The Fontainebleau Sandstone: bleaching, silicification and calcite precipitation under periglacial conditions

Médard Thiry (1) , Romain Millot (2) , Christophe Innocent (2) , Christine Franke (1)

1 GEOSCIENCES - Centre de Géosciences
2 BRGM - Bureau de Recherches Géologiques et Minières (BRGM)

It will be show that the typical Fontainebleau Sandstone facies are in fact secondary facies, developed post‐deposition, and related to surface exposure of the formation. Quartzite pans, white sands, and even the mineralogical composition are related to subsurface (groundwater) alterations connected to geomorphology and result from exposure during the last glacial periods. The Fontainebleau Sandstone example is extremely spectacular. Yet, such alteration features also exist in many other formations and may extent to 50‐100 m depth. One has to be attentive to recognize them. The goal of the field trip is to bring insight into what is related to primary and to secondary processes in the outcropping formation. This is of prime/particular importance for better understanding of the geology of the Fontainebleau Sandstone … and the geochemical processes that led to make up theses exceptionally pure sandstones mined to be used in all high quality and luxury glass manufactories throughout Europe. and supply guides for prospecting such sand ores. But in a general way, when such alteration happen, it is essential to recognize them before any geochemical study because interpretations, in particular isotopic interpretations may completely diverge as a primary or a secondary hypothesis is envisaged for some facies … like marine deposits vs groundwater precipitation, dry and warm vs glacial climates, sedimentary deposition vs acidic leaching, etc.

Mots clés: sandstone; bleaching; silicification; weathering; groundwater; sand calcite; Fontainebleau; France

Domaines: Sciences de la Terre

Médard Thiry, Romain Millot, Christophe Innocent, Christine Franke. The Fontainebleau Sandstone: bleaching, silicification and calcite precipitation under periglacial conditions. [Research Report] RS150901MTHI, Centre de Géosciences, Ecole des Mines de Paris, Fontainebleau, France. 2015, 26 p. ⟨hal-01236712⟩