Main lower Mesozoic opening stages of the Ksour basin (Saharan Atlas, Algeria)

Introduction and geological setting The Ksour mountains (northwest Algeria) belong to the western part of the Saharan Atlas. They are located between the High Plateau to the north, the South Atlas Front to the south and the M oroccan High Atlas to the west (fig. 1a). During the last decades, the Ksour mountains were the focus of several stud ies, mainly based on sedimentary analyses [Cornet, 1952; Bassoulet, 1973; B aiche, 1975; Ameur, 1978; Air Ouali, 1991; Mekahli, 1995; Elmi et al., 1998 ]. These studies indicate that the Ksour mountains correspond to an old asy mmetric rift, trending NE-SW, which underwent extension during the Triassic and Liassic [Ait Ouali,1991; Frizon de Lamotte et nl., 2000]. In order to precise the timing of the rifting processes we have analysed su bsurface data (seismic lines and borehole data) from the Sonatrach Company. Data analysis and results The seismic lines are from several seismic survey s, which have been conducted in the Ksour Mountains between 1971 and 1986. The seismic lines are mainly located in the pre-Atlas domain and along the South Atlas Front. In order to precise the spatial evolution of the lithost ratigraphic sequences along a N-S transect, we have used data from several bore-holes implanted in the region. In addition, these data were useful to constrain the seismic sequences correlation and allow to distinguish in the north a subsiding area corresponding to the preAtlas, where a thick liassi c sequence has been deposited. Moreover in the EBDI-RGT1 boreholes, a thinn ing of the Middle to Upper Liassic and of the Aalenian to Bajocian series o ccur. This thinning would be related with the early mobility of salt series . The maximum filling is observed in the Atlas trough where the thickness o f the Jurassic and Cretaceous series is about 6000m. Towards the south (Ker dacha), the Liassic succession is extremely reduced, with a littoral featur e. From the seismic profiles analysis, we first point out the first opening st age of the Ksour basin, which occurred in the Upper Triassic period. Block tilting marks this tectonic period. The second opening stage occurs in the Liassic period. The depocenters are slightly different from the previous ones. Some volcanic manifestations are also observed. During the Tertiary times a tectonic inversion affects the basin [Vially er al., 1994; Bracene et al.. 1998; Frizon de Lamotte et al., 2001]. Discussion-Conclusion This study allows to point out the subsurface tectoni c structures involved in the different opening stages of the Ksour basin. T he seismic profiles show first the extensional tectonics of the Triassic an d the Liassic periods and second the salt diapirism and associated volcanic rocks. Previous studies often notice the important role of strike slip fau lting. This is not observed on the seismic lines. The geological data prese nted here display the difference in the sedimentary filling between the two troughs of the basin, the 'PreAtlas' and the 'Atlas' where the deposits ar e more important. Comparisons between the Ksour basin, the Moroccan basins and the Sahara pla tform, suggest a similar geodynamical evolution of the whole Atlas system. However, we do not have evidence for the compressional events pointed out i n Morocco.