Abstract

The Devonian crops out in Libya on the western and southern edge of the Gargaf or Djebel Fezzan high along the western and eastern flanks of the Mourzouk basin and in the Koufra basin: Dohone, Djebel Aouenat and Djebel Hauaisc.

The Devonian is known from wells in southern Tunisia, in the Ghadames basin in Libya, in eastern Tripolitania, in the Zegher and around the rim of the Mourzouk basin.

The Devonian is subdivided into lithologic units which are frequently extrapolated from one basin to another but whose position in relation to the reference stages is still imprecise.

The argillaceous-sandy interbeds of the upper Acacus could represent, in part, the equivalent of the post-Ludlow-Gedinnian passage zone. The Devonian itself is subdivided into three major units:

1—The Tadrart Formation, a more or less coarse massive sandstone, suggesting a transgressive character as indicated by local unconformities on the edge of the old highs. It is assigned to the Siegenian on the analogy of its facies and the same stratigraphic position of a sandstone unit of the same age in the Algerian Sahara.

2—A conformable more marine argillaceous-sandy sequence is attributed to the Emsian (s.l.): Emgayet Formation in the Ghadames basin (this sequences continues into the lower part of the Middle Devonian in the Mourzouk basin), Ouan Kasa Formation on the western flank and Emi Magri on the eastern flank.

3—The Aouinet-Ouenine Formation, shales with sandstone intercalations, extends from the Middle to the Upper Devonian. Conformable with the previous formation in the Ghadames basin, it becomes transgressive in southwestern Libya. This transgression is diachronous and can be as late as Frasnian. In these areas, a tectonic phase precedes the deposition of the Aouinet-Ouenine Formation which can be unconformable on even the Cambro-Ordovician. The conditions of sedimentation of the Aouinet-Ouenine are the most marine of the whole cycle, but in its upper part a regressive phase occurs, generally represented by sand, which marks the close of the Devonian in the Sahara.

In the Koufra basin, the Devonian is wholly clastic: above the Tadrart sandstones, the Dinem, Blita and Dalma Formations are represented by fine sandstones and silts.

In general, the marine character of the sedimentation is progressively more accentuated from southeast to northwest, from the wholly clastic facies of the Kourfra region to the more marine, more argillaceous and sometimes organogenic facies of southern Tunisia.