Cathodoluminescence and electron probe micro-analysis of silcretes and puddingstones: towards a better understanding of silcretization and sourcing quartzitic materials
Résumé
We have investigated quartz cement in silcretes and puddingstones from uppermost Paleocene to lowermost Eocene, terrestrial to coastal and shallow marine sediments in north France and Belgium using cathodoluminescence (CL) and trace element electron probe micro-analysis (EPMA). The syntaxial cement overgrowing quartz grains exhibits mostly dark to dark-blue and yellow-brown CL In Ti-rich silcretes. The fine-grained cement that is found capping flint pebbles and as illuviated material in intergranular porosity has a typical milky-white to yellow CL. EPMA analyses of Ti, Fe, Al and K indicate that substituting Al and K are enriched in dark-blue CL quartz with most measured Al concentrations ranging from 1000 to 5000 ppm. Detrital grains and syntaxial quartz with yellow-brown CL typically have <100 ppm Al. We interpret the Al-rich syntaxial cement as indicative of silicification under acidic conditions. Sub-aerial exposition of sediments containing both acid-liberating (pyrite) and Al-rich (feldspar and clays) minerals would provide a favourable, but not exclusive mechanism for the formation of silcretes. Our analyses furthermore suggest that silicification occurred under the influence of a fluctuating water table, with restricted chemical mobility during low water table episodes. In Ti-rich silcretes, up to 8% Ti is concentrated along with subordinate Fe and Al in a heterogeneous, microcrystalline cement, which typically forms illuviated structures. This Ti-rich cement predates syntaxial quartz overgrowth and indicates vadose processes as well. The CL and trace-element characteristics of silcretes are potential fingerprints, which could be used for provenance analysis.