Sur l'evolution des paleo-environnements marins paleogenes des bordures occidentales du Detroit Siculo-Tunisien et leurs rapports avec les fluctuations du paleo-ocean mondial

Abstract

The study of benthic and planktonic Foraminifera along five sections distributed over the Jebel Abderrahman anticlinal limits, has made it possible to reexamine and precise the biostratigraphy of the Palaeogene series which are visible on outcrops in the heart of the Cap Bon anticline. We show that the studied series starts with the Souar Clay in the Middle Eocene, at the level of the Globigerinatheka subconglobata subconglobata zone (Bolli, 1972) and ends with the Mahmoud Clay in the Middle Miocene at the level of Orbulina suturalis zone (Cati et al., 1968). Thus, we confirm that the lumachellic bar called “Burdigalian Bar” (Aïn Grab Formation) is in fact Langhian. The paleo-ecological interpretation of the results allows us to precise the position of the Cap Bon marine environments within the framework of Palaeogene paleo-oceanography known up to now. The Eocene series of Jebel Abderrahman are sedimented just below the hinge line of the Halk El Menzel platform defined by Bismuth and Bonnefous (1981) within a transition area between temperate zones and tropical zones. This area was inundated more often and more abundantly by tropical waters than the Possagno (Italy) and the Prérif regions. This fact reflects the determining influence of the Palaeogene Tethyan paleo-currents along the southwestern borders of the Sicilian Channel, securing the transfers of Indian Ocean waters towards the Atlantic Basin. The importance of some clay formations embedded in the “Souar Clays” can be interpreted by the thermic variations and fluctuations of the paleogeoide which affected the world paleo-ocean during the Middle and Late Eocene. In general, the development of carbonates is correlated with a period of high sea level which separates two rapid falls of eustatic origin. Thus, the “Reineche calcareous clay” is associated with the beginning of the period of high sea level, occurring between the rapid falls at the base of the Globorotalia lehneri zone and at the top of the Truncorotaloides rohri zone. The sedimentary cycles of the Jebel Abderrahman reflect the eustatic cycles and supercycles. The “Fortuna Sandtones”, built up of detritic material with a complex history, contain a microfauna which shows that this classic formation is an example of rhexistatic resumption which is synchronous with the world-ocean cooling and took place after the rapid sea-level lowering marking the top of the Eocene. We show that the fall of the base level of rivers due to this paleogeoid variation, increased the resumption of erosion which had started under the influence of climatic and/or tectonic evolution. This leads us to reflect on: (1) the significance of the complete Oligocene detritic series in the Sicilian Channel and on all the borders of the Western Mediterranean Basin; and (2) the geodynamic evolution of the Pantelleria Rift during the Oligocene (thermal doming). The Cap Bon Peninsula and surrounding areas are favorable zones to analyse the eustatic fluctuations of the paleogeoid in the Tethys during the Cenozoic.