In north-central Arizona, the Moenkopi Formation of Triassic age consists of generally unfossiliferous red mudstones, siltstones, gypsum, and sandstones that contain abundant sedimentary structures, such as ripple marks, cross-stratification, ripple laminae, salt crystal casts, mud cracks, sole marks, parting lineation, and core-and-shell structures. Three informal members were established for this study: the lower member, the lower massive sandstone, and the upper member. Flaser, wavy, and lenticular bedding, bimodal distribution of ripple laminae dips, parallel ripple marks dominant over cuspate ripple marks, gypsum beds and veins, salt crystal casts, and lack of channel deposits are the suite of sedimentary features that are interpreted to indicate a tidal-flat environment during deposition of the lower member. The very fine grained lower massive sandstone can be divided into four subunits, which were formed by a transgression-regression of the sea. Wavy and ripple laminated beds in subunit one were probably deposited in very shallow water. Medium-scale wedge-planar and trough sets of cross strata with mean dip directions to the southeast make up subunit 2 and indicate megaripples formed by longshore drift. Subunit 3 consists of lenticular, wavy, pod-shaped beds that were created in water shallower than that for subunit 2. Continuous, large-scale, low-angle cross strata of uniform thickness and medium-scale wedge-planar and trough sets of cross strata characterize subunit 4 and are typical of beach deposits. The significant sedimentary features in the upper member are unimodal distribution of ripple laminae dips, cuspate ripple marks dominant over parallel ripple marks, channel deposits with shallow trough cross strata, an increase in the number and thickness of sandstone and siltstone beds compared with the lower member, plus vertebrate bones, tracks, and plant impressions. This suite of features indicates a flood-plain environment. Early in Moenkopi time, north-central Arizona was a tidal flat and sabkha. The sea to the west fluctuated east and west and finally transgressed over the entire area. As the sea regressed, a beach formed, and rivers flowing from the east deposited sediment on a westward-prograding flood plain. In the northern, southern, and central portions of the region, sabkhas existed for a time during regression. At the end of Moenkopi time, the entire area was a flood plain. Considering the association of red beds and evaporites, the absence of fossils in the lower member and the lower massive sandstone, the paleowind directions, and the theory of continental drift, the climate during early and middle Moenkopi time was probably hot and arid. The influx of sandstones, the presence of Calamites (?) impressions, and trackways and bones of amphibians in the upper member suggest that the climate became more humid at the end of Moenkopi time.

Ce mémoire aborde l'hydrogéologie de cette plaine de la Tunisie orientale sous les aspects : géomorphologique, climatique et geochimique.

Le bassin versant de la Sebkha d'Oran , allongé du Nord-Est au Sud-Ouest ,forme un bassin sédimentaire sublittoral ; bordé au Nord par les massifs littoraux (Murdjadjo) et au Sud par les monts Tessalas. L'étude hydro climatologique confirme un climat de type seml aride. Les données de la pluviométrie, de l'évapotranspiration et du ruissellement, ont permis l'établissement d'un bilan hydrologique. La structure géologique détermine deux grands ensembles: - un substratum ante-synchro-nappe affleurant dans Ies zones hautes. - un remplissage néogène dans le bassin. Les calcaires messiniens constituent le meilleur réservoir hydrogéologique de la région. La corrélation des forages nous a permis de confirmer l'absence de leur continuité sous la sebkha. Dans la plaine de Mleta ceci a été confirmé par des sondages électriques. L'étude piézométrique de la nappe alluviale révèle une importante sédimentation par les calcaires messiniens au Nord de la sebkha et une alimentation par les cônes de déjection des oueds dans la Mleta.L'écoulement de la nappe se fait globalement en direction de la sebkha. L' interprétation des résultats des essais de pompage a permis de déterminer partiellement les caractéristiques hydrodynamiques de certains aquifères. L'étude physico-chimique des eaux de la nappe entre Oran et Boutlelis permet une bonne distinction entre les eaux peu minéralisées, à faciès bicarbonaté calcique ou magnesien des calcaires et les eaux très minéralisées, à faciès chloruré soidique des alluvions . L'étude des sources indique la forte in fluence des évaporites dans les Tessalas. Les analyses de teneur en Tritium des eaux de certaines sources nous ont renseigné sur leur mode de circulation. Les mécanismes contrôlant la salinisation du bassin versant ont été détaillés; il en ressort que la sebkha n'est pas à l'origine de cette salinisation mais en serait plutôt la conséquence.