Abstract

Two sedimentary profiles, at 18 km and at 125 km from the Rhône River mouth were selected to study the steps of glauconitization inside green (glaucony) grains, in a temperate marine environment. The proximal site is characterized by a greater content of alpine minerals (chlorite, illite) than the distal one. In the proximal site, pellets at a younger level demonstrate a largely inherited mineral composition. In the older level, representing the most advanced stage of glauconitization, there is a mixed-layer glauconite-nontronite (G-N) phase with 20% glauconite-80% nontronite in dark green grains. In the distal site, more evolved glaucony has up to 92% glauconite in the G-N. In spite of this difference, the glauconitization process developed similarly in the two sites. The relatively low Fe content in the mud-matrix does not reflect the concentration of this element inside the grains. During diagenesis, most of the inherited minerals disappear, making way for neoformed mixed-layer glauconite-nontronite.

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