghro Massif, and the mesozoic-Cenozoic domain (alpine) of the High Atlas to the North. The oldest Saghro basement outcrops in the window of Imiter and is characterized by twocomplexes of the middle Neoproterozoic or Cryogenian (NP2-PII): i) the sandy-pelitic metasediments, metamorphosed and folded at a regional scale during the panafrican orogeny, ii) the calc-alkaline intrusive bodies at low metamorphism of Igoudrane (677± 19Ma). This basement is unconformably covered by thick volcanic and volcano-elastic formations (Group of Ouarzazate) and cut by the unmetamorphosed and unfolded subvolcanic and intrusive bodies of Oussilkane, Bou Teglimt, Touazzakt, Arharrhitz and Bou Gafer, that can be related to the upper Neoproterozoic or Ediacaran (NP3-PIII). The upper neoproterozoic magmatism forms a post collisional sequence characterized by high K calc-alkaline content. The Saghro Precambrian is cut by a system of faults, frequently mineralized, and by dykes of basalt, andesite and ryolite. Among these, a certain importance has the large doleritic dyke, of uncertain age, which cuts the NP2 metasediments and the overlying volcanic sequences. Though with petrography and geochemistry similar to the Jurassic dolerite dyke (Foum Zguid type) that cuts the Ordovician of Arg n'Ouchne, this dolerite appears to be in turn cut by the precambrian caldera of Tizi n'Tasettift. The large underground and open air mine of Imiter is located in the northwestern part of the Precambrian basement: it is a hydrothermal deposit at Ag-Hg related to an East-West fault zone and to the emplacement of rhyolitic bodies of uppermost neoproterozoic age (553Ma). The paleozoic sequence of Imtir unconforrnably rests on the Proterozoic. It includes formations of ages ranging from the Cambrian to the Carboniferous. The sedimentary and tectonic analysis indicates a sedimentary context of a silico-clastic passive margin, controlled predominantly by eustatic (and glacial eustatic) changes, and by local tectonic movements (extensional). This double control is at the origin of several local stratigraphic hiatuses (lower Cambrian, Siluro- Devonian, upper Carboniferous), or regional (upper Cambrian/Tremadocian, upper Ashgillian), and of a sequential stacking, often of the third order. The tectonic control is due to a system of faults, probably inherited from the Proterozoic, organized in a main network trending ENE-WSW to E-W, a secondary system with a NE-SW to NNE-SSW direction, and a minor NW-SE one. From the upper Devonian, the extensional block tectonics caused a strong displacement of this northern margin of the paleozoic platform. In the NE part of the sheet is located a very unstable depocenter, limited to the South by the Saghro high and to the West by the margins of the Imider threshold. After the upper Visean, the stratigraphic sequence is involved in a regime of polyphasic hercynian tectonics, in which the major phase causes the inversion of the pre-existing paleozoic faults and some South verging thrusts, but without large displacement. During this shortening, the network of secondary faults trending ESE-WNW and NE-SW, are sometimes reactivated with minor offset, respectively dextral and sinistral. Subsequently, the region has been involved into two late fracturing episodes, one originating ESE-WNW structures and the other ENE-WSW to E-W elements. The first episode is related to the dextral late hercynian transcurrent phase, well known in Morocco, the second one probably reflects the ougartian shortening. After the Hercynian the Anti-Atlas has been emerged until the Cretaceous. But the Paleozoic of Imitir is cut by a pre cretaceous NE-SW dyke (of Arg n'Ouchene or of Foum Zguid), injected of microgranite (late or post hercynian) and later by liassic dolerites. It has also been affected by normal faults, parallel to the dyke or trending ENE-WSW, and by a sinistral reactivation. These movements can be related to the atlasic mesozoic transtensional phases. Later, the Imtir paleozoic domain remained stable, up to the Quaternary, when some neo-tectonic movements can be detected in the directional changes of the major hydrographic network. The meso-Cenozoic sequence is made up by continental and marine sediments which outcrop in a vast area, covering one fourth of the sheet. The lower part of the sequence, liassic, outcrops in limited extension in the northwestern corner of the sheet. It is represented by the filling of basins developed during the atlasic rifting phase. The upper part of the sequence, Cretaceous to Pliocene, represents the filling of the Ouarzazate basin. A hiatus separates the jurassic from the cretaceous deposition, which starts in the Cenomanian with a continental sequence and continues in the turonian sea. In the upper Cretaceous the sequence shows a syntectonic character and is made up continental formations alternated with platform marine deposits of variable bathymetry. The upper paleocene and middle eocene rocks, unconformable overlapping the continental-marine sequence of the middle-upper Cretaceous, are predominantly made of coastal marine sediments. In the upper Eocene the rocks are predominantly continental. At present, due to the alpine orogeny, the Ouarzazate basin is at the front of the High Atlas, which limits it to the North and partially overthrusts it. Towards the South, the cretaceous to quaternary formations that fill it, unconformably rest on the Anti-Atlas paleozoic and precambrian rocks.