108
CLAUDE J. GRENOT
This diagram, which is mostly representative of the western portion of
the platform, is often reduced to only three divisions as far as the eastern
portion is concerned, either because of nonexistence of Paleozoic formations there or of their fusion with the base of the secondary series.
The subflooring—or ancient floor—of the Sahara is composed of Precambrian rock formations with local appearance of often important masses
of granite. With the exception of a chain of small isolated Precambrian
formations, located in the axial portion of the Anti-Atlas, one can say that
the large flush formations of the ancient floor are found for the most part
in the vicinity and along the Tropic, forming a protruding median hump,
encroached with ennoyages (self-engorgements) which roughly divide the
Sahara into a septentrional and a meridional zone.
This median hump is from west to east composed of
1. The Reguibat Shield (or crystalline region), large anticlinal loop, sort
of dorsum stretching out over more than 1500 km, from the southwest to the northeast and splitting diagonally the entire western
portion of the Sahara
2. The Tuareg Shield with its principal massif, Ahaggar (or Hoggar)
and its two satellites, Adrar of Iforas in the southwest and Air in the
southeast
3. The Tebbou Shield, smaller in size, which forms the subfloor of the
volcanic massif of the Tibesti, and flush with it mostly in the northern part
Furthermore, one finds a series of small flush formations of the ancient
floor arising from the Nubian sandstone cover, located in the Libyan
Desert.
Finally, largest of them all is the Arabo-Nubian Shield, split in the middle by the Red Sea.
On top of the Precambian flattened subflooring the Paleozoic formations
appear irregular and erratic (tassilian unconformity). Although in certain
areas they exhibit sharply pronounced folds, nevertheless we never find
any metamorphism or even true granitic intrusions. In the northwest, and
up until the Carboniferous, these formations are of a maritime origin; but,
towards the south and the east, the importance of maritime faciès progressively decreases, and the eastern Sahara, apart from a few exceptions,
has remained wholly continental and this is true during the entire course
of all the primary era.
In the western Sahara, the Paleozoic regions are composed of folded
zones of various dimensions (Anti-Atlas, Ougarta chains, Zemmour) or
of vast syneclises (shallow basins)—such as the Tindouf and Taoudenni—surrounding the Reguibat Shield.
CLAUDE J. GRENOT
This diagram, which is mostly representative of the western portion of
the platform, is often reduced to only three divisions as far as the eastern
portion is concerned, either because of nonexistence of Paleozoic formations there or of their fusion with the base of the secondary series.
The subflooring—or ancient floor—of the Sahara is composed of Precambrian rock formations with local appearance of often important masses
of granite. With the exception of a chain of small isolated Precambrian
formations, located in the axial portion of the Anti-Atlas, one can say that
the large flush formations of the ancient floor are found for the most part
in the vicinity and along the Tropic, forming a protruding median hump,
encroached with ennoyages (self-engorgements) which roughly divide the
Sahara into a septentrional and a meridional zone.
This median hump is from west to east composed of
1. The Reguibat Shield (or crystalline region), large anticlinal loop, sort
of dorsum stretching out over more than 1500 km, from the southwest to the northeast and splitting diagonally the entire western
portion of the Sahara
2. The Tuareg Shield with its principal massif, Ahaggar (or Hoggar)
and its two satellites, Adrar of Iforas in the southwest and Air in the
southeast
3. The Tebbou Shield, smaller in size, which forms the subfloor of the
volcanic massif of the Tibesti, and flush with it mostly in the northern part
Furthermore, one finds a series of small flush formations of the ancient
floor arising from the Nubian sandstone cover, located in the Libyan
Desert.
Finally, largest of them all is the Arabo-Nubian Shield, split in the middle by the Red Sea.
On top of the Precambian flattened subflooring the Paleozoic formations
appear irregular and erratic (tassilian unconformity). Although in certain
areas they exhibit sharply pronounced folds, nevertheless we never find
any metamorphism or even true granitic intrusions. In the northwest, and
up until the Carboniferous, these formations are of a maritime origin; but,
towards the south and the east, the importance of maritime faciès progressively decreases, and the eastern Sahara, apart from a few exceptions,
has remained wholly continental and this is true during the entire course
of all the primary era.
In the western Sahara, the Paleozoic regions are composed of folded
zones of various dimensions (Anti-Atlas, Ougarta chains, Zemmour) or
of vast syneclises (shallow basins)—such as the Tindouf and Taoudenni—surrounding the Reguibat Shield.