Les Terrasses Historiques Du Bassin De Kasserine (Tunisie Centrale) the Historical Terraces of the Basin of Kasserine (Central Tunisia)
Total Page:16
File Type:pdf, Size:1020Kb
Géomorphologie : relief, processus, environnement, 2008, n° 3, p. 201 Les terrasses historiques du bassin de Kasserine (Tunisie centrale) The Historical terraces of the basin of Kasserine (Central Tunisia) Amor Bkhairi* et Mohamed Raouf Karray* Résumé Dans le bassin de Kasserine, les dépôts historiques atteignent plusieurs mètres d’épaisseur et représentent une superficie importante le long de l’oued Htab et de ses affluents méridionaux. En certains endroits, ils recouvrent des sédiments allant de l’Holocène moyen au Pléistocène supérieur et ils forment aussi trois terrasses emboîtées dans la terrasse principale d’âge fini-pléistocène à holocène moyen. Les sédiments de ces terrasses sont riches en matériaux datables au 14C (Helix, charbon de bois et ossements) ainsi qu’en vestiges archéologiques (silex taillés, céramiques et structures romaines), permettant d’établir un calage chronologique et des corrélations. Ce travail précise les âges relatifs et absolus des dépôts et des formes, établit la chrono-stratigraphie des derniers 2 300 ans et esquisse l’étude morphodynamique de leur évolution. Il apporte de nouvelles précisions sur la signification anthropique et climatique de la haute terrasse historique du bassin de Kasserine. Mots clés : terrasses historiques, datation 14C, céramiques romaines, bassin de Kasserine, Tunisie. Abstract In the Kasserine basin, several meters thick deposits of Historic age are widespread along the Htab Wadi and its southern tributaries. In places, they cover Late Pleistocene to mid Holocene sediments and form also three terraces built inside the main late Pleistocene to mid Holocene terrace. The deposits of the three terraces are rich in 14C dateable materials (Helix, charcoal and bones) and archaeo- logical artefacts (flint tools and Roman ceramics and walls) that allow relative dating and correlations. This paper presents the absolute and relative ages of deposits and terraces, establishes a chrono-stratigraphy for the last 2300 years and outlines the emplace- ment processes and geomorphological evolution. The present study gives new data on the anthropogenic and climatic significance of the Historic terrace of the Kasserine basin. Key words: Historic terraces, 14C dating, Roman ceramics, basin of Kasserine, Tunisia. Abridged English version relations between sections, propose a chrono-stratigraphy and clarify their relationship with the earlier Upper Pleis- The middle course of the Htab Wadi drains the 21.5 km tocene – mid Holocene deposits and landforms. long and 8 km wide Kasserine basin, which has an area of Lying below the main terrace (T), there are three morpho- ca. 140 km2 (fig. 1). This morpho-tectonic basin contains logical terraces of Historic age in the basin. The deposits of Miocene to Quaternary sediments (Roumiguières and Uguet, the first terrace (T1) were set down during a long time from 1946; Chihi, 1984 and 1995). The Htab Wadi discordantly the Upper Pleistocene to the Roman period. Two younger crosses the basin in a shallow but wide corridor containing alluvial terraces (T2 and T3) formed by successive incision Upper Pleistocene, Holocene and Historic alluvial deposits and re-sedimentation. during Historic and Modern times. (Hentati, 1977; Bannour and Bouallègui, 1979; Bannour Stratigraphic sections of the main terrace (T) of the Htab and Bonvallot, 1981; Medus and Laval, 1997; Oueslati, Wadi in Bled Haria yielded 14C dates of 21,225 ± 995 and 1988). The present study describes four stratigraphic sec- 11,873 ± 764 years cal. BP in the lower part, which lies tions of Historic deposits and their landforms. Thanks to 14C in unconformity above the Upper Pleistocene deposits. Cap- dating and new stratigraphic find of archeological lithic sian artefacts (of early to mid Holocene age) were found in tools, pottery and structures, it is possible to establish cor- the upper part of this unit. The section is capped by surface * Laboratoire de cartographie géomorphologique des milieux, des environnements et des dynamiques, faculté des Sciences humaines et sociales, université de Tunis I, 94 boulevard 9 avril 1938, Tunis 1007. Courriel : [email protected], [email protected] Amor Bkhairi et Mohamed Raouf Karray deposits, in which Roman walls were found (fig. 2). Other A new phase of incision I3 followed the 5th century AD, so outcrops of terrace T along the Htab Wadi near Kasserine that the previous accumulation formed a distinct terrace town have provided close to the base a 14C date of 11,173 ± from the T1 higher one. The change of morphogenesis coin- 764 years cal. BP, while the upper sediments contain Cap- cides with the tendency to a wetter climate between the 5th sian artefacts giving an age of 6,000 years BP. and 14th centuries AD (Brun, 1985; Bourgou, 1991). How- In the