Quaternary International 93–94 (2002) 109–125 Sedimentation and soil formation phases in the Ghardimaou Basin (northern Tunisia) during the Holocene Christoph Zielhofera,*, Dominik Fausta, Fernando Diaz del Olmob, Rafael Baena Escuderob a TU Dresden, D-01062 Dresden, Germany b Universidad de Sevilla, E-41004 Sevilla, Spain Abstract Chronological and stratigraphical interpretations as well as 14C-dates and geoarchaeological investigations of detailed profiles from the mid-Medjerda valley (northern Tunisia) allow us to reconstruct one late Pleistocene and four Holocene sedimentation cycles within the floodplain area. Initial results from pedological examinations, including thin sections, indicate a latest Pleistocene and three Holocene soil formation periods. Our observations from the Medjerda valley are discussed in relation to current research in the southwestern Mediterranean region. The Crise Romaine—shown in the headwaters of the Medjerda river system by thick cobble accumulations—is exhibited in the basin region by a clear accentuation of the water level amplitude. Catastrophic flooding can be observed for the first time in the late Roman period. After soil formation during Middle Ages, ending at about 450 BP, flooding again increased. r 2002 Elsevier Science Ltd and INQUA. All rights reserved. 1. Introduction slope of the Tunisian Tell region, whose hills in the Kroumir Mountains reach to over 1200 m. A much During the DFG-Project, ‘‘Reconstruction of Holo- greater catchment area lies in the south, drained by the cene environmental conditions in North-Tunisia based main tributaries Oued Melleque, Oued Tessa and Oued on sedimentological analysis and pedogenetic findings Siliana (Fig. 1). They drain the north-facing slopes and from the mid-Medjerda valley’’, detailed profiles of piedmonts of the semiarid Dorsal Mountains. The Oued outer bank positions of the Oued Medjerda were carried Medjerda itself rises in the semiarid Atlas Mountains of out. From chronological and stratigraphical interpreta- eastern Algeria. In the area of the upper middle course, tions we are able to reconstruct the water level before the river is joined by the main southern amplitude of the river during the Holocene. Miner- tributaries, it passes through the Ghardimaou Basin alogical, palaeomagnetic, pedological and geoarchaeo- (Fig. 2), a tectonic depression with 8–10 m thick logical examinations, as well as 14C-dates, allow Holocene floodplain sediments. The Ghardimaou Basin derivation of soil formation and sedimentation cycles. has been settled since at least the latest Palaeolithics, as The initial results of the ongoing project broaden shown by many artifacts found within the sediment knowledge of morphodynamics during the Holocene in layers. northern Tunisia and allow us, to some extent, to draw parallels to comparable work in the West-Mediterra- nean region. 3. Methods 2. Area of investigation The profiles we have introduced are limited to the Ghardimaou Basin (Fig. 3) and are the result of a 6- The Oued Medjerda is the main water course in week stay in the project area itself. Profiles taken from North-Tunisia. The northern catchment area includes the eastern linked Jendouba Basin (Fig. 3) remain at this the subhumid to Mediterranean-humid south facing point unexamined, as the complex fluvial dynamics of the tributaries hinder the recognition of sedimentation *Corresponding author. series during the Holocene. 1040-6182/02/$ - see front matter r 2002 Elsevier Science Ltd and INQUA. All rights reserved. PII: S 1040-6182(02)00010-1 110 C. Zielhofer et al. / Quaternary International 93–94 (2002) 109–125 4. Key profiles in the Ghardimaou Basin Catchment area of the Oued Medjerda The most important profiles for the Ghardimaou Basin are those of (OM) Chemtou (CH) and Sidi Abdallah (SA) (Fig. 3). After the initial observations of the outer s tain bank positions within the basin, these profiles were Tabarka un Mo ir chosen for detailed soil sampling and analysis. These m ou Kr three exposures allowed observation of the sedimentation Tunis and soil formation spectrum during the Holocene in a somewhat representative and compressed form. 4.1. Oued Mliz Profile (OM) Sousse Kairouan ins nta This site lies approx. 500 m northeast of the village of ou l M OM. The village takes its name from the small, southern sa or D tributary, which enters the Ghardimaou Basin in this area. Thus the exposure lies in the interlocking area of Oued Medjerda as well as that of OM. The actual Tunisia location is an outer bank position of the OM directly N before it joins the Oued Medjerda (Fig. 3). In the OM, 16 sediment layers were defined according to field observations and laboratory analysis (Fig. 4), 050100 km grouped into five main sedimentation series (Fig. 7). With the exception of the first incomplete series (S1), the basal sequences of series S2–S4 mainly show sandy to Fig. 1. Catchment area of the Oued Medjerda. silty soil texture, with the upper parts of the individual series fining upwards to clayey sediments. Series S1 (980–620 cm) consists of, from