Development of tightly cemented sandstone lenses in uncemented sand: Example of the Fontainebleau Sand (Oligocene) in the Paris Basin

In the Fontainebleau Sand (Oligucene), superposed lenses of sandstone resul t from a silicification process controlled by the watertable during the rec ent geomorphologic evolution of the landscape. The most outstanding feature of these silicified bodies is the contrast they show between the very hard , tightly cemented sandstone and the Loose and permeable embedding sand, Th is pattern raises the question of the growth mechanism of the lenses. The lenses are composed of tightly quartz-cemented sandstone with well-deve loped quartz overgrowths. Cathodoluminescence of the sandstones shows detri tal grains with subeuhedral quartz overgrowths and isopachous quartz rims s urrounding the detrital grains or the overgrowths, The isopachous rims sugg est amorphous or poorly ordered silica deposits that later recrystallized i nto quartz, The syntaxial over-growths and the silica deposits alternate in a sequential way on a centimeter scale and reflect variations in the physi cochemical characteristics of the feeding groundwater, The mechanism of the silica deposition is probably complex and can only be hypothesized. Whatev er the mechanism, it appears from the arrangement of the sandstone lavers t hat silica precipitation occurred near the watertable and at the interface between regional groundwater and local recharge water. Silica precipitation along an interface may explain the sharp boundary between cemented sandsto ne and Loose sand. The cementation was modeled with the coupled reaction-transport code METIS. The model was constrained with permeability values, hydraulic gradient, an d dissolved silica contents measured in the Fontainebleau Sand, The simulat ion reproduced the characteristics and morphologies shown by the sandstone lenses in the field, It shows the importance of a high groundwater how rate to provide the silica necessary for cementation of the sandstones in a tim e span compatible with the geological constraints. The conventionally accep ted kinetics of quartz precipitation did not result in simulating cementati on of the sandstone lenses in a geologically reasonable time frame. To over come this constraint, it was necessary to increase the kinetic reaction rat e about 1,000 times, which agrees with the amorphous silica deposits observ ed in the Fontainebleau Sandstones.