Palaeoecologie des formations a rudistes du cretace superieur - l'exemple du sud-est de la France

Abstract

In the Upper Cretaceous of southeastern France, the development of rudist formations occured between the Upper Albian and the Senonian. The diversity of biocoenoses is such that a detailed palaeoecological study is possible, of which the principal character is as follows: 1. (1) Conditions of development. The existence of colonial corals and Chaetetida in the rudist biocoenoses indicates a warm sea, which the sedimentology and the palynology of local terrigenuos facies seem to confirm. The rudist communities appear to be stenobath (infralittoral) and stenohaline. These formations are established on slightly subsident platforms which are generally separated from the basin by a flexure. The intense terrigenous sedimentation during this time impedes the great extension of communities. One observes the absence of primary dolomitic deposition in the internal zones. 2. (2) Characteristics. The preponderance of rudists in the population seems due on the one hand to the evolutive “explosion” of this group at this time and on the other, to the acquisition by these organisms of biological characteristics adapted to a reef environment. These advantages enabled them to replace the corals. The rudist shells are fixed either on hard supports or partially implanted in soft substrates of a calcareous nature. Adaptation to the environment is conditioned by biological forms with fixed valves. Between the open sea area and the coast, it is possible to distinguish in a rudist formation several ecological and sedimentological zones. The individualization of these zones depends upon hydrodynamism, terrigenous influence and the morphology of the substratum. The enduring permanence of the rudist communities during the Upper Cretaceous seems to have been due to the functioning of a double stabilizing mechanism: increased evolutive potential and tolerance of the populations to environmental perturbations. A comparison with living reefs shows a close relationship with regard to the developmental climatic and edaphic conditions and the horizontal distribution of the populations. The differences are of a biological nature: the preponderance of the rudist over the colonial corals and the rarity of wave-resistant structures.