Devonian stratigraphy and depositional environments in the southern Illizi Basin (Algerian Sahara)

The Illizi Basin is the easternmost one of a series of cratonic basins of the Algerian Sahara which extend along the northern margins of the Precambrian Reguibat and Hoggar massifs. Devonian rocks crop out along the erosional southern border of the basin and can be roughly divided into three major lithostratigraphic units: (1) Lower Devonian sandstones (upper part of the Tassilis externes), (2) Middle Devonian carbonates (Illizi Formation, introduced in this article) and (3) Upper Devonian shales (Tin Meras Formation). In the present study emphasis was laid on the Middle Devonian carbonates which were dated by conodonts and correlated in 12 sections over a W–E distance of about 350 km. The sequence shows a lateral transition from condensed series in the west, deposited on a submarine high (Amguid Ridge), into a shallow basin (Illizi Basin) which was additionally differentiated into minor ridges and sub-basins. The eastern portion of the basin is characterized by an enhanced clastic input from the Tihemboka Ridge which separates the Illizi Basin from the Murzuq Basin in western Libya. The differential subsidence during the Devonian shows that the Illizi Basin is primarily a depositional structure which subsequently became more accentuated after the Variscan orogenesis at the Carboniferous/Permian transition.