Relations between shallow cataclastic faulting and cementation in porous sandstones: First insight from a groundwater environmental context
Résumé
The interplay between fault zone cataclasis and cementation is important since both processes can drastically reduce the permeability of faults in porous sandstones. Yet the prediction of fault cementation in high-porosity sandstone reservoirs remains elusive. Nevertheless, this process has rarely been investigated in shallowly buried faults (<2 km; T°<80 °C) where its sealing capacity could be acquired early in the geological history of a reservoir. In this paper, the macro- and microscopic analysis of a fault zone in the porous Cenomanian quartz arenite sands of Provence (France) shows that silica diagenesis occurs in the most intensely-deformed cataclastic parts of the fault zone. This fault zone shows 19–48% of its total thickness occupied by low-porosity quartz-cemented cataclastic shear bands whose porosities range from 0 – ca. 5%. The analysis of the weathering profile around the fault zone reveals the presence of groundwater silcretes in the form of tabular, tightly silicified concretions cross-cut by the fault. Detailed transmitted light, cold-cathodoluminescence and scanning electron microscopy analyses of the silica cements (from the fault and the silcrete) reveal that all the silica cements originate from groundwater diagenetic processes. This study therefore shows that silica cementation can occur specifically in fault zones and as groundwater silcrete in the shallow context of a groundwater system, generated at the vicinity of an erosional unconformity.