Essai d'interprétation des coupes du Quaternaire récent de l'Oued El Akarit (Sud Tunisien)

Abstract

The cross -sections of the river El Akarit are of considerable importance because they explain the geomorphological and palae-climatical evolution of south-Tunisia for 40.000 years. The present study has permitted us to distinguish the succession of two sedimentary sequences separated by an unconformity formed by the channeling action of running waters and already observed by W.D. Page (1972). Thanks to a multidisciplinary collaboration successive environments of these two sequences have been studied in detail. The first, at the end of late Pleistocene begins with a wet period whose age is uncertain in spite of carbon 14 dating tests and the presence of «mounsterian» levels. The microfauna especially the Foraminifera indicate variable salted ponds with relation to probable transgression of the sea (about 27.000 BP ?). The second sedimentary sequence also begins with hydromorphic deposits, more typical of euryhaline environments with thalassoid affinities according to the microfauna, the Molluscs and probably the microflora. This period is between 8700 and 7400 BP. Here also the pollens indicate a dominant steppe-like vegetation with a slight increase of trees at about 6500-6000 BP, just before the general deposits of gypsum-dusts and the beginning of a sebkha period. The resumption of stream waters and the deposit of loess indicate the end of the extreme dry period and the return to a steppe-like vegetation nearly 3500 years ago. For 2 or 3 millennial even when the marine level remained high, we noted a very speedy gullying of about 8 to 10 m, along this valley. In addition to this paleoclimatic reconstitution this study calls into question the interpretation and the datation of glacis 2-the major shape of south-Tunisian piedmonts-and chiefly the assumption of the tectonic stability of the Oudref threshold between the chotts and the gulf of Gabès.

The upper pleistocene loess of southern Tunisia: A statement

“…Various absolute dates (on charcoal and molluscs) confirm the reality of this period (Brosche and Molle, 1975;Rognon et al, 1983).…”

“…These palaeosols are Upper Pleistocene in age (Figure 2 ) and their almost complete absence in the upper light-coloured loess deposits is a mark of the well-known trend towards desiccation in this region since the beginning of the Lateglacial (Rognon, 1980(Rognon, 1985.…”

“…Slope and alluvial processes reworked those loesses and formed a low-level sandy-silty deposit along the main wadis Rognon et al, 1983).…”

Évolution holocène de la Tunisie saharienne et présaharienne

“…La série chronologique la plus complète, mais aussi la plus complexe, est celle de l'oued el Akarit (FONTES et ai, 1983, ROGNON et al, 1983. Elle comporte une croûte gypseuse peu après 5 995± 50 B.P., indication d'une phase de morphogenèse éolienne due à un assèchement (ROGNON et al, 1983).…”

“…Elle comporte une croûte gypseuse peu après 5 995± 50 B.P., indication d'une phase de morphogenèse éolienne due à un assèchement (ROGNON et al, 1983). Cependant, l'accumulation se poursuit jusque vers 3 680 ± 160 B.P., non sous forme de loess (idem), mais de limons fluviatiles, en même temps que les débits de l'oued augmentent à partir de 3 910 B.P., (ZOUARI, 1988, p.150).…”