| Author||G. de Koning|
|Title||Geologie des Ida ou Zal (Maroc) Stratigraphie,petrographie et tectonique de la partie SW du bloc occidental du massif ancien du Haut Atlas|
|Journal||Leidse Geologische Mededelingen|
|Abstract||The present publication deals with the geology of the area known as the Ida ou Zal, situated principally in the Palaeozoic of the Western High Atlas mountains in Morocco. The area extends grosso modo between Argana (N), Sidi Idir (E), Menizla (S) and Tirkou (W); its centre lies at 34°10 N—12°65 W.|
The stratigraphical sequence covers formations from — as we assume — the Infracambrian (loc. Adoudounien) up to and including the Trias.
In the so-called Caledonian geosyncline, extending in a roughly E—W direction along the northern edge of the African shield, series of Older Palaeozoic sediments of some 10 km thickness were deposited starting with the Basal-formation (?) of the Adoudounian (?) and reaching up to and including the Upper-Ordovician (Caradoc?).
In the Adoudounian it came to a vehement volcanism with lava extrusions probably of a basaltic or andesitic composition. The Georgian s.s. consists of marbles and limy shales, for the greater part presenting the appearance of what the French call „calcaires scoriacés”. As to the origin of this extraordinary type of rock the present author puts forward a tectonic explanation. A volcanic activity with submarine extrusions of spilitic lavas took place at the end of the Georgian, probably continuing into the beginning of the Acadian. The Lower Acadian is characterised by re-sedimented tuffs, showing graded beddings. The Sardic tectonic phase reveals itself by at least a disconformity that includes the Potsdamian; a slight angular unconformity between the Ordovician and the Acadian seems probable.
Gothlandian (Wenlock-Ludlow) and Devonian (Siegenian) only occur in overthrust sheets.
The autochthonous was folded by a Hercynian phase; the tectonic elements are directed N—S in the Western part of the area, curving to neary NE—SW in the East. The author presumes that this folding is related directly to that of the Anti Atlas, where it is attributed to the Second Erzgebirge folding-phase. The palaeozoic of the Western High Atlas may probably be considered as the Northern flank of a wide anticlinorium, the Southern offshoots of the Anti Atlas forming the Southward dipping flank; the Precambrian outcrops in the core. The curving of the tectonic elements in the Western High Atlas and in the Western Anti Atlas is analogous. A hypothesis is set forward that explains the curving through an adaptation of the tectonical elements — whilst folding — to the NW corner of the African shield.
Overthrusts developed, probably as a result of later Hercynian tectonic movements, i. c. one of the Asturian phases. A large overthrust can be followed in the field from the extreme SE of the area to its Western part. As far as could be observed the overthrust is mainly composed of Georgian and Acadian p.p. the sandy formation, presumably Acadian, in the present publication known as “Acadien gréseux probable”. Overthrusted Gothlandian and Devonian appear in the Western part of the area, partly between, partly in front of the principal mass of the overthrust. The writer supposes that the root of the overthrust will lie somewhere to the south east. A subsiding basin was formed during the greater part of the Stephanian and the beginning of the Autunian, in which lacustrine sediments — first red, later greenish — were deposited, covering a pre-existing relief. The total thickness amounts to at least 1500 m, the formation bears all characteristics of a post-orogenetic suite. It is taken that the lake can be considered as an Intramontane trough (Umbgrove) ; the longitudinal extension was parallel to the direction of the structural elements of the basement i. e. N—S. It is assumed that the whole width of the area was part of the basin.
The area was lifted and slightly folded in about E—W direction by the Post-Autunien tectonic phase (Saalian phase). The „Agadir ou Anzizen fault” originated during or after the folding; the author assumes that at this stage other E—W faults came into being as well. In the case of the Agadir ou Anzizen fault, the southern block subsided with respect to the northern block. In the area all of the Stephano-Autunien sediments N of the fault were eroded. There the Trias covers directly the pre-Stephanian Palaeozoic, while on the other side the Trias overlies Stephano-Autunien, from which it is separated by an angular unconformity.
The Trias has a typical molasse fades; it is completely composed of red sediments, attaining a thickness of about 2400 m, not counting the conglomerats at its base. The source of the sedimentary material probably must be sought in the southeast. According to the writers conception the first Triassic deposits were deltaic in a lake, covering a pre-existing relief. After the lake had been filled-up, a sandy and marly sedimentation developed, partly in lagoonal environment. The Triassic sedimentary sequence ended with the deposition of red clays that were overlain by basaltic lavas.
The dioritic volcanic vent near Zarhenrhin has probably been one of the feeders of the extrusions. The dioritic dyke system too, is probably connected with the Late-Triassic volcanic period. Perhaps there exists a relation between the iron mineralisation near Agadir ou Anzizen and the occurrence of the dioritic dykes.
The Alpine orogeny slightly folded the Triassic with an E—W direction. Principally however the Alpine tectonics revealed itself in faulting: all of the E—W faults show an Alpine shift, possibly a rejuvenation of a late-Hercynian fault system.
An erosion-plane was formed during a period of quiet, on grounds of analogy supposedly the “Surface préhammadienne”. An erosion plane on a lower level may probably be taken for the “Surface villafranchienne”. Between these two the configuration of the topography permits to distinguish some relics of intermediate levels, indicating the action of tectonic pulsations. Terraces were formed as a consequence of climatical changes during the Pleistocene.
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