Elements of geology for tourists in a hurry
On a local geological map we see 3 main groups:
- The Haouz
- The High Atlas of Marrakech
- The Jebilets
The city of Marrakech is built on a large sloping surface from the feet of the High Atlas in the south to the valley of the Wadi Tensift: the plain, or rather the piedmont, of Haouz. To the north extend the peeled hills of Jebilets crossed by the road to Casablanca.
The Haouz plain is made up of the products of the erosion of the High Atlas mountains by torrents. These torrential alluviums are made up of gravel and gravel and silt, particularly suitable for the cultivation of fruit trees.
The Jebilet are mainly made up of old detrital sediments deposited in the primary era, then folded and faulted and recrystallized, predominantly gray to yellowish in color. They represent the heavily eroded remains of an ancient mountain range.
The High Atlas of Marrakech is a young mountain range, contemporary with the Alps in Europe. It consists of sedimentary rocks of reddish color (sandstone, sandstone clays) or yellowish-white (limestone) mainly deposited in the marine environment during the secondary era and which were folded and fractured during the tertiary era. These rocks are based on an older set of rather grayish to yellowish color, which was folded and fractured during the primary era like the Jebilet. This “Hercynian base” outcrops in the center of the chain, as in Tizi n’Tichka, Jebel Toubkal, it includes grayish schists and in particular granites.
Elements of geology for tourists in a hurry
- The geology of the Marrakech region provides a good illustration of continental drift (plate tectonics) which can be summarized as follows:
- – the continental masses move away (extension): the liberated spaces fill with marine sediments
- – the continental masses approach and collide (convergence and compression): the sediments fold, fracture, and form a mountain chain.
In the Marrakech region, the first well-known mountain range was formed during the Precambrian era, around 1 billion years ago. It was eroded during a large part of the primary era during a period of extension and provided the materials for marine sediments of the primary era, here especially detrital sediments (pebbles, sands, etc.)
At the end of the primary era, the continental masses approach, and the sediments are deformed and build a new mountain chain, the Hercynian chain, in Europe and Morocco. Its remains are still clearly visible in the Jebilet massif and in the axis of the High Atlas of Marrakech.
Primary lands deformed by the Hercynian orogeny:
- 1. Sandstone and primary folded schites
- 2. Flaw in a pleated ensemble.
In turn, the latter is eroded during the extension period of the secondary era; marine spaces are filled with debris (detrital sediments) and limestone rocks produced by the activity of living marine beings (corals). Extension fractures allow volcanic products to rise (basalt flows).
The red layers of the Permo-Triassic:
- 3 and 4. the “Grés de l’Oukaïmeden”
- 5. Fine clayey sandstone (siltstone) and sandstone
- 6. sloping surface of the Oukaïmeden sandstone, red siltstones, and gray basalt flow.
Finally, the convergence of the continents during the tertiary era produced the uplifting of the alpine chain which extends over Europe and North Africa and of which the High Atlas of Marrakech is a part.
The current Quaternary era is again a period of calm with the erosion of the reliefs and accumulation of detrital at their feet and beyond.
The 3 roads which from Marrakech enter the High Atlas provide a good example of the different rocks constituting the High Atlas.
1.Ourika Valley, Oukaimeden: the red rocks of the Permo-Triassic (end of Primary-beginning of Secondary) are dominant. The sandstones from cliffs. Above the fine clayey sandstones (siltstones) lie flows of altered yellowish-gray basaltic lava.
- 2. Tizi n’Tichka road: it crosses the red layers, basalts, and very deformed dark schists and sandstones of the Primary
- 3. Tizi n’Test route: first small massifs of primary rocks (Guemassa), then the red series and Primary rocks as on the Tizi n’Tichka route.
Fossils and minerals
- Primary: trilobites (actually collected further south in the same land)
- Secondary: quartz geodes (amethyst) in the volcanic flows of the Triassic