Weathering morphologies of the Fontainebleau Sandstone and ...

M. Thiry Weathering morphologies of the Fontainebleau Sandstone and related silica mobilityWeathering morphologies of the FontainebleauSandstone and related silica mobilityKeywords: sandstone; weathering; morphology; Paris Basin; FranceMédard THIRYGéoscience - École des Mines de Paris35, rue St Honoré, F-77305 Fontainebleaumedard.thiry@ensmp.frIntroductionThe Fontainebleau Sandstone is formed of verytightly cemented sandstone lenses (Thiry &Maréchal 2001) that form spectacular elongatedridges, up to 10 km long and 0.5 km wide.Denudation of the quartzite pans lead to a highlycontrasted landscape, with sandstone ridgestowering the sandy depressions by 50 to 80 m.Weathering of the sandstone brings up a varietyof characteristic morphologies by dissolution ofthe primary quartz cement, but also by precipitationof secondary silica. Silica movement in thesandstone landscape can be followed from centimetricto plurimetric scales. A first aemp to linkthe morphologies with geochemical processes hasbeen done by Thiry et al. (1984).Morphologies of the ridgesThe sandstone ridges are characterized by adiscontinuous sand cover on which grow Calluna,Molinia as well as small birches. Peaty depressionsand small ponds develop between the baresandstones that outcrop like "whale backs". Wetzones and ponds extend to the edge of the ridges,pointing out that the sandstone pans are primarlyfairly impermeable and have no open fractures.At the edges of the ridges, drawing of the sandby erosion brings up yielding and fracturing ofthe sandstone pan. Decametric blocks are releasedand slip along the slope, labyrinth-like widecorridors form between the blocks (Fig. 1 & 2A).The fractured blocks are roughly parallelipipedic.Subhorizontal fractures also develop and cut thesandstone in 0.1 to 0.2 m-tick superposed slabs.These fractures develop only in the lower part ofthe sandstone pan and are strictly limited to theridge escarpments, leading to typical morphologiesof stacked plates (Fig. 2B).Vertical and horizontal fractures are oen coveredwith a lustreous silica skin (Fig. 2D). The silicaskins don’t show in the sandstones exposed inopen pits, they develop only at outcrops and arebeer expressed and thicker towards the edge ofthe ridges. These skins are not true silica depositsbut rather result from silica impregnating thefracture walls with small quartz crystals and/ormicroquartz. Some fractures also show iron oxydedeposits. The silica and iron oxyde deposit in thefractures helps to the impermeability of the ridgesand the development of ponds until their edges.Breakup of the ridgesThe dismantling of the sandstone pan at the edgeof the ridges gives way to masses of rockfall onthe sandy slopes, called rock chaos. These areprismatic blocks at the head and become moreand more rounded, with dome-like outlines,away from the ridge escarpment. Several typicalmorphologies develop.Calcareous sandstone and"sponge-rocks"In places, the upperpart of the sandstone pancontains calcareous nodules related to formercalcareous palaeosols and rhyzolithes. At outcrop,these sandstones develop numerous centi- todecimetric hollows by dissolution of the calciticcement (Fig. 2C). The exposed nodules aredissolved, while those isolated within the siliceoussandstone remain unaltered.Ferrantia • 44 / 200547

M. Thiry Weathering morphologies of the Fontainebleau Sandstone and related silica mobilityRIDGE"whale backs" of bare sandstonediscontinuous sand coverESCARPMENTdrawing of the sand - breakup of theridge - labyrinth-like corridorsSLOPErock chaos – dome-likeboulders with silica crusts50-100 mFig. 1: Schematic sketch showing the dismantling of the Fontainebleau Sandstone ridges by vertical and horizontalfractures and development of the dome-like shaped boulders on the sand slopes.Dome-like morphologies with silicacrustsAer break up, the blocks acquire rounded shapes.In this way, at the ridge escarpment, the sandstoneplates resulting from horizontal fracturing becomerounded at their upper edge, whereas their lowerplane show a prominent lipp-like flange. The silicadeposits formed in the fractures are put into reliefby this weathering.The more massive facies also round out and formdome-shaped boulders at the top of the ridges and onthe rockfall on the sandy slopes. Boulders diminishregularly in size away from the escarpment and thedomes become coated with a 0.5 to 2 cm-thick silicacrust, harder than the sandstone body (Fig. 2E &2F). This silica crust is restricted to the top of thedomes, and is lacking on the lower parts and on theoverhanging surfaces. It is put into relief by differentialalteration at the periphery of the domes.Thin sections across the silica crust show that theovergrowthed quartz grains of the sandstone aresplit off by tiny cracks, from 1 to 20 µm-wide,along the grain contacts, mostly parallel to the crustsurface. Quartz grains are only seldom fractured.These cracks are cemented by brown to clear opalwhich forms the harder and less alterable crust at thetop of the domes. Nor empty cracks, nor cracks withclay or organic-rich illuviations are never observed.This may indicate that opal precipation itself bringsup the disjunction of the quartz grains.Polygonal dissolution groovesPolygonal networks of grooves develop on thedome flanks and on the overhanging surfaces(Fig. 2G). Such polygonal grooves never impressin the silica crusts capping the domes and coatingthe fractures. The network is generally isometric,forming polygones of about 5 to 10 cm in diameter,they may become in horizontal elongated. Inplaces, the layout of the overhanging surfaces ofthe sandstone blocks, and even larger pans, clearlypoint out that these surfaces are related to a formerlandsurface supporting a paleosol which has beenstripped off by erosion. Thus development of thepolygonal networks of grooves may be relatedto processes occuring within the soil and not incontact with the atmosphere.Thin sections show development of large intergranularvoids which come with preferentialdissolution of the quartz overgrowths. Thesevoids remain empty, or may contain tiny quartzchips oen blended with iron oxydes. No preexistingstructure within the sandstone is relatedto the development of the dissolution voids, northe polygonal grooves.Nevertheless, if the development of the polygonalgrooves indicates weathering-dissolution rocesses,the mechanism leading to the polygonal structureremains unknown.48 Ferrantia • 44 / 2005

