Weathering morphologies of the Fontainebleau Sandstone and ...

M. Thiry Weathering morphologies of the Fontainebleau Sandstone and related silica mobilityDissolution bowls with silica flangeCircular decimetric-sized bowls oen form on thebare sandstones flats of the ridges, as well as on thedomes (Fig. 2H). These bowls are water filled andfrequently overflow during autumn and wintertime. During spring and summer, their water levelvaries and they oen oen oen oen dry up completly. The boom boom boom boomis generally covered by an organic-rich sand, andwater pH is about 5.The mouth of these bowls is systematicallyunderlined by an indurated roll. A silica crustalso oen oen lines the inside of the bowls, above themean water level. The boom boom boom of the bowls, nearlyflat, is devoid of silica crust and on the contraryshows grooves about 1 cm-deep and which formpolygonal networks. In this way these smallweathering features are highly interesting : theyshow in the same structure silica leaching anddeposition. Some tilted blocks show several gener-ations of dissolution bowls : the younger onesbeing in "living" position, whereas the older ones,that have undergone the tipping, do not longerkeep water.the chelates formed in the top soils and also fromleaching of biogenic silica related to the vegetationcover. Silica deposition in form of opal occurs abovethe ground level, in the outcropping sandstones .These opal-enriched crusts form at the top of thesandstone domes and around the temporary waterbowls. These deposits most probably originatefrom the pore water of the sandstone, pore waterrising up to surface by capillarity and concentratingunder severe evaporating conditions that occur onthe bare stone. The climate in Fontainebleau (meanannual temperatures of 10.2°C and 722 mm rainfallin 180 days)a priori seems not very favourable,sandstone ridge1 mSilica mobilty and weatheringrateIn the Fontainebleau Forest, the sandstonesdo not display any mechanical, nor aeolian orglacial erosion feature. The morphologies of thesandstones mainly result from silica dissolutionand to a lesser extent from silica deposition.The silica dissolution first wears away the edgesof the angular fractured blocks, which becomerounded, and lastly leads to the development ofvery regular dome-shaped boulders. Further disso-lution arise with the hollowing of alveoles andbowls. These dissolutions occur on outcroppingblocks, above the ground level. Other major lution features are related to the formation ofoverhangs which bring up mushroom-like rockstopped by larger domes (Fig. 2G), or even linearconcave features on massive sandstone pans. Theselaer laer laer laer features, which oen oen oen oen show typical polygonalgrooves, may form at depth, on buried or partlyburied sandstones, in contact with the soil. Organiccompounds in soils andpound deposits mayfavour silica dissolution and chelation (Benne (Bennedisso-1991).In parallel, quartz crystallization occurs at depth,in the fractures, oen oen oen oen together with iron oxydesdeposition, beneath the discontinuous sand coverand the podzolic soils of the ridges. Due to thelack of feldspars and clay minerals in the upperleached Fontainebleau Sand, the deposited silicamay mainly originate at depth from destruction ofsandstone boulderdissolution bowlorganic-rich soilFontainebleau SandstoneFontainebleau Sandwaterlevel0,5 mprimary shape of thesandstone block0,2 msilica depositionsilica dissolutionsilica migrationFig. 3: Sketches of silica mobility during weathering ofthe Fontainebleau Sandstone. Silica dissolution occurson the bare sandstone, but is enhanced in contact withsoils and in the pounds, due to complexation with organiccompounds.50 Ferrantia • 44 / 2005