Abstract
In the Fontainebleau Sand (Oligocene), superposed lenses of quartzite are restricted to outcrops and structurally low zones of the formation. The quartzite lenses result from a silicification process controlled by water-table levels during the recent geomorphologic evolution of the landscape. The quartzite lenses pinch out rapidly under the limestone-capped plateaus and are related to the development of a bleached sand profile. Above the water table, the sand is bleached, and the former quartzite lenses are corroded along their margins. Moreover, flint pebbles are dissolved, and primary smectite and illite are weathered to kaolinite. Dissolved silica can thus reach supersaturated levels, allowing quartz precipitation. Quartz grains are overgrown at the base of the profile in zones of ground-water outflow related to spring lines. The adjustment of the water table during the landscape incision led to formation of quartzite lenses at deeper levels. This silicification process has no climatic significance but is dependent on the stability of the landscape and on its progressive dissection. The rate of quartz cementation, indicated by geomorphologic considerations, is consistent with hydraulic and geochemical calculations; it appears to be very rapid, approximately 30,000 yr for a quartzite lens.
