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The main exposures of "Cambro-Ordovicien" rocks in Al Jazair (Algeria) are in Es Sahara in the Chaines d'Ougarta and around the northern Ahaggar; insignificant outcrops occur in the Atlas Ranges. Subsurface "Cambro-Ordovicien" rocks south of the Atlas Ranges underlie about 900,000 sq km (347,000 sq mi) in Al Jazair and produce oil at Hassi Messaoud and elsewhere.
"Cambro-Ordovicien" formations are more than 5,000 m thick in the Chaines d'Ougarta, where the Ougarta Sandstone Group overlies Precambrian rhyolites and crystalline rocks and is overlain by Silurian graptolitic mudstones.
North of the Ahaggar, and forming conspicuous cliff rims, the Inner Tassili Sandstone Group forms extensive outcrops of quartzite, sandstone, mudstone, and conglomerate about 500-700 m thick. No pre-Tremadocian (Late Cambrian) rocks have been proved in these sequences. Mudstone with Silurian graptolites overlies the Inner Tassili Sandstone Group unconformably and beneath the group are poorly known Cambrian and late Precambrian sedimentary rocks overlying a crystalline basement.
The biota is remarkably monotonous and only 120 forms had been recognized by 1969. It is sporadically distributed and great thicknesses are barren of fossils, except for the extremely common (?)worm tube Tigillites. This contrasts with the prolific Cambrian and Ordovician biota of Morocco and Europe, and is possibly related to the location of polar ice caps over Es Sahara during those times. Late Ordovician glacial and periglacial deposits are magnificently exposed around the Ahaggar, and Early Cambrian glacial deposits have been claimed to occur.
Lithologies, sedimentary structures, and biota indicate that a wide shallow shelf prevailed through much of the Cambrian and Ordovician, except in the Ougarta trough, which may have been connected to the Moroccan miogeosyncline. The post-Tremadocian to pre-Llandoverian deposits are thin at outcrop and in the subsurface northward from the Ahaggar to Tunisia. The sediments, derived mainly from the south, may represent a molasse facies of the mountainous Chaine pharusienne of the Ahaggar region formed during the Pan-African orogeny.
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