QUATERNARY PALEO-ENVIRONMENTAL EVOLUTION OF THE LAGOON OF TUNIS (GULF OF TUNIS, TUNISIA)

The eastern coast of Tunisia represents a marginal-littoral area with lagoons and coastal sebkhas including that of Tunis. The latter, located in the northern part of this coast, corresponds to an old bay which was widely open on the Gulf of Tunis around 190,000 yr BP (Zaara et al., 2010). Nowadays, it is separated from the Mediterranean Sea by a narrow and sandy beach barrier stretching from the mouth of Oued Meliane to the Punic harbour of Carthage. This beach barrier is mainly fed by the Medjerda and Miliane sediments and also by the sediments from the erosion of the Sidi Bou Said cliff. The hydrodynamic analysis shows that the input of marine waters in this area mainly takes place through the Kheireddine (N) and Rades (S) passes. Furthermore, the Tunis lagoon also receives water from small rivers such as O. El Grich and O. El Gsab. Therefore, the solid and liquid contributions are small and the filling rate is slow. This study is part of a research program which tries to ascertain the interactions between fauna and its environment during Quaternary climatic variations. For that purpose, a 55 meter-long core was drilled in the alluvial deposits of the north-eastern edge of the lagoon, and subjected to a high resolution sampling. The collected material was used for sedimentological, mineralogical and paleontological studies in order to identify the main conditions of this wetland formation, to follow the temporal distribution of the sedimentary stock, and to reconstruct its paleoenvironmental evolution. Sedimentological analyses reveal an essentially marine sedimentation, consisting of muddy clayey silt at the base, indicating a relatively quiet depositional environment, followed by a higher energy one leading to sandy and sandy-silty sediment. Mineralogically, these sediments consist mainly of calcite, quartz and aragonite, accompanied by low levels of magnesium calcite, dolomite and feldspar. The fraction of clay minerals in the lagoon of Tunis is made of smectite, illite, kaolinite and chlorite. The distribution of these minerals along the stratigraphic column reflects the nature and importance of detrital supplies under the influence of a predominantly hot and wet climate, probably with alternating wet and dry seasons. In addition, the faunal associations (corals, ostracods and benthic foraminifera) allowed monitoring the depositional environment. They indicate a true circa-littoral shallow marine environment, evolving towards an open lagoon. The U-Th dating of the Balanophyllia regia corals collected at different depths of the stratigraphic column reveals an age ranging from 165 to 205 ky. These results indicate a Middle Pleistocene age for the lowermost part of the section, while the uppermost 10 meters correspond to the Holocene period. This evolution is related to the importance of the detrital contributions of the Medjerda and Meliane wadis and the eustatic variations of the sea level during the Quaternary.