The Lias-Dogger transition in the Tunisian ridge. New stratigraphical data and Tethyan distension up to the Upper Toarcian

In the Jebel Bou Kornine of Hammam Lif (northern Tunisia), the transition between Lias and Dogger is located in a carbonate formation that is approximately 150 m thick (Kef El Orma Formation). Based on ammonite faunas, its age can be determined as Upper Toarcian (Aalensis Zone) up to Lower Bajocian (Propinquans Zone). Four successive conglomeratic units are recognized in this formation. The two earlier ones are situated in the Uppermost Toarcian (Aalensis Zone, Lugdunensis Subzone), while the later two are in the Opalinum Zone of the Lower Aalenian (Opalinum and Comptum Subzones). These deposits, which associate carbonate-bearing conglomerates with laminated/slumped calcarenites, result from a gravitational flow on a palaeoslope induced by extensive palaeofaults. They testify to a tectonic instability during the Upper Toarcian, causing the formation of two distinct palaeostructural units: the 'Tunisian trough' and the 'Tunisian ridge' carbonated platform, both belonging to the Maghrebian passive margin of the Tethys. We must insist on the part played, as soon as the Upper Toarcian, by the polyphased tilted-blocks tectonics on the deposition of the different conglomeratic units of the Bou Kornine Jebel.Original Abstract: Dans le djebel Bou Kornine d'Hammam Lif (Tunisie septentrionale), le passage du Lias au Dogger s'effectue au sein d'une epaisse formation carbonatee (environ 150 m), la formation Kef El Orma. Son age Toarcien superieur, zone a Aalensis, a Bajocien inferieur, zone a Propinquans, a pu etre bien etabli par les ammonites. Quatre assises conglomeratiques successives sont reperees pour la premiere fois dans cette formation. Les deux premieres, d'age Toarcien terminal, se placent dans la partie superieure de la zone a Aalensis (sous-zone a Lugdunensis). Les deux dernieres s'echelonnent dans l'Aalenien inferieur, de la sous-zone a Opalinum a la sous-zone a Comptum (zone a Opalinum). Ces depots associant des conglomerats carbonates et des calcarenites laminees et < slumpees >, resultent d'un ecoulement gravitaire sur une paleopente induite par des paleofailles en extension. Ils temoignent des le Toarcien superieur, d'une instabilite tectonique jouant entre deux unites paleostructurales distinctes, le < Sillon tunisien > et la Dorsale tunisienne, appartenant tous deux a la marge passive maghrebine de la Tethys. Nous insistons sur le role joue, des le Toarcien superieur, par la tectonique distensive polyphasee