Les affleurements de "series intermediaires" dans la region de Silet (Ahaggar occidental, Sahara central)

Abstract Poorly fossiliferous detrital and volcanic beds of the so-called "intermediate series" outcrop in the eastern part of the Pharusian (Precambrian) trough in the western Ahaggar mountains and on the western border of the Suggarian (Precambrian) horst of the central Ahaggar mountains (Algeria) in the central Sahara. They are unmetamorphosed or only slightly metamorphosed, and rest unconformably on either the Pharusian schists or on Suggarian granite. Their relationship to the Cambro-Ordovician cover is not known, for in this area the latter consists only of a few scattered buttes. The intermediate series can be distinguished easily from the Tassilian sandstones and conglomerates on the basis of lithology and structure. There are some similarities to the Nigritian of the Adrar des Iforas region, but in the absence of paleontologic evidence and absolute age determinations, correlation even between closely spaced outcrops is purely hypothetical.