Description, lateral evolution, and geodynamic meaning of a sedimentary unconformity between C2 and C3 units of the Lower Devonian in southern area of the Ahara Spur (southeast Algerian Sahara)

Abstract

Sedimentological unconformities have been considered useful in many recent studies for stratigraphic sequential analysis or in the analysis of general geological topics, as their characteristics impact both sedimentology, paleo-environment, and diagenesis. An interesting example is given by the unconformity between C2 and C3 units of the Siegenian period around the southern boundary of the Ahara spur in the southeast Algerian Sahara. Usually, the limit between C2 and C3 units is plotted as the top of the more shaly deposits of the C2 unit. However, when C3 unit becomes shaly and/or C2 becomes gritty, this limit becomes unclear. In terms of sedimentology, the characteristics of this limit are peculiar in that they correspond to fine deposit levels underlined by ferruginous incrustations so that ferruginous hardpan is generated. It therefore constitutes a sedimentary unconformity assimilated to a lateritic soil. The spatial variability is marked by a strong ratio of spherules in the C2 unit, which attests of a probable relationship between climate and pedogenesis. This unconformity covers the entire studied region. It corresponds to gully erosion in the further south regions. At the northern, as it was distal marine area, it is difficult to locate and trace this boundary. Practically, this boundary as ferruginous hardpan can complicate drilling by introducing slow-drilling and making them unsuitable reservoir