Lithostratigraphy and geodynamic evolution of Lower Cretaceous sedimentary depocenter infilling,in Western Saharan Atlas (Algeria)

The Lower Cretaceous series, in the Western Saharan Atlas, shows important detritic deposits, particularly continental to nearshore red sandstones. Lithologically, it is an alternation of red clay and sandstone intercalated with some carbonate levels at the lower part of the series. These series can be subdivided into three lithostratigraphic units corresponding to Valanginian-Hauterivian, Barremian, and Aptian-Lower Albian. The lithostratigraphic analysis used to reconstitute the important subsidence in the Lower Cretaceous period, particularly in the Lower Albian. The Lower Cretaceous series could be considered as a geodynamic model characterized by individualization of independent depocenter filled with different detritic material and separated by Jurassic structures. This new situation appeared in the beginning of Lower Cretaceous (Valanginian-Hauterivian), following the Neocimmeriean tectonic phase, and it is increasing in the Early Albian, where differences of thicknesses are more important and allow the distinction of the depocenter. The biostratigraphic analysis of the clay samples yielded some foraminifera and palynomorphs. Benthic and planktonic foraminifera confirmed the Hauterivian age of the basal formation, also, two dominant forms of palynomorphs were recognized; according to the large stratigraphic distribution of the identified forms, it can be attributed to the Lower Cretaceous.     

Tectonique quaternaire et plis de rampe dans le golfe d'Hammamet (offshore tunisien)

The Jriba trough is an Upper Miocene graben located within the Tunisian offshore Gulf of Hammamet area, east of the Atlas front. This distensive structure suffered a compressive event during the Early Quaternary (Villafranchian). The Jriba structure was previously interpreted as ‘flower structure’, which possibly complicated by halokinetics movements. A new analysis of a set of seismic lines crossing the Jriba trough allows us to propose a new tectonic model where the Villafranchian deformation is characterized by (1) occurrence of a decollement level cutting Messinian to Pliocene layers; and (2) the growth of fault-related folds (fault-propagation fold). The NE–SW Miocene, inherited normal faults, locate the position of the ramps and folds whereas the NW–SE inherited normal faults are reactivated as tear faults. These NW–SE tear faults define various domains of different shortening values (one kilometre at maximum). To cite this article: M. Ben Romdhane et al., C. R. Geoscience 338 (2006).     

Modeling of Mrihla anticline (Central Tunisian Atlas), combination fault propagation fold and halokenitic model

The modeling study on the Mrihla anticline was carried out using two techniques, i.e., excess area law and balanced cross section. The results show that this structure is likely affected by at least two compressive phases. The interpretation of surface (bedding dips, thickness, lithology, etc.) and subsurface (seismic lines) data along the cross section indicates that the Mrihla structure has a shortening of about 525 m that is evolved above a detachment layer formed by gypsum Triassic formation. The top of this layer is situated at a depth of about 3,890 m from the top of the Aptian dolomitic level, known in Central Tunisian Atlas as Serdj Formation. The kinematic of the investigated structure is a combination of two deformation models. The first is the halokenitic model, defined by the flowing of Triassic ductile material upward toward the surface through the deep Mrihla fault, which is parallel to Mrihla anticline. The second is the fault propagation fold model, characterized by thin-skin deformation mechanics in relation with the movement of the Mrihla fault.     

The Laalam (Algeria) damaging landslide triggered by a moderate earthquake (M<Subscript>w</Subscript> = 5.2)

On March 20th, 2006, a moderate earthquake (M_w = 5.2) struck the Laalam (NE-Algeria). The damage was centred in the Laalam village where four deaths, 68 injured and more than 40 housing units destroyed making dozens of peoples homeless. Damage and casualties are directly related to an earthquake-induced landslide occurred at 10 km far from the epicenter (ISC relocation). The landslide rupture, mapped by using a ground based survey, is striking NE-SW along a 1-km length rupture with a 45 cm of lateral displacement. The rupture plan is dipping 40° at the surface and showing horizontal striations. A 75 cm of pre-2006 lateral displacement is also observed indicating a repetitive behaviour of the landslide. The last time where the landslide was activated extends back to 1974 following another moderate earthquake. This landslide reminds us the high level hazard of induced ground failures due slope instabilities in mountainous zones of seismically active areas.