Late Cretaceous heterozoan carbonates: palaeoenvironmental setting, relationships with rudist carbonates (Provence, south-east France)

During Late Cretaceous (Coniacian-Early Santonian) times, heterozoan-rich bioclastic carbonates prevailed in the Provence Platform area. They substituted for rudist-rich carbonates, which flourished during the Late Turonian in the same area. This article presents a detailed sedimentological study of these heterozoan-rich carbonates, carried out in the Méjean syncline, west of Marseille. At this locality, the thickness of the bioclastic series exceeds 500 m. The heterozoan carbonates are referred mainly to packstones and grainstones, organized in meter-thick, cross-bedded sedimentary strata. Microfacies analysis has shown that bryozoans, red algae and echinoderms are the main skeletal constituents mixed with reworked fragments of rudists and benthic foraminifera. Detritic elements are mainly represented by quartz and glauconite grains. In the study area, heterozoan carbonates underwent a complex diagenetic evolution. Iron precipitation occurred early in environments with high carbonate productivity and a high hydrodynamic level. Early-marine cements developed mainly in the upper portions of shallowing-upward parasequences during intervals with decreasing sedimentation rates. Late burial cements occurred sequentially as syntaxial rim cements, dentate calcite crystals and blocky calcite. During the mesogenetic burial history, microstylolitisation developed in the packstones. Deposition of the heterozoan carbonates began during the transgressive Coniacian cycle, following a relative sea-level fall recorded in the Late Turonian. Bioclastic sedimentation rates during the Coniacian vary between 74 and 88 m/Myr, a value similar to that recorded in the Holocene Australian and New Zealand shelves. Coniacian-Santonian heterozoan carbonates had a widespread palaeogeographic distribution in south-east France and Sardinia. The availability of hard substrates due to a high hydrodynamic level during the Coniacian transgression was a major cause for the development of the heterozoan association as a whole. Likewise, iron-rich terrigenous supply and upwelling patterns probably enhanced nutrients on the shelf and favoured in turn the development of suspension-feeders and mesotrophic organisms. Possible open marine, relatively deep cool-water conditions were responsible for the deposition of the heterozoan association versus the photozoan rudist association during the Coniacian. This latter association progressively recovered and thrived when shallow marine warm-water conditions re-established during the Santonian. Comparisons are made with modern analogue environments in which heterozoan dominated cool-water carbonate sedimentation occur.