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

Abstract

Algeria 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.