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Journal of the Geological Society; 1980; v. 137; issue.6; p. 705-721;
DOI: 10.1144/gsjgs.137.6.0705
© 1980 Geological Society of London

Article

Geochemistry of phosphatic chalks and hardgrounds from the Santonian to early Campanian (Cretaceous) of northern France

Ian Jarvis

Phosphatic chalks occur in erosional troughs cut into soft white chalks of late Coniacian to Santonian age and based by strongly lithified and mineralized hardgrounds. The dominant mineralogy consists of low-Mg calcite, carbonate-fluorapatite, {alpha}-quartz, glauconite and iron oxides. Lithification is geochemically discernible ~90 cm below the surface of the basal hardgrounds and results in a calcite-associated enrichment in Fe, Mn and Mg but depletion in Sr. Factor analysis indicates 4 major controls on the element distributions—carbonate-fluorapatite, clay minerals and silica, lithification, and iron minerals. White chalks are geochemically similar to modern coccolith oozes but hardgrounds have been indurated by the addition of a primary low-Mg calcite cement. Glauconitization was generally restricted to replacement of clay minerals during the early stages of hardground development while lithified carbonate remained in contact with seawater. Phosphatization occurred by the replacement of low-Mg calcite during burial of the hardground by an organic rich sediment. Mineralization was intimately related to organic processes.





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