Geology of Morocco

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The oldest rocks in Morocco are the Jbel Ouiharem augen gneiss and Oued Assemlil gneiss. The augen gneisses and metadolerite of the Zenaga Series experienced composite foliation, likely related to an ancient orogeny. The Zenaga Series is intruded by Paleoproterozoic granitoids, giving a young age constraint within the Precambrian. Granites from the Anti-Atlas Mountains yielded similar ages.

North of Kerdous, the Quartzite Series formed thick quartzite layers, along with siltstone, pelite sandstones and conglomerates in the Neoproterozoic, with intercalated stromatolite limestones. Sills and laccoliths of dolerite and a gabbro tholeiitic magma series intruded the Quartzite Series along sedimentary bedding.

Morocco was affected by the Pan-African orogeny, which produced the Ouarzazate Series molasse deposit.^[1]

The Adoudounian Series overlies the Ouarzazate and marks the start of the Cambrian, formed in parallel with the rapid expansion of multicellular life. The base of series is conglomerate, followed by marl, sandstones and additional carbonate sequences. The Amouslek Formation, within the Adounian Series is made up of shale and limestone and is laden with trilobite and archaeocyathid fossils, from an Early Cambrian shallow marine environment. The Goulimine Quartzitic Series from the Middle Cambrian also contains trilobites, although the Late Cambrian is not exposed.

Sandstones, micaceous clays and some limestones form Ordovician strata overly the Cambrian, containing trilobite and graptolite fossils.^[2] The Late Ordovician and the start of the Silurian is marked by a glaciation, recorded in tillite. Silurian strata is common in the central Anti-Atlas, represented by sandstones, shales and dark mudstones that sometimes contain carbonate nodules. Black shales in the eastern Anti-Atlas host some graptolites, lamellibranch and nautiloid fossils. Devonian mudstones with limestone beds unconformably overly the Late Silurian in the western Anti-Atlas, with brachiopod, conodont and tentaculite fossils, while basalts are found in the east. Carboniferous form the northern edge of the Tindouf Basin and cuesta hills in the Draa plains. Condensed limestone, with cephalopod fossils from the period, was uplifted as platforms in the central and eastern Anti-Atlas.

The Anti-Atlas did not experience significant tectonic changes during the Hercynian orogeny, as Euramerica and Gondwana collided to form the supercontinent Pangaea. The mountains have very little, if any metamorphism, from the time and no Hercynian granites.

The Meseta Domain, taking its name from Spain's Meseta Central inner plateau is an area of stable Paleozoic rock that was never affected by the Hercynian orogeny and was later covered by Mesozoic and Cenozoic sedimentary rocks. Forming Morocco's Central Massif, the Meseta Domain completely conceals Precambrian rocks, although boreholes have found Neoproterozoic rocks in a Meseta anticline. The domain is split in two by the Middle Atlas fold belt. The Western Meseta has relatively little sedimentary cover and well-developed massifs, while the Eastern Meseta spanning the border with Algeria has numerous, small Paleozoic massifs. Continuing from the Neoproterozoic until the Middle Devonian, western Morocco and the Anti-Atlas had the same depositional environment—molasse redbed deposition and post-orogeny volcanism. Southern Morocco was flooded by a massive shallow marine shelf, building up significant carbonates, mixed with continental sediments pouring in from inland areas now in the Sahara. In the Late Devonian, western Morocco and the Anti-Atlas split up into fault-bounded basins, which deformed during the Hercynian orogeny. ^[3]

The Western High Atlas preserves evidence of the formation of the Atlantic margin in North Africa. In the Late Triassic, rifting in central Pangaea began to form the Atlantic Ocean. Large alluvial fans began to fill the down-dropped grabens with fluvial sandstones, mudstones and conglomerates, intercalated with evaporite sequences of dolomite, halite and gypsum. A tholeiitic magma series formed dolerite, capping off the Triassic sequence. Clastic sediments continued to deposit into the Jurassic. Simultaneously, in the Middle Jurassic, limestone shoals formed on fault blocks and chaotic olistostrome slump deposits accumulated limestone fragments in nearby deep water.

The Rifo-Tellian Domain (also known as the Rif Domain) today extends the entire length of the Maghreb and is closely related to the Baetic System mountains in southern Spain. The sediments in the Rif Mountains deposited in the present location of Tunisia in the Triassic, as part of a microcontinent.

A large scale marine transgression in the Cretaceous, timed with subsidence in the region led to the maximum extent of seas in Morocco. By the end of the Cretaceous, a marine regression dropped sea levels in the area as the Atlas Mountains began to rise. River delta fans prograded filled in the Atlas gulf from east to west. Border faults formed, thrusting pieces of Mesozoic rock onto neighboring platforms. The uplifted sedimentary rocks in the trough began to erode into new alluvial fans, filling in marginal foredeep areas.^[4]

The Rif microcontinent shifted westward and collided with the African Plate in the Oligocene and the Miocene, generated the complex Rift overthrust. Seismic studies have found that Carnian sandstones, mudstones and conglomerates from the Triassic, lie unconformably atop the microcontinent's crystalline basement rock in north Morocco. The Miocene and Pliocene marls and carbonates of the Doukkala sub-basin are overlain by Quaternary rocks. Further west, Quaternary rocks cover Middle Cretaceous sedimentary rocks, which lie unconformably atop Triassic and Paleozoic units.^[5]