Cathodoluminescence and electron probe microanalysis of quartz cements in puddingstones and silcretes from Belgium and northern France

Silicification processes in silcretes and puddingstones are still a matter of debate because of the great variety of potential silica sources in the starting sediment or pediment (quartz grains, biogenic particles, clays and other silicates, etc.) Yet understanding these processes is crucial in deciphering the environmental significance of silcretes and related rocks, especially when they are related to periods of climate change such as the PETM (Paleocene-Eocene Thermal Maximum).
We are currently investigating the quartz cement in silcretes and puddingstones from Upper Normandy, Northern Paris Basin and Southern Belgium using cathodoluminescence (CL) and electron probe micro-analysis (EPMA). These techniques were chosen because of their high sensitivity to the presence of trace-elements (CL) and high spatial resolution (EPMA).
The syntaxial cement overgrowing the quartz grains in quartzitic silcretes (or in quartzitic matrix in puddingstones) has variable CL characteristics. In this cement, quartz most frequently displays dark or yellow-brown CL colors. In only one out of the 17 sample localities a distinct green CL emission was detected (Beaumont sandstone). The more common dark and yellow-brown CL colors are usually distributed in alternating bands corresponding to growth zoning or, less frequently, sector zoning. No general trend was noticed, except that the dark CL is often found as earliest cement. In the Grandglise sandstone, only this dark CL cement is observed and in the nearby Roucourt sandstone, which is related stratigraphically, it is overgrown by a yellow-brown cement. The fine-grained Ti-rich cement, which is found capping flint pebbles in puddingstones or filling intergranular porosity in quartzites has a typical white CL. However, it is not clear whether this is a true CL emission or the result of light reflection due to the presence of minute quartz and Ti-oxide grains. As a first result of this study, the distinction of silcretes based on their CL characteristics opens new perspective in terms of provenance analysis of erratic silcrete blocks and archeological materials.
EPMA analysis focused on trace-elements Ti, Fe, Al and K in cement with different CL emission. This was facilitated by fitting a high-sensitivity CCD camera on the microprobe, which allowed capturing full-color CL micrographs with sufficient resolution with respect to the analytical spot size (ca. 2µm). Results for the Grandglise and Roucourt sandstones indicate that dark CL cements are markedly enriched in Al, with ca. 4000 weight ppm, in K (1000 ppm) and Fe (200-1000 ppm). In the yellow-brown CL overgrowth, Al concentration falls down to ~100 ppm while Fe concentration only slightly decreases. Ti remains under detection limit (40 ppm). As dissolving aluminium requires very low pH, it is hypothesized that acidic conditions prevailed in the earliest silicification stage in these sandstones. Acidification of pore fluids was due to subaerial exposition of the sediment, leading to pyrite oxidation, which is evidenced by the presence of jarosite (Grandglise) or cubic/framboidal molds (Terramesnil). Weathered K-feldspar and glauconite grains are occasionally observed, which accounts for the presence of Al, K and Fe in quartz cement. Other silica sources such as fine-grained clay minerals could have contributed as well. The decrease in Al content in later, yellow-brown CL quartz cement, and the presence of meniscus-like coatings of ferruginous clay is interpreted as an increase in pH with time due to progressive exhaustion of pyrite. This resulted in less aggressive weathering conditions which prevented silicate destabilization and hence silicification.