base to top: * OM1: silty clay, very calcareous, hydromorphic features (weak staining of slightly developed aggre- gate surfaces); * In situ fire-site, 11,440750 BP (13,790–13,160 Cal BP; Beta-135718); * OM2: silty clay, very calcareous, slightly aggregated, hydromorphic features. Series S2 begins with rhythmic lamination of silty– clayey to medium-sandy sediments (OM3). In some places gravel layers (OM4; loamy sand, very calcareous) are embedded. The overlying OM5 layer is clay loam, Fig. 2. Ghardimaou Basin—view from the Northeast (photo). extreme calcareous, subangular blocky (7.5 to 10 YR 6/4, light brown to light yellowish-brown). This unit is transitional along a diffuse contact to clearly aggre- gated, subangular blocky, calcareous, humic, silty clay Previous stratigraphical and chronological analysis (OM6). S2 closes with sediment (OM5 to 6) indicating of sedimentation in the Ghardimaou Basin is based on soil formation beyond the degree of simple humic the dating of many artifacts and pottery fragments, 14C- enrichment. The light reddish-brown color and the dating (AMS) of charcoal (calibrated after Stuiver et al., subangular blocky structure of the clay loam (OM5) 1998), pedological and sedimentological field observa- show slight rubefaction. Bean-sized calcareous concre- tions (color, soil texture, soil structure, bioturbation, tions in the lower part of S2 indicate calcareous humic content, rubefaction, calcareous concretions, precipitation during soil formation. etc.), as well as comparison of individual profiles. Series S3 (440–250 cm) consists of (Fig. 4): Fortunately, the charcoal samples consisted almost entirely of in situ fire-sites, so that we can assume that * OM7: silty clay, very calcareous, prismatic structure, we dated reliable material. Palaeomagnetic analysis of hydromorphic features (2.5 Y 5/4, light olive-brown), the samples is still ongoing. 30–160 cm thickness; C. Zielhofer et al. / Quaternary International 93–94 (2002) 109–125 111 Fig. 3. Area of investigation and key profiles. 112 C. Zielhofer et al. / Quaternary International 93–94 (2002) 109–125 * OM8: loam, very calcareous; slow accumulation under lagoonal conditions. This * OM9: silty clay, very calcareous; accumulation covers a great part of the floodplain, * OM10: silty clay, humic, very calcareous, angular mostly situated away from the main riverbed. The blocky; soil sediment. humic, silty clay (OM10) is solely a re-deposited soil, as in situ soil formation processes cannot be confirmed Thin section analysis, conducted by N. Gunster,. from the micromorphology. Physical Geography, Bonn University, confirms the The transition to series S4 is indicated by a marked hydromorphic character of OM7. Soil texture, color, change in the soil texture to fine-sandy (OM11) as well hydromorphic features and prismatic structure indicate as by the presence of numerous Roman pottery Fig. 4. (a) OM Profile: mainly composed of fine sediments, shows a flat and even bedding of the layers. The black horizon (OM6) is seen as a humic in situ soil formation. According to our results obtained in southern Spain we would suppose this soil to having been formed during Neolithics. Below the dark humic horizon we observed a slight reddening (OM5) and some calcareous concretions (OM3). In the upper part of the sequence three colored horizons can be distinguished. The upper dark one (OM14) and the brown layer (OM12) seem to be a redeposition of eroded soils. The sequence ends with laminated latest sedimentation (OM15) that happened during the last 400 years. (b) OM Profile (photo). C. Zielhofer et al. / Quaternary International 93–94 (2002) 109–125 113 the CH does not lie directly in the meandering belt of the Oued Medjerda, but on an outer bank position of the small northern tributary Oued Melah, shortly after it enters the northern edge of the Ghardimaou Basin. The profile represents the interlocking sedimentation area of both rivers. Fig. 5 shows the CH containing exposures from inner bank to outer bank positions. Altogether 23 layers, including some which are only fluviatile facies, are divided into—similar to the OM Profile—five main sedimentation series (Fig. 7: S1–S5). Series S1 (850–510 cm) consists of * CH1: silty clay loam, calcareous, angular blocky; calcareous concretions; * CH2: silty clay loam, noncalcareous aggregate surfaces, angular blocky; * CH3: clay, noncalcareous, angular blocky, clay films; B-horizon (5–7.5 YR 4/6, yellowish-red to strong brown); * CH4: silty clay, calcareous; * CH5: in situ cultural layer (Iberomaurusien! ); 11,160750 BP (13,360–12,990 Cal BP; Beta-135726). In the lowermost part of series S1, a reddish soil has developed (CH1–CH3), whose humic horizon has been eroded. The clay-rich B-horizon (CH3) is fully decalci- fied, has a markedly angular blocky structure and displays clay films on aggregate faces. The subsoil (CH1) Fig. 4 (continued). exhibits bean-sized calcareous concretions in a calcic, grayish-brown soil matrix. Series S1 concludes with an in situ cultural layer (CH5) with a thickness of between 50 and 70 cm.