Une urbanisation linéaire, dynamique demographique et mutations spatiales dans la peripherie littorale algeroise : Cas de la commune de heuraoua

De nouvelles dynamiques caractérisent le monde d’aujourd’hui. Certaines ont donné lieu à de nombreux processus géographiques, parmi lesquels figure la littoralisation. L’Algérie connaît une dynamique démographique qui a renforcé la concentration des hommes et de leurs activités sur sa frange littorale où se localisent trois des quatre métropoles nationales, Oran, Annaba et Alger. L’empreinte du développement de ces métropoles, notamment celui d’Alger correspond à la croissance de son agglomération, durant ces deux dernières décennies, par l'extension de son tissu urbain sur sa périphérie. Ce processus d’artificialisation du sol a marqué la morpho-logie de toute la région algéroise et provoqué, entre autres, la dégra-dation environnementale et la diminution des ressources naturelles de son littoral défini, pourtant, comme un des espaces sensibles, par la politique d’aménagement du territoire en Algérie. Les communes périphériques de l’agglomération algéroise connais-sent une forte dynamique démographique, qui a donné lieu à diverses mutations spatiales, notamment dans la partie orientale du littoral algérois.Mots Clefs :Dynamique démographique, Structure agraire, Périurba-nisatio

Karstic geomorphology of carbonate Ouarsenis Piedmont (Boukadir region, Chelif) in Algeria: The role of the Messinian Salinity Crisis

peer reviewedAlgeria displays various karst landscapes due to its diversity in lithology, relief, age and climate. On 16th June 1988, in the northwest of Algeria, a 60 m wide sudden collapse occurred in the Chelif Basin about 1 km north of a marginal carbonate platform. Despite this large event and visible karst dissolutions in the platform, this region has not been classified among the karst areas of Algeria yet. Our study focuses on this Messinian carbonates that form the northern piedmont of the Ouarsenis Mountain range and are covered to the north by Plio-Quaternary deposits. The geological and geomorphological data that we collected reveal that the present-day karstification is limited at the outcropping surface. Present-day carbonate dissolution is impeded by the absence of a topsoil supplying CO2 and by the presence of a calcrete improving the drainage. Although dissolution at depth is generally diffuse due to the porous and friable nature of the carbonates, two factors can, on the contrary, concentrate water infiltration: the presence of a network of more or less subvertical fractures and, occasionally, at the surface, the absence of calcrete, independently of the fracturing nodes There are few ponors and sinkholes present. The endokarst is still present as evidenced by rare caves. In epikarst, solution pipes and shelter caves are prevalent. The later results from differential weathering in relation with the carbonate facies and the progressive calcrete cementation in valleys and slopes during river incision. Near valley bottom, shelters are arranged in steps like terraces. This morphology is related to base-level lowering in relation with the deformation and uplift of the Ouarsenis piedmont. Near the southern edge of the Chelif Basin, deep (>55m) karstic voids are present and associated with paleo-valley incision presently burried by Plio-Quaternary deposits. We interpreted the large 1988 collapse in relation with this paleokarst and propose that its triggering was partly induced by a lowering of the aquifers due to a deficit of precipitation. The deep paleokarst formation and the buried river incision are attributed to the low base-level during the Messinian Salinity Crisis (5.97–5.33 Ma). We evidenced an upper karstic dissolution level filled that is attributed to a per ascendum evolution of the karstic phreatic network in relation with the following Pliocene aggradation

Karstic phenomena of the BOUKADIR-Chlef. Geological, hydrogeological and mineralogical characterization

Calcareous environments occupy 10% of the surface of the globe (continents and seas),. The fragility, dissolution and rapid evolution of these formations constitute, in addition to the occurence of the karstic forms, a major risk and a potential hazard for the equilibrium of these grounds and more specifically for the infrastructures. Carbonate rocks are also present in the Mediterranean Basin and represents, depending on the country, about 30 to 70% of the surface area. The karstic formations also occur in Algeria. Among the best examples observed across the national territory are the ones found in: Tlemcen (karstification well developed at depth) and Saida (karstification well developed at the surface), the calcareous ridge of Djurdjura, Jijel... The region of Boukadir situated in the northern piedmont of Ouarsenis is the location of karstic forms and has witnessed the occurence a large collapse of the national road RN4 linking the wilaya of Algiers to Oran in 1988. However, this region has never been defined as a karstic region. For a better understanding of these phenomena, it’s essential to catalog and create an inventory of karstic forms developed in this region, to analyze in detail the geology and hydrogeology, using different tools such as : geological maps, drilling and different stratigraphic logs, aerial photography, hydrogeological and piezometric data, and of course, confirm all with a mineralogical study which is based on a morphoscopic analysis of thin sections and a diffractometric examination (DRX) of the samples collected around this locality. The Analysis and evaluation of this hazard is therefore dependent on the knowledge and study of the geology and hydrogeology of these lands, and especially the detection of the location of this phenomenon

Le risque karstique à BOUKADIR (Chlef-Algérie) et son potentiel impact socio-économique

