Skeletal assemblages and terrigenous input in the Eocene carbonate systems of the Nummulitic Limestone (NW Europe)

Abstract

Terrigenous input is often considered detrimental for carbonate producing organisms, however, the common occurrence of mixed siliciclastic–bioclastic deposits indicates that the relationship between carbonate factories and terrigenous fluxes is a complex issue. To investigate this subject, we analyzed the skeletal assemblages of the Paleogene Alpine foreland basin in a wide area encompassing NW Italy and SE France. Four different sections, Mortola, Loreto, Braux and Lauzanier, deposited between the Bartonian and the Priabonian, were studied in detail and, based on microfacies analysis, six main biofacies were recognized: i) nummulitid biofacies and ii) acervulinid and coralline algal biofacies related to shallow water; iii) nummulitid and orthophragminid biofacies and iv) coralline-algal branches and large benthic foraminifera biofacies related to intermediate depth; v) orthophragminid biofacies and vi) orthophragminid and coralline algal biofacies related to deeper settings. Thin sections and X-ray diffraction analyses show that these biofacies can be related to two major carbonate factories. The former was dominated by free-living benthic foraminifera and was characterized by a relevant terrigenous fraction, indicating free-living benthic foraminifera as the most terrigenous-tolerant group of carbonate producers of the Nummulitic Limestone system. The latter was dominated by encrusting acervulinids and coralline algae and thrived far-off major terrigenous sources. Conversely, recent and Neogene coralline algae are known to be able to tolerate high sedimentation rates. The distribution of coralline-algal-rich skeletal assemblages in the Nummulitic Limestone thus hints that Eocene coralline algae might have been fundamentally different (probably less adaptable) than their more modern counterparts.