Riadh Ahmadi

Geochemical and isotopic study of the multilayer aquifer system in the Moulares-Redayef basin, southern Tunisia / Etude géochimique et isotopique du système aquifère multicouche du bassin de Moulares-Redayef, sud tunisien

Abstract Major ion geochemistry, and water molecule isotopes (18O, 2H) and radiogenic carbon (14C) of dissolved inorganic carbon (DIC) were used to investigate the hydrodynamic functioning of the multilayer aquifer system in the Moulares-Redayef basin, southern Tunisia. The groundwater of different aquifer levels is characterized by sulphate to calcium sulphate water type. The major geochemical processes in the aquifer system are evaporite mineral dissolution and mixing. The isotopic study allows two groundwater types to be identified: an old palaeoclimatic groundwater, marked by low 14C activity and relatively depleted stable isotope (18O and 2H) content characterizes the shallowest aquifers of the Plio-Quaternary and Miocene formations; however, a recent groundwater, distinguished by relatively high 14C activity and slightly enriched 18O and 2H content, characterizes the deep Upper Cretaceous artesian aquifer. In addition to these two water groups, other groundwaters are identified, indicating a mixing effect.

Groundwater evolution of the Continental Intercalaire aquifer of Southern Tunisia and a part of Southern Algeria: use of geochemical and isotopic indicators

AbstractThe expansion of irrigated agriculture and the overexploitation of groundwater aquifers are leading to saltwater intrusion, severe deterioration of groundwater quality and soil subsidence at arid areas. The geochemical processes taking place along an 800 km flow line in the non-carbonate Continental Intercalaire aquifer (CI) in North Africa are described using chemical (major and trace element) and isotopic indicators. The aquifer is hydraulically continuous from the Atlas Mountains in Algeria to the Chotts of Tunisia and the geochemical evidence corroborates this. The CI aquifer of North Africa is one of the largest confined aquifers in the world. The aquifer is hydraulically continuous from the Atlas Mountains in Algeria (recharge area) to the Chotts of Tunisia (discharge area) and the geochemical evidence corroborates this. The isotopic study (Delta18O, Delta2H) permits classifying groundwater into three groups. The first group is characterized by low 3H concentrations, low 14C activities and d...

Growth-strata geometry in fault-propagation folds: a case study from the Gafsa basin, southern Tunisian Atlas

The structural and sedimentological study of fault-propagation folds in Southern Tunisia highlights a special geometry of the growth strata (strata deposited simultaneously with the formation or growth of a fold). This distinct geometry is visible in the uppermost growth-strata beds and consists of one flank with unconformity as opposed to the other flank with perfect conformity. This geometry can be explained by the mechanism of fault-propagation folding, with asymmetrical flank dips and hinge migration kinematics. This kinematics was originally predicted by the fault-propagation fold model, which facilitates the study of this special geometry in a narrow domain of sedimentation-to-shortening ratios. A plot projection provides a generalisation of the results of all types of fault-propagation folds by revealing the expected geometry of the growth strata. This study constitutes one of the most complete examples of kinematic model validation on a field scale.

Folds Kinematics in “Fold-and-Thrust Belts” the “Hinge Migration” Question, a Review

In Fold-and-Thrust Belt domains, fold amplification may occur either according to hinge migration process (i.e., hinges move relative to the affected material) or by limb rotation around fixed hinges. The validation of the law grow by hinge migration rather than by limb rotation, consequently, almost all fold kinematic models use the hinge migration process. We show that at least four independant approaches should a priori allow to demonstrate the occurrence of hinge migration mechanisms in nature: i) growth stratal geometry, ii) specific collapse-structures analysis (flaps), iii) morphological imprints of fold growth, and iv) microtectonic signatures. In practice, we observe that the latter is inoperative. We show, through this bibliographic review, that few hinge kinematics could be explicitly established from field observations, and conclude that, i) both expected kinematic behaviours are encountered and can coexist, ii) mobile hinges are extremely scarce in the literature, such that a paradoxal and theoretically unexpected overrepresentation of fixed hinge cases is observed. We discuss this latter point by reconsidering the efficiency of each approach and, without negating limb rotation existence, we propose explanations based on a methodological bias favouring the underestimation of hinge migration frequency.