Facies succession, biostratigraphy and rudist faunas of Coniacian to Santonian platform deposits in the Sant Corneli anticline (Southern Central Pyrenees)

The main lithostratigraphical units of the northern flank of the Sant Corneli anticline are briefly described and discussed. Some new units are erected and, one of these, the Montagut limestones unit group together some latter members: Montagut Mb., Clot de Moreu Mb. and El Grau Mb. A biostratigraphic survey of Coniacian and Santonian outcrops from the St Corneli anticline northern flank is presented. The age of the carbonate platform facies is based on their precise correlation bed by bed with slope facies showing abundant inoceramid, ammonite, and some planktonic foraminifera faunas. The compilation of all these data allow us to differentiate some biozones/associations ranging from the Middle Coniacian to the Upper Santonian. On the basis of large collections of rudists, thirty species of Hippuritidae Gray, Radiolitidae d'Orbigny, Requieniidae Douvillé, Plagioptychidae Douvillé and Caprotinidae Gray are discussed, ranging from the upper part of the Middle Coniacian to the Upper Santonian. Some observations on patterns of rudist evolution are recorded. The kind of morphological change between species and its stratigraphical position have been recognised, allowing us to define more precisely the boundary between species. The evolution of the arête cardinale in a rudist lineage is the best character to differentiate species. And, this better understanding of the boundary between species is the key to use rudists as biostratigraphic tools.