Ecological successions of rudist communities: A sedimentological and palaeoecological analysis of upper Cenomanian rudist assemblages from the South-Provence Carbonate Platform (SE France)

Abstract

Carbonate platforms developed in the southwestern Provence area during the late Cretaceous on the rims of a narrow deep E-W oriented basin. The upper Cenomanian lower Member Fontblanche 1 of the Fontblanche Limestones Formation is exceptionally rich in rudists. This member is well exposed in two quarries with flat outcrop surfaces, thanks to sawn surfaces, that allow 3D investigation of its litho- and biofacies and sedimentary structures. The paper describes the sedimentary sequences of each quarry from a quantitative study of both sedimentary and palaeontological data, both macroscopic and microscopic, derived from quadra counts in the field and thin section analyses. The sequences are one to several metres thick and each of them shows a basal drowning and a final emersion surface with evidence of reworking and dissolution. The microfacies are dominated by floatstone texture, most common grains being bioclasts derived from biocorrosion of rudist shells and benthic foraminifers. The matrix is made mostly of microbioclasts and micrite, resulting from a strong microborers activity. Fossil organisms are regarded as autochthonous or parautochthonous. The overall depositional environment was shallow marine with a maximum depth of a few metres in an inner, peritidal, sheltered and quiet platform. We found a link between the sedimentary facies and rudist ecotypes. The statistical analysis of the fossil counts, using multivariate statistics and clustering, differentiates four fossil assemblages, namely the Chondrodonta assemblage, the Apricardia-Sauvagesiinae assemblage, the Caprotina assemblage, and the Nerinea assemblage, which are considered to represent biotic communities. The replacement of communities through Fontblanche 1 and within the elementary sequences illustrates a relationship between the benthic communities, the degree of biocorrosion, the sedimentary texture, and the sedimentation rate so that ecological processes affect the sedimentary dynamics which in turn affects communities through feedback mechanism. Such an intimate link between the sedimentary facies and ecosystems provides evidence that biotic parameters played an important role in the rudist-rich facies succession.