middle part of the Seid Wadi (fig. 5A), the base of ever, this main phase of incision can be related to man T1 presents Upper Pleistocene deposits with Mousterian influence, namely the abandonment of the Roman land-use artefacts (80 – 40 ka), which are overlain in unconformity after the 6th century (Hitchner, 1989). by deposits containing artefacts attributed to the Capsian A short period of accumulation is responsible for the allu- interval (9 – 6 ka). Close to the base of upper units, a 14C vial cover of T2 that contains worn Roman ceramics and date of 1,948 ± 236 cal. BP was obtained. charcoal. This relatively coarse cover suggests a rather short Radiocarbon dating of the lower layers of the upper His- wet pulsation that did not allow thick aggradation or con- toric T1 terrace (fig. 2, Htab Wadi, fig. 4B Ali Seida Wadi, spicuous soil formation. A fourth phase of incision I4 followed and fig. 5, Seid Wadi) gives ages between 2,439 ± 288 and and created the intermediate historic terrace T2. It started 1,948 ± 236 years cal. BP. The youngest age was obtained before 609 ± 40 years cal. BP and correlates with the end in a Roman soil. (Hamza, 1987, 1988; Bourgou and Oues- of the Medieval Age. During this incision phase, the river bed lati, 1987; Ballais, 1991; Oueslati, 1995; Faust et al., slightly incised the base of Upper Pleistocene deposits 2004). The chronology of middle and upper parts of the T1 (fig. 4). A final phase of minor accumulation started at the terrace is based on the abundance of Roman ceramics from end of the Medieval period (609 ± 40 years cal. BP, fig. 5) the 4th – 6th centuries A.D. (fig. 3 and fig. 5), that is the and has continued to the Modern period. Its surface deposits latest Roman period. The deposits of the middle Historic T2 contain 20th century plastic objects and metal cans (Oues- terrace are scarce and not yet well dated, although they fol- lati, 1988). Currently and during the last several decades the low the end of the Roman period and the formation of T1 river has slightly incised these modern deposits and formed (fig. 4B). The lower Historic T3 terrace is partly composed a low terrace T3. This tendency to linear incision and asso- of coarse deposits (fig. 3 and fig. 5). The base is dated from ciated badland gully erosion is characteristic of the current 609 ± 40 years cal. BP at Wadi Seid. The T3 terrace con- morphogenic system of Central Tunisia (Hamza, 1988; tains many occupancy evidences:burned stones, small bones Zeraï, 2006). and charcoal. In addition to the three main terraces there is a recent alluvial accumulation of limited extent associated Introduction with some meandering bends of the Ali Seida and Oudien Tlou Wadi. En Tunisie centrale, le cours moyen de l’oued Htab draine The study of stratigraphic sections of Historic alluviums le bassin de Kasserine (fig. 1) dont la forme s’allonge sur reveasl a succession of nine phases of incision/erosion and 21,5 km et s’élargit à 8 km, couvrant ainsi près de 140 km2. sedimentation following the formation of the upper Pleis- Dominé par les reliefs énergiques des jebels Chambi tocene – mid Holocene accumulation (fig. 6). The nine (1 544 m), Semmama (1 314 m), Selloum (1 248 m) et phases indicate variations in climatic and anthropogenic Maargaba (814 m), ce bassin correspond à un fossé d’effon- influences. A Neolithic palaeosoil with a 14C date of 4,473 ± drement rempli de sédiments néogènes et quaternaires (Rou- 502 years cal. BP (fig. 4B) can be taken as a marker for the miguières et Uguet, 1946 ; Chihi, 1984, 1995). Le fond du end of the early Holocene sedimentation and the start of these bassin, d’une altitude moyenne variant entre 630 et 650 m, nine phases. est une vaste plaine où affleurent des dépôts attribués au Phase 1 (I1b, fig. 6) is a post-Neolithic phase of incision Pléistocène supérieur et à l’Holocène (Hentati, 1977 ; Ban- identified also by Ritchie (1984), Coudé-Gaussen (1984), nour et Bouallègui, 1979 ; Bannour et Bonvallot, 1981 ; Ballais et al. (1988) and Bourgou (1991). It was followed by Medus et Laval, 1997) ainsi que des dépôts d’âge historique aggradation characterised by coarse deposits (fig. 4B) (Oueslati, 1988). indicating a tendency to large floods. After a small incision La première étude géomorphologique concernant les I2, within the higher Historic terrace, a phase of relative dépôts historiques dans le bassin de Kasserine est due à biostasy occurs between 2,439 and 1,948 BP, characterised H. G.Molle et K. U. Brosche (1976). Elle a permis la data- by slow aggradation and soil formation, later exploited by tion de dizaines d’échantillons prélevés dans les séries the Romans. This relative stability is probably due to rain- quaternaires des piémonts du Jbel Chambi et du Jbel Mrhila fall optimum and widespread natural vegetation combined et a indiqué l’existence de dépôts historiques datés sur des with inefficient human impact on the morphogenic system. hélicidés de 1955 ± 145 ans BP.