M. Thiry <strong>Wea<strong>the</strong>ring</strong> <strong>morphologies</strong> <strong>of</strong> <strong>the</strong> <strong>Fontainebleau</strong> <strong>S<strong>and</strong>stone</strong> <strong>and</strong> related silica mobilityDissolution bowls with silica flangeCircular decimetric-sized bowls oen form on <strong>the</strong>bare s<strong>and</strong>stones flats <strong>of</strong> <strong>the</strong> ridges, as well as on <strong>the</strong>domes (Fig. 2H). These bowls are water filled <strong>and</strong>frequently overflow during autumn <strong>and</strong> wintertime. During spring <strong>and</strong> summer, <strong>the</strong>ir water levelvaries <strong>and</strong> <strong>the</strong>y oen oen oen oen dry up completly. The boom boom boom boomis generally covered by an organic-rich s<strong>and</strong>, <strong>and</strong>water pH is about 5.The mouth <strong>of</strong> <strong>the</strong>se bowls is systematicallyunderlined by an indurated roll. A silica crustalso oen oen lines <strong>the</strong> inside <strong>of</strong> <strong>the</strong> bowls, above <strong>the</strong>mean water level. The boom boom boom <strong>of</strong> <strong>the</strong> bowls, nearlyflat, is devoid <strong>of</strong> silica crust <strong>and</strong> on <strong>the</strong> contraryshows grooves about 1 cm-deep <strong>and</strong> which formpolygonal networks. In this way <strong>the</strong>se smallwea<strong>the</strong>ring features are highly interesting : <strong>the</strong>yshow in <strong>the</strong> same structure silica leaching <strong>and</strong>deposition. Some tilted blocks show several gener-ations <strong>of</strong> dissolution bowls : <strong>the</strong> younger onesbeing in "living" position, whereas <strong>the</strong> older ones,that have undergone <strong>the</strong> tipping, do not longerkeep water.<strong>the</strong> chelates formed in <strong>the</strong> top soils <strong>and</strong> also fromleaching <strong>of</strong> biogenic silica related to <strong>the</strong> vegetationcover. Silica deposition in form <strong>of</strong> opal occurs above<strong>the</strong> ground level, in <strong>the</strong> outcropping s<strong>and</strong>stones .These opal-enriched crusts form at <strong>the</strong> top <strong>of</strong> <strong>the</strong>s<strong>and</strong>stone domes <strong>and</strong> around <strong>the</strong> temporary waterbowls. These deposits most probably originatefrom <strong>the</strong> pore water <strong>of</strong> <strong>the</strong> s<strong>and</strong>stone, pore waterrising up to surface by capillarity <strong>and</strong> concentratingunder severe evaporating conditions that occur on<strong>the</strong> bare stone. The climate in <strong>Fontainebleau</strong> (meanannual temperatures <strong>of</strong> 10.2°C <strong>and</strong> 722 mm rainfallin 180 days)a priori seems not very favourable,s<strong>and</strong>stone ridge1 mSilica mobilty <strong>and</strong> wea<strong>the</strong>ringrateIn <strong>the</strong> <strong>Fontainebleau</strong> Forest, <strong>the</strong> s<strong>and</strong>stonesdo not display any mechanical, nor aeolian orglacial erosion feature. The <strong>morphologies</strong> <strong>of</strong> <strong>the</strong>s<strong>and</strong>stones mainly result from silica dissolution<strong>and</strong> to a lesser extent from silica deposition.The silica dissolution first wears away <strong>the</strong> edges<strong>of</strong> <strong>the</strong> angular fractured blocks, which becomerounded, <strong>and</strong> lastly leads to <strong>the</strong> development <strong>of</strong>very regular dome-shaped boulders. Fur<strong>the</strong>r disso-lution arise with <strong>the</strong> hollowing <strong>of</strong> alveoles <strong>and</strong>bowls. These dissolutions occur on outcroppingblocks, above <strong>the</strong> ground level. O<strong>the</strong>r major lution features are related to <strong>the</strong> formation <strong>of</strong>overhangs which bring up mushroom-like rockstopped by larger domes (Fig. 2G), or even linearconcave features on massive s<strong>and</strong>stone pans. Theselaer laer laer laer features, which oen oen oen oen show typical polygonalgrooves, may form at depth, on buried or partlyburied s<strong>and</strong>stones, in contact with <strong>the</strong> soil. Organiccompounds in soils <strong>and</strong>pound deposits mayfavour silica dissolution <strong>and</strong> chelation (Benne (Bennedisso-1991).In parallel, quartz crystallization occurs at depth,in <strong>the</strong> fractures, oen oen oen oen toge<strong>the</strong>r with iron oxydesdeposition, beneath <strong>the</strong> discontinuous s<strong>and</strong> cover<strong>and</strong> <strong>the</strong> podzolic soils <strong>of</strong> <strong>the</strong> ridges. Due to <strong>the</strong>lack <strong>of</strong> feldspars <strong>and</strong> clay minerals in <strong>the</strong> upperleached <strong>Fontainebleau</strong> S<strong>and</strong>, <strong>the</strong> deposited silicamay mainly originate at depth from destruction <strong>of</strong>s<strong>and</strong>stone boulderdissolution bowlorganic-rich soil<strong>Fontainebleau</strong> <strong>S<strong>and</strong>stone</strong><strong>Fontainebleau</strong> S<strong>and</strong>waterlevel0,5 mprimary shape <strong>of</strong> <strong>the</strong>s<strong>and</strong>stone block0,2 msilica depositionsilica dissolutionsilica migrationFig. 3: Sketches <strong>of</strong> silica mobility during wea<strong>the</strong>ring <strong>of</strong><strong>the</strong> <strong>Fontainebleau</strong> <strong>S<strong>and</strong>stone</strong>. Silica dissolution occurson <strong>the</strong> bare s<strong>and</strong>stone, but is enhanced in contact withsoils <strong>and</strong> in <strong>the</strong> pounds, due to complexation with organiccompounds.50 Ferrantia • 44 / 2005
M. Thiry <strong>Wea<strong>the</strong>ring</strong> <strong>morphologies</strong> <strong>of</strong> <strong>the</strong> <strong>Fontainebleau</strong> <strong>S<strong>and</strong>stone</strong> <strong>and</strong> related silica mobilitybut evaporation is never<strong>the</strong>less important on <strong>the</strong>bare s<strong>and</strong>stones that become very warm in sunnysummer days.In this manner, development <strong>of</strong> <strong>the</strong> typical<strong>morphologies</strong> <strong>of</strong> <strong>the</strong> <strong>Fontainebleau</strong> <strong>S<strong>and</strong>stone</strong>results from concomitant <strong>and</strong> contrasted wea<strong>the</strong>ringprocesses: silica dissolution during wetperiods or seasons alternates with local silicadeposition during times <strong>of</strong> dryness. Differentdissolution <strong>and</strong> precipitation mechanisms occurat depth, beneath or in soils horizons, or at <strong>the</strong>surface in contact with <strong>the</strong> atmosphere (Fig. 3).Silica dissolution is enhanced in <strong>the</strong> soilhorizons by formation <strong>of</strong> compexes with organiccompounds <strong>and</strong> lastingless <strong>of</strong> humidity. Silicadeposition occurs at depth in <strong>the</strong> s<strong>and</strong>stone pansby destruction <strong>of</strong> organic complexes <strong>and</strong> at surfaceby solution concentration during dryness. Never<strong>the</strong>less,<strong>the</strong> final mass balance is a massive loss <strong>of</strong>silica which is exported through groundwater.Even on <strong>the</strong> very steep <strong>and</strong> unstable s<strong>and</strong>y slopesbeneath <strong>the</strong> s<strong>and</strong>stone ridge escarpements, all<strong>the</strong> s<strong>and</strong>stone blocks <strong>and</strong> boulders display <strong>the</strong>se<strong>morphologies</strong>, which moreover are in "active"position. The tilted blocks rapidely recoverequilibrium <strong>morphologies</strong>. The s<strong>and</strong>stone ering , thus is a current <strong>and</strong> rapid weath-phenomenon.ReferencesBenne Benne Benne P. C. 1991. - Quartz dissolution in organic-rich aqueous systems. Geochimica et chimica Acta 55: 1782-1797.Thiry M. & Maréchal B. 2001. - Development<strong>of</strong> tightly cemented s<strong>and</strong>stone lenses inuncemented s<strong>and</strong>: example <strong>of</strong> <strong>the</strong> Fontaine-bleau S<strong>and</strong> (Oligocene) in <strong>the</strong> Paris Basin.Journal <strong>of</strong> Sedimentary Research 71/3: 473-483.Thiry M., Paziera J.P. & Schmi Schmi Schmi J.M. 1984. - Silici-fication et désilicification des grès et des sablesde <strong>Fontainebleau</strong>. Evolutions morphologiquesdes grès dans les sables et à l’affleurement. Bull.Inf. Géol. Bassin Paris, 21/2: 23-32.Thiry M. & Schmi Schmi Schmi J.-M. 2005. - Les rochers de<strong>Fontainebleau</strong> : pourquoi et comment cesformes? Internet: hp://www.cig.ensmp.fr/hp://www.cig.ensmp.fr/hp://www.cig.ensmp.fr/hp://www.cig.ensmp.fr/~thiry/FBL_rochers/fbl-rochers-00.htmCosmo-[15.05.2005].Ferrantia • 44 / 200551
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