Organic facies as a tool for recording eustatic variations in marine fine-grained carbonates—example of the Berriasian stratotype at Berrias (Ardèche, SE France)

Abstract

Sediments of the Berriasian stratotype in southeastern France consist of marine, fine-grained carbonates deposited on an outer shelf at the western edge of the Vocontian Trough. The limestones are monotonous, but display some erosional surfaces and a condensed interval. They are well dated by ammonites and calpionellids, except at the base of the section where the boundary with the Tithonian is not clearly established. Despite their very high carbonate content, the limestones yielded enough organic residue to allow a study of palynofacies variations in time. Organic content consists of two fractions: one is made of terrestrial components, represented essentially by humic fragments of the inertinite and vitrinite maceral groups and accessorily by pollen and spores; the other comprises marine components, essentially dinocysts and accessorily foraminiferal linings. Absence of amorphous organic matter and the oxidized nature of dinocysts (observable in UV light) indicate that aerobic conditions prevailed throughout the Berriasian, probably associated with a low sedimentation rate. The following qualitative and quantitative criteria, indicating distal-proximal trends, were used to convert palynofacies observations into a tentative palaeoenvironmental curve: ratio of continental to marine organic constituents, diversity of dinocysts, degree of biodegradation affecting humic fragments, proportion of blade-shaped inertinite, and finally size, sorting and rounding of equidimensional inertinite particles. The palaeoenvironmental curve derived from organic matter shows oscillations between more proximal (i.e. shallower water) and more distal (i.e. deeper water) conditions. These variations, combined with biostratigraphic information and the few significant sedimentological data, can be interpreted in terms of sequence stratigraphy and compared with the global sea-level curve published earlier. The fairly good correlation between this global curve and that established from palynofacies demonstrates the influence of relative sea-level on the organic facies of the Berriasian carbonates, and the contribution of palynofacies studies to sequence stratigraphy.