The contact of lower Devonian sandstone and shaly sandstone with middle Devonian shaly limestone is exposed at several points in the vicinity of Fort-de-Polignac in the Sahara. The lower Devonian beds show large lateral variations. Instead of the Givetian (upper middle Devonian) transgressing the lower Devonian, as previously supposed, the transgression began in the center of the basin in the Couvinian or Eifelian, i.e., lower middle Devonian, spread progressively eastward and westward, and did not reach the Oued Samene area until upper Couvinian. The submergence of the lower part of Tihemboka, west of In Akaouet, did not occur until Frasnian (lower upper Devonian).

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