CARTE GÉOLOGIQUE DU MAROC AU 1/50 000: FEUILLE BOUMALNE

The Boumalne sheet covers the northern margin of the Anti-Atlas, which is made out of the precambrian Saghro Massif and its paleozoic cover, the western part of the Ouarzazate Basin and the southern margin of the central High Atlas. The oldest Saghro basement outcrops in the Boumalne window and is characterized by two different units referable to the middle Neoproterozoic or Cryogenian (NP2-PII) : i) arenaceous-pelitic metasediments with few flows and dikes of tholeitic basalt, metamorphosed and folded at a regional scale during the panafrican orogeny ; ii) some intrusive calc-alkaline bodies, slightly metamorphosed (Imider : 675 Ma, Tiboulkhirine : 645 Ma). This middle neoproterozoic basement is discordantly covered by thick volcanics and volcano-clastics (Group of Ouarzazate), and truncated by the subvolcanic and intrusive bodies of Tiounouine and of Aguensou, which are not metamorphosed nor folded, and can be related to the upper Neoproterozoic or Ediacaran (NP3-PIII). The upper neoproterozoic magmatism forms a post-collisional sequence characterized by calc-alkaline affinity with high content of K. The Precambrian of the Saghro is cut by a network of frequently mineralized faults and by dikes of various nature and at its margins is unconformably covered by the paleozoic sediments. The Paleozoic corresponds to a reduced sedimentary sequence (middle Cambrian - lower and middle Ordovician) which outcrops only in the eastern part of the sheet. The cambroordovician sequence corresponds to platform silico-clastic deposits with alternating arenaceous and areno-pelitic formations, therefore indicating a bathymetry varying between intertidal and open marine. Its base, lacking the lower Cambrian, is unconformable on the proterozoic basement and the Ordovician is separated from the middle Cambrian by the classic upper cambrian/tremadocian hiatus. Towards the top, the Ordovician and the upper Paleozoic are cut by the unconformity following the hercynian orogeny, which is marked only by an alternation of sequences and by minor deformations. The Paleozoic rapidly disappears towards the West and the upper unconformity directly cuts the Saghro basement. This area without Paleozoic is called “Imider threshold”, and it appears of having had an important role in the tectono-sedimentary control and in the absence of the paleozoic sequence of the northern Saghro. The meso-cenozoic sequence is made of marine and continental sediments which indicate well defined paleogeographic and structural environments. They outcrop in a large area covering about three quarters of the map. The lower part of the sequence, Jurassic in âge, carbonatic and deposited in a marine environment, represents the filling of basins formed in the “Atlasic“ rifting phase, while the upper part, Cretaceous to Pliocene in âge, represents both the cover of the High Atlas and the filling of the Ouarzazate basin. A hiatus separates the jurassic from the cretaceous deposition, which starts in the Cenomanian with a continental sequence extending in the turonian ocean. In the upper Cretaceous the sequence indicates syntectonic characters and corresponds to an alternation of continental and marine platform formations deposited at variable depths. The upper paleocene to middle eocene sediments, which unconformably lie on the middle-upper cretaceous continental- marine sequence, are characterized, in the majority, by coastal marine sediments. In the upper Eocene, the sediments are predominantly continental. At present, due to the alpine orogeny, the Ouarzazate basin rests at the front of the High Atlas, which limits and partially overthrusts it at its northern edge. The cretaceous to quaternary formations that fill it towards the South, lie unconformably on paleozoic and precambrian rocks of the Anti-Atlas.