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The groundwater lying under the small hills around Les Camoins (Marseilles, South France) is sulphated and sulphurous. For more than one hundred years, this water has been used for the hydropathic establishment of ’’Les Camoins-les-Bains’’. It is mineralized when passing through a rocky massive, formed during the Stampian (Oligocene times), and composed of gypseous and carbonated laminites, in which organic beds and native sulphur were observed.

The study of mineral and organic materials, along the outcrops and into the old sulphur mine, showed that bacterial activity was the most important factor explaining the deposit. The comparison between original sediments and similar deposits generated under actual bioclimatic conditions allowed to propose a naturalistic model for this Stampian sedimentation.

We hypothezise that the deposit was formed in a shallow continental lagoon of sulphated and biologically stratified water, which dried periodically under contrasted tropical climate. Photosynthetic sulphur bacteria activity, resulting in an intracelullar accumulation of elemental sulphur, was, at time intervals, ehanced by sudden, short and strong dystrophic crisis, enriching the sediment with both organic matter and sulphur. During the arid conditions that ended each crisis, elemental sulphur was protected from any microbial reduction by a layer of gypsum.

Such bioclimatic conditions were rarely encountered, and this may explain the scarcity of mining sites for Stampian sedimentary sulphur in France: the four existing are located in the Mediterranean area.