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Geological Society, London, Special Publications; 1989; v. 46; p. 165-173;
DOI: 10.1144/GSL.SP.1989.046.01.15
© 1989 Geological Society of London

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Oolitization processes in Palaeozoic ironstones of France, Algeria and Libya

J-J. Chauvel & S. Guerrak

Centre Armorician d’Etude Structurale des Socles, Université de Rennes I, Laboratoire de Pétrologie Sédimentaire, 35042 Rennes Cédex, France
Office Nationale de la Géologie, 18A Avenue Mustapha El Ouali, Alger, Algeria

On the basis of the study of Palaeozoic ironstones from France, Algeria and Libya, some oolitization processes are documented, particularly intrasedimentary accretion and ‘snow-ball’ type accretion. In a single ooid, successive layers of the cortex can have been formed by different processes. Chemical analysis of an ooid cortex can help to reconstruct its history. Chamositic layers are always weathered in their outer part, with the leaching of aluminium and magnesium. Haematitic layers result from iron, and often titanium concentration in the weathered outer parts of primary chamositic layers. Crystallization of siderite appears to be late diagenetic, and often took place at the same time as, or after, the development of quartz overgrowths.