Geology Reference
In-Depth Information
2
Geological Setting
of the above cited domains. Its northern segments
(e.g., Essaouira Basin) have been more or less
deformed during the Atlas and Rif folding events.
In contrast, its southern part (Tarfaya-Laayoune-
Boujdour Basin) remained virtually undeformed and
includes one of the most ancient passive margins
preserved the world over.
Morocco comprises six major geological domains (Michard
et al.
2008
) that are, from north to south (Fig.
1
):
(i) The Rif Belt, which is a subduction-collision Ceno-
zoic belt belonging to the large Alpine system of the
Mediterranean domain.
(ii) The Atlas-Meseta domain, which consists of strongly
deformed Palaeozoic basement, namely the Meseta
Variscan Belt, overlain by aMesozoic-Cenozoic cover,
either thick and folded in the Atlas mountains, or thin
and tabular in the plateau areas of the Western and
Eastern Mesetas. The Meseta Variscan Belt connects
with the southern branch of the Variscan Belt of Europe
(Michard et al.
2010
) and displays a dominant west
verging polarity.
(iii) The Anti-Atlas domain, south of the High Atlas in the
sub-Saharan region, typi
3
Geosites and Geoheritage Trails
Description
The region illustrated in this paper extends over ca 700 km
NNE-SSW from Tan-Tan, an old city at the entrance of
the Saharan regions, to Dakhla and Awsard close to the
Mauritanian border (Fig.
2
). The geological domains repre-
sented are the Anti-Atlas fold belt, the Mauritanide thrust belt,
the West African Craton (Reguibat shield and Tindouf-Zag
basin) and the Tarfaya-Boujdour Coastal Basin. As the study
area is oriented SW-NE, we selected two trails oriented
broadly NW-SE that cross all the geological domains, one in
the southernmost region, fromDakhla to Awsard (trail A), and
the other in the north, from El Ouatia to Mseid at the southern
part of the Anti-Atlas (trail B). Both are located along paved
roads, in areas of moderate relief, with altitudes ranging from
a few tens metres to about 300 m above sea level. Likewise,
both trails include comfortable tourist facilities at their start-
ing points, which correspond to the attractive sea resorts of
Dakhla and El Ouatia (also named Tan-Tan plage).
The choice of these two geotrails across the Southern
provinces is justi
ed by a thick, poorly to
moderately folded Palaeozoic succession overlying a
Precambrian basement
that crops out
in several
(Fig.
2
) and
showing strong imprints of the Neoproterozoic
Pan-African Orogeny (Gasquet et al.
2008
). The Pal-
aeozoic fold belt here displays a SE verging polarity; it
has been overlain by a thin Mesozoic-Cenozoic cover
before being gently uplifted and eroded contempora-
neously with the High Atlas mountains uplift
(Missenard et al.
2006
; Guimer
à
et al.
2011
). Further
in the south-east in Algerian territory, the eastern
Anti-Atlas structures continue in the Ougarta Chain.
(iv) The Moroccan Mauritanides of the Adrar Souttouf
(Oulad Dlim) massif and Dhlou fold belt extending to
the west Saharan region and forming a strip of
deformed Precambrian and Paleozoic terranes thrust
eastward onto the undeformed cratonic areas. The
Mauritanide Belt belongs to the large Appalachian-
Alleghenian-Variscan Orogen as the Meseta Belt,
from which it is separated by the Palaeozoic South
Meseta Fault, broadly superimposed by theMesozoic-
Cenozoic South Atlas Fault.
(v) The West African Craton is extensive in the Saharan
regions. This major domain has remained basically
undeformed since ca 2 Ga. It includes the Pre-
cambrian outcrops of the Reguibat Shield or Arch
and the Neoproterozoic to Cenozoic sequences of the
Tindouf and Taoudenni Basins, north and south of
the Reguibat Shield, respectively. The northeastern
part of the Reguibat Shield is affected by the Ebur-
nian Orogeny (
antiformal
inliers or
“
boutonni
è
res
”
ed because of their rich and varied geodi-
versity. The Dakhla-Awsard trail offers the unique opportu-
nity to walk on 3 Ga-old gneisses in the Moroccan territory.
Near Awsard, a didactic cross-section shows the transgres-
sion of Upper Ordovician (445 Ma) periglacial deposits
directly on top of the Archaean gneisses (ca 3 Ga), which is
one of the largest stratigraphic unconformities in Africa.
Similarly, the trail offers the unique opportunity in Morocco
to cross the tectonic system of the Mauritanide crystalline
nappes, which formed due to the collision of North America
against Africa during the Variscan orogeny (350
330 Ma). In
contrast, the Tan-Tan geotrail crosses a quite distinct tectonic
system of the Variscan Belt, typical of the Anti-Atlas and
characterized by kilometer-scale cylindrical folds formed at
the expense of a thick Palaeozoic sedimentary pile.
Finally, both geotrails include geosites showing the
Atlantic margin sediments that accumulated on top of the
Variscan basement during the Pangaea rifting (250
-
-
200 Ma)
and subsequent opening of the Atlantic Ocean. However, the
southern geotrail mainly illustrates the youngest levels of
this sedimentary wedge, namely the Cenozoic deposits,
whereas the northern geotrail also gives the opportunity to
2 Ga) whereas its southwestern part
*
3 Ga-old Archaean terranes.
(vi) The Atlantic passive margin and related Coastal
Basins developed from the Triassic onwards west
consists of
*