Boukadir, the municipality of the Wilaya of Chlef located west of Algiers has never been defined as a karstic region of Algeria despite the formation of a large collapse sinkhole in June 1988. The sinkhole formed a crater with 60m wide and 35 m deep across the national road RN4 linking the Wilaya of Algiers to Oran. The municipality of Boukadir is located near southern edge of a 20 km wide plain flooded by the Chlef River bounded to the south by the Ouarsenis mountain range. The northern piedmont of the Ouarsenis Range is composed of Lithothamnion limestones of the tertiary that form a 25° dipping structural surface. Traces of karstification were founded in these Lithothamnion limestones outcropping at the surface. At the level of the plain and of the collapse sinkhole, the carbonate rocks are covered with younger sediments (quaternary alluviums). In order to objectively evaluate the hazard in the covered karst, we combine a range of boreholes to unravel the geometry of the carbonates and its cover. The characteristics of the limestones are evaluated combining XRD, XRF and thin sections in order to evaluate its dissolution potential. The socio-economic context is reviewed (population, industry, agriculture...) to discuss the karstic risk

Evaluation du risque karstique combinant des données géomorphologiques et géologiques dans la région de BOUKADIR (Chlef, Algérie)

In 1988, a large collapse crater of 60m in diameter and 35m of deep occurred in the national road RN4 linking the wilaya of Algiers to Oran, exactly in the region of Boukadir located in the northern piedmont of Ouarsenis in Algeria. The cover collapse sinkhole is located in the Boukadir plain, where the Chlef river is running, at the foot of the 723m high Oursenis Mt, composed of Lithothamnion limestones of the Messinian (uppermost stage of the Miocene). In Algeria, there are many karst areas like Tlemcen (karstification well developed at depth), Saida (karstification well developed at the surface), or the calcareous ridge of Djurdjura, Jijel... But the area of Boukadir has never been defined as a karstic region despite the 1988 sinkhole and the various karstic forms that we mapped in Oursenis Mt at the surface.We focus in this study on the Boukadir plain at the foot of the Oursenis range, where the carbonate rocks are covered by younger sediments, and where the infrastructures (RN4, highway from the east of Algeria to the West) and villages (Boukadir, 41,655 inhabitants) are located. To assess karstic hazard and related risk for the two major infractructures running at the foot of the Ouarsenis Mt, we combine geological, geomorphological and hydrological data. The examination of the geomorphology using aerial photographs, DEM and satellite images reveals the absence of any subsidence sinkhole, which is related to the fact that the covering sediments contain a significant amount of clay. Indeed the Lithothamnion limestones are covered by the marine marls of the Astien deposited during the Pliocene and clayey continental deposits of the Villafranchien formation (Upper Pliocene). The combination of surface geology with boreholes in the Boukadir plain reveals that the reef Lithothamnion limestsones does to extend across the Boukadir plain. The karstic hazard is thus restricted to a 2km wide narrow band at the foot of the Oursenis Mt

Karstic hazard of the BOUKADIR region (Chlef-Algeria)

La commune de Boukadir, (ex Charon) dans la wilaya de Chlef au nord-ouest d’Algérie, n’a jamais été classée parmi les régions karstiques d'Algérie malgré la grande doline d’effondrement en juin 1988 de 60m de diamètre et 35m de profondeur sur la route nationale. Boukadir se trouve au piémont nord de la montagne de l’Ouarsenis, sur la bordure sud de la plaine de 20 km de large du bas Chelif traversée par Oued Chlef. Le piémont septentrional de la chaîne de Ouarsenis est composé de calcaire à Lithothamniée subhorizontal d’âge miocène supérieur (messinien). Au niveau de la plaine et de l'effondrement, les roches carbonatées sont recouvertes de sédiments plio-quaternaires, là où les infrastructures (RN4, autoroute Est-Ouest d’Algérie A1) et villages (Boukadir, 41 655 habitants) sont situés. Afin d'évaluer le risque karstique, nous combinons différentes approches..

Le karst messinien de la région de Boukadir (Algérie) : origine des cavités et des effondrements.

peer reviewe

Modélisation de l'altération des calcaires messiniens à Lithothamnium: implication sur les conditions paléoclimatiques

Limestones are prevalent in Algeria. These formations since their deposition have been affected by a range of weathering, dissolution and recrystallization processes dependant of the paleoclimatic conditions they sustained. These transformations also affect the potential hazard that these karstic terrains represent. We focus here on the Boukadir Region situated at the foot of the Ouarsenis Mountain (Fig. 1). The region comprises the Ouarsenis northern piedmont composed of ~15o north dipping lithothamnium limestones of the Messinian period that rest unconformably upon blue marls of the upper Miocene, and to the south the E-W striking lower Chlef Basin filled by Plio-Quaternary sediments and flooded by the Chlef River. The lithothamnium carbonates rocks form a major deep aquifer in the Basin. The Basin is crossed along its southern edge by the Relizane strike-slip fault. In June 1988, there was a large collapse sinkhole of 60 m in diameter and 35 m of deep that occurred along the national road RN4 near the southern edge of the basin (LCTP, 1989). Drilling shows that sinkhole can be associated to lithothamnium limestones that were covered by more than 61 m of sediments at that location. No other sinkhole formed since this accident. In this study we investigate the weathering pattern of the lithothamnium limestones to unravel the likelihood of formation of large sinkholes in this formation. Boreholes and quarries show that the Messinian Calcareous limestones of the Boukadir region are deeply weathered and partly recrystallized calcareous rocks; the weathering affects its entire thickness reaching a maximum of 200 m. This weathering pattern is not visible a few kilometres more to the east, in limestones having a similar origin. To unravel the specific paleoclimatic conditions that these limestones sustained, we combine field work and on selected samples, petrographic thin section and mineralogical (XRD) analysis and SEM observations. Field work in quarries and in the wadi shows that the lithothamnium limestones are composed of altering sandstone rich beds with a calcareous cement and bioconstructions rich beds. .