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Le Devonien du Sahara Algerien
The Devonian is present over the greater part of the Saharan platform, north of the ancient massifs of the central Sahara from the boundary with Morocco and Mauretania in the west to the Lybian frontier in the east.
In the Algerian Sahara the Devonian crops out in the mountains of Ougarta, Gourara and Touat; it encircles the Eglabs massif and Azzel Matti, Asejrad, Tassedit, the Amguid region and Ajjers county.
It has been penetrated by boreholes in the main Saharan basins, but it can be missing as a result of later erosion.
The Devonian in the Algerian Sahara is essentially subdivided into lithologic units in each basin but their position with respect to the standard chronographic scale is often imprecise.
1—At the base a shale-sand sequence called “passage beds”, which in both its sedimentary and its faunal (post Ludlovian-Gedinnian) aspects shows a gradual transition from the Silurian to the Devonian.
2—A sandy sequence with a Siegenian fauna in the west, which passes into locally transgressive massive sandstones of more or less coarse grain towards the external Tassilis and the basins of the east.
3—An argillaceous sequence with interbedded fine sandstone and rare limestone beds having a lower Emsian to lower Couvinien fauna.
4—A sequence of alternating limestones and shales of open marine character which are very fossiliferous. The age of this sequence is from upper Emsian to Givetian.
5—A thick argillaceous sequence with intercalations of sandstones and limestones, the latter being in “Griotte” facies in the Ougarta. This sequence is locally transgressive at the base but marks the start of a regression at its top and is of Famennian age.
6—A final general sandstone episode extending over the whole Sahara, which expresses the regression that marks the end of the Devonian cycle. The age of this episode is from Upper Famennian to Lower Tournaisian.
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