Geology Reference
In-Depth Information
G
eosite A4
(some 40 km south-east of A2) is located at
the base of an isolated rock (the
The structural complexity of the metagabbro unit is much
more visible in the southeastern Adrar Souttouf (Entajat)
massif, due to higher elevation and more deeply incised
relief (Rjimati et al.
2011a
).
) close to the road
to Awsard (Fig.
4
c). This site exposes the continental facies
of the Pliocene-Quaternary deposits, rich in fossil
Helix
shells and casts. The Miocene marls of the
geosite A3
(Fig.
5
f) are lacking here beneath the Pliocene-Quaternary
sands, indicating that the Miocene shoreline was located
between
geosites A3
and
A4
.
“
Sphinx
”
3.1.3 Awsard Geosites
Four geosites situated in the vicinity of the Awsard city
centre (Fig.
7
) will allow visitors to discover exceptional
outcrops with no equivalent elsewhere in northern Morocco.
Two of them (
geosites A7
,
A8
) are located in the Reguibat
shield; another one (
geosite A9
) is on top of the Palaeozoic
autochthonous cover beneath the front of the Mauritanide
nappes, and the last site (
geosite A10
) is located within the
nappe stack.
The Awsard site sensu stricto (
geosite A7
) is located
immediately to the east of the village along the Tichla road.
Typical banded and foliated Archaean orthogneiss crops out
on both sides of the road. Similar metagranites have been
dated some 200 km farther in the south (Tasiast area) at
2.97
3.1.2 Midway Geosites A5 and A6
It is necessary to stop about 50 km SW of the
in
order to locate the inner boundary of the Dakhla sedimentary
basin (southern part of the Tarfaya-Boujdour basin). There,
the
geosite A5
shows Lower Cretaceous red beds with a
shallow northeast dip. This is the upper, outcropping part of
the sandy continental formation that contains the Dakhla
groundwater. On a small hill next to the road, one may
observe Neolithic engravings on the red sandstone blocks
(Fig.
6
a). In map view (Fig.
3
), it must be noticed that Upper
Cretaceous deposits were not identi
“
Sphinx
”
ed between the Lower
Cretaceous and Cenozoic formations of this southern area,
contrasting with the stratigraphy of the Tarfaya-Boujdour
Basin farther in the north (see below). The unconformity at
the base of the Lower Cretaceous continental sandstones
(green color on Fig.
4
) that directly overlie the Variscan
nappes of the Adrar Souttouf Massif (deep or light violet
colors) should be noted here: this is the signature of a long
period of erosion after the Variscan Orogeny (Early
Carboniferous, ca 330 Ma) and before the Lower Cretaceous
(ca 130 Ma).
The geosite A6
is located 55 km farther in the southeast
where the road to Awsard crosses the largest metagabbro
complex of the Adrar Souttouf massif. This site marks the
axis of the Mauritanide nappes in southern Morocco.
The outcrops display typical metagabbro facies (Fig.
6
b, c).
2.93 Ga (Mesoarchean; Key et al.
2008
). A few hun-
dred metres north of the road, a cluster of rounded hills
(Fig.
8
a) belong to a circular intrusion of ultrapotassic sye-
nite. This kalsilite and nepheline-aegyrine-bearing syenite is
undeformed and shows a vertical magmatic lineation
(Fig.
8
b), consistent with its relatively young age (2.46 Ga;
Bea et al.
2013
). Both the foliated granites and at least part
of the syenite are crosscut by a basic dyke swarm (Fig.
7
).
Geosite A8
is located within the Archaean basement ca
10 km north of Awsard city, close to the Dakhla road at the
southern tip of the Dliyat En
-
Sour ridge. It displays wonderful
outcrops of heterogeneous migmatites, with large blocks of
basic rocks (Fig.
8
c) surrounded and crosscut by granite veins.
According to their orientation, these veins are either stretched
or folded, which records the high-temperature deformation of
the rocks in the partially melted Archaean crust.
'
Fig. 6 a
Neolithic engravings on Lower Cretaceous sandstones, geosite A4;
b
Metabasite, and
c
metagabbro of the Adrar Souttouf massif along
the Dakhla-Awsard road
