Zagrarni, MF; Negra, MH; Melki, S
In Central Tunisia, Turonian rudist limestones exhibit planar - convex and subsymmetric lenticular bodies. The latter are rich in complete to fragmented rudists, corals, red algae and sponges. Petrographical and sedimentological analysis have allowed three main types of rudist limestones to be distinguished. Type 1 rudist limestones which form the upper part of Unit B1 of the Bireno Member, consist of micritic to slightly bioclastic limestones, containing rare complete rudists. On a microscopic scale, the limestones correspond to wackestones - packstones rich in fragmented hippuritids and radiolitids, peloids and scarce red algae debris. In terms of paleoenvironment, these limestones suggest accumulation of rudist fragments in a shallow marine and moderately energetic environment. Type 2 deposits which form the lower part of Unit 133 of the Bireno Member, are mainly composed of entire joined rudists organised in carbonate sequences separated by erosive bases. Each sequence starts with bioclastic limestones rich in rudist debris passing vertically into rudist limestones containing entire and joined rudists in inclined growth position, overlain by rudist limestones in which rudists are in vertical growth position. Type 2 rudist limestones directly overlie packstones-grainstones rich in rudist debris showing cross bedding and other current structures suggesting storm deposits. In terms of paleoenvironment, the inclined orientation of rudists and the frequency of rudist fragments indicate episodic high energy conditions. Type 3 rudist limestones which constitute the upper part of the Unit 133 of the Bireno Member, contain an association of rudists, corals, red algae and other components, suggesting relatively open marine conditions which are preferentially provided by palaeohighs such as horsts. In some areas ( e.g. Ain el Kohol, north western part of Jebel Bireno), rudist lithofacies preferentially developed on palaeotopographic highs, limited on both sides by normal faults. All the distinguished rudist limestones suggest shallow water open marine conditions in a carbonate platform domain. However, local tectonic-related processes could have generated a compartmentalisation of this previous platform domain into normal faults sensibly orientated N110 to N130, constituted the most elevated areas favourable for the installation of rudist-coral congregations. On the other hand, global changes such as eustatic processes appear to have been significant during the Turonian interval. Similar facies rich in rudists and corals which have been described in other localities, especially in North Africa and Europe, appear to have a widely extended occurrence.