M. Thiry Weathering morphologies of the Fontainebleau Sandstone and related silica mobilityA ~ 50 m B 1 m C0,1 mD 0,1 m E 0,5 m F0,5 mG0,5 mH0,1 mFig. 2: A. Aerial view of the breakup of sandstone ridges with formation of the rock chaos. B. Subhorizontalfractures at the ridge escarpment, the upper part of the sandstone pan is not fractured and displays dome-likemorphologies. C. “Sponge-rock” resulting from the dissolution of calcareous nodules. D. Lustreous silica skin on anuneven vertical fracture. E & F. Dome-like shaped boulders capped with a well developed silica crust. G. Mushroom-likerock resulting from enhanced sandstone weathering at the level of former soil horizons which have beeneroded. Note the associated polygonal network of dissolution grooves. H. Dissolution bowl showing the induratedsilica lipp at its mouth.Ferrantia • 44 / 200549

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

M. Thiry Weathering morphologies of the Fontainebleau Sandstone and related silica mobilitybut evaporation is nevertheless important on thebare sandstones that become very warm in sunnysummer days.In this manner, development of the typicalmorphologies of the Fontainebleau Sandstoneresults from concomitant and contrasted weatheringprocesses: silica dissolution during wetperiods or seasons alternates with local silicadeposition during times of dryness. Differentdissolution and precipitation mechanisms occurat depth, beneath or in soils horizons, or at thesurface in contact with the atmosphere (Fig. 3).Silica dissolution is enhanced in the soilhorizons by formation of compexes with organiccompounds and lastingless of humidity. Silicadeposition occurs at depth in the sandstone pansby destruction of organic complexes and at surfaceby solution concentration during dryness. Nevertheless,the final mass balance is a massive loss ofsilica which is exported through groundwater.Even on the very steep and unstable sandy slopesbeneath the sandstone ridge escarpements, allthe sandstone blocks and boulders display thesemorphologies, which moreover are in "active"position. The tilted blocks rapidely recoverequilibrium morphologies. The sandstone ering , thus is a current and 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. - Developmentof tightly cemented sandstone lenses inuncemented sand: example of the Fontaine-bleau Sand (Oligocene) in the Paris Basin.Journal of 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 Fontainebleau. 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 deFontainebleau : 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

M. Thiry Weathering morphologies of the Fontainebleau Sandstone and related silica mobilityRésumé de la présentationMorphologies des Grès de Fontainebleau et mobilité de silice associéeMots-clés: grès; altération; morphologie; Bassin de Paris; FranceL’altération des Grès de Fontainebleau conduit à desmorphologies variées par dissolution du ciment dequartz primaire, mais aussi par précipitation de silicesecondaire. Le mouvement de la silice dans le paysagegréseux peut être suivi de l’échelle centimétrique à plurimétrique.Les fractures verticales et horizontales qui seforment en bordure des platières sont souvent couvertespar des placages de silice lustrée n’apparaissant que surles grès à l’affleurement et jamais sur les grès recoupéspar les carrières. Ces placages sont de la silice secondairedéposée en profondeur dans les dalles de grès à l’affleurement,nourris par des dissolutions de silice qui se fonten surface dans les sols podzoliques. A l’affleurement, lessurfaces dénudées des grès acquièrent des morphologiescaractéristiques en dôme. Les dômes montrent deuxmorphologies de surface: des «crevasses» polygonalesde dissolution sur les flancs des dômes et une «croûte»indurée sur leur sommet. Des dissolutions de silice sefont sur les flancs des dômes où l’eau ruisselle, alors quede la silice secondaire, sous forme d’opale, précipite sousla surface sommitale où l’eau des pores se concentre parévaporation. Des vasques de dissolution, circulaires, detaille décimétrique, se forment au sommet des platièresgréseuses et des dômes. Ces vasques, périodiquementremplies et asséchées, montrent aussi deux comportementsdistincts de la silice: dissolution sur le fond descreux et dépôt de silice secondaire dans un bourreletinduré sur l’ouverture de la vasque. Certains blocstournés montrent différentes générations de vasquesde dissolution. Les vasques les plus récentes étant enposition de «vie», alors que les plus anciennes, qui ontsubi une rotation, ne retiennent plus d’eau. Le développementde ces processus d’altération sur des pentessableuses, instables et à forte déclivité, indique que cesaltérations sont récentes et que leur vitesse d’évolutionest rapide.52 Ferrantia • 44 / 2005