[en] 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.
