Geology Reference
In-Depth Information
Fig. 20 a
Jbel Tazout cuesta (Tournaisian) as seen from Mseied,
b
Jbel Ouarziz cuesta (Visean) south of Mseied
whose apparent thickness is increased by NE-trending folds
(Rjimati et al.
2011a
). These folds are the south-easternmost
record of the Variscan deformation. Just in front of Mseied
village (
geosite B9
), the Tournaisian (lowermost Carbonifer-
ous) sandstones and limestones form amonocline (Jbel Tazout
cuesta) dipping weakly southeastward (Fig.
20
a). This marks
the entrance into the cratonic and undeformed part of the
Tindouf-Zag Basin sensu lato, the deformed part of which
corresponds to the Anti-Atlas domain. A few hundred metres
farther in the southeast, the Visean (upper Lower Carbonif-
erous) horizontal limestones of the Jbel Ouarkziz (Fig.
20
b)
characterize the cratonic domain. Farther in the east but not
seen from here, the Tindouf-Zag Basin also contains Late
Carboniferous deposits (Fig.
15
), composed of continental
clastic sediments that correspond to the erosion of the recently
uplifted Variscan belt (Cavaroc et al.
1976
).
by a thin Palaeozoic cover (Fig.
21
, pro
le A). Again, the
WAC is encountered farther in the north at Mseied (Fig.
21
,
pro
le B, East) with a younger continental basement
(Eburnian, ca 2 Ga) and a much thicker Palaeozoic cover
(Tindouf-Zag cratonic basin).
The less popular concept of
metacraton
: this is illustrated
in the Bas Draa massif (Fig.
21
, pro
le B East), at the
northern rim of the WAC (Eburnian basement) modi
ed
by the Pan-African orogenic cycle (extension, collision,
post-orogenic magmatism and volcano-clastic accumu-
lation). The same metacratonic border of the WAC was
deformed again during the Variscan orogeny,
rst during
the Cambrian extension and then during the Carbonif-
erous collision.
The concepts of
orogeny
and
orogenic cycle
: these are
well illustrated along two transects of the Variscan Belt
(Fig.
21
, pro
les A and B East). The Cambrian rifting
and Ordovician-Devonian subsidence of the continental
crust clearly appears in the Bas Draa transect. Evidence
of collision tectonics is conspicuous in both the southern
and northern transects (Mauritanide nappes west of
Awsard, thick-skinned shortening of the Bas Draa area
with inversion of the palaeomargin normal faults and
folding of the thick overlying sediments).
4
Discussion and Conclusions
Two potential geotrails have been described above, which
altogether involve 19 geosites offering scenic landscapes and/
or excellent outcrops (Table
1
). The Dakhla
-
Awsard trail
“
A
”
traverses the southernmost part of
the Moroccan
Southern Provinces, whereas the El Ouatia
-
Tan-Tan
-
Mseied
Plate tectonics
: this is inevitably called upon to explain
the Variscan collision (closure of the Rheic paleo-ocean
between Laurussia and Gondwana) and correlative Pan-
gaea building, whereas the outcrops and well data pre-
sented next to El Ouatia illustrate the Pangaea breakup
process, which formed the Triassic rift basin and the
Jurassic passive margin.
trail
traverses their northern border.
Considering the international signi
“
B
”
cance of the trails, we
may emphasize that they involve three major geotectonic
structures, namely the West African Craton (WAC), the
Variscan Belt and the Atlantic continental margin. The
WAC is one of the largest cratons of Africa and extends
southward up to the Ivory Coast. The Variscan Belt and the
superimposed Atlantic margin extend in several West Afri-
can and European countries, and have their counterpart in
the eastern countries of America.
From a pedagogical point of view, several major geo-
logical concepts can be introduced into the educational panel
at the proposed varied geosites along the trails, as follows:
In the stratigraphic domain, the concept of major
uncon-
formity
:
rstly by the Upper
Ordovician transgression onto the Archaean basement,
due to the Hirnantian glaciation, and secondly by the
Lower Cretaceous red bed deposits on the deeply eroded
continental domain and adjacent passive margin (Fig.
21
).
this is illustrated twice,
Varied petrographic facies
: these are encountered from
one geosite to the other, ranging from fossiliferous cal-
carenites or limestones to low-grade metagreywackes, to
high temperature migmatites.
The concept of
continental craton
: this is encountered
rst
at Awsard, where the WAC is represented by the Reguibat
Shield
, made up of Archaean rocks (ca 3 Ga) and overlain
