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Geological Society, London, Special Publications; 2007; v. 275; p. 17-28;
DOI: 10.1144/GSL.SP.2007.275.01.02
© 2007 Geological Society of London

Lower Cambrian shelled phosphorites from the northern Montagne Noire, France

Sébastien Clausen1 & J. Javier Álvaro1,2

1 Laboratoire de Paléontologie et Paléogéographie du Paléozoïque, UMR 8014 CNRS, Université des Sciences et Technologies de Lille, 59655-Villeneuve d'Ascq, France (e-mail: Sebastien.Clausen{at}univ-lille1.fr)
2 Departamento Ciencias de la Tierra, Universidad de Zaragoza, Ciudad Universitaria, 50009-Zaragoza, Spain

Shelled phosphorites of Early Cambrian age are common in the Avène-Mendic autochthonous unit (Marcory Formation) and the Melagues nappe (‘Heraultia beds’ of the Lastours Formation), northern Montagne Noire (France). Palaeogeographically, the concentration of phosphate took place along the shelf edge between a stable inner platform (southern Montagne Noire) and an unstable slope-to-basin sea floor preserved in the northern Montagne Noire. Petrography, back-scattered SEM (scanning election microscopy) and elemental mapping by EDS (energy dispersive system) show that the phosphorites were generated by repeated alternations of low sedimentation rates and condensation forming hardgrounds, in situ early diagenetic phosphogenesis, winnowing and polyphase reworking of previously phosphatized skeletons and hardground-derived clasts. The succession of repeated cycles of sedimentation, phosphate concentration and reworking led to multi event phosphate deposits rich in allochthonous particles. Associated accumulations of exhumed and reworked pyrite clasts reflect final deposition in a mainly dysaerobic substrate.